Monday, January 31, 2011

Kepler habitable worlds around M-dwarf stars to be announced on Wed.

I was excited about the discovery of Gliese 581g a few months ago.  Well, it turns out that planet may have been a statistical glitch and not actually exist.  But that's okay.  Because on Wednesday, the Kepler mission will announce their results from the first six months of observation (the release of the data has been delayed for a year from when it was captured because of researcher politics).
A planet like Earth around a star like our sun would need to have an orbit like Earth's to be habitable.  By habitable, I mean not too hot (like Venus) or too cold (like Mars). But M dwarf stars (red dwarfs) are much smaller and cooler than the sun, so a planet getting the right amount of heat would be much closer, so that it's year would be about 1 earth-month long.
Kepler is observing 2460 M dwarf stars. (From this paper.) Of those stars, between 1 and 3 percent will have their solar systems lined up in the right way so that the planets will pass in front of the star. That makes about 50 systems. Of these, not all the systems will have a rocky planet with a gravity near our own in the habitable zone. But it seems reasonable to suspect that at least 10% of them will.  Since Kepler has been observing for six months, all 5 of these planets will be observed crossing their star at least three times. M dwarfs are more variable than the sun, so the data is more noisy, but the more transits are observed, the more sure we can be that a planet really is in the place we think it is.
So my prediction is that on February second, the Kepler mission will announce not just one, but five potentially habitable worlds.
These worlds are likely tidally locked, so one side is always facing the star and the other is always facing away, in the same way that the moon is tidally locked to the Earth, so that we always see only one face. So I would say that the most likely candidates to actually be habitable (by Earthlings) would be ones with a little higher gravity than earth, so that they can sustain a thicker atmosphere better able to distribute heat around the planet.  Since the greenhouse effect would make such planets hotter on average, we should choose the ones that are receiving a little less heat from their stars than Earth gets from the Sun.

Friday, January 14, 2011

Winter and Summer

(With apologies to Mr. Whelan.)