Just like home, yet light years from Earth - Mild temps but does planet have water?
By Jeremy Manier
Copyright © 2007, Chicago Tribune
Published April 25, 2007
European astronomers say they have found the first Earth-sized planet beyond this solar system with temperatures mild enough to allow liquid water -- a crucial step toward answering whether our cradle of life is unique in the universe.
The planet circles the star Gliese 581 in the Libra constellation, and at 20 light years away is among the 100 stars closest to Earth. Dubbed Gliese 581c, the planet orbits very close to its star -- closer than Mercury is to our sun. But astronomers with the European Southern Observatory say the star is dim enough that average temperatures on the planet would fall in the range of an ordinary Chicago spring day.
If the planet has water -- a big unknown -- its size and climate could make it habitable, experts said. The planet appears to be about 50 percent larger than Earth and has five times more mass, making it one of the smallest far-off planets ever detected.
The conditions look promising enough that officials with the California-based SETI Institute, which looks for signs of radio communication from alien civilizations, said they hope to give the planet a fresh look this summer. Previous radio observations of Gliese 581 in the 1990s turned up nothing unusual.
But the finding is a milestone in any case because it suggests that Earth-like planets may be common throughout the universe, astronomers said. Our galaxy alone could be home to 100 million habitable planets, if such worlds are as easy to spot as the new study indicates.
"This is a marvelous discovery," said astronomer Geoff Marcy, a principal investigator for the California and Carnegie Planet Search. Marcy's group had been racing the Europeans to find the first potentially Earth-like planet.
Next year NASA plans to launch the Kepler probe, designed to find even more Earth-sized planets outside our solar system. That probe will survey thousands of stars in hopes of catching sight of planets that cross in front of them.
As with nearly all such planetary discoveries, astronomers could not observe Gliese 581 directly because it is invisible in the glare of its sun. Instead, the European group led by Stephane Udry and Michael Mayor calculated the planet's presence from its tiny gravitational tug on the star.
The research group released its results Tuesday night and has submitted the work for publication in the journal Astronomy and Astrophysics.
The hunt for far-flung planets has made dizzying progress since 1995, when Mayor and his Swiss colleagues discovered a large extrasolar planet circling a star called 51 Pegasi.
The method they use is straightforward but requires exquisitely precise measurements: Astronomers must scrutinize stars for wobbles caused by the gravity of their planets. Because most planets are tiny compared with the stars they orbit, the wobble can be extremely small.
At first, scientific critics wondered whether the wobbles were merely byproducts of the stars' natural oscillations or resulted from companion stars and not planets. But experts said the techniques have been accepted and are responsible for the discovery of 227 distant planets.
Until the last few years, the only planets found in this way were huge gas giants, similar to Jupiter or even larger. None approached Earth's relatively small size.
To find Gliese 581c, Mayor's group used a cutting-edge planet-seeking instrument at an observatory in La Silla, Chile. The telescope has a device called a spectrograph that can detect small shifts in a star's light as it moves in relation to Earth. When the star edges toward Earth, its light shifts to the blue end of the spectrum; when the star moves away, the light undergoes a red shift.
In the case of Gliese 581c, the planet's pull on the star produced minute shifts in light. The star's movement was as slow as 2 meters per second -- about the speed of a person walking in a park.
"We're to the point where we can see if stars are strolling toward us or strolling away from us," said Steve Vogt, a professor of astronomy and astrophysics at the University of California at Santa Cruz and a member of the California-Carnegie planet search team.
Vogt called the achievement "a landmark discovery."
If it is habitable, Gliese 581c would be a bizarre home for life. It orbits so close to the star that its year is about 13 days long, and it is probably locked with one face constantly facing its sun. On the light side of the planet the sun would dominate the sky, but because it is a relatively weak red dwarf star, average temperatures on the planet would range between 32 and 104 degrees Fahrenheit.
No one knows if the planet has water, or even if it has a solid surface. Some experts said the planet could be a smaller version of gaseous planets like Neptune, with a deep atmosphere and little chance for life to take hold. But models of planetary formation suggest that's unlikely, said Bruce Jakosky, a planetary scientist at the University of Colorado.
"We would lean pretty strongly toward the idea that this has to be a rocky object," said Jakosky, who specializes in astrobiology -- the study of how life might arise on other planets.
The odds so far suggest the galaxy may be littered with such worlds. Planets have been detected near about 5 percent of the stars at which astronomers have looked, and the discovery of Gliese 581c suggests at least 1 percent of those stars have Earth-like planets. So amid the 200 billion stars of the Milky Way galaxy, some 100 million may have planets that meet the bare requirements for life.
Beyond listening for radio transmissions, learning more about those planets will be difficult. It may be decades or longer before astronomers learn how to cancel out the glare of stars and directly observe far-off planets. And although Gliese 581 is our close neighbor in galactic terms, it would take thousands of years for the fastest modern spacecraft to reach it.
The greatest value of planet hunting may lie beyond science and in the realm of philosophy, said Vogt of UC-Santa Cruz. Peering into the void of space so far away, we're really looking for reflections of our origins.
"This is how I connect with the reason for our being," Vogt said. "Are we really rare, a freak of nature, or are there lots of places like this? I think this helps us get purchase on what this life means, and what's our place in the universe."