Session 2

Geoff Marcy is next talking about radial velocity measurements of 52 planets orbiting 22 stars from Kepler, half of which have asteroseismology measurements. The revealed masses and densities of some of these planets appear to be negative, because the team declined to enforce positivity in the MCMC posteriors in order to avoid biasing the overall statistics of the sample.  That is, many of the planets have not been detected in radial velocities.  

Some of the detections are consistent with rocky planets, densities of a few g/cc, others are clearly gaseous (about 1.5 g/cc).  The bigger planets are lower density, consistent with earlier transiting work and the smaller (below 2 Earth radii) planets are “mostly rocky planets” (where the “mostly” is deliberately ambiguously modifying “rocky” and “planets”, or possibly both).  Above 100 Earth masses, the trend reverses, as we transition from planets to brown dwarfs.

Geoff says 21 years ago Dave summoned the nascent radial velocity community here, and Geoff has his transparencies for his talk at that meeting with him!  Back in 1992, there were no BDs and no exoplanets.  Shows a an image of the 429 RVs for HD 114762 presented there.  Dave’s introductory remarks state that the “conventional wisdom” is that giant planets will only be found in decade-long orbits, so this will be a long-term project.  

Geoff cites, still from that conference, Bill Cochran’s First Commandment of Planet Detection: Thou shalt not embarrass thyself and they colleagues by claiming false planets.

  

The following events occurred amid a jocular mood:  “And now a controversial topic”: “Who may name a planet?”  Marcy says that anyone may suggest a planet name, and so he solicits nominations from the gallery for names for HD 114762b.  “Latham’s planet” is loudly and prominently shouted from many points of the room (and may or may not have come from plants who were sitting next to me prior to the talk).  Marcy quickly closes the nomination and holds a lightning-fast vote (“all in favor” gets hands; a vote for “all opposed” is not taken).  Marcy’s next slide:

 “HD 114762b is hereafter known as: Latham’s planet*” (“*=pending IAU approval, of course”).  

Next up is Leslie Rogers showing how Kepler has filled out the mass-radius diagram below 10 Earth radii and 50 Earth masses, including adding a new dimension of incident flux, allowing us to study retention and survival of atmospheres.  Describes the problem of inverting mass and radius measurements with uncertainties into a probability of being “rocky”.  Finds a dividing line around 1.5-2 Earth radii between rocky vs. non-rocky planets, consistent with Geoff’s talk.

Leslie finds that the boundary between “rocky” and “non-rocky” can be tightly constrained between 1.7-1.8 Earth radii in a simple, one-parameter model where “rockiness” is a simple function of radius.  In a two-parameter model with a transition region, values up to 2 Earth radii are allowed.

Finishes with a Latham memory, despite not having ever worked with him.  Dave gave many conference highlight talks at many of her first conferences, and she saw him as a “a kind senior authority figure”.  It became her aspiration to “make it into one of your summary highlight reels”. :)

Next up is Ruth Murray-Clay giving an overview of the physical processes that we have to worry about for these lower mass planet atmospheres. Shows fraction of mass lost over 10 Gyr at 0.05 AU.  HD 209458 loses 1% of its mass, which is not a big deal because it is made entirely of gas.  For a super-Earth, this is a substantial fraction of its atmosphere.

Two kinds of escape: kinetic, and hydrodynamic, regulated by the Jeans escape limit and hydrodynamic escape limits, respectively.  UV photons heat atmospheres through ionization.

3 fates for this energy: radiated away in place, conducted lower in the atmosphere, then radiated, or it could drive an outflow; all of these could be observable.  Ly-α and H₃⁺.  Low-T planets will have molecules that are good radiators, so the energy will come out as radiation.  For hot Jupiters, the result could be an energy-limited outflow.