January 07, 2002

Strange Object Found, Defying Ideas of Solar System Formation

by Robert Roy Britt

WASHINGTON D.C. - Solar system creation theorists got more to chew on Monday when astronomers announced the discovery a huge object called a brown dwarf orbiting a star nearly as closely as Saturn is to our Sun.

Added to recent findings of extrasolar planetary systems that are markedly unlike the one around the Sun, the new finding makes our solar system look like an oddball in the galaxy.

Brown dwarfs are large balls of gas, much more massive than Jupiter but not heavy enough to generate the thermonuclear fusion that powers a star. In recent years, these strange, in-between objects have been found in so many bizarre configurations that researchers are scrambling to figure out whether they are dealing with one class of object or several.

Lone brown dwarfs have been spotted wandering through space fairly nearby. Others have been detected at vast distances from other stars, forming in nests. Brown dwarfs might even spawn their own planetary systems.

And scientists have struggled and argued over the specific differences between brown dwarfs and planets, especially how and where they are born. Finding a brown dwarf in a region around a star thought to be reserved for planets will only exacerbate ongoing disagreements.

"This finding suggests that a diversity of processes act to populate the outer regions of other solar systems," said Michael Liu, a University of Hawaii astronomer who led the team that made the discovery.

Liu and his colleagues, a group of venerable planet hunters, found the object using the Gemini North and Keck telescopes at Mauna Kea, Hawaii. The results were presented here at a meeting of the American Astronomical Society.

So close

The brown dwarf is just 14 Astronomical Units away from the star (1 AU is the distance from Earth to the Sun). Saturn is 10 AU from the Sun, and Uranus, the next planet out, is 19 AU. The object is between 55 and 78 times as massive as Jupiter, which is the largest planet in our solar system.

Researchers say the configuration of a star and such a large, close companion can't be explained by current theories of solar system formation.

Stars are created by the gravitational collapse of a cloud of material. Planets, however, are thought to develop out of a flat, rotating disk of material left over from the creation of a star. Planet evolve when smaller rocks, like comets and asteroids, get together. Jupiter and the other gas giant planets probably have origins in such rocky cores, which grew large enough to attract a huge gas envelope, most astronomers believe.

"This [brown dwarf] companion is probably too massive to have formed the way we believe that planets do," Liu said.

Many stars also are known to form as pairs, orbiting each other.

There's little question that the newly found object is a brown dwarf, said Alan Boss, a planet formation theorist at the Carnegie Institution. It probably formed out of the same cloud of material from which the star formed, he said.

The presence of the brown dwarf might inhibit the formation of planets, said Boss, who was not involved in the research. Or, he said, it might actually foster the development of planets, according to competing theoretical models.

Further study of this and similar systems will be needed to learn how they form, the researchers agreed.

Hunting planets

Hints of the brown dwarf were initially spotted a decade ago by a team of extrasolar planet hunters.

Geoff Marcy and Debra Fischer at the University of California at Berkeley, along with Paul Butler of the Carnegie Institution, have helped find many of the 80 or so known extrasolar planets. Their technique, which notes slight wobbles in a star caused by large, closely-orbiting planets, could not definitively identify what they suspected was a brown dwarf farther out from a star called HR 7672, which is 58 light-years away from Earth.

Current techniques cannot find small planets, either. The extrasolar planets that have been identified in the past 5 years are all very large. Most are bigger than Jupiter and orbit very close to their host stars. Some are as close as Mercury is to our Sun.

Already, astronomers were struggling to explain how planets so large could exist so close to stars. And they wonder whether this is normal, or whether other solar systems like ours might be the norm but simply have eluded detection so far.

The answer bears on whether there are other Earth-like planets out there, which might support life.

Answers won't come, however, until technology allows for the discovery of small extrasolar planets, assuming they exist. One space-based observatory that would do this, called Kepler, was approved by NASA in late December and is scheduled to launch in 2006. Kepler would succeed by doing its observing from above the blurring effects of Earth's atmosphere.

Other researchers are working to stretch ground-based observation techniques beyond current limits to allow for the discovery of smaller planets and of large objects orbiting far from stars. The new discovery represents one of these efforts.

How it was found

Brown dwarfs generate enough heat to shine, but only dimly and in infrared light.

The newly spotted brown dwarf was found using a relatively new technique called adaptive optics, which corrects for some of the blurring effects of Earth's atmosphere and allows faint objects to be photographed with resolution that can be 10 times better.

Liu imaged the primary star HR 7672 (also called 15 Sge) last summer, and he noticed a dim spot of light next to it, described as being akin to a firefly next to a bright searchlight. Follow-up images, using adaptive optics, showed the faint object moved on the sky along with the primary star, proving that it was in fact a companion and not a background star.

HR 7672 is slightly younger than our Sun, but otherwise is seen as a twin to the Sun, Liu said. The researchers are now looking for similar groupings of other stars and brown dwarfs.

"Now that we know brown dwarfs exist in the region of giant planet formation, we would like to understand how often these oddball pairings occur in the universe, and what that can tell us about the alternate and divergent ways in which solar systems form around Sun-like stars," Liu said.

James Graham and James Lloyd, also from the University of California at Berkeley, contributed to the study.

Next up: Jupiter-sized planets

Boss said the results were a tantalizing appetizer of a feast of observations expected in the near future from adaptive optics systems.

"It's basically changing an astronomical image from something that looks like a very fuzzy impressionistic painting into something that looks like a photo-realist painting," Boss said.

Ray Jayawardhana, a researcher at the University of California, Berkeley, said he expects the technique to provide direct images of Jupiter-sized planets around other stars soon, possibly within two years. Several groups are competing to achieve that goal, he said, and already there are candidate
objects that they are examining more closely.

"It might seem crazy," Jayawardhana said, but it's not."

File Date: 01.08.02