Expert: Philip Stahl - 12/7/2011

Question
Hi,

A new Earth-like planet has been discovered, c.600 light years away. Based on the information in these 2 articles

http://www.telegraph.co.uk/science/space/8939138/Kepler-22b-the-new-Earth-could-

http://www.telegraph.co.uk/science/space/8937466/Nasa-discovers-new-Earth-Kepler


, what would you say the average global temperature of that planet is likely to be? Also, if you could give me any other estimates of habitability(eg:- the planet's gravity, whether humans could survive the (greater?)air-pressure at sea-level given the planet's greater size than Earth etc. etc.), based on the above information(even just vague, calculated  guesses if the above data is incomplete), I would be most grateful.

Thanks,
Geoff.

Answer
Hello,

The mean temp. of this new planet has been given as about 72F, but so far none of the sources I've seen indicate how it was obtained, or estimated. It may be a best guess based on some key assumptions, including that a certain type of atmosphere exists analogous to our own. The truth, however, is that we don't *know* that.

The info so far is that Kepler 22b that orbits a "sunlike star" in the Constellation Cygnus, 600 light years distant. But only two datums are available: the period of the planet - 290 days, and the diameter, estimated to be 2.4 times that of Earth, or about 19,000 miles in diameter.

Assuming the planet is orbiting a sun-like star (essentially a "twin" of our Sun to hear the planet finders tell it) then we can use Kepler's 3rd law (a^3 ~ P^2) to obtain the semi-major axis of the orbit, or the mean distance from its sun. This would work out to be about 0.86 AU or 79 million miles. Roughly 7 million miles beyond the mean distance of Venus from our Sun.
(Bear in mind this doesn't mean Kepler 22b is a hellhole like Venus, because Venus' conditions are not derived from its distance from the Sun, but by an incipient runaway greenhouse effect - which is why we want to avoid it here on Earth!)

What we don't know is the **mass**, and that's important because if the density isn't as high as for the Earth (5.5 g/cm^3) it may not be habitable by ordinary mammalian bipeds because of not being able to retain an atmosphere.  We can make the calculations here, assuming the planet is a near sphere, and taking its effective radius as 1.5 x 10^7 m so its volume will be: V = 1.4 x 10^22 m^3. Then for an Earth-like density (D), the mass would be:

M = D x V = (5500 kg/ m^3) (1.4 x 10^22 m^3) = 7.7 x 10^25 kg

The surface gravity, or acceleration of gravity at the surface, can be found from:

g = G M/ R^2

where G is the Newtonian gravitatonal constant, G = 6.7 x 10^-11 N-m^2/kg^2

And g = 10.2 m/s^2

Now, we know the weight is defined: w = mg

And an easy way to arrive at this is simply to apply a ratio of the planet's surface gravity to Earth's which in this case is:

g'/ g = (10.2) / (9.8) = 1.04

In other words, to find a person's weight on the new planet (again, using the assumption it has Earth's density) simply multiply one's Earth weight by a factor 4.85. Hence, a 100 pound woman will weigh 104 pounds, and a 200 pound man will weigh 208 lbs, etc..

Of course, the converse might also be true, and this exciting new world might be a gas planet, with no definable surface but only layers of differing atmospheric density, which is not even comprised of oxygen, but more likely methane and ammonia.

Obviously, if the assumptions above are correct, e.g. it has an Earthlike density, then the air pressure at a putativer sea level would also be similar to Earth's (76 cm of Mercury, or about 10^5 N/m^2), but this is assuming an atmosphere analogous to ours which may not be the case at all.

My best guess then is that little in the way of any educated guess can be made as to Kepler 22b's habitability until such time we find out its mass. Once we know that we can perhaps eliminate whether it is a gas planet, and confirm (more or less) that it actually has a solid surface on which life forms can exist.