New software finds smallest extrasolar planet yet

A team of researchers recently revealed it has discovered the smallest planet yet found orbiting a normal star outside the solar system thanks to a new computer program developed under the guidance of a NASA astronomer.

The team studied irregularities in the motion of two known giant extrasolar planets circling the star Gliese 876 and its motion to deduce the presence of a third planet in that system, according to a scientific paper submitted for publication in the Astrophysical Journal. An extrasolar planet is a planet that orbits any star other than the sun.

“The mathematical techniques are very complicated, but suffice it to say the new computer program includes the interactions of the planets with one another, not just their individual interactions with the star,” said team member Jack Lissauer, an astronomer at NASA Ames Research Center in California’s Silicon Valley. “Because the interactions of the two biggest planets with each other are so large, this effect needed to be accounted for in order to reveal the existence of the third (smaller) planet,” Lissauer noted.

The newly discovered planet is about seven-and-one half times more massive than Earth and may well have twice its radius. One of the two giant planets orbiting Gliese 876 has a mass half that of Jupiter. The other giant planet is more than twice as massive as Jupiter, according to astronomers. The red star is an M-type, which is considerably cooler than the sun and about a third of its mass.

The new computer program uses ‘dynamical modeling,’ which predicts the motion of planets around a star. This technique accounts for the gravitational tugging of planets on one another.

“This calculation is done on a computer workstation,” Lissauer explained. “We did the dynamical modeling at NASA Ames and at the University of California at Santa Cruz (UCSC) supported by a university/NASA Ames consortium agreement,” Lissauer added. The California-Carnegie Planet Search Team — led by Geoffrey Marcy, University of California, Berkeley; and Paul Butler, Carnegie Institution of Washington — made the observations.

“Eugenio Rivera of UCSC developed the program,” Lissauer said. He guided Rivera, who conducted his doctoral thesis research at NASA Ames with Lissauer. “Without this sophisticated computer program, the small, third planet would have taken much longer to discover,” Lissauer added.

“We’re going to observe this star to see if we can find out more information about its three known planets, and whether there may be more distant planets in that system,” Lissauer said. “We are also going to apply these techniques to other stars to attempt to discover planets even more like Earth,” he added.

Because the newly discovered planet orbits the star Gliese 876, it has been designated as Gliese 876 d. The star is about 15 light years from Earth. The team is only able to determine the planet’s mass, but not its composition, according to Mark Marley of NASA Ames. Marley is not a member of the team, but is one of Lissauer’s colleagues.

All known planets smaller than the newly discovered world are solid bodies, and those larger are gaseous.

According to the team, their data indicate the planet very closely orbits the star, as close as 10 stellar radii, which is less than a tenth of Mercury’s orbit about the sun. Gliese 876 d zips around its star every two days.

“This discovery whets our collective appetite for discovery of even smaller planets,” Marley said. “This extrasolar planet detection gives us confidence that even lower mass, more clearly Earth-like planets await discovery by even more capable ground and space-based techniques,” he said.

NASA’s Kepler mission, scheduled to launch in June 2008, will be able to detect true Earth analogs – planets not only the size of Earth but at the same distance from their stars as Earth is from the sun.

In addition to Lissauer of NASA Ames, the team includes Geoffrey Marcy, University of California, Berkeley; Paul Butler, Carnegie Institution of Washington; Eugenio Rivera, Steven Vogt and Gregory Laughlin, University of California, Santa Cruz, Calif.; Debra Fisher, San Francisco State University; and Timothy Brown, National Center for Atmospheric Research, Boulder, Colo. The team’s observations were conducted at the Keck Observatory in Hawaii.

From JPL

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