Earthlike Planet Found Orbiting Star Near Our Solar System
ARLINGTON, Virginia, June 14, 2005 (ENS) - For the first time, a team of astronomers has found a rocky, Earthlike planet orbiting a star not much different from our Sun.
"This planet will be historic," 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."
Marcy and three other members of the team presented their findings Monday during a press conference at the National Science Foundation in Arlington.
The smallest planet ever detected beyond our solar system, it is about seven and a half times as massive as Earth, with less than twice the radius, the scientists said. All of the nearly 150 other extrasolar planets discovered to date around normal stars have been larger than Uranus, an ice giant in our solar system 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 the University of California, Santa Cruz.
The newly discovered planet orbits the star Gliese 876, just 15 light years away and located in the constellation Aquarius. The planet whips around its star every 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) temperatures far too hot for life as we know it, the scientists said.
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.
UC Santa Cruz postdoctoral researcher Eugenio Rivera, and theoretical astronomer Jack Lissauer of NASA's Ames Research Center have been analyzing data from the Keck Observatory on the Gliese 876 system in order to model the unusual motions of the two known planets.
"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 Observatory'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 UC Santa Cruz to upgrade the spectrometer last August.
"It is the higher precision data from the upgraded HIRES that gives us confidence in this result," said Paul Butler of the Carnegie Institution of Washington, a member of the team that introduced the new planet to the world Monday in Arlington.
The team now has convincing data for a planet orbiting very close to Gliese 876, at a distance of 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."
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 one meter per second 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.
The team plans to continue to observe the star Gliese 876, but is interested in finding other terrestrial planets among the 150 or more M dwarf planets they observe regularly with the Keck telescope.
"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," predicted Butler.
Marcy 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."