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Alien Earths: As Common as Dirt?

Scott Gaudi has a simple answer when asked about the number of Earth-like planets in the universe. "They're everywhere. Common as dirt," says Gaudi, an astronomer at Ohio State University. He spoke on 15 February at the AAAS meeting in Chicago during a session titled "From Enlightenment to Lunar Theories to Extrasolar Planets."

Gaudi pointed out a half-hidden statistic near the end of a recent paper by Michel Mayor of the Geneva Observatory in Switzerland: For stars like the sun, the likelihood of finding planets similar to Earth in orbits very close to the star is about three in 10. That doesn't take into account Earth-like planets that may lurk further away from their host stars. The actual odds probably are much higher, Mayor noted.

The research, published in Astronomy & Astrophysics, is good news for those astronomers looking for planets in the "habitable zone," the place around the star where researchers think life could exist. It's also helpful information for telescopes that look for planets similar to Earth -- including NASA's upcoming launch of Kepler, a satellite designed to locate and study such planets.

The idea of finding a planet around a star other than the sun was considered impossible just 15 years ago. The only planets observed at that time were those in the solar system, and based on those observations astronomers came up with a theory of planet formation. Now, scientists know differently. "It turns out all of our expectations were wrong. Mother Nature is a lot more inventive than we are," Gaudi said.

Astronomers now know of more than 340 worlds orbiting stars other than the sun. The origin of many of those doesn't fit the previous formation theory. In fact, scientists can't explain how most of these newly discovered planets formed, according to Christophe Lovis of the University of Geneva.

Lovis explained that to detect faraway planets, one can carefully monitor the movements of host stars. Just as ripples coming from a point on a lake's surface prove there's a disturbance at that point, Lovis notes that planets can be detected by the tiny, ripple-like changes they make on a star's orbit.

Researchers like Mayor and his team have honed this technique to allow them to see smaller ripples caused by planets a few times the size of Earth. But the question remains: How much like Earth are these planets? Do they harbor water and the possibility for life?

Using this method alone, astronomers can't tell. To find out more about the planet's properties, scientists much catch it as it passes in front of its host star, which is only possible if the star and planet are in just the right alignment for observers on Earth.

Kepler is designed specifically to find such planets. Future NASA missions, more powerful than Kepler, may be able to discern the ingredients of the planets' atmospheres -- and detect possible chemical imprints of life.

The realization that rocky planets probably are common pays tribute to the tremendous advances in planet-finding capabilities, Gaudi said.

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Jessica Kloss is a senior astrophysics major at Princeton University. She loves reporting on science -- especially astronomy -- for the general public. Reach her at jkloss@princeton.edu