University Of Arizona Scientists Are First To Discover Debris Disk Around Star Orbited By Planet

The disk of material is around 55 Cancri, a sun-like star, barely visible to the human eye, about 40 light years away in the constellation Cancer. The disk of material around the star looks similar in many ways to our solar system's Kuiper belt, a ring of comets and dusty debris left over from the formation of the planets, the scientists say.

"And, for all we know, there could be other similarities in this system yet to be discovered," said David E. Trilling of the Lunar and Planetary Laboratory at The University of Arizona in Tucson. Trilling and UA planetary sciences Professor Robert H. Brown report on their discovery in the current (Oct. 22) issue of the journal Nature.

San Francisco State University astronomers two years ago discovered a planet orbiting 55 Cancri. They used the radial velocity technique for their observations, a technique that detects gravity-induced wobble in the movement of stars. The technique does not show how the orbital plane of the system is inclined to the Earth, so the astronomers could only calculate a minimum mass for the stellar companion. The mass of the companion object, "55 Cancri b," was determined to be from about the mass of Jupiter to 100 or more Jupiter masses -- in which case the object would be a star, not a planet.

55 Cancri b is ten times closer to its star than the Earth is to the sun. Direct imaging of a planet so close to a star is not yet possible.

Trilling and Brown used NASA's Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, with "Co Co," or the Cold Coronagraph, an instrument that Brown designed, developed and built for this specific telescope. A coronagraph blocks light from the central star so observers can image the region near the star at high sensitivity. Co Co is superb at masking starlight.

The Arizona scientists viewed 55 Cancri at infrared wavelengths -- 1.5 microns to 2.4 microns -- and discovered the circumstellar disk. The inner edge of the disk is probably closer than 27 astronomical units (AU) from the star -- at which point the coronagraph mask cut off their view -- and probably extends farther than 44 AU, Trilling said.

(One astronomical unit is the distance from Earth to the sun. Jupiter is five AU from the sun; Pluto is 40 AU from the sun. Scientists usually consider the inner edge of the Kuiper belt to begin at about 50 AU. How far it extends is unknown. Estimates are that the outer edge of the Kuiper belt extends from between 100 AU to l,000 AU.)

"The disk we have found is similar in extent to our solar system's Kuiper belt, and has a spectral signature similar to some Kuiper Belt Objects, suggesting similar compositions," Trilling said. He and Brown compared spectra from the 55 Cancri disk to spectra from Pluto, the largest and closest of the 60 known Kuiper Belt Objects. Their data are consistent with the presence of methane ice, a hydrocarbon found on Pluto and in the icy, organic-rich Kuiper belt, Trilling said.

"Further, by directly imaging this dust disk, we have determined the inclination of the 55 Cancri system relative to observers on Earth. Given this inclination, we constrain the mass of the planet to be around 1.9 times the mass of Jupiter.

"By determining that the companion is a planet and not a star, we have extended the idea that the 55 Cancri system is, in many ways, a near analog for our solar system. This detection is the first time that a circumstellar disk has been found around a star with a known planetary companion; both are expected to be present in mature planet-bearing solar systems."

Trilling, a planetary sciences graduate student, won the 1998 Kuiper Award for his work on the formation and evolution of planetary systems. Brown's theoretical and observational research recently has focused on searches for and studies of planets orbiting nearby stars, as well as the icy surfaces in the outer solar system, particularly Triton, Pluto and the Kuiper belt.

The researchers were not surprised that 55 Cancri b - a planet about twice the size of Jupiter - is so close to its star. The planet is about 50 times closer than Jupiter is to the sun.

Their search for dust disks around solar-type stars orbited by planets has been guided by a powerful UA theory group studying ideas of extra-solar planet formation. The group includes Trilling, Jonathan Lunine and William Hubbard of the UA Lunar and Planetary Laboratory (LPL); Adam Burrows of the UA Steward Observatory; Tristan Guillot and Didier Saumon, formerly post-doctoral researchers at the LPL; and Willy Benz, formerly of the UA Steward Observatory, now with the University of Bern, Switzerland.

The theory explains why planets migrate inward toward their central stars after they form, and it predicts that the migration process creates a circumstellar disk that should be relatively bright and massive enough to detect.

The discovery of a Kuiper belt-like disk around 55 Cancri and its known planet strengthens the idea that our galaxy holds many other solar systems like our own, Trilling said.

"To know that there is this analog for our solar system of course implies that there are others, that this isn't the only one," he said.

"The more analogs we find, the more data we can interpret for better theories and then the more we can observe and figure out how planets and solar systems form.

"Can we learn what governs planet formation, including the question of how did Earth form? That's the question," Trilling said.

*** NOTE TO EDITORS: The color image released today can be viewed on the World Wide Web at: http://science.opi.arizona.edu.