Last month I was at the meeting on Exoplanet atmospheres in Heidelberg (http://www.mpia-hd.mpg.de/exoplanets2012/). The conference took place at the MPIA in their new and shiny M51-shaped Center for Astronomy Education and Outreach, complete with a very impressive planetarium.
Summary of the meeting? Well, the theorists took a bashing, and the observers took a bashing, so it was a bit of a sobering wakeup call for everyone. I took it as a sign of growth and maturity within the exoplanet atmosphere community.
Drake Deming set the tone early with the first talk, introducing “the score card”. In it, among the numerous observations and claims about exoplanet atmospheres published in recent years, very few are ranked as reliable by many in the community. It is also still widely agreed that observations are leading theory. Thus, the over-confident fitting of detailed atmosphere models to current observation is also judged to be very fragile by many. In this context, the usefulness of “retrieval” methods – inferring abundances and temperature profiles from a few Spitzer passband measurements – was brought up and debated several times.
The lessons weren’t just for transits either. There was a nice talk by Artie Hatzes about RV searches for exoplanets and GJ 581, bottom line there was that systematics and short-term observations should be treated with caution.
My impression is that as a result of this “sobering up”, the community has realized that the characterization of exo-Earths and the search and identification of biomarkers is not as close as we thought or perhaps hoped. Sarah Seager presented theoretical work on possible biomarker signatures, with the overall task of identifying a possibly inhabited planet. To me the problem looks daunting, with Sarah emphasizing serval times that there will always be false positives to any biosignature. Dimitar Sasselov gave a wonderful talk on his approach (complete with props of haze-generating beer bottles), which is to nearly forget about observations at this point as there is so much theoretical work to be done on even basic planetary structure.
Partially as a result, the focus of exoplanet atmosphere research seemed to move back a bit toward hot Jupiters, less culturally significant maybe, but perhaps a bit more accessible to reproducible observations. There was a lot of ground-based activity shown (lots of great talks and posters) as most groups are trying to tweak their local imagers and spectrographs to get transmission and emission spectra of hot Jupiters. This is definitely a clear and exciting way forward for the next few years as we wait for JWST, although most people don’t realize it yet, as all us such observers are in the middle of battling their own sub-mmag systematic errors and there is little yet in the way of published results.
Often in exoplanet meetings, one planet turns out to be the “star” of the meeting, gathering disproportionate attention. In Heidelberg, that planet was WASP-12 b.
WASP-12 b is the prototype “very hot Jupiter”, orbiting so close to its parent star that it is pulled into a tear-drop shape, and with an atmospheric temperature above 2000 K. The Spitzer eclipse data showed some anomaly that Nikku Madhusudhan et al. interpreted (with a variant of the “retrieval” method, see above) as a sure sign of a very elevated carbon-to-oxygen ratio in the planet (see the “diamond planet” media excitement).
At the meeting, Ian Crossfield showed that a M0V dwarf stands very close to WASP-12, diluting the transit flux measured. This changes everything: all measurements of the atmosphere of WASP-12 are affected, particularly in the infrared, where transit depths and eclipses depths have been underestimated by about 10%.
Actually, in my own talk I was presenting our initial result on WASP-12 b with the Hubble Space Telescope. My striking conclusion was that the transit radius of the planet was much higher in the visible than in the infrared. However, this can entirely be explained by the M dwarf companion. So it goes.
Ironically, when I went back to the acquisition images the HST takes before switching on the spectroscopic mode that we are using, there it was: I could clearly see the stellar companion.