Astronomers have discovered a planet orbiting a red-dwarf star called Gliese 581, located 20 light years from Earth in the constellation Libra, that might have liquid water on its surface.
If they're right, this would be the first planet found outside our solar system that would be capable of sustaining life -- or at least life as we know it.
The discovery was announced today in a paper published in the Astrophysical Journal by Steven Vogt, a professor of astronomy and astrophysics at U.C. Santa Cruz, and Paul Butler of the Carnegie Institution in Washington.
It was based on 11 years' worth of observations through the HIRES spectrometer at the Keck Observatory in Hawaii.
From NASA, which funded the research along with the National Science Foundation:
The spectrometer allows precise measurements of a star's radial velocity (its motion along the line of sight from Earth), which can reveal the presence of planets. The gravitational tug of an orbiting planet causes periodic changes in the radial velocity of the host star. Multiple planets induce complex wobbles in the star's motion, and astronomers use sophisticated analyses to detect planets and determine their orbits and masses.
...The new planet designated Gliese 581g has a mass three to four times that of Earth and orbits its star in just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and enough gravity to hold on to an atmosphere.
But the "g" planet that the astronomers are excited about orbits the star in the "Goldilocks" zone -- a distance that's not too close and not too far, but just right for sustaining water.
Gliese 581g does not rotate on its axis like Earth -- the planet is "tidally locked" to the star so that the same side is always facing the star. Half the planet is always dark and half is always light, but water could exist on the border between the two halves, the astronomers say.
Also, even though the g planet's orbit is much shorter than Earth's -- 37 days compared to 365 days, shorter even than Mercury's orbit -- it shouldn't be unduly hot because, as a red dwarf, Gliese 581 is much cooler than the sun.
Here's an interesting video from NASA on the discovery: