Astronomers announced this week that features observed in the dust swirling around the nearby star Vega may be the signatures of an unseen planet in an eccentric orbit around the star. The report was presented by David Wilner, Matt Holman, Paul Ho and Marc Kuchner of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA to the American Astronomical Society meeting in Washington, DC. This result shows that the gravitational effects of extrasolar planets on circumstellar dust may be used to infer their existence and orbital properties.

Two giant clumps of dust in the ring surrounding Vega may be signatures of an unseen planet orbiting the star in an eccentric orbit. Observations at millimeter wavelengths indicate that infalling dust is trapped in dynamical resonances by the gravitational influence of a planetary body. Such effects may offer an additional method of detecting and studying extrasolar planets to complement radial velocity programs. Credit: Dr. David Wilner and David Aguilar, Harvard-Smithsonian Center for Astrophysics

Vega, located 25 light years away in the constellation Lyra, is the brightest star in the summer sky. Observations of Vega in 1983 with the Infrared Astronomy Satellite provided the first evidence for large dust particles around another star, probably debris related to the formation of planets. This discovery likely inspired Carl Sagan to place the alien listening post at Vega in his novel Contact.

In our Solar System, dust particles created by asteroid collisions and the evaporation of comets spiral in toward the Sun. The gravity of the planets affects the distribution of these dust particles. The Earth, for example, traps dust in a series of dynamical resonances that produce a ring of enhanced density along the Earth's orbit. When viewed from afar, the signatures of extrasolar planets imprinted on circumstellar dust may be the most conspicuous evidence of their existence besides their gravitational influence on their central stars. The dust clouds are much easier to detect than the planets because of their much larger surface area. It's akin to seeing the wake of a boat from a plane when the boat itself is too small to be visible.

The new observations of Vega's dust cloud were made at a wavelength of 1.3 millimeters with the Plateau de Bure Interferometer (PdBI) of the Institut de RadioAstronomie Millimetrique (IRAM), an array of five 15-meter (49-foot) diameter antennas located in the French Alps. The observations are sensitive to structures as small as 20 astronomical units (AU) at Vega, or roughly the size of Saturn's orbit.

An important advantage of observing at millimeter wavelengths is that the contrast between the star and dust particles is much smaller than in the optical. So it is much easier to detect the faint dust signal near the bright star. Observations at these wavelengths in 1998 from the James Clerk Maxwell Telescope were the first to show structure in Vega's dust cloud and hint at the presence of a planet.

Because Vega is viewed nearly pole-on, it presents a perfect target for more detailed study of features in its dust cloud. The new high- resolution observations detect two prominent peaks of dust emission, one offset 60 AU to the southwest of the star, and the other offset 75 AU to the northeast. "These offset emission peaks are naturally explained by the dynamical influence of an unseen planet in an eccentric orbit," said Dr. Wilner, who leads the CfA team.

A massive planet on an eccentric orbit within an inspiralling dust cloud does not create a simple ring like the Earth. Instead, calculations by the astronomers show that an eccentric planet traps dust in two main concentrations at different distances from the star, at positions outside the planet orbit that are generally not in line with the star.

This phenomenon is robust; computer simulations show that it appears over a wide range of planet masses and orbital eccentricities. It is not observed in our Solar System because the orbits of the planets in our Solar System do not have large enough eccentricities.

"Since precision radial velocity surveys indicate that massive extrasolar planets often follow highly eccentric orbits, asymmetric dust concentrations may be common features of extrasolar planetary systems," said Dr. Holman.

More observations are necessary to fully understand the nature of the structure in Vega's dust cloud. Physical scenarios other than the resonant interactions of a planet might create a dust peak, like the recent collision of very large asteroids. For two such major collisions to happen on opposite sides of Vega at nearly the same time is extremely unlikely, but it cannot be ruled out with the current data.

An important prediction of the eccentric planet idea is that the dust concentrations will appear to rotate around Vega at half of the planet's orbital speed.

These motions might be detected in images made over a period of years with a new generation of sensitive telescopes, including the Smithsonian Astrophysical Observatory's Submillimeter Array now nearing completion on the summit of Mauna Kea, Hawaii.

This work was supported by the Smithsonian Astrophysical Observatory and a grant from NASA's Origin of Solar Systems Program. We acknowledge the IRAM staff from the Plateau de Bure and from Grenoble. IRAM is an international institute for research in millimeter astronomy funded by the Centre National de la Recherche Scientifique, France, the Max Planck Gesellschaft, Germany, and the Instituto Geografico Nacional, Spain.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into seven research divisions study the origin, evolution, and ultimate fate of the universe.