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June 13, 2005

Astronomers discover the most Earthlike planet yet outside the solar system

By Robert Sanders and Tim Stephens

A team of astronomers has reached a major milestone in the search for Earthlike planets with the discovery of the smallest planet ever detected beyond our solar system. About seven and a half times as massive as Earth, with less than twice the radius, it may be the first rocky planet ever found orbiting a normal star not much different from our Sun.

New planet

In this artist's conception, the newly discovered planet is shown as a hot, rocky, geologically active world glowing in the deep red light of its nearby parent star, the M dwarf Gliese 876. The heat and the reddish light are among the few things about the new planet that are certain; depending on the thickness and composition of its atmosphere--if any--it could range from being a barren, cratered ball of rock like Mercury or the Moon, to being a featureless, cloud-shrouded cueball like Venus.
Photo: Trent Schindler, National Science Foundation

All of the nearly 150 other extrasolar planets discovered to date around normal stars have been larger than Uranus, an ice giant about 15 times the mass of the Earth.

"We keep pushing the limits of what we can detect, and we're getting closer and closer to finding Earths," said team member Steven Vogt, a professor of astronomy and astrophysics at UCSC.

The new planet orbits the star Gliese 876, just 15 light-years away and located in the constellation Aquarius. This star also harbors two larger, Jupiter-size planets.

The smaller planet whips around the star in a mere two days, and is so close to the star's surface that its temperature probably tops 200 to 400 degrees Celsius (400 to 750 degrees Fahrenheit)--ovenlike temperatures far too hot for life as we know it.

Nevertheless, the ability to detect the tiny wobble that the planet induces in the star gives astronomers confidence that they will be able to detect even smaller rocky planets in orbits more hospitable to life. Though the team has no proof that this new planet is rocky, its low mass precludes it from retaining gas, like Jupiter. And because it is so close to the star, it is unlikely to be a cold ice giant like Neptune. Three other purported rocky planets have been reported, but they orbit a pulsar, the flashing corpse of an exploded star.

"This planet will be historic," said team leader Geoffrey Marcy, professor of astronomy at UC 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."

Marcy and three other members of the team--Paul Butler of the Carnegie Institution of Washington, UCSC postdoctoral researcher Eugenio J. Rivera, and theoretical astronomer Jack Lissauer of NASA Ames Research Center--presented their findings June 13 during a press conference at the National Science Foundation in Arlington, Va. The research, conducted at the Keck Observatory in Hawaii, was supported by NSF, the National Aeronautics and Space Administration, and the University of California.

A paper detailing the results has been submitted to the Astrophysical Journal. Coauthors on the paper are Vogt and Gregory Laughlin of UCSC, Debra Fischer of San Francisco State University, and Timothy M. Brown of the National Center for Atmospheric Research in Boulder, Colo.

Gliese 876 (or GJ 876) is a red, M dwarf star--the most common type of star in the galaxy--and, at about one-third the mass of the Sun, the smallest star around which planets have been discovered. The Marcy team detected the first planet in 1998, a gas giant about twice the mass of Jupiter. In 2001 they reported a second planet, another gas giant about half the mass of Jupiter. The two are in resonant orbits, the outer planet taking 60 days to orbit the star, twice the period of the inner giant planet.

Rivera and Lissauer have been analyzing Keck data on the Gliese 876 system in order to model the unusual motions of the two known planets, and three years ago got an inkling that there might be a smaller, third planet orbiting the star. In fact, if they hadn't taken account of the resonant interaction between the two known planets, they never would have seen the third planet.

"We had a model for the two planets interacting with one another, but when we looked at the difference between the two-planet model and the actual data, we found a signature that could be interpreted as a third planet," Lissauer said.

A three-planet model consistently gave a better fit to the data, added Rivera. "But because the signal from this third planet was not very strong, we were very cautious about announcing a new planet until we had more data," he said.

Recent improvements to the Keck Telescope's high-resolution spectrometer (HIRES) provided crucial new data. Vogt, who designed and built HIRES, worked with the technical staff in the UC Observatories/Lick Observatory Laboratories at UCSC to upgrade the spectrometer's CCD (charge coupled device) detectors last August.

"It is the higher precision data from the upgraded HIRES that gives us confidence in this result," Butler said.

The team now has convincing data for a planet orbiting very close to the star, at a distance of about 10 stellar radii. That's less than one-tenth the size of Mercury's orbit in our solar system.

"In a two-day orbit, it's about 200 degrees Celsius too hot for liquid water," Butler said. "That tends to lead us to the conclusion that the most probable composition of this thing is like the inner planets of this solar system--a nickel/iron rock, a rocky planet, a terrestrial planet."

"The planet's mass is enough that it is capable of holding onto an atmosphere," noted Laughlin, an assistant professor of astronomy and astrophysics at UCSC. "It would still be considered a rocky planet, probably with an iron core and a silicate 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."

With the HIRES upgrade and improved computer software, the team can now measure the Doppler velocity of a star to within 1 meter per second--human walking speed--instead of the previous precision of 3 meters per second. This improved sensitivity will allow the planet-hunting team to detect the gravitational effect of an Earthlike planet within the habitable zone of M dwarf stars like Gliese 876.

"We are pushing a whole new regime at Keck to achieve 1 m/s precision, triple our old precision, that should also allow us to see these Earth-mass planets around Sunlike stars within the next few years," Butler said.

"Our UC Santa Cruz and Lick Observatory team has done an enormous amount of optical, technical, and detector work to make the Keck Telescope a rocky planet hunter, the best one in the world," Marcy added.

Lissauer also is pleased by another feat reported in the Astrophysical Journal paper. For the first time, he, Rivera, and Laughlin have determined the line-of-sight inclination of the orbit of the stellar system solely from the observed Doppler wobble of the star. Using dynamical models of how the two Jupiter-size planets interact, they were able to calculate the masses of the two giant planets from the observed shapes and precession rates of their orbits. (Precession is the slow turning of the long axis of a planet's elliptical orbit.) The researchers showed that the orbital plane is tilted at an angle of 40 degrees to our line of sight. This allowed the team to estimate the most likely mass of the third planet as seven and a half Earth masses.

"There's more dynamical modeling involved in this study than in any previous study, much more," Lissauer said.

The team plans to continue to observe the star Gliese 876, but is eager to find other terrestrial planets among the 150 or more M dwarf planets they observe regularly with Keck.

"So far we find almost no Jupiter-mass planets among the M dwarf stars we've been observing, which suggests that, instead, there is going to be a big population of smaller-mass planets," Butler noted.

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