The first planet orbiting the red dwarf star Gliese 581 was discovered in 2005. Since then, five more have been added. There are now three planets in this system with the tag “could be habitable.” The most recently verified is Gliese 581g, which has the distinction of sitting more or less in the middle of what astrobiologists call the habitable zone of the system. (The zone has also recently acquired the quirky moniker of the Goldilocks Zone because it’s neither too hot nor too cold.)

Let’s make this clear: ‘habitable’ doesn’t mean the planet is fit for intelligent (or even human) life. It could be any kind of life, viral or bacterial most likely – if there is any life at all. The discovery of a planet in the middle of a habitable zone in no way constitutes proof that there is actually life there. For such proof is needed, at least, detection of water or especially oxygen in an atmosphere. That hasn’t happened yet.

The discovery of the Gliese 581 planets is something of a triumph for astronomy – big telescopes, very tiny measurements, many years of data, and lots of patience. The Gliese planets were found by measuring the changes in the wavelength of light emitted by the star as the planets ‘tug’ on it. This is called detection by the radial velocity method (or Doppler method due to the measurement of the color shift). The latest finding is to be published in Astrophysical Journal and can be currently seen at arXiv [The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581

Two other conceivably habitable planets exist in the margins of the habitable zone of this system. They have been controversial as habitable since their discovery. Gliese 581g is perhaps less controversial, but there are some life-unfriendly things about the planet:

- It is probably tidally locked to its star, like our Moon is locked to Earth. That means one side is always hot and the other side very cold. It’s a precarious situation for life. Can there be an atmosphere? How is heat distributed? If life exists, it might be in the terminator zone, the permanent boundary between light and dark. This has been the subject of much science fiction; but there’s no scientific evidence of life in such zones.
- The planet is in the middle of the habitable zone, but that’s based on models and speculation. That such a zone exists, we don’t know for sure.
- It appears to be a rocky planet like Earth, but that doesn’t mean that it has the right elements for life – especially water.
- There is no evidence as yet for water on the planet. Detection of water isn’t the sine qua none of life, but it’s a good start.
- Because the Doppler method of detection was used, the true mass of the planet is not known. It could be much bigger, having gravity problematic for life.

The planet’s star, Gliese 581 is 20 light years distant, almost a neighbor in an interstellar way. It is a red dwarf, meaning it has a mass less than one-half of our Sun and is relatively cool (4,000 degrees Kelvin). Red dwarves are very common in our galaxy, and they have the distinction of continuing to burn hydrogen for a very long time, as in longer than the life of the known universe. Put these two things together, and the probability for planets around many such red dwarves seems quite high.

That means it's arguably more significant than the discovery of Gliese 581g is how quickly the planet was identified. There are now six verified planets of Gliese 581, all with nicely round (stable) orbits. The evidence is starting to accumulate that planets like this are common – translation into galactic terms: there are billions of them.

The researchers also explored the implications of this discovery with respect to the number of stars that are likely to have at least one potentially habitable planet. Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.

"If these are rare, we shouldn't have found one so quickly and so nearby," Vogt said. "The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy."

[Source: EurekAlert]