First “potentially habitable” exoplanet discovered
By Darren Quick
September 30, 2010
If you’re looking to get away from it all then Gliese 581g might just fit the bill. But be prepared to pack enough for the trip that, even on a rocket traveling 30,000 km per second (18,640 miles per second), would take 200 years. Gliese 581g is the first exoplanet discovered that sits in an area where water could exist on the planet’s surface. If confirmed, this would make it the most Earth-like exoplanet yet discovered and the first strong case for a “potentially habitable” one.
Gliese 581g is located 20 light years from Earth, orbiting the nearby (in astronomical terms) red dwarf star Gliese 581. It, along with the discovery of another new planet, brings the total number of known planets around this star to six – the most yet discovered in a planetary system outside our own. Like our solar system, the planets around Gliese 581 have a nearly-circular orbit.
Full of potentialBut perhaps put off packing your bags just yet. To astronomers, a “potentially habitable” planet isn’t necessarily one where humans would thrive. Rather, it refers to a planet that could sustain life. Actual habitability depends on many factors, but having liquid water and an atmosphere are among the most important.
With a mass three to four times that of Earth, Gliese 581g 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 onto an atmosphere.
However, the planet is tidally locked to the star, meaning that one side is always facing the star, while the other side is in perpetual darkness. This means that the most habitable zone on the planet’s surface would be the line between shadow and light known as “the terminator”.
A long time comingGliese 581g’s discovery by a team of planet hunters from the University of California (UC) Santa Cruz and the Carnegie Institution of Washington was the result of more than a decade of observations using the W. M. Keck Observatory in Hawaii, one of the world’s largest optical telescopes.
Using the HIRES spectrometer on the Keck I Telescope, the team was able to precisely measure the star’s motion along the line of sight from Earth, and detect the new planet using the radial velocity method. This is when the gravitational tug of an orbiting planet causes periodic changes in the radial velocity of the host star.
"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."
Two previously detected planets orbiting Gliese 581 lie at the edges of the habitable zone. One, Gliese 581c, is on the hot side and the other, Gliese 581d, is on the cold side. While some astronomers still think that planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are skeptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.