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SFSU 'planet hunters' do it again

August 31, 2004

Image of an artist's rendering of the "Neptune-Class" planet orbiting the star GJ 436Members of the team that distinguished San Francisco State University in 1996 by being the first to detect planets outside of our solar system have done themselves even better. The planet hunters have just detected some even smaller extra-solar planets. Comparable in mass to Neptune, these are the smallest extra-solar planets ever detected. The discovery of the two "Neptune-Class" planets promises to intensify the search for even smaller extra-solar planets.

The team, funded jointly by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA), are all San Francisco State University family. Geoffrey Marcy is an adjunct professor of physics at SFSU and professor of astronomy at University of California at Berkeley; Debra Fischer is an SFSU assistant professor of astronomy; and Paul Butler is an SFSU alum (BA, BS, MS) and staff scientist at the Carnegie Institute in Washington, D.C.

"Planet hunters" all over the world will no doubt be delighted with this latest discovery. It increases the prospect that even smaller and more extra-solar planets will be detected in the future.

"Although lower-mass planets tend to be harder to detect than their higher-mass cousins, the statistics to date suggest that they occur more frequently," says Marcy. "And if that is the case, it suggests that we may soon be seeing more Neptunes ... and that Earth-sized planets, if we can ever detect them, may be downright abundant." While Neptune is 17 times the mass of Earth, "it's tiny in comparison to the 120-plus extra-solar planets that have been discovered so far," Marcy adds. "Virtually all of these objects are considerably heftier than our own solar system's heavy weights."

Photo of planet finder Debra Fischer, SFSU alum and assistant professor of astronomyThe team uses a detection strategy that Marcy and Butler helped pioneer over a decade ago and is credited with the majority of the exoplanet detections to date. Instead of trying for an actual image -- even the biggest exoplanet is much too faint and far away for that -- they monitored a long list of candidate stars, looking for wobbles that might be caused by the gravitational tug of an orbiting planet. The wobbles would show up as subtle "Doppler shifts" in the starlight. By observing those shifts over a period of years, the astronomers could then infer the planet's approximate mass and orbital size and period.

In July 2003, the team discovered a periodic wobble in a low mass star 33 light years away from Earth. Another 12 months of careful observation confirmed their suspicion. The mass, now known as "Gliese 436," has a Neptune-sized planet of at least 21 times the size of Earth. This planet speeds through a circular orbit once every 2.64 days.

For more on the 'Neptune-Class" planets, see

-- Denize Springer


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Last modified August 31, 2004 by University Communications