Transiting exoplanets in SETI

I feel that it is quite appropriate for me to review this paper by David Kipping two days after we conducted an observation of 12 transiting Kepler planets from Green Bank Telescope in association with Breakthrough Listen, based on the principle outlined in this paper.

The paper talks about using lasers to cloak the presence of a planet during its transit. However, in this blog I shall not talk about a civilization trying to mask its presence but its attempts to broadcast itself. The paper proposes the principle of a temporal Schelling point in our search for ETI. The question often arises of the best time to search. Since there is no real special time, this paper suggests that the transit of an exoplanet around its host star could be one. If there is a beacon on the night side of the planet, then it would sweep out an arc as the planet revolves around its star. This beacon would be visible from our line of sight when the planet transits the star, if it is directly aimed at its sub – stellar point. This beacon if broadcast continuously would be visible to observers periodically with every transit. Doing this during a transit is an interesting proposition since transits allow for us to also measure the atmospheric composition of planets using spectroscopy. Further, in the near future we should be able to map the longitudinal heat profile as well as atmospheric composition of planets using phase curve spectroscopy. This would provide for definite clues of bio-signatures.

However, the beacon might not necessarily be on a planet which is inhabited by the ETI. The beacon can be on the closest planet, since that would have the highest probability for transiting in a randomly oriented system.

I think this paper is important in acknowledging the special place transits occupy in the optical astronomy, and subsequently extending it to SETI. Its ideas about a civilization using this phenomenon to hide its presence or beam out and advertise itself are novel, and can be one of the anomalies being considered in Wright et al. 2015 (GHat 4).

Playing Games with Lasers (Beaming Manhattan into the Void)

Everyone has been in a situation where they need to make themselves conspicuous. Proponents of SETI have often provided novel solutions to ensure an observer would readily identify their planet as one hosting life. The answer can be condensed to a basic principle: do something unnatural at the exact moment someone is observing you. David Kipping, an astronomer at Columbia University, who searches for planets and moons beyond our solar system, believes lasers can be used by ETI to serve as a beacon or mask a planet entirely. In a recent paper, Kipping and a graduate student argue that artificial transit profiles can be feasibly generated using laser emission. Unlike optical SETI, which focuses on pulses of light, Kipping believes the transit can be a useful signal to or cloak from Earth (see Movie 1).

Movie 1. Alex Teachey on Cloaking Planets
One of the co-authors of this paper sets out to describe how lasers could be used to cloak a transit. The timing of this video showed poor foresight (April Fool’s Day….). A secondary video by Alex provides answers to some common questions from YouTubers.

The use of transits in SETI goes back to the pre-Kepler days, when Luc Arnold first proposed distinguishing a transiting mega-structure from a natural body. Cloaking a planet requires many assumptions. Kipping ask us to consider an arbitrarily advanced civilization that discover all “nearby” habitable planets along their ecliptic plane. Kipping assumes the inhabitants would know which of these planets could observe their transits and, through some machinations privy only to ETI, such civilization would decide to prevent detection by these planets using the transit method. Kipping et. al. dismiss a previous suggestion of a mega-structure, arguing a powerful laser would be “technologically more feasible”. After performing a few calculations, Kipping et. al. argue a ~60 MW laser would serve as an optical, “broad-band” cloak and prevent detection from a mission such as Kepler. A laser, while monochromatic, could in theory serve to effectively mask a transit, as shown in Figure 1. Kipping et. al. argue that a laser array on the surface of a planet would be difficult and that instead ETI could place an array of lasers in space (colloquially known as a weapon). The authors aptly refuse to compare either solution. A similar and energetically cheaper alternative would be to use lasers to block out the absorption lines of biosignatures.

Figure 1. The Strange World of David Kipping
Both images are from Kipping et. al. 2016. On the left: Cloaking of a Transit Signal. The top panel shows the unaltered transit for various missions. The middle panel is the power profile of a 600 nm laser array designed to cloak the Earth. The bottom panel shows what an observer would detect. On the right: Using Transits as a Beacon.  The top panel shows the power profile of a laser array designed to broadcast the Earth. The bottom panel shows the transit signature an observer would detect. The laser makes for very unnatural signatures that distinguish it from orbiting planets.

In addition to cloaking, Kipping et. al. briefly discuss signaling via lasers. Broadcasting would be much cheaper, as it would not have to be broadband. The ingress and egress could be altered with lasers as shown in Figure 1. Another possibility, is to use lasers to etch intriguing patterns during the light curve. Kipping has stated:

You can make your transit look strange, have bumps and wiggles, maybe even the New York City skyline—whatever you want.

Savvy extraterrestrial scientists could use a deformed transit as a beacon to announce their existence (see Figure 2). By Kipping’s hypothesis, ETI no longer required planet-size megastructures, such as a rotating triangle or louvres, to produce unnatural transit signatures.

Figure 2. Laser Doodles
Going from top to bottom: (i) An unperturbed transit showing how a star dims slightly when an orbiting planet passes in front of it. (ii) A transit showing different shapes due to a laser array aimed toward an observer. This example shows the New York City skyline. (iii) The ideal beacon would be a square. This is a simple shape that would never occur naturally (yay limb darkening) and would require a laser only at ingress and egress.  Source: David Kipping

The reader is left with many questions and a sense of unease given all the assumptions. The ETI in question is apparently aware of all habitable planets in its ecliptic plane and capable of generating an array of lasers to block its transit. This is an act in vain if said planets use other techniques (i.e. direct imaging or radial velocity) to detect said planet. Kipping et. al. acknowledge this:

Transits are not the only method to discover planets and thus a truly xenophobic civilization may conclude that even a perfect and chromatic transit cloak would be ultimately defeated by observation of the planet using radial velocities. In this sense, the biocloak is perhaps the most effective strategy since certainly the transit and radial velocity measurements would appear compatible. However, even here, direct imaging would reveal a strong discrepancy in terms of the atmospheric interpretation and thus overcome the cloak.

A large part of this paper was to discuss how a transit could be cloaked, only to have that entire hypothesis appear to be an act in vain. The discussion on broadcasting with a strange transit signature is not fundamentally new. This blogger is left pondering the purpose of this paper. The authors themselves have dismissed the efficacy of cloaking and suggest we search for strange transits, something proposed by Arnold a decade earlier. Even if one were to assume cloaking to be efficient, SETI has predominantly concerned itself with civilizations indifferent to outside observers. After all, one could always invoke any arbitrary set of conditions or technology that would make a civilization impossible to detect. While the method of using lasers is novel, the rest of the paper reminds astronomers to search for strange transit signatures. Believe this requires strong priors and an indifference to all the assumptions. Kipping himself expects detections “on the order of a few dozen” and this blogger wishes him the best in his future endeavors.

Spin ‘er up, and call it alien!

Harwit describes a characteristic of photons that most, including this blog writer, did not know actually existed: orbital angular momentum. On top of this description, he also explains that we are able to incite large values of this momentum on photons ourselves and that we can (sort of) measure this value. Since nature doesn’t make photons with such high orbital angular momentum, such a detection would be an indication of artificial origin.

I happened to read this entire paper before realizing that the important bits were in Section 4.3, but oh well. The authors bring up interesting astrophysical applications aside from SETI, including probing a turbulent medium for inhomogeneities and studying different characteristics of black holes. According to the authors “radiation with high values of photon orbital angular momentum might have [significant advantages] for communication and quantum computing.” Apparently, taking this additional spin into considerations, one is able to encode more bits of information than previously (which makes sense given the additional degree of freedom). I think this paper is fantastic! It’s a good idea that is now

I must confess, this paper basically goes completely over my head, which I suppose could be formed as a sort of critique. I think it is important in science in general, but also in something as interdisciplinary as SETI to be clear in one’s writing and to lead the reading through all of your arguments, even if this means sometimes being repetitive or dumbing down your work. Although fields such as astronomy are marked by academics making things excessively convoluted to make them seem above the populace, I’d like to believe those days were over, and should have been over by 2003. Someone with a degree in physics or astronomy should be able to understand this paper, and although it could be my lack of coffee, Harwit should have written his paper at a more comprehendible level.

Given my lack of complete understanding of this paper and the fact that this paper is now 15 years old, most of my musing might not be all that interesting. For instance, how far have we actually progressed in this? Can we now readily inflict orbital spin on a photon and then detect it? Can we, with our current technology, encode messages in these photons, send them, and then later detect them?

A quick Google search has showed me that Wikipedia is, once again, a bro. Preliminary tests of radio and microwave photons showed that we are able to transmit 32 gigabits per second over the air, and 2.5 terabits of data per second through optical fibers. This is fantastic and amazing; this would revolutionize the telecommunications industry around the world! It reminds me of Artemis by Andy Weir, but with less mafia. Unfortunately, we have apparently not yet figured out how to reliably measure the orbital angular momentum. Since orbital angular momentum can have as many states as it wants, there is no device that can separate out more than two modes. A diffractive holographic filter is promising, but this idea is still being investigated.

Reaction to Hippke 2017 (Non-EM Carriers): Is the SETI search too narrow-minded?

Since the conception of communications with extraterrestrial civilizations in the late fifties (Cocconi & Morrison 1959), the overwhelming majority of SETI endeavors have centered on electromagnetic communication systems, often in one narrow fraction of the entire spectrum. Hippke is aware of the potential shortcomings of such an approach and presents the possibility of alternatives, not just to microwave emission as in his previous work (Hippke 2017), but to electromagnetism as a medium for information carrying in general. In particular, he examines the merits and shortcomings of a variety of non-EM carriers such as electrons, protons, neutrinos, gravity waves, and occulting megastructures. Vetting based on energy efficiency and data rates, Hippke places these alternative channels in competition with EM-based communications. For transiting megastructures, Hippke fails to find a way for this method to be competitive when it comes to target communication with high data rates, and so tepidly dismisses them. He also quickly rules out charged particles, particles with short lifetimes, and heavy particles due to interstellar magnetism, longevity, and energy requirements, respectively. He is also critical of gravitational waves as a medium for signal carrying as their artificial production is extremely resource intensive and wasteful. Lastly he examines neutrino based communication, which fails due to issues with focusing when compared to photons and size requirements of detectors. All of his conclusions are based on current knowledge of physics, and so the possibility is open that with an improvement in knowledge, some of these avenues may potentially become viable again. He has framed this investigation to work within the confines of what is currently understood. With these limitations, he concludes that the best medium for point-to-point communications is still electromagnetic radiation, at around the 1nm scale. If the assumption of preference for speed is relaxed, then the best alternative would be inscribed matter, or probes carrying vast databases of information. This paper was a novel contribution to SETI because it is one of the first attempts at an exhaustive analysis of alternative modes of communication. Scientists can often times get caught up in the present paradigm, and so it is beneficial to get a fresh perspective on the issue from someone who is not formally scientifically trained and thus potentially not subject to the same prior perceptions. His conclusions also vindicate the thinking behind the Pioneer and Voyager plaques and records, since physical media transported on long timescales is shown to be one of the preferred methods of communication. The potential this paper had to to retroactively dismiss all of our previous SETI efforts as foolishly narrow-minded or misguided should not be discounted. While we will continue to perform SETI in the radio and microwave, we should always be open to the possibility of alternative means of communication, and at the very least entertain a more expanded search of the electromagnetic spectrum when designing future SETI surveys.

A Way to Find Big ETI Laser Pointers

In Wright et al. (2014), a new, versatile optical SETI instrument is described that can search for direct evidence of interstellar communications via pulsed near-infrared signals. The article is in an SPIE (Society of Photo-Optical Instrumentation Engineers) conference proceedings paper and focuses heavily on the physical design of the detector and optics.

Modern high-powered lasers can easily outshine our Sun (for limited frequency ranges and times). As such, it is obvious that, if advanced ETI exists and wanted to communicate (or just broadcast its presence) specifically with us, it could easily do so with lasers. According to the article, the largest lasers on Earth are detectable with meter-class telescopes up to thousands of light years away. This is because, unlike radio signals, optical beams can be finely focused, providing a high received power flux for each amount of transmission energy.

Fast NIR pulses searches are an underexplored area not just for SETI, but for astronomy in general. Astrophysically-based nanosecond optical pulses are supposedly very rare, but this instrument is also planned to observed possible pulse sources such as pulsars, black holes, cataclysmic variables, gamma-ray bursters and active galactic nuclei.

The instrument works by utilizing commercial-off-the-shelf (COTS) products including NIR photon counters that, for the first time, can be very fast, wide bandwidth, high-gain, low noise and cheap. The light from the sky comes in and the NIR light gets split into two independent detectors (to help eliminate false positive detections) while the optical light goes to a guide camera so that they know that the telescope is pointed at the right place. Their setup is capable of recording the time of arrival of signals down to the nanosecond and it is planned to be used on the 1-meter Nickel telescope at Lick Observatory in California.

MASERS, LASERS, ETI, and other fun initialisms

A reoccurring thought while I read this paper was “What the hell are masers?” I just kind of assumed they would be defined *somewhere.* Well, they’re not. So here’s what masers are: “microwave amplification by stimulated emission of radiation.” So lasers, but specifically in the microwave. The acronym was coined in 1953 when the first maser (I think) was successfully operational. Later, an “optical maser” was successfully created in 1960. The optical maser was first envisioned in 1957, and the term LASER (*light* amplification by stimulated emission of radiation) was coined that same year. Apparently Charles H Townes made the first ammonia maser:

Given this information, it makes sense that Townes would look into additional uses for his optical maser (from now on, just laser). At the time, SETI was new and exciting (well new-ish; the idea of contacting ET species had existed for centuries), and was not weighed down by the “giggle factor” that it experiences today. An inventor could write a paper like this, and receive mostly positive support for the idea without anyone calling BS or science-fiction on the idea. This paper, I believe, marks the first discussion of contacting ET species with lasers and possibly detecting such signals. These ideas have now been integrated, and other papers written on them, but the first to propose it is always the coolest (right?). Townes computes that with modern (from 1961) technology, we could already detect specific laser signals, and postulates that with only a bit more time (and narrow-band optical receivers) other laser signals would have high enough signal-to-noise to be detected.

One last note is I enjoy how Schwartz and Townes end their paper. The paper is fairly technical and a nice proof of concept, but they end the paper with a quick SETI discussion, saying that searches should go beyond the waterhole, that UCE and IR are absorbed by most atmospheres (so not to really bother with those), but also that a civilization more advanced than our own could have technology that we currently rule out as unfeasible. I do appreciate this throw-in, since we only ever look for traces that could have potentially been left by humans, since we need to set restrictions to actually make a search, but it is nice to acknowledge that other civilizations could be unlike us, and therefore could be communicating in ways unimaginable to us.

The first proposal for Optical SETI

This 1961 paper by R.N. Schwartz and Charles Townes, discusses using Optical Masers (Lasers) for communication across interstellar distances. I feel that it is worth noting that this falls closely on the Cocconi and Morrison paper of 1959 which first suggested the water-hole in the radio as the ideal place to look for, for intelligent extra terrestrial (ETI) civilization.

The authors talk about the recent discovery of ruby optical Masers  by Townes. Since the M in Masers is for Microwave, optical Masers, were soon called Lasers or Light Amplification by Simulated Emission of Radiation.

The authors consider using Optical Masers (Lasers) on two different systems and compare the two. One is a laser on a 200 inch telescope (like the 200 inch Hale Telescope), whereas the other is 25 individual 4 inch  telescopes with Lasers pointed in the same direction. They consider atmospheric seeing as a limiting factor and hence consider that the 25 individual small telescopes might be a better option. I think this paper was really advanced for its time, since 4 years after the launch of Sputnik (1957) it considers the use of Adaptive optics and space telescopes.

It also considers the detectability of Lasers using 1961 technology levels for laser power and detectability. The paper also talks about high resolution spectrometers which could spectrally resolve the laser and hence detect that this artificial beacon outshines the host star. This would be a hallmark of its artificial origins.

The paper concludes by noting that the water hole in the radio should not be the only region where we look for interstellar communication. It also mentions that an advanced ETI might develop capabilities that we have ruled out and consider impractical.

Optical SETI is not exactly a novel approach, but one that has not yet been pursued in earnest. There have been recent efforts by Andrew Howard, Shelley Wright, Nathaniel Tellis in this direction. We must take advantage of the vast resources that are plowed by the astronomical community in this direction and utilize the instruments, development and data sets that exist as a product of this.



Response to Schwartz and Townes (1961)

The authors propose the use of optical/near-IR masers as an alternative to radio transmissions for the purposes of searching for extraterrestrial intelligences (ETIs). In that way, they are suggesting new search methods.

As of 1961 (when this paper was published), the “[development of maser oscillators in the optical/near-IR spectral region which would allow transmission across several light-years]” was on the horizon. Interestingly, the authors state that such masers could have been thoroughly developed even 30 years earlier, suggesting that other ETIs may have pursued such avenues (as opposed to radio transmission).

Technology in 1961 suggested that the continuous operation of high power masers was entirely within the realm of possibility, and the outlook has only brightened in the years since then. One issue relates to the directability of such a maser, and the authors suggest that the problem can be overcome by employing masers in tandem with optical systems. They recommend a coordinated system of 25 masers as the optimal configuration.

There are two important factors that must be considered when evaluating the detectability of a maser signal: (1) it must produce a sufficiently large photon flux and (2) it must be distinguishable from the astronomical background. The authors argue that the first condition can be easily satisfied, and, again, the outlook has only become more optimistic over the past half century. The second condition requires more consideration. Due to the small separation between Earth and the Sun, it is likely that a maser signal cannot be spatially separated from the light of the host star around which the signal is originating. The authors suggest transmitting far away from the peak energy output of the Sun (~5000 Angstrom), i.e., either the extreme violet (shortward of ~2000 Angstrom) or in the near-IR. The former choice suffers from limited atmospheric transmission, while the latter suffers more from the diffraction limit at longer wavelengths (if such a limit is applicable for the system being employed). The authors also suggest transmitting in strong absorption features, e.g., the Ca II H or K lines. Since 1961, our knowledge of stellar populations and corresponding exoplanet systems has greatly improved, so perhaps it is more useful to optimize the transmission interval for a typical M dwarf (as opposed to the Sun, which is a G dwarf).

This study proposes a very novel idea. When considering how to find an ETI (and by extension, in trying to envision how they might attempt interstellar communication) it is important to broaden our perspective and consider all possibilities.

X-Rays = Best Rays

This paper contains an argument for X-ray SETI. I will admit that I was skeptical when I read the abstract – after all, X-ray photons are much higher energy than radio photons and thus the typical logic of energy efficiency (of the transmitter) does not apply.

The paper speaks about the “streetlight effect” – an observational bias that causes you to “look where the light is good” aka. to search where it’s easiest (cheapest, already available data, good quality data given your technology, etc. etc. etc.). So, to try to preempt the gradual growing of our “streetlight” and cut to the chase, so to speak, the authors wanted to derive a physical optimum instead of just looking at the current technological “sweet spot”.

The Streetlight Effect in comic form

In this paper, the authors only considered photons, which is a choice that I’ll comment on at the end of this post.

They decide that a minimum wavelength for communication is probably about an atomic width; they adopt 1 nm as their order of magnitude value. The choice of this wavelength is dictated by how smooth we could possibly make a physical receiver surface.

The actual focusing of wavelengths of this scale is typically done with X-ray grazing mirrors that involve multiple mirrors in the design, but they are expensive to build. Because of this, the authors also discuss as-of-yet unknown alloys that could be used in a single mirror design and focusing with EM fields (not possible now, needs too much energy, but maybe in the future?).

The authors make the point that the advantage of X-rays is the amount of data that you can send and the tightness of the beam that you can create (both functions of the shorter wavelength). With a tighter beam, the pointings that you choose have to be proportionally more precise, even down to having to account for a planet’s position around a star.

Another benefit of the 1 nm wavelength choice is that it works at all distances, even when extinction is considered. Gamma rays are even better in this regard (they are barely extinct by anything), but they also would require instrument precisions that exceed the physical limitations described in the early part of the paper.

Finally, if you assume that each photon carries 1 bit of information, the authors find that you can get reasonable data rates in the megabit per second-year range, which would be sufficient for substantial communications. They propose searching for intentional communications in the existing Newton-XMM X-ray data. They note, however, that we have the technology to create pulses that are orders of magnitude shorter than we can detect with current technology, and that the time domain constraint might make us miss potential signals.

A Few Final Thoughts

  • “Ephemerides sharing is likely to be a small but significant component of all interstellar communications” – how best to share ephemerides in a general way, with the least assumptions possible (Schelling Points?) might be an interesting topic in CETI.
  • Choosing to look at only photons is a fair choice, but I can’t help but wonder if this is analogous to placing a physical constraint on the giant wheat triangle proposed by Gauss by setting it at an Earth diameter. It’s a physical limit for the method, but it means nothing if the method itself is (in hindsight) quaint/silly/outdated. Maybe photons will be a quaint/silly/outdated mode three centuries from now, and this is just a pointless thought experiment. I have a little bit more faith in EM communication than that, but it is something to consider.

Reaction to Wright (2017) (Incomplete)

Humans have occupied the Earth for less than 0.1% of its total existence. Given the vastness of time and the incompleteness of the geologic record, is it possible that the Earth had independently produced another intelligent species in the remote past? By extension, given the lack of fullness of knowledge regarding the deep history of our solar system, could other worlds have been host to such intelligences? And if it is possible, is it a worthy endeavor to pursue answers to these questions? This is the situation presented to us by astronomer Jason Wright in his paper regarding the possibility of what he calls prior indigeneous intelligent species, that is, intelligences which arose organically (i.e., not from other star systems) within our own solar system.

For many modern astrobiologists, there is hope that extant or extinct life will be discovered on perhaps Mars or the moons of the gas giants, but Wright is distinct in his pursuit of the possibility of the development of a more complex lifeform in the solar system. He considers a variety of plausible lines of evidence we could follow to establish whether or not the Earth or any other body of the solar system could have harbored an intelligence in the distant past. In the case of the Earth, it is difficult to conceptualize a way for artifacts or other signatures could be preserved on lengths of time comparable to the age of the Earth, but one strong marker would be unnatural isotope ratios discovered in sedimentary layers indicative of nuclear activity. Since some radioactive atoms have half-lives of billions of years, their signal should still be active even given the eons. For other bodies, those with geologic activity and atmospheres tend to continually renew their surfaces wherease those without are at the very least impacted frequently by micrometeorites and infrequently by asteroids. This reduces the probability that any actual physical relicts would ever be detected. Also, relics in orbit would tend to decay or scatter or collide with other objects, and hence are unlikely to survive the temporal expanse.

Even with the apparent implausibility of any success, I would agree with the author and argue that there is a small place for this area of inquiry so long as it does not detract from the pursuit of more secure science. If indigenous species’ artifact searches can be performed in the background using data that is already acquired, then I feel that there is no harm in at least exhausting the possibilities.