Cambridge, MA — Astronomers are announcing today 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
is being 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.

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.

EDITORS: A full-color artist’s concept of the Vega dust ring is online
at http://cfa-www.harvard.edu/cfa/ep/pressrel/wilner_images.html