Wednesday, 7 December 2011

A new Earth?

The latest scientific story to grab the headlines is the discovery of planet Kepler-22b, an apparent 'twin' of the Earth. This is certainly a fascinating and important discovery, but the matter has been overstated.

Kepler-22b, located 600 light years away on the edge of Cygnus, is the most Earth-like planet so far discovered beyond our solar system. It is the first planet of a comparable size to our own to be confirmed to exist within its system's habitable zone. Smaller planets, closer to that of the Earth, have been detected, but so far all orbiting their primaries at close proximity that would likely make them too hot for life. Kepler-22 is a Class-G star, much like our own, but a little smaller and dimmer; the planet lies 15% closer to its star than we do to our Sun, putting it in a similar position heat- and radiation-wise.
So, the position is a good one, giving a potentially temperate environment for the planet. Yet the 'twin' aspect of Kepler-22b to Earth has been overstated by the stories. The planet has a radius about 2.5 times greater than the Earth's. That's a pretty good approximation of the Earth, in the scheme of things, but not quite what I'd call a twin. As a contrast, look at Venus. Venus is almost an exact match for Earth, size-wise, and is very similar in terms of structure and composition. It's also within the solar habitability zone, just about, so you would expect an environment very similar to that of the Earth. Yet Venus is a broiling hellhole, and any human being who set foot there would be immediately choked, crushed and boiled to death. Venus is a much better match for the Earth than Kepler-22b, but it's hardly a promising colony site.

The nature of Kepler-22b will remain a mystery until estimations of mass and density can be derived. It's probably what planet-hunters call a super-Earth, a planet more massive than our own yet smaller than a gas giant. If it's a rocky planet, like our own, it would have a mass around fourteen Earth masses, and over double the gravity. Enough to retain a thick atmosphere, which, depending on the composition, would retain heat as much as, or considerably more than the Earth's. Wihout an atmosphere it would be far colder - the only reason the Earth is as warm as it is, is due to the greenhouse effect. With a gravity like that, an atmosphere looks likely. Of course, this is no guarantee that the atmosphere will be a nice, transparent oxygen-nitrogen mix like ours. I return your attention to Venus, with its thick layers of carbon dioxide laden with sulphuric acid.

Indeed, this is assuming K-22b is a rocky planet at all. It may well be gaseous, in spite of being a good deal smaller than the gas planets in our own system. Such planets are not precluded by the 'super-Earth' model, which is a misleading term, and includes planets very unlike the Earth. Such a planet is more of a 'gas dwarf' than a gas giant, perhaps similar to the 'hot Neptune' type planets that have recently been detected in other systems. A third possibility is an ocean planet, something that has been theorised but never seen. Such a planet would have a small, rocky-metallic core, surrounded by an extremely deep water ocean, which would itself be enclosed in either a thick atmosphere or a shell of ice.

Finding a planet as Earthlike as K-22b is a major breakthrough, but what everyone is really looking for is a planet similar enough to be a potential habitat for life. Both a rocky world and an ocean planet in the habitable zone are potential abodes for carbon-based life (who knows, maybe a gas dwarf would be too, although it would have to be a very different sort of life). So, there's a slim chance of encountering some alien organisms. At a distance of 600 ly, K-22b is far too distant to even hope of sending a probe to in any kind of forseeable future, although it might be worth training our radio telescopes on it in case of any transmissions from native Keplerites.

Still, this is a fascinating step forward, and shows that such Earthlike worlds do exist in the greater galaxy. It's surely only a matter of time before a planet even more like our own comes to light - and perhaps rather closer to home.

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