By Ron Cowen
Like bugs glued to a phonograph record, the solar system’s planets all orbit the sun in nearly the same plane. A new finding shatters the notion that planetary systems around other stars all have a similarly flattened arrangement. Newly reported measurements reveal that the two outermost planets known to circle a nearby sunlike star called Upsilon Andromedae are wildly misaligned, orbiting the star in different planes separated by 30 degrees.
The observations include ground-based measurements of the back-and-forth motion, or wobble, of Upsilon Andromedae due to the tug of its orbiting planets. But most critical were Hubble Space Telescope observations that tracked the two-dimensional motion of the star as it pirouetted across the sky, orbiting the center of mass of its planetary system.
The new measurements are the first to accurately determine the angle between the orbits of two extrasolar planets circling a sunlike star, says Barbara McArthur of the University of Texas at Austin. She and her colleagues describe the findings in the June 1 Astrophysical Journal. The researchers are also scheduled to describe their study May 24 at a meeting of the American Astronomical Society in Miami.
The finding, says theorist Greg Laughlin of the University of California, Santa Cruz, is yet another reminder that the architecture of the solar system, while not necessarily rare, may be downright foreign compared to the arrangement of planets around other stars.
“A lot of things had to go right in our solar system to lead to the kind of environment that we have today,” says Laughlin.
The observations suggest that something violent must have happened early in the history of the Upsilon Andromedae system, notes McArthur. Theorists believe that planets form as gravity gathers together gas and dust within a flattened disk of material circling a young star. Gravitational interactions between the two massive outer planets of Upsilon Andromedae, or between one of them and a more remote body, could have stirred up a planetary system that was initially as flat as the solar system’s, she and her colleagues conjecture.
Alternatively, a complicated dance between one of the planets, Upsilon Andromedae, and a third object — perhaps a companion star — “could have pumped up the mutual inclinations of the planets,” says astrophysicist Sara Seager of MIT. Indeed, McArthur’s team notes that Upsilon Andromedae posseses such a companion.
Residing about 45 light-years from Earth, Upsilon Andromedae was the first multiplanet system found around a sunlike star and has been studied extensively since its discovery in the 1990s. The innermost planet lies too close to the star for the inclination of its orbit to be determined; the two outer, giant planets are those whose orbits lie in planes that are 30 degrees apart.
McArthur and her colleagues have obtained new, more accurate mass measurements for both of the massive outer bodies. The closer one to Upsilon Andromedae weighs the equivalent of 13.98 Jupiters and lies slightly farther from its star than Mars’ average distance from the sun. The second orb weighs the equivalent of 10.25 Jupiters and lies a little closer to the star than Jupiter’s distance from the sun.
NASA’s Kepler spacecraft, which hunts for planets that pass in front of a parent star, is likely to produce “a burst of new data” on the orbits of extrasolar planetary systems, McArthur says.
In the meantime, says Seager, the Upsilon Andromedae planetary system “will continue to be a feast for theorists.”
