"Earth's Bigger Cousin" Found Outside Solar System

Brian Handwerk
for National Geographic News

June 15, 2005

A team of scientific "planet hunters" has identified the most Earthlike planet ever found outside our solar system. The rocky "super-Earth" is 7.5 times more massive than our own planet.

"This planet answers an ancient question," said team leader Geoffrey Marcy, professor of astronomy at the University of California, Berkeley. "Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earthlike planets. Now, for the first time, we have evidence for a rocky planet around a normal star."

The as-yet-unnamed new world orbits Gliese 876, a star that is about one-third the mass of our own sun. The planet orbits much closer to the star than our system's innermost planet, Mercury, does to our sun. As a result, the newfound body takes just under 48 hours to circle Gliese 876—giving the planet a year that is equivalent to about two Earth days.

Two Jupiterlike planets also circle Gliese 876, which is the smallest known star to host orbiting planets. The entire system is located just 15 light-years from Earth in the direction of the constellation Aquarius.

Though larger than Earth, the new planet's mass is small compared to the masses of gas-giant planets, which are more commonly detected. This lower mass first alerted scientists to the presence of a possibly rocky planet much like our own.

"This is the smallest extrasolar planet yet detected and the first of a new class of rocky terrestrial planets," said Paul Butler in a statement. Butler is an astronomer with the Carnegie Institution in Washington, D.C. Terrestrial planets are relatively Earthlike; in our own system Mercury, Venus, and Mars are considered terrestrial.

"It's like Earth's bigger cousin," Butler added.

Earths Aplenty?

A decade ago extrasolar planets—planets found outside the solar system—were virtually unknown. Today more than 150 have been discovered orbiting stars similar to our own sun, and the number is steadily rising.

Previously, all of the planets found orbiting sunlike stars resembled the gas giants of our solar system: Jupiter, Saturn, Uranus, and Neptune. Just three rocky planets were discovered previously, and those were all orbiting a pulsar—the twinkling corpse of a collapsed star.

None of the distant extrasolar planets, not even the gas giants, are visible, even with the most advanced telescopes. To spot these planets, scientists rely on indirect methods of detection, such as the "wobble" a planet's gravity induces in the star it orbits.

Such techniques generally make smaller planets difficult to detect. The new planet, though, was located successfully after scientists refined the "wobble" technique.

"We keep pushing the limits of what we can detect, and we're getting closer and closer to finding Earths," team member Steven Vogt said in a statement. Vogt is a professor of astronomy and astrophysics at the University of California, Santa Cruz.

The search for other "Earths" is often tied to the quest for life on other planets. Scientists theorize that many Earthlike planets may exist in orbits around distant stars. (See "Many "Earths" Are Out There, Study Says.")

Some planets could even orbit in a so-called Goldilocks Zone (not too hot, not too cold). There, a planet's surface temperature could be just right to sustain liquid water—and possibly life as we know it.

Such life is extremely unlikely on the new "super Earth," however. The planet orbits so close to Gliese 876 that its surface temperatures are estimated at 400 to 750 degrees Fahrenheit (200 to 400 degrees Celsius).

Direct observation of the new world is not yet possible, leading scientists to speculate about its composition.

"The planet's mass could easily hold onto an atmosphere," said Gregory Laughlin of the Lick Observatory at the University of California, Santa Cruz, in a media release. "It would still be considered a rocky planet, probably with an iron core and a silicon mantle. It could even have a dense steamy water layer.

"I think what we are seeing here is something that's intermediate between a true terrestrial planet like the Earth and a hot version of the ice giants Uranus and Neptune," Laughlin added.

The newfound planet was identified at the W.M. Keck Observatory on Hawaii's Mauna Kea volcano. Scientists announced the discovery Monday at the National Science Foundation in Arlington, Virginia.

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