Signs of a 'something' circling star.

Signs of a "something' circling a star

It is apparently too low in mass to be a star, yet too warm to be what we think of as a planet--if, that is, it is there at all. But observations made less than three months ago of a star known as Giclas 29-38 have provided perhaps the most tantalizing evidence yet for the existence of some kind of "substellar object' circling a star other than our own sun.

"I'm not really ready to say, "I think we got one,'' said Benjamin Zuckerman of the University of California at Los Angeles last week at the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Pasadena, Calif. In fact, according to Zuckerman and Eric E. Becklin of the University of Hawaii's Institute for Astronomy in Honolulu, "there is at the moment not a single confirmed example of an extra-solar object, either isolated or in orbit around a star, that is unambiguously substellar.'

What the two scientists do have is measurements, taken on Aug. 23 and 24 from the NASA Infrared Telescope Facility at Mauna Kea Observatory in Hawaii, indicating that Giclas 29-38 is unusually bright at one particular wavelength (3.5 microns) for a star of its type. Located about 50 light-years away, or a bit less than 300 trillion miles, it is a "white dwarf' star, most of whose infrared spectrum is just about what one would expect from such a star having a temperature of 11,500 kelvins. But there is a little excess-- something left over.

A likely source of such infrared excess is energy given off by nearby solid material that has been heated by the star. The material could be in the form of tiny dust grains, perhaps flowing out from the star itself or else in a disk shape that some researchers believe could be a planetary system in the early stages of its formation. Such a disk was discovered in 1984 around the star Beta Pictoris (SN: 10/20/84, p.244). For various reasons, however, Zuckerman and Becklin note in the Nov. 12 NATURE (whose publication of their report was timed to match the Pasadena meeting), both sources of particles seem unlikely in the case of Giclas 29-38.

Another possibility, they suggest, could be "a true planet, such as Jupiter,' orbiting close to the star. Such a planet, however, would probably have the same side always facing the star, producing one hot and one cold hemisphere that would show up as a pattern in some of the infrared emissions. No pattern of this kind has been observed, according to the researchers.

"The most natural explanation,' the scientists maintain, is a "substellar brown dwarf' in orbit around the star. A so-called brown dwarf, so far only a hypothesis, would be an object whose mass is too low for the compression caused by its own self-gravity to trigger thermonuclear fusion at its heart, but high enough, in the view of Zuckerman and Becklin, for it to be capable of burning deuterium into helium. This would presumably give it something between 1 and 8 percent of the mass of the sun. In 1984, a brown dwarf was reported to be orbiting the star Van Biesbroeck 8 (SN: 12/15/84, p.373), but the spectral measurements that led to its discovery could not be repeated.

If a brown dwarf indeed circles Giclas 29-38, it should have between 4 and 8 percent of the sun's mass (or roughly 40 to 80 times the mass of Jupiter), about 15 percent of the sun's diameter, a faint 0.005 percent of its luminosity and a temperature of about 1,200 kelvins, the researchers estimate. "And unlike some previous claims,' Zuckerman said at the meeting, "I'm confident that this signal is not going to go away.'

Also during the gathering, Bradford A. Smith of the University of Arizona in Tucson reported that the disk of particles around Beta Pictoris is both larger and less symmetrical than previously thought, spanning about 900 astronomical units in one direction and about 1,100 in another. This asymmetry, suggested Daniel P. Whitmire of the University of Southwestern Louisiana in Lafayette, might be due to the presence of a brown dwarf moving around it in a noncircular orbit.