June 26, 1998
New Planet Detected Around a Star 15 Light-Years Away
By JOHN NOBLE WILFORD
he discoveries of planets beyond the solar system, stimulating renewed speculation of other possible worlds throbbing with life, are now drawing closer in cosmic terms to the world of their discoverers.
The latest detection, made this month by American astronomers, is of a planet about twice the mass of Jupiter that is orbiting the star Gliese 876, one of the Sun's nearest neighbors at a distance of only 15 light-years. Previous confirmed extrasolar planets have been observed at stars from 40 light-years to 80 light-years away.
The discovery at Gliese 876 adds another element of surprise and surmise to the continuing quest for other planetary systems. The large object is orbiting a red dwarf star less than one-third the mass of the Sun.
As the first planet found orbiting a low-mass star, the most common type of star in the Milky Way galaxy, the discovery encourages astronomers to think that other planetary systems may be anything but rare.
They are not confined to Sun-like stars, where at least 11 planets have already been detected in the last three years. No sooner had astronomers taken a look at a low-mass star than they found it had a planetary companion. Could it be that planets are a common phenomenon of stars, large and small?
Time, and improved observation instruments, should yield the answer. Then it may be possible to compare the solar system with other families of planets and learn if it is unique in some ways, typical, or one of many manifestations of nature's variety.
A team of astronomers led by Dr. Geoffrey Marcy, who has made most of the extrasolar planet discoveries, reported the new finding on Monday at a symposium of the International Astronomical Union in Victoria, British Columbia. Within hours, European astronomers sent a message to a colleague at the symposium saying they had confirmed the observations.
"This is something we really weren't sure we could expect," Marcy said in a telephone interview Thursday. "It is the closest star around which a planet has been found and by far the lowest-mass star with a planet. Now we have to consider the possibility that planets may be common around stars with a wide range of properties."
Dr. Stephen Maran, an astronomer and spokesman for the American Astronomical Society, said this was one of the most significant discoveries since the first detection of planets around other stars, beginning in 1995.
"If red dwarfs have planets, there must be a vast number of planets in the galaxy," he said. "It's also important because red dwarf stars have very long lives, which means more time for life to have a chance to evolve on any planets where conditions are favorable to life."
Theorists like Dr. Frederic Rasio of the Massachusetts Institute of Technology were especially intrigued by the orbit of the newly discovered planet. It completes a trip around the star every 61 days, traveling a much more eccentric orbit than any of the Sun's planets. Its elongated orbit takes the object, probably a huge sphere of cold gases, closer to its star than Mercury gets to the Sun.
The most reasonable explanation for such an orbit, Rasio said in an interview, is that Gliese 876 has at least two or three large planetary companions, and their gravitational interactions create a "dynamical instability."
As a result, in the early stages of the planetary system, one planet probably got a gravitational kick to the outer fringes of the star's vicinity, while the one now being observed was kicked in toward the star itself.
By contrast, the solar system -- with its planets in more or less circular orbits -- has enjoyed a more serene history.
"The solar system is looking more and more like an extremely unusual planetary system," Rasio said. "It's been stable a very, very long time, which is probably why we are here."
The existence of the new planet was first detected at the Lick Observatory near San Jose, Calif., and then studied in more revealing detail at the more powerful Keck Observatory on Mauna Kea in Hawaii.
Marcy is an astronomer at San Francisco State University and the University of California at Berkeley. The other team members are Dr. R. Paul Butler of the Anglo-Australian Observatory near Sydney, Dr. Steven Vogt of the University of California at Santa Cruz and Dr. Debra Fischer of San Francisco State.
Independent evidence of the new planet's existence was provided by a European group led by Dr. Xavier Delfosse of the Geneva Observatory in Switzerland. Their work was done at the Haute-Provence Observatory in France and the European Southern Observatory in La Serena, Chile.
As with other recently discovered planets, the object orbiting Gliese 876 has not been seen or photographed. Its presence is indicated by the gravitational tug on its parent star, which causes it to wobble enough to be recorded in spectrographic measurements of the faint starlight.
In its next issue, the weekly magazine Science News will report on preliminary calculations by Dr. Didier Saumon, an astrophysicist at Vanderbilt University in Nashville, Tenn., which indicate that the surface of the presumably gaseous Gliese planet has a temperature well below the freezing point of water. But in warmer layers not far below the surface, he said, water could exist as liquid droplets.
In addition to confirming the Gliese 876 findings, European astronomers reported evidence this week of planets orbiting three other stars, HR5568, HR7875 and HR8100, one with a relatively low mass. Two of them are also relatively nearby, at distances of 20 light-years and 25 light-years. And the astronomers told colleagues that they may soon be ready to announce two more discoveries.
But the new European observations have yet to be confirmed by independent research and some scientists questioned the validity of at least one of the planet candidates.
Marcy said his group was monitoring more than 400 stars within about 100 light-years of Earth; several show signs of wobbling that could indicate planetary systems.
Dr. Stuart Weidenschilling, a theorist at the Planetary Science Institute in Tucson, Ariz., agreed with Rasio of MIT that the extremely eccentric orbits of some of these planets indicated that their stars have at least one other planet that has not been detected. Soon, astronomers might be studying systems of planets orbiting other stars.
In October 1995, Dr. Michel Mayor and Dr. Didier Queloz of the Geneva Observatory made the first discovery of a planet of a normal star, 51 Pegasi, 40 light-years away. Marcy and Butler weighed in a few weeks later with confirmation of the 51 Pegasi discovery and evidence of two other stars with large planets, the first of their string of successes.
But even with Keck's powerful telescopes, astronomers have trouble finding planets much smaller than Jupiter, the solar system's largest planet. NASA is planning to build spacecraft telescopes that should be able to observe Earth-size planets around normal stars.