Three "Super-Earths" Found Orbiting Sun-Like Star

Anne Minard
for National Geographic News
June 16, 2008
A trio of "super-Earths" have been found near a sun-like star, a team of European astronomers announced today.

The planets orbiting the star HD 40307—which is 42 light-years away—were found using an advanced "planet searcher" instrument at the European Southern Observatory in La Silla, Chile, the French and Swiss astronomers said.

The part of the sky being studied contains 45 potential planets that are smaller than 30 times the mass of Earth, the astronomers said. Most of them orbit HD 40307 quickly—every 50 days or less.

"We are convinced that there are plenty of planets everywhere," said Didier Queloz, a member of the research team from the Observatoire de Genève in Switzerland.

The discovery is creating a buzz throughout the astronomy community.

David Charbonneau, an exoplanet expert from Harvard University who was not involved with the new find, said it heralds a new age: "We have entered the Epoch of the Super-Earths," he said.

The announcement of HD 40307's planets came out of an international workshop called Extra Solar Super-Earths in Nantes, France.

Rapid Discovery

The study of exoplanets really didn't get under way until 1995, Queloz said.

That's about the time Queloz and his colleague Michel Mayor, also from Observatoire de Genève, discovered a planet around the star 51 Pegasi. Since then more than 270 exoplanets have been found, most of them around sun-like stars.

Many are giants like Jupiter or Saturn, and current statistics show that about 1 out of 14 stars harbors that kind of planet.

(Related story: "Top Five Stars That May Support Life Announced" [February 23, 2006])

Queloz said the largest extrasolar planets—gas giants as massive as Jupiter—were found first because they were easiest to detect.

"The biggest ones are certainly the most difficult to form, he added. Everyone is kind of expecting that there are many more small mass planets than big planets."

Better Eyes

The astronomers say better technology developed in recent years will enable researchers to see ever-smaller planets around other sun-like stars.

Now super-Earths—planets that are more massive than Earth but less massive than Uranus and Neptune—mark the threshold for detection.

The three newly discovered planets are 4.2, 6.7, and 9.4 times more massive then Earth and orbit the star in periods of 4.3, 9.6, and 20.4 days, respectively. Their orbits cause a disruption in the motion of their parent star, which allowed the researchers to infer their presence using a popular technique called the "wobble method."

"The perturbations induced by the planets are really tiny," said team member François Bouchy, from the Institut d'Astrophysique de Paris in France.

"The mass of the smallest planets is one hundred thousand times smaller than that of the star, and only the high sensitivity of HARPS made it possible to detect them," he said, referring to the High Accuracy Radial Velocity Planet Searcher instrument, which attaches to the observatory's telescope.

Study team member Queloz said its only a matter of time before astronomy's eyes get sharp enough to find planets no bigger than Earth.

Closer to Life

At the same conference, the team of astronomers announced the discovery of two other planetary systems, also found with the HARPS instrument.

One of these systems has seven times the mass of Earth and orbits the star HD 181433 in just under ten days. That star also hosts a Jupiter-like planet with an orbit that takes close to three years.

The other system contains a planet that is 22 times more massive than Earth, with an orbital period of four days, as well as a Saturn-like planet with a three-year orbit.

"Clearly these planets are only the tip of the iceberg," study team member Mayor said.

Drake Deming, a scientist at the Goddard Space Flight Center in Maryland, called the new study a major breakthrough in exoplanet research.

"It is among the super-Earths that many workers in this field hope to find the first evidence for biomarkers, and thus the first evidence for extrasolar life," he said.

To find biological indications of life, he said, astronomers will need to know something about the atmospheres of the potential planets. And that work requires planets that pass in front of their parent stars.

"Since only a relatively small fraction of planets will transit, finding large numbers of them will be of tremendous benefit," Deming said.

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