Sunday, January 11, 2015

Why Kepler-442b is probably habitable

Last week NASA announced that eight more of the most promising Kepler planetary candidates had been confirmed. The most interesting of these is Kepler-442b. I'll call it 442b for short. It was previously known as KOI-4742.01. The KOI designation includes things that haven't been confirmed as a planet yet.

442b is the only planet in its system we've discovered, though there may be smaller planets or planets with a year longer than 3 years. We wouldn't have been able to detect those other planets in the system, because they haven't crossed their star from our viewpoint yet, and we've only been looking carefully at this star for the past few years. The 'b' designation means it's the first body in the system detected other than the star. The prevailing theory says that it is important for Earthlike planets to have a few gas giants in their system to sweep up asteroids, so that fewer will hit the planet and prevent life from developing, so we'll want to keep watching this star to see if we can see something like Jupiter for that system.

442b orbits an orange star. This is somewhere between a red dwarf star and a yellow star, like our sun. It means the star is smaller and dimmer than the sun. Kepler orbits closer to its star than Earth does to the Sun: its year is only 112.30 days long (this is one value we know very accurately.) This is close enough that the planet gets about two thirds of the amount of sunlight as Earth does. That still seems pretty bad-- Mars gets 44% as much sunlight as Earth does, and it is really cold-- but luckily, 442b is larger than the Earth. Its radius is a third larger than Earth's radius (another value we can measure directly.) Assuming a composition similar to Earth, it would have a mass about 2.3 times that of Earth, and it's surface gravity would be about 1.3 times that of Earth. More gravity means the planet can hold on to a thicker atmosphere, which means greenhouse effects increase the overall heat of the planet. There's a lot of uncertainty in the temperature, so it could be either too cold, too hot, or just right for humans. On the other hand, the poles would be colder and the equator warmer, just like on Earth, so it seems more likely than not there would be somewhere on the planet we could be comfortable. This is why I say 442b is the most interesting of the ones discovered in this batch: there is another planet (Kepler-438b) even closer in size to Earth, but it orbits much closer to its own star, with a year of just 30 days, and since it is larger than Earth and gets more sunlight than Earth, I would expect it to look more like Venus than like Earth. Also, with a 112 day year, you don't have the risk of tidal locking that you did with some of the earlier discovered 'habitable' exoplanets.

On larger worlds, the theory says oceans would tend to take longer to develop, though they would be deeper and last longer.

442b is 1300 light-years away. 

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