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Brown Dwarf Seen Around Sun-like Star
Astronomers have spotted a brown dwarf in images of a nearby sun-like star.
by Vanessa Thomas

15 Sge Brown Dwarf
A brown dwarf about 65 times the mass of Jupiter orbits a sun-like star 57.7 light-years from Earth.
Gemini Observatory
Less than 58 light-years from Earth, a brown dwarf orbits a star much like our own sun. A team of astronomers presented images of this newfound substellar object at the American Astronomical Society's annual meeting in Washington, D.C., today.

While astronomers have imaged nearby brown dwarfs before, none of the photogenic "failed stars" orbit as close to a main-sequence star as this one. (In fact, some brown dwarfs don't orbit stars at all.) If the newly imaged brown dwarf was part of our solar system, it would orbit between Saturn and Uranus. It orbits approximately 14 astronomical units from a star called HR 7672, located just outside of the Summer Triangle in the small constellation Sagitta the Arrow. Commonly known as 15 Sge, the parent star is a G-type star like our sun. Between one and three billion years old, 15 Sge is a bit younger than our star.

15 Sge Starchart
The star 15 Sge appears in the sky near Altair in the constellation Sagitta.
Gemini Observatory
"This discovery implies that brown dwarf companions to average, sun-like stars exist at a separation comparable to the distance between the sun and the outer planets in our solar system," said Michael Liu of the University of Hawaii's Institute for Astronomy.

Liu and colleagues from the University of California at Berkeley and the Carnegie Institution estimate that the brown dwarf circling 15 Sge has between 55 and 78 times the mass of Jupiter.

"This companion is probably too massive to have formed the way we believe planets do," Liu said. "This finding suggests that a diversity of processes act to populate the outer regions of other solar systems. The parent star is very similar to our sun, yet it has a brown dwarf companion whose mass is dozens of times the combined mass of all the planets in our solar system."

The first clues suggesting 15 Sge might have a substellar companion came from Lick Observatory data collected about ten years ago, when Paul Butler of the Carnegie Institution and Berkeley astronomers Geoff Marcy and Debra Fischer obtained radial-velocity measurements of 15 Sge. The method, used to detect extrasolar planets, suggested that there was an unseen companion too large to be considered a planet.

15 Sge and Brown Dwarf
The faint brown dwarf is seen at about the 7 o'clock position below the star 15 Sge.
Gemini Observatory / University of Hawaii Institute for Astronomy / Michael Liu / NSF
When Liu imaged 15 Sge with the 8.1-meter Gemini North telescope and its adaptive optics system in the summer of 2001, he spotted a faint object near the star. To make sure the unidentified object wasn't just a background star, Liu used the 10-meter Keck II and its adaptive optics system to monitor 15 Sge over the next six months. The fainter object moved with 15 Sge, proving that it orbited the star. The companion's spectrum confirmed that it had a very cool temperature, typical of brown dwarfs.

"Only by using adaptive optics to produce very sharp images could we have found this companion," Liu states. "It is too faint and too close to its parent star to be seen otherwise."

Liu's team is now trying to find substellar objects around other stars. "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 says.

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