Three planets in the Kepler-11 system as they simultaneously transit their star as imagined by by a NASA artist (Image credit: NASA). From http://ciera.northwestern.edu/Research/highlights/research_highlights.php#ForeignWorlds.

Great finish to the meeting, and thankfully no big disasters at my special session.

Lots of excellent talks, but the talk that stood out for me was Sarah Ballard’s, in which she addressed an impressively simple but compelling question: Is there a difference between planetary systems where we’ve only found one planet and systems where we’ve found more than one?

This question is important because such a difference could point to different formation and/or evolutionary processes in these systems, and so comparison of these systems could elucidate subtle but significant aspects of planet formation.

In fact, Ballard did find the two types of planetary system are different, and that, for some reason, about half of M-dwarf stars that host planets have only one.

She also found modest but intriguing differences in the stars that host single planet: their features suggest they may be older than stars with multi-planet systems. Does that mean that single-planet stars used to have multiple planets but enough time has passed that the system became dynamically unstable, leaving behind a single planet?

Blue glacial ice. From http://upload.wikimedia.org/wikipedia/commons/1/10/JoekullsarlonBlueBlockOfIce.jpg.

I really enjoyed Aomawa Shields‘s dissertation talk in the “Extrasolar Planets: Host Stars and Interactions” session, in which she discussed how different stellar types could influence the climates of putative Earth-like planets.

She highlighted how the ice-albedo feedback would operate differently on planets orbiting M-dwarfs as compared to those orbiting F-stars. Since they are so cool, M-dwarfs shine primarily in infrared (IR) wavelengths, while F-stars are much hotter and emit in the visible and ultraviolet (UV). At the same time, water ice primarily absorbs IR but reflects visible light.

Therefore, around an M-dwarf, the ice on an Earth-like planet’s surface would absorb a lot of the stellar insolation, heating the planet, while around an F-star, the ice would reflect it, keeping the planet cool. As a consequence, Shields argued that M-dwarf planets have climates more stable against global ice ages than F-star planets. So although there may be other challenges to life on an M-dwarf planet, climate stability is probably not one of them.