Astronomers spy familiar planetary system

Studying a star in the Big Dipper, astronomers have for the first time found a planetary system that reminds them of home.

The planetary system--actually a pair of planets--orbits 47 Ursae Majoris, a sunlike star that lies just 51 light-years from Earth and is visible to the naked eye. Both planets, like most in our solar system, have nearly circular orbits. Moreover, the location and mass of the outer planet is similar to that of Jupiter in our solar system.

The planets are most likely giant balls of gas, so it's unlikely either could support life. However, their circular orbits increase the odds that an inner, Earthlike planet could also reside there, the astronomers calculate.

"We seem to be edging closer and closer to the Holy Grail of extrasolar planetary systems" with one or more planets capable of supporting life, comments Alan P. Boss of the Carnegie Institution of Washington (D.C.), a theorist who wasn't part of the study.

Veteran planet hunters Geoffrey W. Marcy of the University of California, Berkeley and R. Paul Butler of the Carnegie Institution, along with their colleagues including Debra Fischer of Berkeley, announced the finding this week.

The researchers base their results on a now-standard technique for deducing the presence of unseen planets (SN: 8/8/98, p. 88). The tug of an orbiting planet causes a star to wobble ever so slightly. The star's motion causes the starlight observed on Earth to periodically shift in frequency, a change that astronomers pick up with a sensitive spectrograph. The method most easily detects bodies that exert the greatest tug--massive planets that lie close to their parent star.

Nonetheless, among the nearly 70 extrasolar planets discovered since 1995, astronomers have been surprised to find that many are what they call "hot Jupiters"--massive bodies that come within roasting distance of their parent. It's as if in our own solar system, Jupiter were close enough to graze the sun's hot outer atmosphere.

Other extrasolar planets, in longer orbits, seem no less strange. Some, within a period of a few years, regularly careen from a frigid Marslike distance to a hot Venuslike distance from their star.