Three of the four planets that are known to orbit the red-dwarf star Gliese 581 are depicted in this artist's concept. The small dot near center is Gliese 581d, a planet that lies inside the habitable zone -- the distance from the star where temperatures are just right for liquid water, which is a critical ingredient for life. [ESO]
Moon and Libra
Don't give up just yet, Jodie Foster -- another planet may have more Earth-like qualities than originally thought. New measurements of the planet known as Gliese 581d show that it's in an orbit where conditions are suitable for liquid water -- a key ingredient for life.
Gliese 581d is about seven times at massive as Earth, so it probably consists mainly of rock. When it was discovered in 2007, it was calculated to be too far from its host star for liquid water. But later calculations show that it's inside the star's "habitable zone" -- the distance where conditions are just right for liquid water. That makes Gliese 581d one of the first "water world" candidates among the more than 400 extrasolar planets discovered so far.
Its parent star, Gliese 581, is a red dwarf -- a star that's much smaller, fainter, and cooler than the Sun. So far, astronomers have discovered four planets orbiting the star. The lightest of them is only about twice the mass of Earth. It's so close to the star, though, that it's much too hot for water.
This family of planets is in the constellation Libra. Gliese 581 is far too faint to see without a telescope, but you can see its location by following the Moon tonight. Libra's brightest stars stretch to the upper left of the Moon as they rise in late evening. The more distant one is Zubeneschamali. Gliese 581 is just a couple of degrees away from that -- a star system that could be just right for life.
Script by Mark Seidl, Copyright 2009
For more skywatching tips, astronomy news, and much more, read StarDate magazine.