Gliese 581

Gliese 581 is a red dwarf star about twenty light years from Earth in the constellation Libra.  It is about one hundred time fainter than the faintest stars that we can see with our eyes and about one third the mass of the sun.  So what makes it special?   It turns out that Gliese 581 has six (and counting) planets in orbit around it.   Last to be discovered, Gliese 581g, is at just the right distance from the star so that its surface temperature would allow liquid water, presumably one of the prerequisites for life.  It turns out that this planet is quite interesting.  Its rotation is tidally locked with its orbit, so as one side of the Moon always faces the Earth, one side of the planet always faces its star.  The most comfortable place to live would been along the terminator between night and day.

The planet orbits the star every 37 days at a distance seven times closer than the Earth is to the Sun.  The mass of the planet is three to four times that of Earth, so astronomers guess that it may have an atmosphere with temperatures ranging from -32C on the night side to 71C on the day side.

http://arxiv.org/abs/1009.5733 The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581

http://www.santacruzsentinel.com/localnews/ci_16212943 UCSC astronomer, others discover first habitable planet outside our system