The quest to find Earth-like planets around other stars appears to have taken a significant step forward. Astronomers in Europe have detected a "super Earth" orbiting a dwarf star 20.5 light-years away.
The planet appears to be the most Earth-like of any of the more than 200 planets yet found outside the solar system.
Among these, only a handful of "super Earths" – a moniker for planets weighing in at two to 10 times Earth's mass – have been discovered so far. But to some astronomers, they may hold special significance. In theory, at least, they have the potential to be better platforms for the emergence of life than Earth itself.
The newly discovered planet's diameter is about 50 percent bigger than Earth's and tips the scales at five times Earth's mass, according to the team of scientists, which announced their findings Tuesday. Most intriguingly, it orbits its parent star at the right distance to allow for the possibility that liquid water has pooled and remains stable on its surface.
"The planet should be either rocky ... or covered with oceans" based on modeling studies, notes Stephane Udry, a researcher at the Geneva Observatory and head of the team reporting the results. Its findings are going through peer review for the journal Astronomy & Astrophysics.
To be sure, no water has yet been detected. Indeed, the planet itself has only been observed indirectly – through a slight wobble its gravitational tug imparts to the spectrum of Gliese 581, a star in the constellation Libra.
But researchers say the characteristics they can tease from that wobble point to a rocky planet orbiting once every 12.9 Earth days at a distance of about 6.5 million miles from the star.
If Gliese 581 were as big and hot as our sun, that would spell trouble. The solar system's innermost planet, Mercury, is nearly six times farther away from the sun and it's still a barren hunk of crackling hot rock on the daylight side and a frosty ice box on the night side. The sun's habitable zone is said to begin about 88 million miles from the star and drop off some 155 million miles out (think Earth to Mars).
Benefits of a dwarf star's faintness
But Gliese 581 is a red dwarf, a small type of star that has only a third of the sun's mass and is at least 50 times fainter. So its habitable zone lies much closer to the star than the sun's. The team estimates that the planet they've discovered boasts an average surface temperature of between 0 and 40 degrees C (32 to 104 degrees F.).
"On the treasure map of the universe, one would be tempted to mark this planet with an X," says Xavier Delfosse, a researcher at Joseph Fourier University at Grenoble, France, and another member of the team.
The discovery is "truly awesome," agrees Dimitar Sasselov, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Although he was not part of the team reporting the discovery, he is collaborating with the group on follow-up observations to help pin down the planet's traits with greater precision.
The team used an extremely sensitive planet-hunting detector, dubbed HARP, bolted to the back of the European Southern Observatory's 3.6-meter telescope at La Silla, Chile. HARP is said to be able to detect an object's velocity to within about two miles an hour of its actual value. Two years ago, HARP nabbed Gliese 581's first planet – similar in mass to Neptune. In the new data the team gathered on the second planet, astronomers also have seen strong evidence for a third planet. Preliminary indications suggest it's orbiting every 84 days and is about eight times more massive than Earth.
Along with Dr. Sasselov, the team is using Canada's "MOST" orbiting telescope starting this week to conduct follow-up studies. MOST is small by just about any standard. Its light-gathering mirror is only about 6 inches across, compared with the Hubble Space Telescope's 94-1/2-inch mirror or the Keck Observatory's twin mirrors, each nearly 400 inches across. Indeed, Canada's plucky instrument has earned a water-cooler name: the Humble Space Telescope.
But what it lacks in size it makes up for in sensitivity to subtle changes in starlight, Sasselov explains. The team will use that sensitivity to monitor Gliese 581 for slight changes in its light as its planets swing across its face.
This "transit" technique for detecting planets can add information that will improve estimates of the planets' masses and other characteristics.
"We expect a very small amount of dimming, maybe one-fifth of 1 percent of the light we receive" from the star, he explains. "That's difficult to do from the ground."
Some researchers have suggested that rocky Earth-like planets orbiting so close to a star, even faint ones like Gliese 581, may not be very hospitable.
Big tides could hurt potential for life
Tidal forces between the star and planet could shut down the planet's rotation. It would present the same face to the star, frying half the planet and freezing the other half. But Sasselov suggests that, at least in this case, tidal forces from the two other planets could moderate the star's effect. Moreover, for even simple life to emerge and adapt, it needs a stable environment over tens of thousands to millions of years, he says. Because super Earths are so massive, their gravity can retain its grip on a thick atmosphere. And theory suggests super Earths would have more-active plate tectonics than Earth itself, he says. This shifting and ripping of the planet's crust renews it with molten material and could represent a stable source of carbon dioxide over geological time scales, potentially allowing for the rise of photosynthesis.
These ideas are speculative for now. Nevertheless, super Earths may "change the whole discussion on what constitutes a habitable planet," Sasselov argues, especially those orbiting red drawfs like Gliese 581.