Author Archives: jtw13

Thanks But I’m


Last month marked the 20th anniversary of the publication of Cape Cod Light, a book of poetry by Michael Hattersley, who was my uncle. The cover is a painting by his father, Van, after a painting of Cape Cod Bay.

MICHAEL E. HATTERSLEY

You’ve probably never heard of it. It had a small run, and it’s not in print, so you can’t get a copy even in today’s age on on-demand printing (perhaps someone knows how to change that?).

But in my family we all have a copy, and when one of us quotes a favorite passage the others usually recognize it.

Michael would have been 70 right now, which was an age he had no interest in experiencing. His friend Richard Schneider wrote a piece in memoriam Michael in the Gay and Lesbian Review, where Michael was a frequent contributor. It wonderfully captures what it was like to know Michael. After school in his smoke-filled room in my family’s condo in Boston while the MacNeil Lehrer News Hour droned in the background, and in long discussions over the holidays under the cathedral ceiling in his living room in Provincetown, he taught me what it meant to be an academic, a thinker, and a moral being.

The first poem in the book was written the Christmas before the book was published, and is about turning 50 (a birthday less than a decade in my own future.) His cranky impatience with the ritual commemoration of the passage of time is interrupted by one of his cats—a metaphor, perhaps, for his impulse to write the poem, or the book. It ends in haiku-like meditation on the ambience of winter on the Cape.


Thanks But I’m

Just as happy being left alone here
Believing all dates on the calendar
Haven’t been labeled yet. Today is:
Inventing your birthday again for the fiftieth time
Unless it can be resolved by stealth.
But wait. A large animal
Has entered the picture, intent
On bearing witness by its presence.
It composes itself like a cat,
Means: late in the year,
Evening. Final quarrelsome squibs of sun.
Make something permanent of, say,
The heat cranking up,
A snowplow grating by in the dusk.

Planet-Planet Tides in TRAPPIST-1

Tides are complicated and hard.  When a planet orbits a star, the star raises a bulge on the planet’s surface.  As the planet rotates, this bulge moves across the surface, and because rocks and oceans and atmospheres are viscous this dissipates energy.  The energy comes out of the planet’s rotation, which spins it down.  This is likely why Venus and Mercury rotate so slowly.

Diagram from wikipedia

The planet can also raise tides on the star (as can a moon on its planet), and that tidal bulge gets carried around by the central object’s rotation.  This spins down the central object, and can make the planet (or moon) move inward or outward in its orbit.

Finally, if the planet is in an eccentric orbit, then the magnitude of the tides change.  This dissipates energy, which ultimately comes from the planet’s orbit. This makes the planet slowly circularize its orbit.

Put it all together, and this is why the moon is synchronously rotating around the Earth, moving in a (nearly) circular orbit, slowly moving away from the Earth, and why the Earth’s rotation is slowly slowing down.

Things can get really complicated if you have lots of moons.  The moons of Saturn and Jupiter tug on each other gravitationally, exciting eccentricities that drive tides and keep the bodies warm inside, leading to liquid oceans and volcanism despite the very cold temperatures they would have if they were only heated by sunlight.  Solving all of the resonances and tidal effects in these systems is still an active area of research.

TRAPPIST-1 is a system of at seven close-in planets tightly packed in orbital resonances, much like the moons of Jupiter.  In fact, they have more in common in terms of scale with the Galilean satellites than the Solar System planets:

Comparison of the TRAPPIST-1 system with the inner Solar System and the Galilean Moons of Jupiter

From European Southern Observatory

I think that the TRAPPIST-1 system is exhibiting yet another effect that we haven’t seen before: planet-planet tides. The planets in this system are very tightly packed, and quite massive (much more than the Solar System moons are).  It’s likely that eccentricity tides have made their orbits circular (they have very low eccentricity) and that they are synchronously rotating with the star (or stuck in a spin-orbit resonance of some kind, like Mercury is).

But if that’s true, then when neighboring planets lap each other, the strain caused by their mutual tides are actually of similar magnitude to the strain caused by the eccentricity tides with the star. In fact, if our estimates of the planet masses and eccentricities are right (they’re pretty uncertain, so this is not necessarily a safe assumption) then planet f causes significant tides on planet g that are actually more important than the very small variations in that planet’s tides raised by the star.

This isn’t the case for any moons I know of in the Solar System: although they are more tightly packed than the TRAPPIST-1 system, the Galilean satellites are so much less massive that their tides just can’t compete with that of Jupiter.  The mass ratio for the TRAPPIST-1 system is much more favorable, so the tides may matter.

Now, planet-planet tides can’t matter too much.  If they did, the planets would not be synchronously rotating with their orbits, which would make the stellar tides orders of magnitude stronger, and then the planet-planet tides would be a minor effect.  But if the planets are in a synchronous rotation state, then I think the planet-planet tides at the very least have to be included in the calculations.

After doing some rough reality checks with Eric Ford and Andrew Shannon, I fired up the old RNAAS template at Overleaf and shot another Research Note on the topic off.  You can find it here.

I hope this is right!  This is way outside my training so I’m worried I missed something obvious…

 

The Cosmic Haystack

I taught the first Penn State graduate course in SETI last spring, and I thought it went really well. We read a lot of papers, had great discussions about Schelling Points and all the different ways to search, and got to visit Green Bank to conduct original SETI observations. You can read all about the course at the class website here, along with student reactions to the papers (tweeted out by Sofia Sheikh here).

One of the cuter terms in the syllabus was that any student whose final project got published in a refereed paper would get a retroactive ‘A’. Well, the first automatic ‘A’ has arrived!  Shubham Kanodia and Emily Lubar, following a suggestion by Jill Tarter, calculated rigorous volumes of the Cosmic Haystack, and the paper is now on the arXiv.

Some highlights:

The first instance of the term “Cosmic Haystack” to mean the space through which SETI searches to find alien technology is apparently from this article in the Christian Science Monitor in 1977, as tracked down by @spacearcheology:

It appears in a paper 4 years later in Wolfe et al. (1981) in a NASA report, which includes this figure (drafted by Jill Tarter):

The Cosmic Haystack here is 3 dimensional, “compressed” down from many more for clarity. The rectangles inside the haystack represent relatively narrowband searches that were either sensitive or included many targets (it’s hard to do both).

Today, wide-field instruments and broadband receivers and backends can search the haystack for alien “needles” much faster. The question is: how much have we searched? Some have gotten the wrong idea about the whole endeavor, as we explain in our abstract:

Many articulations of the Fermi Paradox have as a premise, implicitly or explicitly, that humanity has searched for signs of extraterrestrial radio transmissions and concluded that there are few or no obvious ones to be found. Tarter et al. (2010) and others have argued strongly to the contrary: bright and obvious radio beacons might be quite common in the sky, but we would not know it yet because our search completeness to date is so low, akin to having searched a drinking glass’s worth of seawater for evidence of fish in all of Earth’s oceans.

Shubham and Emily decided to construct a radio Cosmic Haystack that included transmitters anywhere, not just around nearby stars, akin to NASA’s old SETI Sky Survey from the 1980’s. We also tried to capture the way that very broadband signals might be detected, and not just the narrow-band signals considered by some of the earliest searches. This required us to include transmission bandwidth as a haystack dimension, and calculate a rough sensitivity function in terms of it. We also wanted to make sure that our haystack boundaries were well defined, and that we were calculating only search space within a well-defined range of parameters, which led to some interesting integrals with lots of cases to consider:

Sensitivity as a function of transmission bandwidth and central frequency for an idealized broadband instrument, with a “ceiling” set by the haystack boundary.

Case six, for very broadband, very bright transmissions.

The bottom line was that we got to calculate what region of parameter space has been searched for transmitters in the Milky Way.  It’s tiny.  Jill Tarter once estimated it as a glass of water out of all the oceans in the world; we got more like a bathtub (despite a much different haystack construction):

We found that the MWA low frequency searches dominated our haystack search volumes, because of their incredible étendu: a very wide field and very good sensitivity. And they did it in just a couple of hours of searching!

We also construct another one-parameter way to calculate search completeness: what density of uniformly spaced megawatt transmitters in the Milky Way can we rule out?  We get about one per 0.27pc for the latest Breakthrough Listen paper (for L-band transmitters). The Galaxy could be filled with more transmitters than stars, and we wouldn’t have found them yet!

So we haven’t done much searching, and people should not use the “failure” of radio SETI to date as evidence that we should stop looking.

But it’s also important not to let the pendulum swing to far the other way! One might see numbers like -18 in the exponent and declare the entire project hopeless.  On the contrary:

  1. New radio technology allows us to search through haystack volumes much faster than before; the Breakthrough Listen program is quickly catching up to the historical NASA programs by our metric, and MWA needed only 2 hours to dominate our calculation.
  2. If we expect to find transmitters near stars, our completeness is many orders of magnitudes higher, because we have concentrated our efforts there
  3. We don’t need to search the entire haystack unless there is nothing to find.  Another way to look at it: you don’t have to drain the oceans to find marine life, unless they are sterile and you are trying to prove that.  A bathtub’s worth of water is probably not enough to find a fish if you randomly sample the ocean, but if you look in the right places it’s about the right order of magnitude of search required.

Hopefully our paper will be a useful extension of Jill Tarter’s work along these lines (see here and here).

We also had a lot of fun figuring out where the phrase “needle in a haystack” comes from which I detailed here. (it’s not Cervantes!)

Finally, a note on how just as every journey begins with a single step, every search begins with comically weak upper limits.  We quote a critical referee on Bachall & Davis’s first neutrino search paper (an upper limit):

Any experiment such as this, which does not have the requisite sensitivity, really has no bearing on the question of the existence of neutrinos. To illustrate my point, one would not write a scientific paper describing an experiment in which an experimenter stood on a mountain and reached for the moon, and concluded that the moon was more than eight feet from the top of the mountain. (Bahcall & Davis 1982, p. 245)

which I also discussed back here. Waste not want not!

 

 

What do SETI terms mean? A committee weighs in.

Inspired by a paper by Iván Almár’s paper on SETI terms:

https://www.sciencedirect.com/science/article/pii/S0094576509003816

I wrote down my opinions on the topic and recommended a taxonomy of SETI in this paper, which I describe in my blog post hereSofia Sheikh presented my suggestions at the Decoding Alien Intelligence workshop:

One of my recommendations was to organize SETI in a taxonomy like this:

After the talk, Frank Drake(!) stood up and recommended that there be an ad hoc committee to discuss SETI terms and recommend what they mean (thanks to Michael Oman-Reagan for providing me the audio!). So, in anticipation of the upcoming NASA Workshop of Technosignatures, in Houston next week, I convened such a committee, including Sofia and me, Jill Tarter, Iván Almár, Kathryn Denning, and Steven Dick.

We didn’t agree on everything, but we came up with a lot of good recommendations and clarifications that I hope NASA will find useful. We did not go with “Communication SETI” because the committee thought it was too reminiscent of METI, and we did not recommend against “civilization” or “colonize” (mostly because both are in wide use and we did not want to endorse major changes without very good reason) but we did note the problems with those terms.

Our final report is here:

https://arxiv.org/abs/1809.06857

and Sofia will present it at the workshop on Wednesday!

Measuring Rocky Exoplanet Compositions with Webb

Back in June 2016 I advertised a postdoctoral position between me and Steve Desch at ASU for someone to work on the problem of exoplanet interior compositions.

Normally, the way we determine the interior composition of exoplanets is a combination of inference, measurement, and guesswork.  In the Solar System we can study the surface compositions of planets directly, get the bulk density by dividing masses by radii cubed, and a sense of internal structure by looking at how the surfaces have changed or what a body’s gravitational field says about the interior mass distribution. It’s amazing how much we can determine about, say, the interior of Europa!

Evidence from NASA’s Galileo mission suggested that there might be a liquid water ocean underneath Europa’s icy crust. Image credit: NASA/JPL

On Earth we can also use seismology to directly probe the density and composition of the Earth’s interior.  Some things we still don’t know to great precision (the water content of Earth’s mantle is still pretty uncertain, though it’s small) but in general we have a good sense of what’s down there, despite having a very limited set of actual samples from far below the surface (from volcanos, mostly).

Based on this we have a good sense of how the planets formed, and so we can make good inferences and guesses about the parts we don’t know about yet. For instance, we suspect that Jupiter formed around a rocky core of about ten times the mass of the Earth; one of the purposes of Juno is to see if we can better determine Jupiter’s inner composition and perhaps check this model prediction.

This artist’s concept shows the pole-to-pole orbits of the NASA’s Juno spacecraft at Jupiter. Image credit: NASA/JPL-Caltech/SwRI

Exoplanets are much harder.  We generally can measure their masses from radial velocities and radii if they transit, and for planets for which we have both we have bulk densities. By analogy with Solar System planets we can then guess that they have similar compositions to those, and from the compositions of their host stars we can make inferences about how they might differ. But getting anything like a mantle or surface sample seems impossible.

Except…

There is a whole class of planet recently discovered by Kepler (Saul Rappaport and Roberto Sanchis Ojeda did a lot of the pioneering work here) that appears to be evaporating. The transit signatures of these things is amazing; they vary in depth (sometimes disappearing entirely!) and don’t have the usual shape of transiting planets. Apparently, these are small rocky bodies  (which have orbital periods of less than a day and which are too small to see transit at all) have gigantic, variable tails of rock that is condensing from a plume of surface material evaporated by the intense heat of their parent star.

Screen shot 2013-03-09 at 5.24.42 PM.png
Transits of KIC 12557548, from Fig. 2 of Rappaport et al. 2012
Here’s an artist’s impression:

Exoplanet KIC 1255b orbits its parent star followed by a comet-like dust tail. Image credit: Maciej Szyszko.

So by now maybe you’ve spotted the opportunity: that background star is a great lamp to pass light through that material so we can figure out what it’s made of!  Is it mantle material? Is it core material?  It it hydrated?

Studies of white dwarf “pollution” can measure the bulk, relative elemental abundances of material from presumably rocky planets that have crashed into the white dwarf, but here we have the opportunity to study chemical composition of one particular layer of a distant rocky exoplanet—we can’t even do this for the Earth!

But will it work? Are the transits deep enough? Can we distinguish different minerals with spectroscopy? Are there any stars bright enough for this?

Yes to all.  ASU/NExSS postdoc Eva Bodman has all the details in her latest paper. It’s an exciting time!

Eppur si muove

We sometimes read in the history of astronomy that it was Bessel that finally proved the Earth moves around the sun with the measurement of the parallax of 61 Cygni, one of the brightest and closest stars to Earth in the Northern Hemisphere.

This is because people early on realized that if the stars are different brightnesses because they are at different distances, and if the Earth really does move around the Sun, then we should see the nearby ones appear to move with respect to the background ones annually as our light of sight towards them changes.  The effect is quite small, and it was challenging for 18th and 19th century astronomers to measure the tiny effect using only their eyes to make measurements (the biggest parallaxes are like a part in a million).

And so astronomers put a lot of effort into this, and indeed eventually Bessel pulled it off. But the clinching observational proof actually came about 100 years earlier, based on those same observations!

Astronomers were expecting to see a “reflex” motion: when the Earth moved in one direction the nearby stars would appear to slightly move in the opposite direction, so when the Earth was at one extreme of its orbit they would appear to be a bit closer to the center of Earth’s orbit (i.e. the Sun) than they should be.  Instead, astronomers kept measuring a much larger than expected motion (around 20 arcseconds instead of less than 1 arcsecond) in the wrong direction: the stars seemed to move towards a point about 90 degrees away from the Sun, towards the ecliptic.

This is what was actually happening, but it was actually pretty confusing at the time.  One way they were measuring positions was by using the zenith as an absolute direction (you can use a plumb or a liquid to determine which direction is straight up) and a telescope to see how close stars got to the zenith as they passed overhead.  So they could only measure one component of the star’s motion, so all they knew is that it was large and had the wrong phase (90 degrees from what was expected).

What was actually going on is that the stars were suffering from aberration. Imagine you have a trash can an you want to collect as much rain as possible.  If there is no wind, you should just keep the can vertical.  But, if you are on the bed of a pickup truck moving at 10 mph, then some of the rain that would have fallen into the can will hit the side instead.  To maximize rain collection, you have to tip the bucket towards the front of the truck (i.e. in the direction of the truck’s motion) to get the rain to go straight down into the can.

Wikicommons illustration of the aberration of starlight. Original here. By Brews ohare, CC BY-SA 3.0.

Similarly with telescopes: to get starlight to go straight down the optical axis of the telescope, you actually have to “lead” the motion of the Earth slightly by pointing in the direction of Earth’s motion.  This effect is equal in radians to the speed of the Earth’s orbit divided by the speed of light, or about 20″.  This is what was being observed, and this is what Bradley finally figured out in 1727.

You can learn more about this history here. Interestingly, Bradley’s original paper describing the aberration and proving the Earth moves has only one citation in ADS!

So while hunting for the proof of Earth’s motion, astronomers actually discovered a much easier to find proof of Earth’s motion!  But it took decades to understand it.

Today, it’s tricky to repeat these measurements with modern equipment. Finding the zenith to a precision of 1″ is not something most observatories are set up to do; we almost never use high precision, absolute positions with respect the ground in astronomy any more (our instruments tend to change their pointing with temperature, humidity, and other factors, so we calibrate them on actual star positions every now and again to keep our pointing models accurate and precise).

But interestingly, there is a way most college observatories and many amateur ones can find an absolute position: star trails!  By turning off tracking and letting the stars trail, one can identify the center of the trails as the true Celestial Pole.  As the Earth goes around the Sun, one can measure star positions with respect to that and detect the aberration, thus proving the Earth orbits the Sun.

Of course, there are lots of other ways to do this: you could build an R~10,000 spectrograph and feed it light from stars on the ecliptic and measure the Doppler shift caused by Earth’s motion, or you could measure the parallax directly with differential astrometry, like Bessel did. But this is a novel solution that actually doesn’t require special equipment of good seeing, just an ordinary camera.

I was going to try this someday, and even wrote up a whole blog post about it, but finally decided that if I haven’t started by now I probably never will, so I’ve “given the idea away” in the form of a Research Note to the AAS (my sixth!).  One reason I never started is that although the equipment needed isn’t special, there are lots of complications.  One is that the Poles move on their own (due to Earth’s axial precession) so you have to remove that effect first.

As part of my plan to learn Python and Astropy I tried to make a figure showing how the apparent position of the true Celestial North Pole moves with respect to the background stars, showing the precession and the aberration.  It was surprisingly tricky!  Plotting things near coordinate singularities is not something most plotting software does well, so in the end I cheated and just plotted things as a plane chart in ecliptic coordinates and drew the Celestial coordinates in by hand.  I think it came out really well:

Figuring out how to calculate the motion of the Celestial Pole (also affected by nutation) was tricky; it turns out Astropy does not expose those functions to the user so I had to cheat.  In the end, I did it two ways that gave the same answer: I asked for the ICRS (astronomical) coordinates of the zenith at the North Pole of Earth.  Astropy converts from geocentric coordinates to barycentric coordinates by correcting for the orientation of the Earth (the axial motion) and the aberration, so this yields the green curve above.  Almost equivalently, one can ask for the Celestial Pole in CIRS coordinates (the intermediate coordinate system between the Earth and Celestrial Sphere) at many times and then ask Astropy to convert these positions to the ICRS frame.   The latter is faster.

To learn more, you can read my research note describing the experiment here.  And if you try this, please let me know!

 

A Needle In A Haystack

Where does the old idiom “finding a needle in a haystack” come from?

According to my physical copy of Bartlett’s Familiar Quotations, the phrase originates with Cervantes in Book III Chapter X of Don Quixote, and indeed most searches online state so with authority.

But I just checked my physical copy and the Project Gutenberg version and not only is there no  Book III, the phrase does not appear!

Some sleuthing of other phrases that do appear reveals that “Book III” refers to what is normally called “Part II”, and indeed there in Chapter X of my copy it reads:

…tracking Dulcinea up and down El Toboso will be as bad as looking for a needle in a haystack or for a scholar in Salmanca.

just as promised.  So why isn’t it in the Project Gutenberg version? There it reads:

looking for Dulcinea will be looking for Marica in Ravena, or the bachelor in Salamanca.

which is not the same thing at all. Indeed, the Spanish original seems to read:

buscar a Dulcinea por el Toboso como a Marica por Rávena, o al bachiller en Salamanca.

So the phrase is actually not in the original!  It seems to be due to Cervantes’ English translators who used the phrase as a more familiar [to English ears] version of “to find Maria in Ravena”.  Bez Thomas helped me to figure this out on the Twitters:

So where does the phrase “needle in a haystack” originate?  The OED has two attestations that predate Cervantes:

c1530   T. More Let. Impugnynge J. Fryth in Wks. (1557) 837/2   “To seke out one lyne in all hys bookes wer to go looke a nedle in a medow.”
1592   R. Greene Quip for Vpstart Courtier sig. Ev   “He…gropeth in the dark to find a needle in a bottle of hay.”

Where a “bottle” here means “bundle”.  Apparently the translators were using a 100+ year old phrase!   The “haystack” version is from later:

1779   W. Rogers in J. Sullivan Jrnls. Mil. Exped. (1887) 262   “But agreeably to the old adage it was similar to looking for needles in a hay stack.”

And there you have it.  Even an authority as solid as Bartlett’s occasionally gets things wrong, so it’s good to check!

 

[Update: Apparently Bartlett’s is full of these errors with respect to Don Quixote: see this article here which details the “haystack” mistake and many more.]

Smooth continuum stars and RNAAS

We can tell what stars are made of by the colors missing from their spectra, but that’s not really true for hot, rapidly rotating stars. These stars lack convective envelopes, so they lack magnetic dynamos, so they do not spin down as they age. As a result of their rapid rotation, the Doppler shift blurs out their lines and its hard to get a precise measurement of their abundances (except hydrogen, whose lines are so deep you can’t miss them, even blurred out).

But one astronomer’s trash is another’s treasure. These rapidly rotating stars make great sources of light for calibrating spectrographs because you can be sure that any spectral features you *do* see are due to your instrument, not the star. And these stars can be very bright, so it’s a quick test.

The problem is that not all hot stars rotate quickly enough to be “good” calibrators. For instance, here’s what a small portion of a rapid rotator looks like:
This star’s spectrum is flat (or slightly sloped) in this small region of the blue.  The overall mountain shape is the response of the spectrograph, which this star lets you model.  The “fuzz” is photon noise—by chance some channels get more photons than others.  The spike in the middle is a “cosmic ray” event—a high energy particle from somewhere in the dome struck the detector and caused a spike.  The only thing here that’s due to the star are some barely perceptible wiggles.

Here’s a “bad” calibrator star, that is not spinning fast enough to be a “good” calibrator:

Not smooth at all!  Those “bites” taken out are due to elements in the star’s atmosphere absorbing certain shades of blue light, and the bowl shape is due to the way different parts of the stellar surface are moving towards and away from us as the star rotates.

So which stars are “good” and which are “bad” for calibrating high resolution spectrographs Published values of their rotation speeds turn out to be an unreliable guide for this, so observers over the years make lists of “good” hot star calibrators.  For instance, when I need a “good” hot star, I ask Howard Isaacson at Berkeley, who has a list carefully compiled by Kelsey Clubb.  At Berkeley, Kelsey Club went through the California Planet Survey’s library of hot star spectra and separated the wheat from the chaff, which is really useful!

This sort of list isn’t usually publishable—it’s not the sort of scientific advance or discovery that usually warrants a peer-reviewed paper.  But it is the sort of thing scientists should share and that Kelsey should get credit for.

Now, thanks to the new AAS journal “Research Notes of the American Astronomical Society”, we have a good way to share the list.  This new journal is not peer reviewed, but it is free, curated, and has a science editor who accepts papers. They can only be 1,000 words, have one figure or table, and they do not have to be new or novel or anything—just interesting.

But why not just put it to the arXiv, and skip the 1,000 word limit thing?  Well, Geoff Bower asked the same thing on the Twitter, and I came up with two big reasons: RNAAS will curate machine-readable tables, which is great, and as a AAS journal, if your result is (unlike this one) newsworthy, it might get picked up by AAS Nova.  Editor Chris Lintott points out a third:

Anyway, as a journal that emphasizes utility and curates tables, it is the perfect place for Kelsey’s list, so that’s where we published it.  You can find it here.

I was actually worried this would happen.  When RNAAS came out, and then when Overleaf linked directly to it for submissions, I got worried I’d like it too much:

and (despite my misspelling of RNAAS) I was right.  I’ve now submitted or supervised five of these. Here are the others:

Tabby’s Star Explanations

‘Oumuamua Is Almost Certainly Interstellar

EPRVIII Instruments

Barycentric Corrections in Python

I may have a problem…

Planets in Clusters

As we study more and more exoplanets, one variable that we have not really gotten a great handle on is age.  There are not many planets orbiting stars with very well constrained ages. We’d like to be able to see how, for instance, young planetary systems differ from old ones to study planet-planet scattering, planetary migration, and other effects.

So there have been many studies of planets orbiting stars in star clusters.  Clusters are great laboratories for stars because the stars formed (mostly) at the same time out of the same stuff. The repurposed Kepler mission K2 was great for this because it looked for planets along the Ecliptic Plane, and by a bizarre coincidence almost every important benchmark cluster is in the ecliptic!

Jason Curtis, NSF postdoctoral fellow a Columbia University

Jason Curtis is a Penn State grad now an NSF postdoctoral fellow at Columbia working on the problem of stellar ages and activity, using the topic of his PhD thesis, the nearby open star cluster Ruprecht 147. He campaigned to get NASA to repoint K2 to make sure it would capture the stars of Ruprecht 147 so we could study its properties (and, you know, maybe find some planets).

And it worked! He has now written up the paper, and you can find it on the arXiv, in particular the new hot Neptune K2-231b.

But even more useful, to my mind, than the 231st K2 planet is that this planet has a well constrained age.  If we get a lot more, we can look for those trends we’d like to study about how systems change with age.  Jason helpfully compiled a list of all known planets in clusters, and there put them together in one big table. I imagine that with TESS we’ll end up with so many you won’t be able to fit them on one page, but for now here they are, with references.  For the full thing with working links, be sure to read the paper!

Table-1d4tdc0

SETI Jargon

SETI has a jargon problem. This is not news; I think everyone in the field appreciates that we need to be more consistent in the words we use.

One reason this matters is that the search for alien technology is really a very broad endeavor (best thought of, I have argued, as a subfield within astrobiology). It includes not just radio astronomy  but infrared astronomy, optical and NIR instrumentation, exoplanets, Earth system science, game theory, social sciences such as anthropology, galactic astrophysics, stellar astrophysics, time-domain astronomy, computer science, multi-messenger astronomy, planetary science, remote sensing, media and communications, law, and political science.

These fields all have their own jargon, and if we want them to be part of SETI we should avoid misappropriating their jargon.  For instance “civilization” has concrete and jargon meaning in anthropology and archaeology. I imagine anthropologists at our meetings wincing every time we use the term in a very different (vaguer, more generic) way than they do.

Indeed, “intelligence” itself is problematic and not an ideal term. The term is nebulous (is an ostrich intelligent? Is a bee colony?) but also presumes much about how an alien species’ psychology works.

Why shouldn’t we assume aliens will be “intelligent” or have “civilizations”? As many have noted, we should not assume that the first extraterrestrial technological species we discover is anything like us: it might not be a collective of individuals, might not be “conscious” in the way we are, might not organize itself with anything like politics, might not be animal, might not be planet-based, etc. etc. etc. Science fiction is filled with potential SETI signals of things that look nothing like “civilizations”, from Hoyle’s Black Cloud to the Borg to the Monolith.

Even more than being careful in not misusing established terms, many have noted that we use many of our own terms inconsistently (Iván Almár has been particularly persuasive on this point, and I’m certainly guilty on this score.)

So, as part of the SETI Institute’s Decoding Alien Intelligence workshop this month and in response to Nathalie Cabrol’s call for white papers an broadening our conception of SETI, I submitted something about how we think about SETI as a field and the jargon we use.

I couldn’t make it, but Penn State graduate student Sofia Sheikh went and presented the paper for me.

My recommendations:

First of all, the field needs a name. The term SETI has variously been used to refer strictly to radio searches, specific NASA programs, to any search for communication, and broader searches, and has been used both to include and to distinguish from efforts such as searches for artifacts.

I agree with Almár that SETI should be the name of the entire field. One problem is that this it includes “intelligence”, which I have just argued is not a good term, but I feel that “SETI” is such a strong “brand” at this point—such a well-known and widely used term—that I think it is best to use it in a jargon sense of “whatever distinguishes technological species from other species that makes them easier to detect because of their technology”.  A rebranding is very unlikely to be successful (I would support it if everyone agreed to start using a single better term).

Having adopted “SETI” as the name of the field because of something like stare decisis, it follows that the term “ETI” is what we are looking for, again in a jargon sense.

I also like Almár’s definition ““the collective name of a number of activities, based on science, aimed to detect messages, signals or traces” of extraterrestrial intelligence.

I also really like the term technosignatures (whose origin I’ve been trying to track down, see this tweet:)

But again, we use it inconsistently.  I prefer the term to include any technological signature, including communication, both because that is the term’s natural meaning and because the contrast with biosignatures helps identify SETI’s place within astrobiology

I like to divide SETI into several classes, including communication SETI and artifact SETI, being the searches for deliberately transmitted information carriers and the effect of technology on the environment.  Other terms for the latter abound (technomarkers, Dysonian SETI, SETA…) and we should settle on one.

Within artifact SETI then there are lots of ways of searching: waste-heat SETI, probe SETI, Solar System SETI, and so on.

METI or active-SETI would also then constitute a subclass of SETI.

Paul Davies coined the term “nature-plus” which is the best term I’ve seen to describe the idea that alien technology could be so advanced that it will look like a force of nature; this could include contamination of stellar abundances, artificially modulated Cepheid variable stars, or even something like Hoyle’s Black Cloud.  It’s not exactly what Davies intended, but its the best label I could find for this kind of search.

Finally, I think it’s important to define a beacon as a signal or artifact meant to be discovered by strangers. The term has been used in other ways, but this is the term’s most natural meaning, and helps us identify which sorts of signal searches can be informed by the concept of a Schelling point. The latter term has many many names in the filed (“strategy of mutual search”, “convergent search strategies,” “attractor for SETI”, “synchrosignals”) generated as people rediscover Schelling’s insight, but we should honor game theory’s prior art here and recognize the value of needing to think about assumptions of common knowledge that we make when looking for “magic frequencies” and such.

Finally, we should avoid terms like “colonize” and “alien race” because of the social baggage they bring along. (This is not because I think we should be PC, but because we should be precise: if you really do mean to project our notions of colonization and race onto alien species with whom we share no evolutionary descent, much less culture, then by all means use those terms).

So that’s my reasoning and set of preferences for jargon, but I recognize this needs to be a collective decision in the community, and I suspect that the final answer will arise collectively and organically, and not by fiat.  Already we’ve received good feedback (CETI is probably its own category, distinct from METI, as Jill Tarter pointed out; “artificial” is a difficult and probably problematic term we should define better, as Frank Drake has pointed out).

Anyway, I think Sofia’s presentation will be public at some point, and the paper is available here at the conference website, and an updated version with the figure below is on the arXiv here.

SETI as Astrobiology-t593u0

Milton’s Cosmology Leaned Heliocentric

It’s a home day for me with the flu, so to recover from a long day of videoconferencing meetings, and because a headache won’t let me concentrate on important stuff, I played around with an idea I wrote up as a high school English project.

Milton wrote the epic Paradise Lost, in which he presents his own cosmology of Heaven, Hell, and Earth. This was an ambitious task, since Dante’s Divine Comedy had set a pretty high standard here.

The geometry of Creation in the Divine Comedy (not to scale) with Hell inside the Earth and heaven outside the final sphere in a Ptolemaic universe.

Milton actually met Galileo in person—he visited Galileo’s estate as a boy when the astronomer was there in his later years under house arrest.

The visit may have left an impression. Milton avoids any reference to where the center of the Solar System is in his cosmology—it’s pretty nebulous exactly how everything is arranged—but in Book 4 the angel Uriel needs to get back to his station in the Sun from Earth. He slid down to Earth on a sunbeam, but fortunately for him the Sun has now set, so the tip of his sunbeam is now up in the air. He just jumps on and slides down:

 …and Uriel to his charge
Returnd on that bright beam, whose point now rais’d 
Bore him slope downward to the Sun now fall’n
Beneath th’ Azores

but how did the Sun get so low? It is “beneath th’ Azores” far to the West:

whither the prime Orb,
Incredible how swift, had thither rowl’d
Diurnal, or this less volubil Earth
By shorter flight to th’ East, had left him there

The “prime Orb” is Ptolemy’s name for the Sun, which had (“incredible how swift”) there “rowl’d Dirunal”—rolled down there over the course of the day.  But Milton did not choose his words lightly.  “Incredible” literally means “beyond credibility”.  He doesn’t buy that it really goes so fast.

Is there an alternative?  Milton dithers.  “Or” he writes “this less volubil Earth” (meaning “less apt to roll” according to the OED) went a much shorter distance the other way and the Sun just stayed where “he” was.  That’s Copernicus’s model for the days!

So he clearly prefers Copernicus’s model, and offers it as an alternative, but opens with the more familiar and established terminology and cosmology of Ptolemy (and Dante).

It’s interesting he didn’t commit.  Perhaps he didn’t want to tie his work to a model that might be wrong, or perhaps he didn’t want to alienate his Ptolemist readership.

I wonder, though, if heliocentrism had been widely accepted a century before, if Paradise Lost would have had a well defined geometry of the universe, like the Divine Comedy famously does, but with Heaven and Hell having distinct places in a heliocentric Solar System?

 

Maunder Minimum Analogs

For a long time after sunspots were discovered telescopically by Galileo, there weren’t any to see. John Eddy has a nice paper on the history of sunspot measurements, showing conclusively that there was an 80-year period, now called the “Maunder Minumum”, in which the Sun just didn’t have any sunspots.

Why not? We’d love to know.  The Solar dynamo is a bit of a mystery, and it just apparently turning off for 80 years is kind of important—it may have even had an effect on climate on Earth, although that case is often overstated (it’s not glaringly obvious from the global temperature anomaly record).

Shivani Shah, Penn State undergrad now applying to a graduate program near you!

Finding another star undergoing such a period would be great. We could study its corona and chromosphere it answer questions like: Is it still undergoing its magnetic cycle, just without sunspots? Is it just an extended minimum that lasts many cycle periods? Or did the dynamo turn off entirely? If we had a sample of them we could ask: is this typical behavior of Sun-like stars? All cycling stars? Just stars of the Sun’s age?

For a while, people looked for extremely inactive “sun-like” stars to find “Maunder minimum stars”, but I showed in my thesis that these stars are in fact not Sun-like (they’re subgiants).

People looked in M67, a cluster filled with Sun-like stars, to find extremely inactive stars there, but Jason Curtis showed that these stars were not actually all that inactive (the ISM got in the way).

For a long time Steve Saar has advocated using time series to see a star do what the Sun apparently did: go from a cycling state to a quiet state (or the reverse).  This means using the 50+ year baseline of activity measurements we have of stars to find a cycling star that transitions into or out of a cycling state to a “flat activity state”.  That would make for a pretty convincing candidate, I think.

Well, now we have one! Shivani Shah has written up the strange case of HD 4915, a Sun-like star that seems to have had its cycle peter out to almost nothing.  Here’s the killer plot:

Activity history of HD 4915. Larger “S-Value”s mean (presumably) more starspots. Note that the star came down from a (presumed) maximum, came up to a second, weaker maximum, then had a very slow rise to what seems to be a very weak maximum. Is the magnetic cycle of this star dying out?

It’s cool to see something that looks a lot like what we’ve been expecting to see for a while.  For comparison, here are the “grand magnetic minima” the Sun has experienced, measured with sunspots:

Sunspot number vs. time for three magnetic state transitions on the Sun, from the Hoyt & Schatten 1998 historical reconstructions. Note that we have reversed the x-axis on the bottom panel.

 

Black dots above are the parts of the Solar record we are suggesting to be analogous to the HD 4915 time series.  The red points are our projections of its future behavior if the analogy is perfect.  So, if this is analogous to the “Dalton Minimum” (no, not that Dalton Minimum) then the next cycle should be rather strong; if it’s truly going into a Maunder Minimum-like state we may not see any activity for another 80 years! So only time will tell if this is right, but I think it’s the best candidate I’ve seen so far.  I hope that by studying this star we can finally crack the nut of what the Sun was doing without sunspots for all those decades!

The paper is on the arXiv here.  Comments welcome!

SETI is Not About Getting Attention

No, this isn’t a post about METI.  This is about an interesting sociological phenomenon about one of the ways in which SETI is marginalized in astronomy.

SETI tends to get media attention, at an amount disproportionate to the amount of SETI work actually done. There are many reasons for this. One is that it is a topic of genuine interest to much of the lay public.  Another is that it is easily sensationalized and conflated with UFOlogy and science fiction by the yellow press.

I’ve had my share of both sides. Take Ross Andersen’s excellent article on Tabby’s Star (which was a scoop; we did not put out a press release or publish anything that triggered it). This story got huge amounts of global media attention, to the point that it appeared on Saturday Night Live and the Tonight Show. This second wave of stories led to the perception that Tabby herself “jumped to aliens” as an explanation, when in fact her paper and press release made no mention of aliens, and the press release announced “comets” as the cause.

The Daily Mail is a sensationalist rag in the UK that seems to have decided I’m their go-to name for all things alien. Sort of like the way that they find a way to shoehorn Stephen Hawking into the headlines of any article they write about space, but to a much smaller degree, they seem to love to claim I’ve found aliens (or that I think I have) when I write the opposite in a public space.  For instance:

  1. I had a press release titled “Search for Advanced Civilizations Beyond Earth Finds Nothing Obvious“.  The Mail Online’s lede was that I had “found 50 galaxies that may contain intelligent alien races.”
  2. I wrote a paper whose premise was that the question of extraterrestrial life (of any kind) in the Solar System is an “open question”, and in particular that the Solar System apparently lacks any alien artifacts.  The Mail Online article’s subheads claimed that I believe that intelligent “aliens either lived on Earth, Venus or Mars billions of years ago.”
  3. And when I wrote a couple of blog posts about how I didn’t think ‘Oumuamua was a great SETI target (but that it should get us thinking about Solar System SETI), they again reversed my meaning and wrote that I claimed ‘Oumuamua “could be an alien spacecraft with broken engines.”

It’s pretty embarrassing to see your work so brazenly sensationalized in the media, but given the Daily Mail’s reputation I’m not sure there’s anything I could have done to prevent it except not talk about SETI at all where it might be overheard. I’ve developed a thick skin about it, but it still smarts to see my name next to pictures of bug-eyed aliens.  I know that colleagues of mine that don’t know the whole story will think less of me because of these false portrayals of me working on “fringe” science or shouting “aliens” at every astronomical anomaly.

Actual image the Daily Mail used in an article quoting me about an asteroid.

So it’s especially galling when my colleagues accuse me of sensationalizing my work or, worse, only working in SETI at all because I’m after media attention. This attitude is probably widespread, because a few fed-up people have lost their cool and announced it several times in rants online; I can only imagine how many more have kept their cool or only said it where I haven’t noticed.  Some examples (names and links omitted to protect the guilty):

  1. About a short ETI discussion in a longer paper (that did not seek or garner any headlines)
    What does the mention of alien civilizations really add to these topics other than an attempt to grab headlines?
    and
    What’s to be gained by a casual mention in the abstract and end of the paper? … this (along with a growing list of other examples in the literature) is an attempt to grab headlines.
  2. Rebutting an argument that there is nothing wrong with seriously discussing SETI angles of astronomical anomalies on social media:
    All fine, but there is another component of this, which is cynical citation of ETI as a simple way of gaining attention. Your discussion assumes earnest and honest motivations. I’m not sure that that is always true.
  3. Starting a discussion about how SETI astronomers need to stop sensationalizing their work:
    most SETI-related news seems to be interfering with conventional scientific discoveries, stealing the limelight – without following basic rules of science
  4. Piling on to that discussion:
    It’s not just SETI you should be dumping on here, if your overall argument is to stop selling bullshit to the media because it’s fun.

At the risk of making an analogy to an infinitely more serious problem, they’re blaming the victims. Here we are getting misquoted and caricatured in the yellow press, and they’re the ones that are offended and embarrassed at what we have put them through. To them, somehow it’s our fault that the Mail misquotes us, and their attitude is that if we didn’t want to be misquoted, then why were we doing that kind of science in the first place? 

The real bad actors here are the yellow journalists, and that is a problem all of us in science and science communication have to deal with all the time; SETI is just a particularly soft target for them.

So, for the record: this kind of media attention is not “fun,” it’s mortifying, and we are not asking for it when we discuss our work in public. Many of the above accusations were surrounded by claims that the writers respect SETI as science, but you don’t really respect scientists’ work if you think it’s irresponsible of them to talk about it out loud, or if you think the only reason they do it is so that they can get their names in the papers.

SETI astronomers have had to deal with conflation with UFOlogy and fringe psuedoscience for decades; I hope that more of our colleagues will recognize that we share their disdain for sensationalism and are pulling in the same direction on the issue of sober science communication about good science.

And I hope that they won’t cast scorn at every SETI paper or reference to ETIs in the literature (“astro-crap” one astronomer called it on Facebook), and not cast aspersions on the authors for working on an important problem (especially junior researchers, who are both the future lifeblood of the field and the most sensitive to these accusations).

SETI gets enough unjustified grief from Congress, the last thing we need is to have to worry about our colleagues in our flanks piling on.

SETI is Part of Astrobiology

What follows is my submission to the National Academies of Sciences, Engineering, and Medicine ad hoc Committee on Astrobiology Science Strategy for Life in the Universe, 2018. It is available as a PDF here.

Please also see Jill Tarter’s companion white paper here.

I. SETI is Part of Astrobiology

“Traditional SETI is not part of astrobiology” declares the NASA Astrobiology Strategy 2015 document (p. 150). This is incorrect.1

Astrobiology is the study of life in the universe, in particular its “origin, evolution, distribution, and future in the universe.” [emphasis mine] Searches for biosignatures are searches for the results of interactions between life and its environment, and could be sensitive to even primitive life on other worlds.  As such, these searches focus on the origin and evolution of life, using past life on Earth as a guide.

But some of the most obvious ways in which Earth is inhabited today are its technosignatures such as radio transmissions, alterations of its atmosphere by industrial pollutants, and probes throughout the Solar System. It seems clear that the future of life on Earth includes the development of ever more obvious technosignatures. Indeed, the NASA Astrobiology Strategy 2015 document acknowledges “the possibility” that such technosignatures exist, but erroneously declares them to be “not part of contemporary SETI,” and mentions them only to declare that we should “be aware of the possibility” and to “be sure to include [technosignatures] as a possible kind of interpretation we should consider as we begin to get data on the exoplanets.”

In other words, while speculation on the nature of biosignatures and the design of multi-billion dollar missions to find those signatures is consistent with NASA’s vision for astrobiology, speculation on the nature of technosignatures and the design of observations to find them is not. The language of the strategy document implies NASA will, at best, tolerate its astrobiologists considering the possibility that anomalies discovered in the hunt for biosignatures might be of technological origin.

But there is no a priori reason to believe that biosignatures should be easier to detect than technosignatures—indeed, we have had the technology to detect strong extraterrestrial radio signals since the first radio SETI searchers were conducted in 1959, and today the scope of possibly detectable technosignatures is much larger than this. Furthermore, intelligent spacefaring life might spread throughout the Galaxy, and so be far more ubiquitous than new sites of abiogenesis. Life might be much easier to find than the NASA strategy assumes.   

Indeed it has been cynically, but not untruthfully, noted that NASA eagerly spends billions of dollars to search for “stupid” life passively waiting to be found, but will spend almost nothing to look for the intelligent life that might, after all, be trying to get our attention. This is especially strange since the discovery of intelligent life would be a much more profound and important scientific discovery than even, say, signs of photosynthesis on Ross 128b.

Further, since technosignatures might be both obvious and obviously artificial SETI also provides a shortcut to establishing that a purported sign of life is not a false positive, a major and pernicious problem in the hunt for biosignatures. SETI thus provides an alternative and possibly more viable path to the discovery of alien life than is reflected in NASA’s astrobiology roadmap. Indeed, this was recognized explicitly in the panel reports of the Astro2010 decadal survey:

Of course, the most certain sign of extraterrestrial life would be a signal indicative of intelligence. [A radio] facility that devoted some time to the search for extraterrestrial intelligence would provide a valuable complement to the efforts suggested by the PSF report on this question. Detecting such a signal is certainly a long shot, but it may prove to be the only definitive evidence for extraterrestrial life. (p.454, Panel Reports—New Worlds, New Horizons in Astronomy & Astrophysics)

II. Why is SETI Neglected in NASA’s Astrobiology Portfolio?

While it is not completely clear why NASA does not include SETI in its astrobiology portfolio, there are several factors that seem likely to be at play.

The first is the risk of public censure: SETI sometimes suffers from a “giggle factor” that leads some to conflate it with “ufology” or campy science fiction. Indeed, such an attitude likely led to the cancelation of the last NASA SETI efforts in the early 1990’s, after grandstanding by US senators denouncing “Martian hunting season at the taxpayer’s expense” (Garber 1999). Such attitudes harm all of science, and the National Academies should be clear that such a “giggle factor” must not be allowed to influence US science priorities.

The second is the erroneous perception that SETI is an all-or-nothing proposition that yields no scientific progress unless and until it succeeds in detecting unambiguous signs of interstellar communication. On the contrary, even with scant funding, SETI has historically been involved in some of the most important discoveries in astrophysics. Not only have the demands of radio SETI led to breakthroughs in radio instrumentation (see, for instance, the new Breakthrough Listen backend at the Green Bank 100-meter telescope, with bandwidth of up to 10 GHz, an ideal Fast Radio Burst detection device; Gajjar et al. 2017), but some of the most famous SETI false positives have proven to be new classes of astrophysical phenomena, including active galactic nuclei (CTA-21 and CTA-102, Kardashev 1964), pulsars (originally, if somewhat facetiously, dubbed “LGM” for “Little Green Men”), and perhaps the still-not-fully-understood “Tabby’s Star” (KIC 8462852, Boyajian et al. 2016, Wright et al. 2016, Wright & Sigurdsson 2017).

Indeed, exactly because SETI seeks signals of obviously artificial origin, it must deal with and examine the rare and poorly understood astrophysical phenomena that dominate its false positives. Anomalies discovered during searches for pulsed and continuous laser emission (Howard et al. 2007, Wright et al. 2014, Tellis & Marcy 2015, 2017) broadband radio signals, large artificial structures (Dyson 1960, Griffith et al. 2015, Wright et al. 2016), and other astrophysical exotica push astrophysics in new and unexpected directions. If there is a perception that SETI little more than the narrow search for strong radio carrier waves producing a long string of null results it is because historically there has been essentially no funding available for anything else.

Third, there is the erroneous perception that, since radio SETI has been active for decades, its failure to date means there is nothing to find. On the contrary, the lack of SETI funding means that only a tiny fraction of the search space open to radio SETI has been explored (Tarter et al. 2010). Indeed, Robert Gray has estimated that the total integration time on the location of the Wow! Signal (the most famous and credible SETI candidate signal to date) is less than 24 hours (see, for instance, Gray et al. 2002). That is, if there is a powerful, unambiguous beacon in that direction with a duty cycle of around one pulse per day, we would not have detected a second pulse yet. Other parts of the sky have even less coverage. The truth is, we only begun to seriously survey the sky even for radio beacons, and other search methods have even less completeness.

Fourth, there is the erroneous perception that SETI will proceed on its own without NASA support. Indeed, the 2015 NASA Astrobiology Roadmap claims that “traditional SETI is…currently well-funded by private sources.”  Even setting aside the non sequitur of considering the amount of private philanthropic funding when assessing the merits of the components of astrobiology, this is not a fair description of the state of the field. While it is true that the Breakthrough Listen Initiative has pledged to spend up to $100 million over 10 years, in truth its spending has been far below that level, and it is focused on a small number of mature search technologies. Beyond this initiative, private benefactors have supported the SETI Institute’s Allen Telescope Array, but not at the level necessary to complete the array or fund its operations.

Fifth, there is the erroneous perception that the search for technosignatures is somehow a more speculative or risky endeavor than the search for biosignatures. We note that the entire field of astrobiology once faced a similar stigma. Chyba & Hand rebutted that perception in 2005:

Astro-physicists…spent decades studying and searching for black holes before accumulating today’s compelling evidence that they exist. The same can be said for the search for room-temperature superconductors, proton decay, violations of special relativity, or for that matter the Higgs boson. Indeed, much of the most important and exciting research in astronomy and physics is concerned exactly with the study of objects or phenomena whose existence has not been demonstrated—and that may, in fact, turn out not to exist. In this sense astrobiology merely confronts what is a familiar, even commonplace situation in many of its sister sciences.

Their rebuttal holds just as well as SETI today. Indeed, Wright & Oman-Reagan (2017) have articulated a detailed analogy between SETI and the relatively uncontroversial search for dark matter particles via direct detection. They argue that unlike with dark matter searches, with SETI, at least, we have the advantage that we know that the targets of our search (spacefaring technological species) arise naturally (because we are one).

Finally, there is an erroneous perception that SETI is exclusively a ground-based radio telescope project with little for NASA to offer. On the contrary, SETI is an interdisciplinary field (Cabrol 2016) and even beyond the potential for NASA’s Deep Space Network to play an important role in the radio component of SETI, archival data from NASA assets have played an important role in SETI for decades: from Solar System SETI using interplanetary cameras, to waste heat searches using IRAS (Carrigan 2009) WISE, Spitzer, and GALEX (Griffith et al. 2015), to searches for artifacts with Kepler (Wright et al. 2016) and Swift (Meng et al. 2017). Future ground-based projects like LSST and space-borne projects like JWST and WFIRST will undoubtably provide additional opportunities SETI research both as ancillary output of legacy and archival programs and through independent SETI projects in their own right.

III. Reinvigorating SETI as a Subfield of Astrobiology

One difficulty SETI faces is a negative feedback between funding and advocacy.

As it stands, SETI is essentially shut out of NASA funding. SETI is not mentioned at all in most NASA proposal solicitations, making any SETI proposal submitted to such a call unlikely to satisfy the merit review criteria. Worse, the only mentions of SETI in the entire 2015, 2016, and 2017 ROSES announcements are under “exclusions,” in the Exobiology section (“Proposals aimed at identification and characterization of signals and/or properties of extrasolar planets that may harbor intelligent life are not solicited at this time”) and the Exoplanets section (as “not within the scope of this program.”) In other words, SETI is ignored entirely in NASA proposal solicitations, except for those most relevant to it, in which cases it is explicitly excluded.

Meanwhile, other parts of astrobiology have flourished under NASA’s aegis, which has incubated strategies for the detection of life elsewhere in the universe, and produced scientists who can advocate for mature roadmaps to the detection of life in the universe as part of NASA’s astrobiology program. But now, twenty years after the last major NASA SETI program was cancelled, there are only a handful of SETI practitioners and virtually no pipeline to train more.

Thus there are only a few well-developed strategies to advocate for, and only a few scientists to advocate for them. This will doubtless be reflected in the number of white papers advocating SETI (like this one) versus those advocating other kinds of astrobiology responsive to the current call. This disparity should not be seen as indicating a lack of intrinsic merit of the endeavor of SETI, but as a sign of neglect of SETI by national funding agencies.

Since SETI is, quite obviously, part of astrobiology, SETI practitioners should at the very least be expressly encouraged to compete on a level playing field with practitioners of other subfields for NASA astrobiology resources.

Doing so will uncork pent-up SETI efforts that will result in significant progress over the next 10 years and beyond. As a fully recognized and funded component of astrobiology, SETI practitioners will be able to develop new search strategies, discover new astrophysical phenomena and, critically, train a new generation of SETI researchers to guide NASA’s astrobiology portfolio to vigorously pursue the discovery of all kinds of life in the universe—both “stupid” and intelligent.

And if, as many suspect, technosignatures prove to be closer to our grasp than biosignatures, then including of SETI in NASA’s astrobiology portfolio will ultimately lead to one of the most profound discoveries in human history, and a reinvigoration of and relevance for NASA not seen since the Apollo era. In retrospect, we will wonder why we were so reluctant to succeed.

IV. Bibliography

  • Boyajian, T. S. et al. 2016, MNRAS 457, 3988
  • Carrigan, R. A, Jr., 2009, The Astrophysical Journal 698, 2075
  • Cabrol, N. A. 2016, Astrobiology, 16, 9
  • Chyba, C. F. & Hand, K. P. 2005 Annu. Rev. Astron. Astrophys. 43, 31–74.
  • Domagal-Goldman S. D. & Wright K. E. Astrobiology. August 2016, 16(8): 561-653. 
  • Dyson, F. 1960 Science 131, 1667
  • Gajjar, V., et al. 2017, The Astronomer’s Telegram, 10675
  • Garber, S.J. 1999. J. Br. Interplanet. Soc. 52, 3–12.
  • Gray, R., et al. 2002, The Astrophysical Journal 578, 967
  • Griffith, R. et al. 2015, The Astrophysical Journal Supplement Series 217, 25
  • Howard, A., et al. 2007, Acta Astronautica 61, 78H
  • Kardashev, N. S. 1964, Soviet Astronomy, 8, 217
  • Meng, H., et al., 2017 The Astrophysical Journal 847, 131
  • Reines, A. E., 2002 Publications of the Astronomical Society of the Pacific 114, 416R
  • Tellis, N. K, & Marcy, G. W., 2015 PASP 127, 540T
  • Tarter, J., et al., 2010 SPIE 781902 http://dx.doi.org/10.1117/12.863128
  • Tellis, N. K, & Marcy, G. W., 2017 The Astronomical Journal 153, 251
  • Wright, S., et al, 2014 SPIE 9147E, 0JW
  • Wright, J. T., et al. 2016, The Astrophysical Journal 816, 17
  • Wright, J. T. & Sigurdsson, S. 2016, The Astrophysical Journal, 829, 3
  • Wright, J. T. & Oman-Reagan, M. P. 2017, Int. Jour. of Astrobiology, arXiv:1708.05318

1 Indeed, broad swaths of the astrobiology community disagree with NASA’s assertion. For instance, SETI was included as a component of astrobiology in The Astrobiology Primer v.2.0 (Domagal-Goldman & Wright 2016), and SETI activities fall under the Carl Sagan Center for astrobiology at the SETI Institute (which, despite the name, conducts a broad range of science, including many sub-fields of astrobiology).

AstroWright Group Science at the 231st AAS Meeting: Thursday

Today it’s stars stars stars! Including a couple of really nifty you-heard-it-here-first results.

10:20 am #303.03 Maryland Ballroom A don’t miss Jacob Luhn talk about jitter in dwarf and subgiant stars. Where does jitter come from in inactive stars? Which stars are least jittery? Jacob has the answers in what we are calling the “amazing jitter plot”.  Don’t miss it!

Poster #349.11: Penn State undergraduate Shivani Shah shows off her thesis work studying magnetic cycles in Sun-like stars.  We were going to look at activity-RV correlations but when we found this star we changed our focus: it appears to be entering a Grand Magnetic Minimum state, similar to the Sun’s Maunder Minimum.  Finding such a star has been a goal of stellar astronomers for decades, and now we think we’ve got a good candidate.  Ask her about it during the 9am and 5:30 poster sessions!  (Oh, and Shivani is applying to graduate school this year, so if you’re on an admissions committee, make sure you talk to her!)

Poster #349.24: AstroWright collaborater Brendan Miller presents his work on Swift X-ray monitoring of the coronae of nearby planet-hosting stars.  X rays are an important consideration in the habitability of planets, and this work helps put things into perspective.

Precise RVs at the 231st AAS Meeting: Tuesday

Good morning!  Here are some abstracts not to miss today:

Oral Presentations

10am, National Harbor 11: #111.01  Jason Eastman will talk about the first year of operations at MINERVA.  Jason and I have collaborated many times on EPRV projects, most notably our barycentric correction routine.  Jason is the project manager for MINERVA, our array of 4 small telescopes at Mt. Hopkins observing nearby bright stars at very high RV precision and with nightly cadence. Jason will talk about how we have addressed the challenges of fully automated robotic operations and what we will be able to accomplish in the coming years with MINERVA.

2pm, National Harbor 11: #128.01 Rob Wittenmyer will talk about MINERVA-Australis at USQ’s Mount Kent Observatory.  Rob is building a sister project in the south to MINERVA, which is now funded and coming along nicely. Together the two MINEVRA projects will monitor the entire sky for rocky planets orbiting our nearest neighbors. (graduating PhDs take note: Rob will be looking for a postdoc to join the team!)

 

Posters

#152.08 Sarah Logsdon will present the NEID Port Adapter. NEID (pronounced noo-id) is the new facility instrument for the WYIN 3.5m at Kitt Peak. It will be an extremely precise RV machine with < 30 cm/s instrumental precision. We are building it at Penn State now, but Sarah is working at NASA Goddard SFC with Michael McElwain on the interface between the telescope and the instrument. This is a crucial component that has to handle all of the guiding, tracking, focusing, and other components of injecting light into the NEID optical fiber to the extremely high precision and stability we required for our RV precision goals.

#152.18 Speaking of Penn State spectrographs, Joe Ninan will present the commissioning results for HPF, our near-infrared precise RV machine on HET. This is, as I like to say “HARPS in the NIR on a 10m”, although all 3 of those are slight exaggerations (1-3 m/s, ZYJ bands, 9m). HPF is commissioning now and Joe has been a crucial member of the commissioning team and will discuss the challenges of EPRV work with NIR detectors, and our solutions to these challenges.

What We’ve Learned About Boyajian’s Star II: Data and Interpretation

Part I here.

It’s been a huge amount of work, but we finally have some conclusions.

First and foremost, the dips have now been observed by an instrument other than Kepler.  So we can firmly rule out instrumental effects! (This was already clear, but it’s now true beyond all doubt for the dips).

Secondly, the dips are clearly chromatic:

Analysis of LCO data by Eva Bodman.

Eva Bodman has done a lot of work to characterize how much deeper the dips are at blue wavelengths than red ones.  If there were opaque objects blocking our view of the light, the star should get equally dim at all wavelengths. Instead, Eva finds that the blue (B) dips are much deeper—about twice as deep—as they are when we look at infrared wavelengths (i’ band, just beyond human vision).

This is consistent with ordinary astrophysical dust, and a major conclusion of our paper: the dips are not caused by opaque macroscopic objects (like megastructures or planets or stars) but by clouds of very small particles of dust (less than 1 micron in typical size). We can also say that these clouds are mostly transparent (“optically thin” in astrophysics parlance).

Secondly, we have spectra from Keck/HIRES both before and during the dips (in-dip spectra kindly contributed by John O’Meara, Jay Farihi, and Seth Redfield; see the black lines in the above figure for when they were taken.  Pre-dip data taken by Andrew Howard and Howard Isaacson.)  The difference between these spectra should bear the spectral fingerprints of whatever is causing the dips!  So, is there atomic gas?  Let’s check the neutral sodium lines (analysis courtesy of Jason Curtis):

The broad sodium line from the star forms a shallow bowl, the sharp features are due to interstellar clouds containing sodium between us and the star.

The black points and red line in the figure above are from before and during the dips, respectively, and the black line at the bottom is the difference.  As you can see, there is no obvious change in the spectrum at all. This strongly suggests that the dust causing the dips is not accompanied by much neutral sodium.

What about hot gas? If it’s really hot there shouldn’t be much in the way of dust, but if it’s warm there should be ionized calcium.  How do the calcium lines look?

The broad calcium line from the star forms a shallow bowl, the sharp features are due to clouds containing calcium between us and the star.

Again, no change, so it looks like there is no additional ionized gas accompanying the dust. So the dust—if that’s what it is—seems to be by itself with no accompanying gas.

In fact Jason Curtis has gone further, and shown that there does not appear to be any change in the stellar lines, either, during a dip, meaning the star is not moving, so does not have a nearby companion orbiting it.

So where are our 10 possibilities?

As I wrote, instrumental effects (#1) are now firmly ruled out.

The hypotheses I found most plausible involving an interstellar gas and dust cloud (#3 and #4), are not looking great. There should have been atomic gas in that case, and we see none.

My favorite (but “less-plausible”) hypothesis #5, a black hole disk, has not been similarly developed, so I think is still in play because we’re not sure what we would have expected to see for that one yet. A cold disk of dust could easily have had all of its gas frozen out onto grain surfaces, I suspect.

The unlikely hypotheses of an orbiting black hole disk (#6), spherical swarm of megastructures (#9) and pulsations (#12) continue to be unlikely.

But now even the more generic “alien megastructures” hypothesis (of any geometry) takes a severe blow from the chromatic nature of the dips: no opaque objects seem to be causing this.  I suspect this will be the big headline here, so let me reemphasize: if the dips had been the same color at all wavelengths, we would have been scratching our heads and this hypothesis would be looking better than before (though still of unclear likelihood).  The fact that the data came in the other way means that we now have no reason to think alien megastructures have anything to do with the dips of Tabby’s Star (Recall that Meng et al. had already come to a similar conclusion with respect to the long-term dimming, but it was the dips that got us thinking along these lines in the first place).

I still like the Solar System cloud idea (#2) but until it is developed to the point where we know what colors of dimming we would expect for a Solar System cloud, it remains of “unclear” plausibility.

The fact that the stellar lines did not change velocity during a dip helps us rule out pulsations (#12, if the star changed size then its atmosphere would be moving and would have a changing radial velocity) as well as close companions. Tabby had already ruled out the nearby companion hypothesis (which is why it wasn’t even on my list) but we now have independent confirmation.

Hypotheses invoking circumstellar material seem to be doing well. Steinn and I were originally pretty down on this class of solutions because of the lack of infrared excess and their inability to explain the long-term dimming, but Metzger et al. and Wyatt et al. (2017) ‘s models have shown how this could be explained, bringing this class of hypothesis up the plausibility scale to near the top (in my mind). To remind you, Wyatt et al. explain both the long- and short-term dimming with circumstellar material, while Metzger et al. have the long-term dimming being intrinsic and the dips due to exocomet-like debris).

We were also down on the family of solutions involving intrinsic variations, and we still don’t think the polar spots model (#11) and stellar cycle model (#10) have high likelihood. But in addition to the Metzger et al. hypothesis, Peter Foukal has developed a model where the entire star gets cooler, and this model is also consistent with the data (I think the dips seem to be too deep in the blue, but formally it’s consistent at the “2-sigma” level). Indeed, Peter himself finds the dips to be less chromatic than we do and very consistent with his model. So I’m ready to promote this class of solution up, in particular because it predicts no absorption features accompanying dimmings, which is indeed exactly what we see.

As for the circumstellar material solutions (“exocomets”), I’m not personally sure why that model does not predict neutral and ionized gas to accompany the dips, but I don’t think anyone has worked it out in detail yet, so it could be easy to explain.

So to recap:

Wyatt et al. and Metzger at al. have developed models involving circumstellar material like exocomets that seem to be consistent with the data we have. Wyatt et al. and Foukal have developed models where the star itself is getting dimmer that also seem supported.  Both classes of model are now at the top of my list, though I still see major problems with both.

Hypotheses invoking intervening material like an interstellar cloud, seem to have taken a blow, though I still want to understand better if they are really ruled out by the lack of gas in the spectra, and whether circumstellar material like exocomets is similarly ruled out. I’m still fond of this solution, but it has gone down a notch in light of the new data.

I think my black hole disk hypothesis is still a dark horse in this race.

And the instrumental effects and alien megastructures hypotheses have been put to bed.

So that’s where we are.  The next highest priorities (in my mind) are to scrutinize the in-dip spectra for any signature of the occulting material (I’m especially curious if the diffuse interstellar bands change depth), and modelers need to make detailed predictions of the atomic and ionized gas that should accompany dust in the exocomet, interstellar cloud, and black hole disk models to see if they can be made consistent with our in-dip spectra.

Onward!

What We’ve Learned About Boyajian’s Star I: Background

For those just catching up on Tabby’s Star, read Kimberly Cartier’s article in Scientific American and my series of blog posts here. And don’t read or trust anything the Daily Mail writes on this (or any other topic involving me).

The star exhibits two unique and very difficult to understand behaviors: the short-term”dips” in brightness (of up to 22%) and long-term brightness variations on years-to-centuries timescales.

Since the Kepler mission stopped observing it, it seems to have continued it slow decline in brightness over the past few years, and that dimming does not seem to be due to solid objects.  But we still don’t have any information about what’s responsible for the dips because we hadn’t been able to see one happening in real time.

But now, thanks to the generous support of our Kickstarter backers, Tabby’s team has been able to pay for year-long monitoring of the star with the Las Cumbres Observatory global telescope network to “catch it in the act” of dipping again so we can study what’s going on.

And in May, it finally happened (when by amazing coincidence I just happened to be at the Breakthrough Listen Lab at UC Berkeley during my sabbatical):

Since then, Tabby’s team has been able to collect a huge amount of data not only from our own organized follow-up efforts, but thanks to he amazing generosity and interest of astronomers around the world who volunteered to observe the star during the dips.  We sincerely appreciate their contributions, and they are all authors of our latest paper.

You can follow every twist and turn of this summer’s activity on the Kickstarter project blog here and follow along with the fans of the project on the Reddit page.  Here’s what’s been going on all summer and fall:

In this plot, the different colors represent different LCO sites where data were taken.  The brightness of the star is on the y-axis, and the date (measured in days with an arbitrary offset astronomers like to use ) is on the x-axis.

One of the rewards for our Kickstarter backers was to name the various dips (they need names!).  After the first (“Elsie”, a nod to Las Cumbres Observatory (“LC”) who was one of our most generous backers), the star continued to oblige with a series of dips. The next dip, “Celeste,” was named as a near reversal of “Elsie” when it looked like the two events might be exhibiting time symmetry (it’s also a nod to team member Angelle Tanner’s mother, who sadly died around the time of the event). The subsequent events have started a theme of “lost cities” which it seems the backers would like to maintain going forward.

After all of that, the star exhibited a strange brightening for a couple of months.

To recap, we were hoping that once we finally caught a dip happening in real time we could see if the dips were the same depth at all wavelengths.  If they were nearly the same, this would suggest that the cause was something opaque, like a disk or (whispering) alien megastructures.

The long-term dimming doesn’t seem to be the same at all wavelengths, which suggests it’s being caused by something like ordinary astronomical dust, but that doesn’t tell us what’s causing the dips (which are what got everyone excited in the first place).

So, what have we found!?!  Well, our paper with a huge author list has been accepted by Astrophysical Journal Letters (thanks to a quick and conscientious referee) and we’re ready to reveal what happened in part II.

 

1I/’Oumuamua updates!

[note: As I wrote in November, I don’t think ‘Oumuamua is an alien spacecraft. While other astronomers have made that suggestion, and while I’m happy to engage in such speculation in a SETI context, I think ‘Oumuamua is interesting in its own right as an asteroid and because of how it is getting us thinking about how to find alien probes in the Solar System.]

Three updates to the ‘Oumuamua story!

First, it appears to be tumbling:

This explains a lot about the confusion over its shape and color. The data keep giving different answers because the object is spinning in a complicated way. To understand deeply, you need a quick primer on principal axes (skip to the slo-mo parts, especially the “unstable” axis around 2:00):

The key is that in space, things generally rotate in a very simple way, about the “principal axis” with the largest moment of inertia (smallest radius).  This is because this is the axis for which a given angular momentum has the least energy, and over time objects will lose energy but not angular momentum.  The Earth, for instance, is oblate, and rotates along the shortest axis it has.

But if you just start something spinning arbitrarily (or, say, you knock it around) or if you start it spinning with some motion along its intermediate axis, it will execute a much more complex motion (around 2:00 in the video above) called tumbling.  It will do this in space until the changing distortions of the body from the changing centrifugal forces eventually cause the rotational energy to dissipate away as waste heat and it ends up a principal axis rotator again (that’s why the Discovery One in 2010:Odyssey Two is spinning that way, along its shortest axis).

The Discovery One is spinning when he Alexei Leonov comes to find it because it had angular momentum but no attitude control, so eventually found the lowest energy state, which was a spin about its shortest axis.

So why is ‘Oumuamua tumbling? It’s unclear, but it may be related to its elongated shape: unlike typical Solar System “rubble pile” asteroids and icy comets, it seems to have more rigidity (apparently not uncommon in smaller Solar System objects), and so it dissipates its rotational energy more slowly—so slowly that it can tumble for a long time.


Second, I wrote a AAS Research Note correcting a small point made by Jean Schneider, who showed that ‘Oumuamua could not have been sent into it’s current orbit via gravitational slingshot with any known planet, or the hypothetical Planet Nine.  I pointed out that in fact there is no way any Solar System object could have done it, hypothetical or not (I supect that this point is trivial to people that think about this for a living, but it is nontheless surprising to those of us who don’t).  I think Alt Mars Crater put it best:

Update:

 


Third, Breakthrough Listen is taking a look (listen?) to see if it is emitting radio waves as one might expect (?) if it is an alien probe:

This is neat! We should be thinking about what we will do if something that looks (more) like an alien craft comes through the Solar System. Now the Breakthrough Listen team has a protocol for tracking Solar System objects with Green Bank and analyzing the data they collect.

Such a discovery would imply that there are lots of these things in the Solar System at any given moment (even if they are deliberately targeting the Sun, they are hard to spot and we’ll miss most of them), and so lots of opportunities to study them.

Why would there be so many of them? Part of the argument that it is possible to settle the entire Galaxy is that exponential growth is possible, because the only limiting resource is the stars (and the material around them) themselves.  Exponential growth can be achieved via Von Neumann probes: self-replicating spacecraft that go to a system, make lots more of themselves, and then go to more systems.

Now even if these have purposes that don’t involve coming near the Sun, you might expect some fraction to eventually go derelict (space is a harsh environment, and an optimal design will likely have a nonzero failure rate). Such derelict craft would, if they are not traveling so fast that they escape the Galaxy, eventually “thermalize” with the stars and end up drifting around like any other interstellar comet or asteroid.

In fact, since they (presumably) no longer have attitude control, one would expect that they would eventually begin to tumble, and if they are very rigid that tumbling might distinguish them from ordinary interstellar asteroids… and in fact, just because their propulsion is broken doesn’t mean that their radio transmitters would be broken…