Planet x

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.

HOW THEY STACK UP: The difference between brown dwarfs and planets.

This image is a merger of the two images above that highlights the brown dwarf image around Sge 15 that was revealed for the first time by Gemini. CREDIT: Gemini Observatory/University of Hawaii Institute for Astronomy/Michael Liu/NSF

Keck adaptive optics image of 15 Sge and its companion, also obtained in the near-infrared. The arrow points to the companion, seen as a close point source. (The streaks of light around the primary star are image artifacts produced by the telescope.) CREDIT: W. M. Keck Observatory/University of Hawaii Institute for Astronomy/Michael Liu

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.

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