In 2010 a team of planet hunters from the University of California (UC) Santa Cruz, and the Carnegie Institution of Washington announced the discovery of a planet with three times the mass of Earth orbiting a nearby star at a distance that places it squarely in the middle of the star's "habitable zone." The planet designated Gliese 581g orbits its red dwarf 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.
This discovery 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. The research, sponsored by NASA and the National Science Foundation, placed the planet in an area where liquid water could exist on the planet's surface. If confirmed, this would be the most Earth-like exoplanet yet discovered and the first strong case for a potentially habitable one.
To astronomers, a "potentially habitable" planet is one that could sustain life, not necessarily one where humans would thrive. Habitability depends on many factors, but having liquid water and an atmosphere are among the most important.
“We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted,” said Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA).
Six percent of red-dwarf stars have habitable, Earth-sized planets, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) have found. Red dwarfs are the most common stars in our galaxy; about 75 percent of the closest stars are red dwarfs. The closest Earth-like planet could be just 13 light-years away, Harvard astronomer and lead author Courtney Dressing calculated. Since red dwarf stars live much longer than Sun-like stars, this discovery raises the interesting possibility that life on such a planet would be much older and more evolved than life on Earth.
Red dwarf stars are smaller, cooler, and fainter than our Sun. An average red dwarf is only one-third as large and one-thousandth as bright as the Sun. The cFa team culled the Kepler catalog of 158,000 target stars to identify all the red dwarfs, then reanalyzed those stars to calculate more accurate sizes and temperatures. They found that almost all of those stars were smaller and cooler than previously thought.
Locating nearby Earth-like worlds may require a dedicated small space telescope, or a large network of ground-based telescopes. Follow-up studies with instruments like the Giant Magellan Telescope and James Webb Space Telescope could tell us whether any warm, transiting planets have an atmosphere and further probe its chemistry.
Fast forward to 2013, Raymond Pierrehumbert at the University of Chicago studied the range of climates that Gliese 581 g might have and "found one that would have a pool of water on one side, making it look like an eyeball." Should future observations disprove the existence of Gliese 581 g, Pierrehumbert says his efforts could help determine the habitability of exo-Earths still to be discovered.
Because Gliese 581 g is tidally locked to its star, with one side always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness, the most habitable zone on the planet's surface would be the line between shadow and light (known as the "terminator").The artist's conception at top of the page shows the newly discovered super-Earth GJ 1214b, which orbits a red dwarf star 40 light-years from our Earth. It was discovered by the MEarth project -- a small fleet of ground-based telescopes no larger than those many amateur astronomers have in their backyards.
Image Credit: David A. Aguilar, CfA and http://rareearth.wikia.com/wiki/User:Beau.TheConsortium