A small planet, a giant leap for planet hunting

In what researchers are calling a milestone, a team of US astronomers has detected what may be the most Earth-like planet yet orbiting a dwarf star 15 light-years away.

Over the past decade, astronomers have found more than 100 planets orbiting nearby stars. But the vast majority have been gas giants several times more massive than Jupiter, our solar system's largest planet. Last year, astronomers reported finding two "exoplanets" that were closer in mass to Jupiter's smaller neighbor, Neptune. The ultimate goal is to find Earth-sized rocky planets, especially at distances from their "suns" that would allow water to exist in liquid form - necessary for organic life to develop.

While the new planet doesn't fit this ultimate category, it represents a breakthrough in astronomers' ability to detect ever-smaller planets from ground-based observatories.

"This planet answers an ancient question," said team leader Geoffrey Marcy, professor of astronomy at the University of California, Berkeley, at a briefing on the results Monday. "Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets. Now, for the first time, we have evidence for a rocky planet around a normal star."

Other members of the team hail from the University of California at Santa Cruz, NASA's Ames Research Center in Mountain View, Calif., the Carnegie Institution of Washington, San Francisco State University, and the National Center for Atmospheric Research in Boulder, Colo.

It's a bizarre world, indeed. The team estimates that the planet is about 7.5 time more massive than Earth. By comparison, Saturn has 100 times Earth's mass. While the planet may well be rocky, the researchers say they can't exclude the possibility that it has a fair bit of gas as well. It orbits Gliese 876, a star in the constellation Aquarius that is only about a third as massive as the sun and much dimmer. The planet whips around the star at a distance of some 2 million miles and completes its "year" in about two Earth days.

The approach the team used for uncovering the planet's presence - the third planet discovered around this particular star - is indirect. They used telltale signatures in the star's light to measure the feeble gravitational wobble the planet imparts as it orbits. Though indirect, the technique allows astronomers to glean enough information about the object to determine some of its basic traits.

The results come at a time of remarkable growth in discoveries of extra-solar planets. In large part, the growth stems from long-term, tedious observations finally bearing fruit. It took eight years of data, new detection hardware on the back end of the 10-meter Keck telescopes atop Hawaii's Mauna Kea, and new data-processing techniques to tease the tiny planet's signal from those imparted by two gas giants also orbiting Gliese 876.

But the growth also comes from more teams and techniques entering the field. One approach involves looking for the dimming a planet imparts when it orbits its sun across our line of sight. The technique not only indicates the presence of a companion, it also can permit the astronomers to spot the planet's light by subtracting the star's component from that of the planet and star combined.

In March, for example, two teams of astronomers from NASA's Goddard Space Flight Center and the Harvard-Smithsonian Center for Astrophysics reported that they had used this approach to tease information about temperatures and rudimentary composition from two extrasolar planets.

Another, more exotic technique, uses subtle changes in light from a planet-harboring star as the light is bent and magnified by the gravity of another star in astronomers' line of sight. Last month, an international team of professional and amateur astronomers headed by astronomer Andrzej Udalski of Warsaw University used this "gravitational micro-lensing" technique to detect a planet around a star 15,000 light years away.

"Today's results are an important step toward answering one of the most profound questions that mankind can ask: Are we alone in the universe?" said Michael Turner, head of the Mathematical and Physical Sciences Directorate at the National Science Foundation.