Goldilocks number 2 has been found: Gliese 581g. The planet is some 20.5 light-years away from Earth, in the constellation of Libra – unsurprisingly a sign that is presently visible in the Zodiac, so if you live somewhere on the northern hemisphere, go take a peek tonight, to see where we may one day all be heading. For Gliese 581g (the 8th planet discovered orbiting the star Gliese 581) is a special place, statistically.
Gliese 581g is much like Earth. It is small and rocky, but heavy enough – and at enough distance from its sun – to contain an atmosphere. The presence of water would therefore also be ‘likely’, as would – of course with lesser certainty – life. Co-discoverer Steven Vogt reportedly named her Zarmina, after his wife. Mrs. Vogt has an average surface temperature that is estimated between minus 31 to minus 12 degrees Celsius. It may sound a bit too chilly, were it not for another interesting characteristic: like our own Moon the planet does not spin around its own axis. This means [if you don't take dimensions too literally, you shouldn't anyway] one side always faces the sun and could have more Earthlike temperatures.
All in all the discovery has been labeled as the first planet with properties that come close to the ones that are needed to sustain human life. But perhaps these planets are not as rare as we may think. They are just very hard to find. Planets do not emit enough light to be visible over the distance of multiple light-years. They are detected through the anomaly their gravity makes in the position of their central sun. Planets make stars wiggle a little. Problem is stars are usually extremely heavy, whereas Earthlike planets are quite small. Jupiter-sized planets are much heavier and a have a bigger influence on their star’s position – so we can spot these quite easily. But Jupiter-sized planets are, well, like Jupiter. Too big, too gassy, uninhabitable.
(c) Rolf Schuttenhelm | www.bitsofscience.org