In the middle of a crazy week at AGU, I finally had a “free” hour with which to watch the Kepler press conference on Kepler 22b. I have to say, the they really did an *outstanding* job. This goes both for the science contained in the announcement, and the announcement itself. The one wish I have is that they’d stop using equilibrium temperature as the metric they use to bound the habitable zone, but I totally understand why they’re doing this. Equilibrium temperature is something they know how to calculate, and that is related to the habitable zone. The issue is twofold: 1.) the use of equilibrium temperature requires knowledge of the planetary albedo (the fraction of incoming light reflected by the planet), and the albedo is totally unconstrained; and 2.) to explain to people why “262 K/-11 C/12 F” is habitable, they have to explain that a greenhouse effect “like the Earth’s” would bring the temperature to a nice, comfortable 72 F. The issue is this is not even an estimate… it’s just “what this planet would be like if it’s like the Earth.”
I plan to head up to the Kepler Science Conference later this week (either tomorrow or Friday) to congratulate the Kepler team… and to talk to some of them about this, and to give them an alternate metric for habitability (instellation, the total energy received from the star). This is a metric that one can also use to constrain the habitable zone, AND plot it against other physical parameters such as planet radius. And what’s best is that it doesn’t require knowledge of anything beyond which the things the Kepler mission (and others) can directly measure. This is what my AGU poster was all about. It turned out to be incredibly timely, as I ended up adding a pushpin for Kepler 22b on the graphs on my poster. It was well received amongst the planetary folks here, but that was preaching to the choir a bit. I hope it’s as well received at the Kepler meeting.
I want to stress that despite my complaints above, I am incredibly excited by this discovery. If Kepler 22b isn’t at the top of the “most likely to be habitable” exoplanet list, it’s very, very close to the top. And while I think it’s too large to be truly Earth-like, it is instead probably something really exotic… like a “warm Neptune” that is inside the habitable zone or a “super-Earth” that is rocky, but that has a mass ~20 times the mass of our planet. Or it could be something else entirely. The point is until we measure the mass of the planet we won’t know what it’s like. And until we get some atmospheric measurements, we won’t know what the surface temperature is like. For example, it could have a runaway greenhouse, and thus be a “super-Venus.” (Cue David’s demand to send a probe there!) Or maybe it’s something I can’t even imagine or explain using the examples we have here in our own solar system. No matter what it is, it is and will continue to be fascinating.
Well done, Kepler team. You continue to make wonderful discoveries. (I can’t tell you how much I’m awaiting confirmation of the planets with Earth-level insolations and < 1.2 Earth radii….)