Distant Planet Spotted Around the Bend

Astronomers have for the first time used the bending of light waves from a star to identify a planet 17,000 light-years away, an achievement that could set the stage for the discovery of more extrasolar planets, especially smaller planets similar in size to Earth.

Researchers have so far identified more than 100 extrasolar planets by observing slight wobbles in a star’s trajectory caused by a planet circling it or by observing small changes in a star’s brightness when a planet passes in front of it.

But those methods work only for planets larger than Jupiter, and most of the planets discovered so far orbit very close to their stars -- indicating that the planetary systems are not very much like our own.

Although the planet discovered using the new technique, called gravitational microlensing, is about the size of Jupiter, the method should work equally well with smaller planets, researchers said. Furthermore, the newly discovered planet, in the constellation Sagittarius, orbits its star at a distance of about three astronomical units (one AU equals the distance from Earth to the sun), and that star is a red dwarf similar to our sun.

The fact that the newly discovered planet orbits its star at such a distance means there could be smaller planets, more like Earth, Venus and Mars, orbiting it more closely, said NASA’s Philippe Crane, who is in charge of the search for such planets.

The new technique relies on the fact that powerful gravitational fields can bend light rays, a phenomenon first predicted by Albert Einstein. When a relatively nearby star passes between an earthbound telescope and a more distant star, its gravity acts like a telescopic lens, bending and focusing light from the distant star so that it appears much brighter than it otherwise would.

About 13 years ago, astronomer Bohdan Paczynski of Princeton University and his student Shude Mao suggested that this microlensing capability could be used to find extrasolar planets.

A smaller object, such as a planet, would have a similar brightening effect. The increase in brightness would be smaller, though still easily detectable.

“I’m thrilled and delighted to see that old idea come true,” Paczynski said Thursday in a telephone news conference announcing the finding. The report will be published in the May 10 issue of Astrophysical Journal Letters.

The discovery was made jointly by two international teams that use large digital detectors attached to telescopes to monitor as many as 200 million stars at a time, looking for changes in brightness.

The two groups together observe about 570 such events each year, according to astronomer Ian Bond of the University of Edinburgh in Scotland, who led the team reporting the new planet. Virtually all of those events, however, occur when a single star acts as a gravitational lens for a second single star.

Last July, however, the team observed an event in which the brightness of the observed star increased twice over a period of about 40 days, indicating the presence of two objects in the system acting as the lens.

Mathematical calculations indicted that the second object had only 0.4% of the mass of the star and must therefore be a planet, and about 1 1/2times the size of Jupiter.

Obtaining the proper astronomical alignment to use gravitational microlensing is a very rare event, Paczynski noted, occurring less than once in every 100 million observations. But the groups are looking at so many stars that he believes they will find many more planets.

“The chances are good we will discover another planet within the next half-year,” he said.