Researchers, including a University of Florida astronomer, found the planet, which has a mass about five times that of Earth, orbiting a small star near the center of the galaxy in the constellation Sagittarius.
Located about three times as far away from its star as the distance from Earth to the sun, it is probably too cold to support life. But its presence suggests there are many other small planets orbiting the star. That makes it likely that at least some are located in the so-called habitable zone, the region around stars where temperatures are moderate enough for liquid water to appear on their surfaces.
"The good thing about this is it shows that planets this size might be quite common in habitable zones," said Stephen Kane, a postdoctoral associate in UF's astronomy department.
Kane co-authored a paper about the discovery set to appear Jan. 26 in the journal Nature.
Since the first planet was discovered outside our solar system in 1992, astronomers have found the vast majority of the 160-plus planets so far with a technique called radial velocity. The technique detects planets that are too faint to be seen with visual telescopes by observing the wobble in the stars induced by the orbiting planet.
Bigger planets have more gravitational pull, inducing bigger, more detectable wobbles. Also, the closer planets lie to the star, the more wobble they cause. As a result, radial velocity tends to turn up the largest, closest, hottest and consequently the most gaseous planets - planets, in other words, that are not good candidates for supporting life.
Astronomers discovered the new, small planet by tapping a completely different stellar phenomenon: galactic microlensing. Most easily observed with small, older stars known as M dwarfs, microlensing occurs when light from a distant star encounters the gravitational field of a closer star as the closer star passes in front or just to the side it. The gravitational field literally bends and magnifies the light.
The effect is a bit like the beam from a searchlight encountering a giant magnifying glass. But, for planet-finders, the key item of interest is that if the parent star is orbited by a planet, the planet's gravitational field also acts as its own little lens -- magnifying some of the distant star's light in a brief but distinctive flicker.
"There's a very subtle effect, a spike, and that's what we're looking for," Kane said.
It's rare for one star to pass so near another star that it causes microlensing to occur. The timing is also brief, with most microlensing events lasting 30 days or less. As a result, astronomers hunting such events focus their searches on the center of the galaxy, where stars are most densely distributed. This area is best viewed from the Southern Hemisphere, so astronomers coordinate observations using multiple telescopes in such places as Australia, Chile, South Africa and the Canary Islands.
"We have a real-time alert system," Kane said. "We've got people in Australia observing, and if they see something strange happen but their source is starting to set, they call up South Africa and say, 'Something's happening here. Point your telescopes at this.'"
In the latest finding, some 73 astronomers affiliated with three independent groups coordinated observations of a microlensing event first identified on July 11, 2005. Nearly a month later, on Aug. 9, the astronomers observed "a short duration deviation from a single-lens light curve ... due to a low-mass planet orbiting the lens star," the Nature paper says.
The planet is at the outer edge of the zone where it can be seen. With a surface temperature below 350 degrees below zero, it is probably rocky and icy. Even if conditions appeared more favorable for life, its location would make it tough to learn more: The planet is located 7,000 parsecs, or nearly 23,000 light years, from Earth.
That said, M dwarfs are the most common type of stars in the galaxy. The fact that microlensing uncovered such a small planet around one M dwarf suggests that there are likely many others, possibly with better conditions, Kane said.
"This has huge implications for finding life," he said.