A LAND NOT SO FAR AWAY: Twenty light-years away, astronomers have located what might be the most hospitable world yet discovered outside our solar system. This artist's conception depicts the Gliese 581 planetary system, with the potentially habitable Gliese 581g in the foreground. Image: Lynette Cook/NSF
After more than a decade of telescopic monitoring, astronomers have added two newfound worlds to a nearby planetary system already known to harbor four other planets, and one of the new discoveries looks to be the kind of place where life might be able to take hold.
"Since the beginning of this hunt we've tried to find planets at about the size of the Earth with temperatures so that water can exist," said one of the researchers, Steven Vogt of the University of California, Santa Cruz, in a Webcast press briefing on September 29. "This is the first exoplanet that really has the right conditions for water to exist in liquid form on its surface." Vogt and his colleagues are set to publish their findings in a future issue of The Astrophysical Journal.
By monitoring a small, nearby star for 11 years with one of the 10-meter Keck telescopes in Hawaii and combining the data with 4.3 years of similar observations published by another team, Vogt and his co-authors found two orbiting planets, with respective masses of at least 3.1 times and seven times the mass of Earth. Both qualify as quite small in the field of known exoplanets, in which most of the hundreds of worlds that have been discovered are giants larger than Jupiter. The planetary system, which encircles the red dwarf star Gliese 581 only 20 light-years away, now ranks among the largest known. (In August it was announced that another planetary system boasts at least five, and possibly seven, worlds.)
Of the four previously known planets orbiting the diminutive star, two bracket what astrobiologists call the habitable zone, or the "Goldilocks zone"—the region of space surrounding a star that is neither too hot nor too cold for liquid water and just possibly life. The smaller of the two new worlds, Gliese 581g, orbits right between those two planets, placing it more squarely in the star's habitable zone. Nevertheless, Earthlings would not mistake Gliese 581g for their home planet—in addition to its so-called super-Earth dimensions, it orbits a star far smaller and dimmer than the sun, and its average surface temperatures would vary dramatically, from well below freezing on its night side to scorching hot on the day side.
But somewhere between those temperature extremes, which Vogt estimated might range from –35 to 70 degrees Celsius, would exist stable climatic bands, which Vogt called "eco-longitudes," within which liquid water might persist. Because the planet is probably tidally locked, showing only one hemisphere to its star just as the moon does to Earth, the temperate band between permanent daylight and permanent night might afford life a toehold. "There is a continuum of temperatures in between that are stable," Vogt said. "You just have to move around on the surface."
The actual surface temperatures of Gliese 581g depend on a number of factors that are currently unknown—such as the planet's reflectivity and the strength of any greenhouse effect it might have. "We can't say anything for sure about its atmosphere or about water," study co-author Paul Butler of the Carnegie Institution of Washington said during the Webcast. But what is known about the planet qualifies it as at least potentially habitable. "Its mass would be sufficient to hold a nice, strong atmosphere like Earth," Butler said, "and there would be places on the surface that would be sufficient for water."
The radial-velocity, or "wobble," technique used to identify the new planets relies on tracking Doppler shifts in the host star's light as orbiting planets tug on the star, drawing it nearer to and then farther from Earth. The radial-velocity method has been an extraordinarily productive technique since it was used to identify the first exoplanet orbiting a sunlike star in 1995, but it yields a partial portrait of an exoplanet—revealing only lower bounds for planetary masses, for instance, which can produce ambiguities about whether a planet is rocky, like Earth, or gaseous, like Jupiter, or whether it is an even larger object such as a brown dwarf. In the case of Gliese 581g, it appears that the stability of the planetary system would be compromised if the planet were much more than 4.3 times as massive as Earth.
To get a better picture of the newfound world, astronomers would need a complementary observation, such as watching a partial eclipse (known as a transit) as the planet passes in front of its star, or making a precision measurement of the star's side-to-side motion in the sky. With a planetary transit, researchers can even identify constituents of a planet's atmosphere that might indicate the presence of biological organisms there. But Gliese 581g does not appear properly aligned to transit its star from Earth's vantage point. And instruments for astrometry, which measure stellar positions on the sky, are not yet up to the task. "We're hopeful that continuing advances in astrometry will lead to a confirmation of this discovery and lead to a more precise mass estimate for this planet," Butler said.
Even so, the new paper presents "a marvelously intriguing result," says Geoff Marcy, a University of California, Berkeley, astronomy professor who has collaborated with Vogt and Butler on numerous planet discoveries in the past but was not involved in the new study. Gliese 581g, Marcy says, "is certainly extraordinary for its low mass and for being in the habitable zone."
"It's a very exciting step forward," says David Charbonneau, an astronomer at the Harvard–Smithsonian Center for Astrophysics, who adds that the group collected some "really beautiful data" over the years. Teasing out the subtle signature of small planets in radial-velocity data takes a wealth of observations, especially when the signal is dominated by larger planets in the system, and others are sure to investigate whether the signature of Gliese 581g is real. "There are competing groups that will do their best to see if they can confirm the signal or not," Charbonneau says. "They're probably running those analyses right now, because they learned about this five minutes ago."
Even if the planet proves out, the question of whether Gliese 581g actually hosts any biological activity will remain open. "Any discussion of life at this point is of course speculative," Butler cautioned. "That being said, on Earth, anywhere you find liquid water you find life in abundance."