It probably wouldn't feel exactly like home. But the planet known as Gliese 581d has a lot more in common with Earth than astronomers first thought.
New measurements of the planet's orbit place it firmly in a region where conditions would be right for liquid water, and thus life as we know it, astronomer Michel Mayor, from Geneva University in Switzerland, announced today.
"It lies in the [life-supporting] habitable zone, and it could have an ocean at its surface," Mayor said during the European Week of Astronomy and Space Science conference, being held this week at the University of Hertfordshire in the U.K.
First discovered in 2007, Gliese 581d was originally calculated to be too far away from its host star—and therefore too cold—to support an ocean.
But Mayor and colleagues now show that the extrasolar planet, or exoplanet, orbits its host in 66.8 days, putting it just inside the cool star's habitable zone.
At the same time, Mayor and colleagues announced that they have spotted a fourth planet orbiting in the Gliese 581 star system—and it's the lightest exoplanet found so far.
The planet, dubbed Gliese 581e, is only about twice the mass of Earth and is the closest planet to the star, completing its orbit in about 3.15 days.
"It brings down the mass [of the lightest known exoplanet] by more than a factor of two. The previous smallest was around five Earth masses," said Andrew Collier Cameron, an astronomer at the University of Saint Andrews in the U.K. who was not involved in the find.
(Related: "Mysterious 'Super Earth' Is Smallest Known Exoplanet?")
Near Neighbor
Gliese 581, a red dwarf star in the constellation Libra, lies around 20.5 light-years from Earth.
"In astronomical terms it is one of our near neighbors, the 87th closest known star system to the sun," said Carole Haswell, an astronomer at the Open University in Milton Keynes, U.K.
Since planets orbiting Gliese 581 are too far away to be seen directly, Mayor and colleagues originally spotted Gliese 581d by searching for tiny wobbles in the host star's motion using the European Southern Observatory (ESO) telescope at La Silla in Chile.
Weighing in at around seven Earth masses, Gliese 581d is unlikely to be made of rocks alone, the team believes.
"We can only speculate at this stage, but it may have a rocky core, encased in an icy layer, with a liquid ocean at the surface and an atmosphere," Mayor said.
Meanwhile, the much smaller and lighter Gliese 581e "probably doesn't look too different to Earth, except that it will be very hot, because it is so close to its host star," said Andrew Norton, an astronomer also at the Open University.
Norton's colleague Haswell added: "It is very exciting that such a promising candidate for an Earthlike planet has been found so close to us. It means there are likely to be many more when we search further."
And the more Earthlike planets there are, the greater the chance of discovering one that harbors life.
"I think it is only a matter of time," Norton said. "If life really does exist elsewhere in the universe, then within the next 10 to 15 years I expect we may see the first signs of life, via spectroscopic signals from exoplanets."
Scientists have made two major discoveries in the search for other Earth-like planets. British astronomers say they have discovered the smallest known Earth-like exoplanet orbiting a living star called Gliese 581 e and also discovered that a nearby planet called Gliese 581 d, discovered in 2007, is in the much sought after 'habitable zone'. Astronomers also believe there is a possibility that 581 d could have liquid oceans on its surface, calling the find "extraordinary."
Artist's impression of the planetary system around the red dwarf Gliese 581. 
"The Holy Grail of current exoplanet research is the detection of a rocky, Earth-like planet in the 'habitable zone'," said astrophysicist at Switzerland's Geneva University, Michel Mayor.

Astronomers say 581 e is 20.5 light years away from Earth and that it's roughly 1.9 times larger than Earth, making it the smallest exoplanet discovered to date. It was found using the High Accuracy Radial Velocity Planet Searcher (HARPS). It orbits too close to its parent star, Gliese 581, which would make it too hot, unable to support life as we know it. It takes just over 3 days for it to orbit the star.

Exoplanet 581 d, however, is another story. Astronomers say this makes it possible to not only support life, but have liquid water in the forms of oceans. 581 d is approximately 7.7 times larger than Earth. Astronomers originally thought it was too cold to have liquid water thus would not be able to host Earth-like life.

"It is very exciting that such a promising candidate for an Earthlike planet has been found so close to us. It means there are likely to be many more when we search further," added Mayor.