By Ron Cowen
The population of extrasolar planets may be an illusion, according to a controversial new study. Nearly half of the so-called planets may be much more massive and mundane—either lightweight stars or stellar wannabes known as brown dwarfs.
The researchers who presented the contentious report this week at a meeting of the American Astronomical Society in Pasadena, Calif., call their results preliminary. Several other scientists, including an astronomer whose team has done a similar analysis, express skepticism.
Still, the study highlights a well-known limitation of the standard technique to detect extrasolar planets: It can only detect the minimum mass of an orbiting object. The actual mass may be much greater.
Planets that lie outside our own solar system are too faint to be easily seen. Instead, astronomers infer their presence through the tug they exert on the star they orbit. As a planet moves about its parent, it pulls the star to and fro. The star’s back-and-forth motion along the line of sight to Earth shows up as a change in its spectrum, or a Doppler shift. As the star moves toward Earth, the light it emits appears shifted to bluer, or shorter, wavelengths; as it recedes, its light gets shifted to redder, or longer, wavelengths.
The method is precise enough to detect objects lighter than Saturn moving in Mercury-like orbits (SN: 4/1/00, p. 220: Available to subscribers at Less Massive than Saturn?). But because the Doppler shift can only measure one component of motion—in and out along the line of sight—the extent to which it can reveal the true mass of the orbiting body depends on the tilt of its orbit relative to the line of sight.
Suppose an object orbits a star so that it never disappears behind the star with respect to an Earthbound observer. In that case, the tug it exerts will primarily move the star across the sky rather than in and out. As a consequence, the Doppler shift will be quite small, and astronomers attempting to gauge the mass using the Doppler method could severely underestimate the weight of the object.
That’s the case for at least 15 of the 50 or so objects that have been deemed extrasolar planets, assert David C. Black of the Lunar and Planetary Institute in Houston, George D. Gatewood of the University of Pittsburgh’s Allegheny Observatory, and Inwoo Han of the Korea Astronomy Observatory in Kyung-Book.
The team arrived at that conclusion after combining the Doppler measurements with data from the European Space Agency’s Hipparcos satellite, which tracked the astrometric, or side-to-side, motion of stars. Astrometric measurements, unlike the Doppler method, determine the orientation of an orbiting body and can therefore gauge its true mass.
Black’s team examined 30 stars purported to have planets with an orbital period of at least 10 days. The side-to-side motion induced by planets with shorter orbits can’t be detected, Black says.
The researchers report that four of the stars have companions hefty enough to qualify as low-mass stars rather than planets. The objects orbiting 11 other stars have masses ranging from 15 to 80 times that of Jupiter, which would make them brown dwarfs—bodies thought to form as stars do but that can’t sustain nuclear burning. Another six stars require further study before the masses of their companions can be accurately measured. Just nine could still be planets.
“I think it is potentially revolutionary work,” says Keith S. Noll of the Space Telescope Science Institute in Baltimore, who heard Black lecture there last month. “If true, the whole field of extrasolar planets will be turned on its head.”
Theorist Alan P. Boss of the Carnegie Institution of Washington (D.C.), however, says he doesn’t believe that the Hipparcos data, even in combination with the Doppler data, have the accuracy required to measure the objects’ true masses. Moreover, the team’s results suggest that planet hunters should find many objects with a minimum mass equal to a brown dwarf’s. Instead, they find a dearth.
Another analysis of Hipparcos’ findings, by Shay Zucker and Tsevi Mazeh of Tel Aviv University, has concluded that the objects orbiting two stars examined by Black’s team—rho Coronae Borealis and HD 10697—are heavier than a planet. But Mazeh says that Black’s conclusion that most of the extrasolar planets are stars or brown dwarfs “is not justified by the data.”
In January, Mazeh plans to hunt for further evidence that rho Coronae Borealis has a companion that’s a star, not a planet. He’ll search for its fingerprint in the spectra recorded by a near-infrared spectrometer on the Keck telescope atop Mauna Kea, Hawaii.
