Astronomers have discovered what may be the first planet ever found beyond our solar system, a massive globe of apparently hot gas circling a nearby star only 21 light years from Earth.

The discovery is the fourth in the last two years showing evidence that other planets exist outside our solar system, and it is the first to pinpoint a single planet circling another star. The orbiting Infrared Astronomical Satellite has found solar systems forming around the stars Vega and Femelhaut, and a ground-based observatory has found a young solar system circling the star Beta Pictoris.

The National Science Foundation said yesterday that astronomers at the University of Arizona detected a planet approximately the size of Jupiter -- the largest of the nine planets circling our sun -- but several times heavier than it in orbit around the faint star Van Briesbroeck 8, apparently at a distance of 600 million miles. That is roughly the distance (500 million miles) at which Jupiter is in orbit around our sun.

The star, named for the Belgian-born American astronomer who discovered it in 1961, is part of the constellation Ophiuchus between Scorpio and Sagittarius. The constellation's brightest stars can be seen in the nighttime summer skies in the Northern Hemisphere.

Astronomers Donald W. McCarthy Jr. and Frank J. Low said in a report to the Astrophysical Journal that the planet apparently has a surface temperature of 2,000 degrees Fahrenheit, which makes it hotter than any planet in our solar system but still too cool to be a star.

"This is certainly not a habitable planet and the star it is going around is not very hot like our sun, so there probably isn't any life there," Dr. McCarthy said. "But it is another step in the direction that should lead to the discovery of systems of planets around other stars."

McCarthy said there is no question that the planet is circling the star and that it is not a background object such as another nearby star just behind Van Briesbroeck 8. Said McCarthy: "The body we have identified as a planet is too dim and too cool to be a star."

Low said the planet "fills a gap we had never been able to fill before. This planet clearly lies between what we see in our own solar system as Jupiter and a real star like our sun."

Low said the planet's surface temperature of 2,000 degrees suggests that the hydrogen isotope deuterium is burning on its surface, a preliminary stage in the birth of a star. To become a star, a planet must reach a much hotter condition in which hydrogen begins to burn in its interior as in a sustained thermonuclear explosion.

"There are many astronomers who believe that deuterium once burned on the surface of Jupiter as it struggled to become a star," Low said. "This planet may be going through the same struggle. . . ."

Deuterium is a rare isotope of hydrogen that is roughly twice as heavy as hydrogen and burns at far lower temperatures than hydrogen. It can be extracted from sea water and is known as heavy water when it is used to slow down the rate of chain reactions in nuclear power plants.

Low said he has little doubt that the object is a planet and not a star: "This is the first time we have clear-cut temperature and size limits on an object that places it unambiguously in the gap" between large planets and small stars.