Orb isn't just another extrasolar planet. (Distant and Strange).

With the discovery since 1995 of more than 100 planets beyond the solar system, finding yet another one might seem more of a yawn than hot news. But the latest detection of an extrasolar planet smashes several records: At 5,000 light-years from Earth, the newfound body lies 30 times farther away than any other detected planet and is the first one found outside our own spiral arm of the Milky Way. The planet lies closer to its parent star than any other known orb does and endures the highest temperature. It's so hot that iron droplets may rain down on its gaseous surface.

Most significantly, the finding marks the first time that astronomers have discovered a planet by observing the body periodically pass in front of the star it orbits, blocking a little of the starlight seen from Earth. This technique, known as the transit method, could expand the search for planets to 100 million distant stars and will be key to searching for Earthlike worlds, says Dimitar D. Sasselov of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

All the extrasolar planets discovered until now have been detected indirectly through their gravitational tug, which induces a wobble in their parent star. Astronomers subsequently observed one of these planets by the transit method (SN: 11/20/99, p. 324). In the new study, reported last week at a meeting of the American Astronomical Society in Seattle, Sasselov and his collaborators did the reverse--they used the transit method to find an extrasolar planet and then confirmed the finding by measuring the parent star's wobble.

The difference is important because the wobble method is so time intensive that it can only search for relatively massive planets around some 40,000 stars within 150 light-years of our solar system. In contrast, the transit technique can hunt planets among a much larger, more distant population of stars. The transit method also yields both the radius and the mass of a planet, while the wobble technique provides just the lower limit of the mass.

The data obtained by Sasselov's team are "a bit noisy," comments Adam S. Burrows of the University of Arizona in Tucson. But he adds that "if this approach proves out, it will open the way to a new means [for planet hunters] to survey the nearest large chunk of the galaxy."

Sasselov's group bases its findings on data from the Optical Gravitational Lensing Experiment (OGLE), in which ground-based telescopes search for planets and other objects by monitoring thousands of stars for telltale changes in brightness. The newly detected planet blocks about 1 percent of its star's light. Spectra taken with the Keck 1 telescope on Hawaii's Mauna Kea confirm that the body, now dubbed OGLE-TR-56b, tugs on its star with a strength indicative of a planet rather than a more massive body. The object has 90 percent of Jupiter's mass and 1.3 times its radius. The planet whips around its star once every 29 hours.

In 2006, NASA plans to launch Kepler, a mission that will use the transit method to search for planets as small as Earth.