Planet or Star?

By Philip Elmer-DeWitt

Excitement over a sighting

Is it a planet or is it a star? Or might it be a brown dwarf? Those questions were being debated throughout the rarefied world of astronomy last week after a University of Arizona team announced the first sighting of a planet outside the solar system. It appeared to be the long-awaited breakthrough hi the search for planetary systems other than our own: a warm ball of gas about the size of Jupiter seen orbiting around Van Biesbroeck 8, a star some 21 light-years from the earth. Since a light-year is the distance light travels in one year at the rate of 186,000 miles per sec., that would put the planet about 123 trillion miles from the earth. Said a confident Donald McCarthy Jr., the astronomer who headed the Arizona team: "It's a large planet orbiting around a small star."

But while McCarthy's fellow scientists were clearly excited by the find, many had doubts about exactly what to call it. Astrophysicist David Black, who heads NASA'S project to search for other planetary systems, theorized that what McCarthy's team had found was actually a pair of diminutive stars, one of which failed to develop fully and became a celestial relic known as a brown dwarf. Benjamin Zuckerman, professor of astronomy at the University of California, Los Angeles, called the discovery "not quite a planet and not quite a star." George Gatewood, director of the University of Pittsburgh's Allegheny Observatory, agreed: "Planet is the wrong word. Call it what you like. It just doesn't seem like a planet." But he added, "If you took a layman by it, it would look to him like a star."

The issue is more than a matter of semantics. Scientists have long suspected that the universe is teeming with distant planets, some of which might support life. Some astronomers have inferred the presence of planet-like bodies by measuring the wobble in the path of certain stars as they travel across the sky; they suggest that the tug of another object's gravity might cause a disturbance in the star's movement. The Infrared Astronomical Satellite in 1983 detected around a few stars great disks of dust and debris that are thought to be spawning grounds for new planets. The fact that astronomers have been unable to see extrasolar planets is hardly surprising, given their relative small size and lack of luminosity. By comparison, hosts of stars, with the light generated by their nuclear fires, are clearly visible against the background of the heavens.

The object discovered by McCarthy and his colleagues--Frank Low of the University of Arizona and Ronald Probst of the Kitt Peak National Observatory in Tucson--has been dubbed VB 8B and is some 600 million miles from the star it orbits. It is visible only through powerful telescopes. Although it is nine-tenths the size of Jupiter, its mass is ten to 50 times greater. It is also a good deal warmer: 2,000DEG F, in contrast to Jupiter's -240DEG, or as Gatewood put it, "as hot as a Pittsburgh blast furnace."

The sighting of VB 8B was made using a technique known as speckle interferometry, which eliminates most of the distortion that the earth's atmosphere causes to the faint emissions of light from so distant a body. Instead of taking one infrared exposure, the astronomers snapped 10,000 quick shots. A computer then blended all these shots into a composite image.

After considering its evidence, McCarthy's team decided to call its find a planet, or "more accurately a brown dwarf." This kind of quasi-star, whose existence was first theorized by Shiv Sharan Kumar, an astronomer at the University of Virginia, has never been observed directly before. Said Kumar of the Arizona sighting: "People wish for planets, but these are not them." -- By Philip Elmer-DeWitt.

Reported by Heyden White/New York and Robert C. Wurmstedt/Tucson

With reporting by Heyden White, Robert C. Wurmstedt