Kepler 22B: Exoplanet Dress-up Doll

It's 600 light years from Earth, orbits a star very similar to our Sun in a period of about 290 days, and has a diameter about two and a half times that of Earth. What is it? It's the NASA Kepler mission's most recent exciting confirmed discovery, the extrasolar-planet (exoplanet) Kepler 22B.

Another real, Earth-sized planet to imagine? Cool! I'm on it….

It's fun to play around with the planetary possibilities, as science fiction writers have done for decades, but having a real find out there to pin our thoughts on is something more. On that blank ball-shaped canvass we can paint whatever atmosphere, hydrosphere, lithosphere--and who knows, biosphere?--we care to imagine, at least until scientific observation starts to fill in those details.

But, Kepler 22B offers something more to our fancy than a mere Earthoid dress-up doll. Being somewhat larger than our world, though still smaller than a Neptune or a Uranus, places it in the category of "super-Earth," a type of planet that we have very little experience with.

What do we know of Kepler 22B beyond the barebone figures revealed by the Kepler spacecraft? In a word, not much. Kepler—a really big camera orbiting the Sun and staring at a patch of 150,000 or so stars in the constellation Cygnus—was designed to detect the presence of Earth-like exoplanets. Using the "transit" method of exoplanet detection, Kepler watches unblinkingly for the slight dimming of a star's light as one of its planets "transits," or moves across, its face.

That dimming can tell us exactly three things: the approximate diameter of the planet by how much of the star's light is blocked, the orbital period of the planet by how often it transits, and the distance between the planet and its star (because once you know the orbital period, you can calculate that distance--or visa versa--as long as you know the mass of the star the planet orbits…which we do).

Being relatively close to the size of the Earth makes Kepler 22B an important find, but maybe more important is the fact of the planet's distance from its star. Kepler's mission isn't merely to find Earth-sized planets, but ones that are within their stars' habitable zones: the right distance so that, given a sufficient atmosphere, liquid water could exist on their surfaces.

Kepler 22B is at that correct distance. Though it is closer to its star than Earth is from the Sun, that star is slightly cooler than the Sun, so its habitable zone is closer in. (Earth is obviously in the Sun's habitable zone; if you're not sure, go get yourself a glass of water.)

But back to what we know, and don't know, about Kepler 22B. Not having a super-Earth in our own Solar System, we don't have an up-close example to study. Is it like Earth, a rocky sphere from the core right up to the visible surface, with an apple-peel thin layer of gas and liquid on top? Or is it more like Uranus, with a solid core deep down with massive layers of fluid and gas upholstered around it? Or something between?

We don't know. Future observations may reveal more about this planet, and others. One day we might know more about Kepler 22B's atmosphere (if it has one) through spectroscopic measurements. If we can make a measurement of the planet's mass, we could calculate its average density and better place it on the spectrum between super-Earth and infra-Uranus.

Were we to travel there, could we land, step outside and breathe the air (as well as strain under a super-Earth's gravity)? Would we sink into the fluid envelope around the hidden core, falling to ever greater depths and atmospheric pressure? Would we find ourselves surrounded by human-sized chimpanzees?

That adventure is yet far in the future…but a lot of fun to imagine in the meantime….