Another Earth?

Exoplanets
Sister Earth
Apr 25th 2007 EARTHFrom Economist.com
Is there anybody out there?
IF EXTRATERRESTRIAL life were to exist, it would need a planet on which to evolve. All but one of 200-or-so planets outside the solar system discovered by astronomers so far would be quite unsuitable. That is because these planets are composed of gas. Yet the one whose discovery was announced in the early hours of Wednesday April 25th is different. Astronomers think it is rocky, like the Earth, and that it may harbour liquid water. This makes it the best candidate yet for supporting life.
The planet in question orbits a star called Gliese 581 that lies a mere 20 light years away in the constellation Libra. The temperature of the sun is such that it supports nuclear fusion that generates bright sunlight. By contrast, Gliese 581 is a red dwarf, so-called because the star is small and the fusion reaction proceeds slowly, creating a dim glow. Nevertheless, because the planet is much closer to its star than the Earth is to the sun, it lies in what astronomers call the "habitable zone"-the region surrounding a star where water would be liquid.
Seeing such faint objects is difficult. Astronomers used to detect them by indirect methods, such as picking up a small wobble in the position of the star that indicated it was being pulled very slightly towards an orbiting planet. New telescopes and techniques have found other exoplanets directly, by sensing a slight fading in the luminosity of the star as the planet crosses its face. But these techniques only work with massive planets and, in general, giant planets are gaseous.
Looking for planets orbiting red dwarfs is easier because the stars are less massive. This not only means that any planets orbit much more closely but also that the wobbles are more readily seen. The researchers—a team of Swiss, French and Portuguese planet-hunters using a telescope based in Chile—used an indirect method called the “radial velocity” technique. This exploits the Doppler effect familiar when a siren changes pitch as the fire engine on which it is mounted speeds past you. Using this technique, changes in the velocity of the star—that is, the wobble caused by the previously unseen planet—can be measured extremely accurately.
The planet, dubbed Gliese 581c, has a radius 50% larger than this planet. It has five times the mass of the Earth and orbits its star every 13 days. The same team of astronomers who discovered it had earlier found another planet, this time a gaseous giant similar to Neptune, orbiting the same star every 5.4 days. They say they have strong evidence for another planet in the same system that has about eight times the mass of the Earth and orbits every 84 days. The evidence is reported in a paper submitted to Astronomy and Astrophysics.
According to theory, a planet the size and mass of Gliese 581c should be rocky, like the Earth. It could, too, be covered in oceans, perhaps completely. The mean temperature on the surface of the planet would be between 0°C and 40°C, making it far more hospitable than either Venus or Mars, Earth’s nearest neighbours.
The race is now on to detect whether the planet has an atmosphere and whether it contains water. Just a fortnight ago, astronomers using the Hubble space telescope identified water vapour in the atmosphere of an exoplanet, albeit a gaseous exoplanet some 150 light years away. The planet, called HD209458b, passes in front of the Earth every three-and-a-half days, making accurate measurements possible over time.
Even if Gliese 581c turns out to be void of little green men, there is time yet. The sun is thought to be about 5 billion years old and halfway through its lifetime as a “main sequence” star. After that it is expected to become a red giant, as the hydrogen that presently comprises it is exhausted and the sun switches to burning helium instead. At that point, the Earth’s atmosphere and water will be boiled away, leaving the planet uninhabitable.

Red dwarfs, meanwhile, burn for hundreds of billions of years. This not only gives plenty of time for life to evolve on the recently discovered planet. It may make the place a useful bolthole in some 5 billion years’ time.