Astronomers find two planetary systems

Planet hunters this week announced they had found two planetary systems, one of which resides only 15 light-years from Earth. Each system features a star roughly similar to the sun and a bizarre entourage that defies ready explanation and may provide fresh insight into planet formation.

Artist’s depiction of the two planets orbiting Gliese 876 (central red dot). Lynette Cook/Science Photo Library, (c) 2001

Among the 50 or so stars where extrasolar planets have been detected, astronomers have reported only one with more than a single planet (SN: 4/17/99, p. 244). The new finds harbor several surprises. One system features an orbiting body so massive it may qualify as a failed star. The other system features two planets with an unusual orbital harmony.

A team led by Geoffrey W. Marcy of the University of California, Berkeley and R. Paul Butler of the Carnegie Institution of Washington (D.C.) detected the planets indirectly with a frequently used technique (see p. 24). The group measured the wobbles that the orbiting bodies produce in their respective suns.

The team had already used this method to discern that a single planet orbits the sunlike star HD 168443, located 123 light-years from Earth. That planet is at least seven times as massive as Jupiter and resides, on average, slightly closer to its parent star than Mercury does to the sun. After further tracking the star’s wobble, the researchers now find that HD 168443 has a second companion. It’s a whopper—at least 17 times as massive as Jupiter and double the minimum mass of any other known extrasolar planet. A team led by Michel Mayor of Geneva Observatory in Sauverny, Switzerland, has also found evidence of this body.

Observations by the Hipparcos satellite indicate the object can’t be more than 40 times Jupiter’s mass, well under the limit for a bona fide star.

According to one criterion, the object is too heavy to be a planet. Any body between 13 and about 100 times Jupiter’s mass burns deuterium at its core and begins to shine as stars do but soon fizzles out. Scientists hold that these failed stars, dubbed brown dwarfs, form as stars do–from the fragmenting of giant clouds of cold gas.

Despite its heft, the newly found companion may be a planet or some strange hybrid, Marcy said at a meeting of the American Astronomical Society in San Diego. The body, he notes, has a planetlike orbit that lies at a slightly greater distance from HD 168443 than Mars’ orbit does from the sun.

On the other hand, it’s unclear whether the object could have assembled as planets do, from material coalescing within the disk of gas and dust that swaddles young stars. Theorist Doug N.C. Lin of UC Santa Cruz suggests the behemoth instead could have formed fully grown if the disk collapsed into chunks.

The two planets orbiting one of Earth’s neighbors, the red dwarf star Gliese 876, profoundly influence each other. Marcy’s team reported in 1998 that one planet, at least 1.8 times Jupiter’s mass, takes some 61 days to orbit the star (SN: 6/27/98, p. 405). The team now finds that a second planet, at least half Jupiter’s mass, lies nearer the star and orbits in just under half that time.

Locked into this pattern, the planets tug on their star in the same direction every 60 days, causing it to wobble with a larger, easier-to-detect amplitude. Such a configuration resembles the orbital synchrony of the Jovian moons Io, Europa, and Ganymede in our solar system.

The elongated path of the inner planet orbiting Gliese 876 provides the first indication that the unseen planet is, as theory predicts, gaseous and not solid, Lin says. The star would have flexed a solid planet, making its orbit much more circular.

The planets’ orbital harmony may enable astronomers to measure the actual mass of each body. Lin speculates that the two haven’t always had this special relationship. The inner one has migrated closer to the parent star and the outer one followed at a more rapid clip. Understanding this system could open windows on the diversity of planet formation in the cosmos, he says.

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