Astronomers Discover Brown Dwarf Orbiting Nearby Star
They found and photographed the object in orbit around a star about 19 light years from Earth in the constellation Lepus.
Brown dwarfs -- objects less massive than stars but more massive than planets -- are thought to be failed stars; they did not start out with enough mass to generate the high temperatures needed to bring about nuclear fusion, which make stars shine.
The discovery marks the first time astronomers can be certain that they have identified a brown dwarf. It was observed at Palomar Observatory in southern California, using an image- sharpening device called the Adaptive Optics Coronagraph, designed and built at Johns Hopkins. Details of the discovery are to be published in the Nov. 30 issue of the British journal Nature.
Astronomers have long been able to observe stars orbiting other stars, in so-called binary star systems, but the Caltech/Hopkins discovery marks the first time that a more planet-like object has been observed in orbit around a star. The team of astronomers, led by Caltech's Tadashi Nakajima, deduce that the object is about 20 times more massive than Jupiter, even though it probably is about the same size as Jupiter.
It orbits a red-colored star called Gliese 229, some 6 trillion miles from Earth.
"This is clearly a brown dwarf," said Samuel Durrance, a Johns Hopkins astrophysicist. The brown dwarf, named GL 229B, is probably similar in many respects to the large gaseous planets in our solar system, he said.
"One difference between planets and brown dwarfs lies in how they formed," said Durrance, who conceived the idea for the Adaptive Optics Coronagraph, which was needed to make the crisp images.
The astronomers suspect that the brown dwarf developed during the normal star-formation process as one of two members of a binary system. The brown dwarf did not acquire enough mass to become a star. Astronomers cannot yet rule out the possibility that GL 229B formed as a super-size planet.
All of the planets in our solar system, including the large gaseous planets like Jupiter and Saturn, are believed to have formed from material in a primeval disk of dust around our newborn Sun. They did not start out as potential stars.
The discovery of GL 229B comes three years into a long-term project to search for "low-mass" companions of stars near Earth. The brown dwarf was first observed in October 1994 using the adaptive optics device and a 60-inch reflecting telescope at Mount Palomar, said David Golimowski, an associate research scientist at Johns Hopkins.
Golimowski, who has been involved in designing and building the coronagraph since its inception, was one of the astronomers who made the historic observation. It took another year to confirm that the object was actually an orbiting companion of the star. Although the astronomers do not know the brown dwarf's orbit, they estimate that its distance from the star is at least the same as Pluto's from the sun, about 4 billion miles.
After making the discovery, the astronomers used the larger 200- inch Hale telescope at Mount Palomar and a spectrograph to analyze the infrared light emitted from the brown dwarf. They found strong evidence of methane in its atmosphere, implying that the surface temperature is no more than 1,300 degrees Fahrenheit. Methane also is present in Jupiter and other giant gaseous planets of our solar system.
Using the Hubble Space Telescope earlier this month, the astronomers found no other brown dwarfs orbiting 1 billion miles or more from the star, but they cannot rule out the possibility of planets the size of Jupiter or smaller orbiting the star.
Astronomers have been trying to spot brown dwarfs for more than 20 years. Their lack of success is partly due to the fact that as brown dwarfs age, they become cooler, fainter, and more difficult to see. An important strategy used by the Caltech/Hopkins group to search for brown dwarfs was to view stars no older than a billion years. Caltech's Nakajima reasoned that, although brown dwarfs of that age would be much fainter than any known star, they would still be bright enough to be spotted.
"Another reason brown dwarfs were not detected years ago is that imaging technology really wasn't up to the task," Golimowski said. With the advent of sophisticated light sensors and adaptive optics, astronomers now have the powerful tools they need to resolve smaller and dimmer objects near stars.
Adaptive optics work by cancelling out the distortion -- or twinkling -- caused by temperature variations in Earth's dynamic atmosphere. Stars appear to twinkle as starlight passes through shifting air pockets that act as roving lenses.
The Hopkins-built device uses a computer to precisely measure the twinkle of starlight. Then a small mirror compensates for the distortion by rapidly tilting and tipping, undoing much of the atmosphere's distorting effect. As a result, the contrast between the images of the faint brown dwarf and its brighter stellar companion was improved. The astronomers then used the coronagraph, which blocks much of the star's glare with an opaque disk, to more easily see the faint brown dwarf orbiting its far brighter companion.
Also involved in designing and building the Adaptive Optics Coronagraph were Johns Hopkins engineer Robert Barkhouser and Mark Clampin, a former Hopkins scientist who is now affiliated with the Space Telescope Science Institute in Baltimore. Other Caltech astronomers involved in the discovery were Ben Oppenheimer, Shrinivas Kulkarni and Keith Matthews. The adaptive optics project has been funded, in part, through a grant from the Seaver Institute in Los Angeles.
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