I remember, as a kid, when the first planets outside our solar system were discovered. But these strange new worlds, inferred largely through watching the gravitationally-induced wobble in “nearby” stars (where “nearby” means stars whose light only took decades or centuries to reach us rather than millennia), were all very different to the planets in our own solar system. Not only did many make Jupiter look like a minnow, they seemed to like getting up close and personal with their parent star. With conjectured surface temperatures hot enough to melt steel, let alone lead, the large number of such planets discovered started to lend some credibility to the Rare Earth hypothesis, which claims that complex life is rare (or even unique to Earth) because planets, like Earth, capable of supporting life are exceedingly uncommon.
As one who has not entirely given up the Star Trek dream of meeting three different alien species a week, therefore, it’s exciting to hear that the most Earth-like planet yet has been discovered. Gliese 581, an otherwise undistinguished little minnow of a red dwarf star about 20 light years up the road from the Sun, has three planets orbiting around it. One of them, with the rather undistinguished name Gliese 581 c, is a planet rather different to the heaving “hot Jupiters” that have been found so far. With a mass roughly 5 times that of Earth, and a diameter conjectured to be about 50% larger, the planet’s orbit is such that liquid water could exist on its surface, and modelling suggests that it will either have a rocky surface like our own Earth, or possibly be covered with oceans.
Nobody has actually seen Gliese 581c; imaging it directly is still beyond the capabilities of our best telescopes. And so we don’t know whether it’s “tidally locked” to its star in the same way the moon is tidally locked to the Earth; if so, it may well be that its sunny side is unbearably hot, its dark side unbearably cold – though, again, computer modelling suggests that an atmosphere might circulate enough heat to avoid that.
To get a decent look at this planet – and the others like it we’ll almost certainly find over the next few years – we’ll probably have to wait a decade or two for NASA’s Terrestrial Planet Finder or the European Space Agency’s Darwin project. But now we at least know that they’ll have some very interesting places to look. Maybe we’ll see some Star Trek aliens waving back down the telescope lens…