Not too hot. Not too cold. Those are the criteria that astronomers have applied in their search for habitable worlds beyond our solar system. Now, an Earth-sized planet has been discovered that apparently meets the Goldilocks test.
The planet is the sixth detected around Gliese 581, a cool red dwarf star located just 20.3 light years away. What makes this planet particularly significant is that it orbits squarely in the middle of the star’s habitable zone – the narrow range of orbital radii where the glow of the star is just warm enough (but not too warm) for liquid water to be stable.
“Hauntingly, this system reminds us of our own solar system in many ways,” says co-discoverer Steve Vogt of the University of California, Santa Cruz, at a news briefing today.
A paper detailing the find by Vogt and Paul Butler of the Carnegie Institution of Washington, along with colleagues, is in press at the Astrophysical Journal. It is based on 11 years of data acquired by the Lick-Carnegie Exoplanet Survey, using the Keck Observatory in Mauna Kea, Hawaii, in combination with an equivalent number of observations made over a four-year period by the High Accuracy Radial velocity Planetary Search (HARPS) project at the European Southern Observatory in La Silla, Chile.
Details about the planet, dubbed Gliese 581g are limited. Because it does not transit (i.e. cross in front of the star) there is no way to independently establish its size. However, the planet’s 36.6 day orbital period can be inferred from a slight back-and-forth wobble of the star, induced by the planet’s gravitational pull. The period translates into an orbital radius that is just under 15% the distance between Earth and the sun. In our solar system such a planet would be broiled by intense heat. However, because Gliese 581 is both smaller and cooler than the Sun, the planet’s “equilibrium” temperature is a balmy 228 K. That is below the freezing point of water, but if the planet has a substantial atmosphere the resulting greenhouse effect could bring that up to Earth-like conditions.
“It seems to have the proper conditions that would be required for life,” Butler observed. He added that red dwarf stars are “virtually immortal”, burning their hydrogen fuel at such a modest pace that Gliese 581 is easily several billion years old, making the odds of life emerging that much greater.
Planets have been discovered in such a favorable range before, but not a planet that is so clearly near Earth’s mass range. Because of the size of the star’s wobble, astronomers can say the planet must be at least three times Earth’s mass. However, if it were any more than about five Earth masses, it would destabilize the other planets previously discovered to be orbiting in the same system. Within that size range, a planet is most likely to be a “super Earth”, composed of the same sorts of rocky material that makes up the terrestrial planets in our own solar system. It would also be expected to have an atmosphere.
Sara Seager, a theoretical astrophysicist who specializes in exoplanets at the Massachusetts Institute of Technology in Cambridge, calls the discovery both “incremental and monumental”. Incremental because it represents just one more object found through the steady application of a technique that planet hunters have mastered over the past two decades. It’s also monumental, she says, because “most would argue this is the first super Earth in a habitable zone”, which means – at least in principle – such a planet could support life.
Seager says she plans to test different possible atmospheric compositions of the planet to see what range of conditions might exist at its surface. Beyond that, it is unlikely that much more can be gleaned only from a mass range and an orbit. To detect a world that can be confirmed to have water and possibly life as well, astronomers must search for planets that cross in front of the stars they orbit. In addition to yielding precise sizes, such detections can sometimes provide direct information about a planet’s atmosphere.
NASA’s Kepler mission, launched in 2009, is currently looking for transiting exo-Earths around sunlike stars. In the future, advanced space telescopes may allow the possibility of directly imaging Earth like worlds.
In the meantime, says Vogt, the discovery of another potentially habitable world within 100 light years of our own increases the likelihood that more such planets exist throughout the galaxy.
Illustration by Lynette Cook