A Harvard professor of Astronomy yesterday questioned the labelling of a recently-discovered object in the solar system as a possible tenth planet.

Brian G. Marsden, professor of Astronomy and researcher at the Harvard-Smithsonian Center for Astrophysics, said a reference in yesterday's New York Times to "Object-Kowal" as a possible tenth planet was "nonsense."

He explained that there are at least three known asteroids between Mars and Jupiter that are larger than Object-Kowal, and they have not been labelled planets.

The discovery of Object-Kowal has raised speculations as to the possibility of an asteroid belt beyond Saturn, Charles Kowal, the object's discover, said yesterday.

But Kowal, a research assistant in the department of astrophysics at the California Institute of Technology, said it would be wrong to call the object an asteroid, because it is different from other known asteroids.

"It is farther out in the system, has a different origin, and is a lot more icy," he said.

The possible icy composition of Object-Kowal, which astronomers have deduced based on the amount of light it reflects, suggests that it may be a comet, Kowal said yesterday. He added, however, he thinks it is too big and smooth to be a comet.

Marsden said, however, "It could be a peculiar kind of comet." He added he could not reject the possibility that it is a comet, because of a theory postulated in the '50s that comets could originate in the vicinity of Uranus and Neptune.

Marsden said he tends to think that Object-Kowal is a type of asteroid.

"I tend to favor the idea that it's in a nearly circular orbit just inside the orbit of Uranus," he said. He added if the orbit is circular it is probably stable, suggesting the object has existed in its present orbit for a long time.

But Eric J. Chaison, assistant professor of Astronomy, said yesterday, "The fact that we haven't seen it yet implies that it has just appeared in the solar system, like a comet."

Marsden explained that it may not have been seen earlier because astronomers were not looking for such slowly moving objects. Object-Kowal moves three times more slowly than asteroids around Jupiter, he said.