RELEASE: 99-127

ASTRONOMERS FIND EVIDENCE OF FIRST PLANET ORBITING A PAIR OF STARS

Astronomers have found evidence of the first known planet orbiting a pair of stars. Previously, planets have been found circling only single stars.

The Microlensing Planet Search (MPS) project, led by David Bennett and Sun Hong Rhie of the University of Notre Dame, South Bend, IN, used a technique called gravitational microlensing that may have revealed a planet about three times the mass of Jupiter orbiting a binary star system. The researchers, who are supported by NASA's Astronomical Search for Origins Program, the National Science Foundation (NSF) and the Research Corporation, report their results in the November 4 issue of Nature.

"Between half and two-thirds of the stars in our solar neighborhood are known to be members of binary or multiple star systems," said Morris Aizenman of NSF's Astronomical Sciences Division. "To find evidence of a planet orbiting a pair of stars means there could be more planetary systems than we previously thought." Astronomers have detected only about 20 planets outside our solar system, all orbiting single stars, although some of those stars are in binary systems.

Gravitational lensing is based on a property first noted by Albert Einstein in the 1930s. When an object such as a star or planet moves in front of a more distant star, the gravity of this star or planet serves as a "lens," magnifying the light from the distant star and making it appear brighter. The Microlensing Planet Search astronomers analyzed data from such an event that occurred in 1997, referred to as MACHO-97-BLG-41 -- the 41^st microlensing event discovered by the Massive Compact Halo Objects (MACHO) collaboration that year. During this 100-day event, the pattern of brightness appeared too complex to be produced by a single-star lens.

While Bennett and his colleagues believe the best model for explaining this microlensing event is a planet orbiting a binary star system, other astronomers have proposed alternative models they believe could also fit the data. One possibility is that the orbital motion of the binary star system itself could have caused the change in the observed brightness of the distant star. Another possibility is that the distant star may itself be part of a binary system. These scenarios will be tested in future observations.

The MACHO project, which is supported by NSF as part of the National Science and Technology Center for Particle Astrophysics at the University of California at Berkeley, routinely makes data

on microlensing events available to other astronomers. MACHO is using microlensing to explore tens of millions of stars in a search for the "dark matter" that dominates the mass of our galaxy. Dark matter is believed to exist because the combined gravity of the known matter in the universe is not enough to account for the observed gravitational effects.

The MPS astronomers are using the microlensing technique to search for planets orbiting stars other than our Sun. For this analysis, they used observations from telescopes at the Mount Stromlo Observatory in Australia and the Wise Observatory in Israel as well as data from the NSF's Cerro-Tololo Inter-American Observatory in Chile. Astronomers at the Wise Observatory co-authored the Nature report.

More information on MPS can be found on the Internet at:

http://bustard.phys.nd.edu/MPS/