First Habitable Earthlike Planet Found, Experts Say

James Owen
for National Geographic News
April 24, 2007

The first known planet beyond the solar system that could harbor life as we know it has been discovered, scientists report.

The most Earthlike planet yet found, it orbits a red dwarf star and likely contains liquid water, said the European astronomers who made the discovery.

The planet is estimated to be only 50 percent larger than Earth, making it the smallest planet yet found outside the solar system, according to a team led by Stephane Udry of the Geneva Observatory in Switzerland.

Known as Gliese 581 c, the newfound world is located in the constellation Libra, some 20.5 light-years away.

The planet is named after the red dwarf star it orbits, Gliese 581, which is among the hundred closest stars to Earth.

Because the planet is 14 times nearer to its star than Earth is to the sun, a year there lasts just 13 days. Gravity on the planet’s surface, though, may be twice as strong as Earth’s gravity.

Despite the close proximity to its parent star, however, Gliese 581 c lies within the relatively cool habitable zone of its solar system. That’s because red dwarfs are relatively small and dim, and are cooler than our sun, the team explained.

The scientists estimated the planet’s surface temperature at between 32 and 104 degrees Fahrenheit (0 and 40 degrees Celsius).

“This means water can exist in liquid form,” Udry said. “If you want life like our own, then you need water.”

The team reports its findings in the journal Astronomy and Astrophysics.

Rock or Oceans

The new world could feature familiar, rocky terrains or be completely covered with oceans, the researchers said.

In either case, Gliese 581 c will likely become a target for missions in search of extraterrestrial life, they added.

“We still have a long way to go before reaching that point. But for sure it’s the best candidate we know of right now,” Udry commented.

“The planet is really close to us,” he said. Still, it would take 20 years to get there if traveling at the speed of light, and another 20 to return.

Gliese 581 c is better suited to life than larger planets like Jupiter, which tend to be dense masses of gas, Udry explained.

(See an interactive map of the solar system.)

“You need a rocky planet to find life—the big giants are not the best places for that,” he said.

More precise instruments have recently enabled astronomers to detect small “exoplanets”—worlds found outside our solar system.

“We started to find them two or three years ago,” Udry said. Thirteen exoplanets that have less than 20 times the weight of Earth have been discovered so far, he noted.

“We found them very easily, so it looks like they are much more numerous than the giant planets we were finding before,” the astronomer said.

Planet Hunter

The new planet was detected using an instrument called a spectrograph at the European Southern Observatory at La Silla, Chile.

Known as the High Accuracy Radial Velocity Planetary Searcher (HARPS), the device is described by the team as a “unique planet-hunting machine.”

It works by detecting the pull of an unseen exoplanet on the star it orbits. An orbiting planet causes its star to wobble slightly, and this effect can be measured by instruments such as HARPS.

Advanced spectrographs are enabling astronomers to detect ever smaller planets, said Michael Perryman of the European Space Agency’s Astrophysics Missions Division in the Netherlands.

“The wobble for these planets that they are detecting now is very, very tiny—about three meters [nine feet] per second, which is about the speed you run at,” Perryman said.

“New planets are being discovered every few weeks or so,” he added. “The interesting development is when you start getting these lower-mass planets closer to [the weight] of the Earth.”

The newfound planet is especially noteworthy, Perryman said.

“As soon as you find a planet at the right distance [from its star] such that liquid water might exist, then you’re saying this is the kind of environment in which one might start looking for life,” he added.

Udry, of the Geneva Observatory, said the goal of future programs is to find a planet and star pairing to match that of Earth and the sun.

“We are now developing instruments which will allow us to find those,” Udry said

“We hope, and even expect, to have these habitable planets all over the place.”

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