UF Team: Dust Disks May Reveal Evidence Of Hard-to-find Planets

December 20, 1999

GAINESVILLE — A team of astronomers from the University of Florida is proposing that it may be possible to find evidence of “hidden” planets by observing the huge disks of dust created when stars are born.

The findings, described in an article appearing today in The Astrophysical Journal, a leading international astronomy publication, may lead to the discovery of planets that are far smaller and farther away from stars than planets so far discovered — including planets whose size and distance from a star could mean they have conditions similar to Earth.

“The big advantage of the work is that it is capable of indicating the presence of planets even if these planets are, like the Earth, very small,” said Stanley Dermott, professor and chairman of the UF department of astronomy.

Astronomers have discovered 27 planets outside the solar system orbiting sun-like stars since the first was found in 1995. In one case, astronomers found the planet because it dimmed the brightness of its parent star as it eclipsed the star during its orbit. More recently, a team of British astronomers last week announced observations of what they believe to be starlight reflected from a large planet. In most other cases, however, astronomers detected the planets with the “wobble” method: They inferred the presence of a planet from its gravitational influence on the movement of its parent star.

The problem with the wobble method is that it works only with very large planets that have orbits very close to stars, said Mark Wyatt, who graduated from UF with a doctorate in astronomy Saturday and was the lead researcher on the UF team.

“If the planet is too small, then the wobble may not be large enough to be detectable,” Wyatt said. “If a planet is a long way away, then the period of wobble may be too long to detect.”

Observations of a young star in the constellation Centaurus approximately 200 light years away led the UF team to propose a new method, Wyatt said.

Like many other stars, the star HR 4796 has a disk of dust particles surrounding it — material left over from when it formed approximately 10 million years ago. Observations of the doughnut-like disk revealed that one side was slightly brighter than the other, Wyatt said. The UF astronomers believe this disparity could be the result of the gravitational impact of an unseen planet shifting the dust disk slightly off center with the star.

“Because the center of the doughnut is offset, one side is closer to the star, so that side is hotter and brighter,” Wyatt said.

HR 4796 lies in what is known as a binary star system, meaning it has a companion star relatively nearby. It’s possible the companion star could be shifting the dust disk as well, Wyatt said. As a result, more observation and research is needed before astronomers can confirm that HR 4796 has a planet nearby, he said.

Wyatt said the wobble method only detects planets at least the size of Jupiter, or about 300 times larger than Earth.

The disk-dust method, by contrast, not only can detect planets as small as 10 times the size of Earth, but it also can detect them much farther away from stars, he said. The planet that may be shifting the dust cloud around HR 4796 could be as far away as 50 astronomical units from its star, Dermott said. One astronomical unit, the distance between the Earth and the sun, equals about 92.7 million miles.

Dermott was among the members of a joint UF-Harvard team that last year announced the discovery of the dust disk around HR 4796 and its companion star. The astronomers said the disk, the first to be discovered around a binary star system, had the right conditions to be forming planets.

Wyatt, who spent three years on the research for his dissertation, was supported in part by a Fulbright Scholarship. He also received NASA funds provided to Dermott for work on the dynamics of dust in the solar system.

The other members of the research team are Dermott; Charles Telesco, a UF astronomy professor; Scott Fisher, a UF graduate student; Keith Grogan, a UF postdoctoral research associate; and Robert Pina, a UF assistant professor of astronomy.