Pluto, the Maybe Planet, Gets Another Look
By JOHN NOBLE WILFORD
Published: November 16, 2004
So maybe Pluto is a planet after all.
Astronomers in recent years were having doubts. Perhaps Pluto, small, distant and poorly observed, should be downgraded from true planet to a mere Kuiper Belt Object, or K.B.O., one of the thousands of icy bodies orbiting beyond Neptune that probably are leftovers from the creation of the outer solar system.
''People were finding all these K.B.O.'s that were huge, literally half the size of Pluto or larger,'' said Dr. John Stansberry, an astronomer at the University of Arizona. ''But those supposed sizes were based on assumptions that K.B.O.'s have low albedos, similar to comets.''
Albedo is a measure of how much light an object reflects. The more light it reflects, the higher its albedo and the smaller its size needs to be.
In a report last week at a meeting of planetary scientists in Louisville, Ky., Dr. Stansberry said a recently discovered K.B.O., first estimated to be 932 miles in diameter, appeared in new observations to be no more than 435 miles wide. It was a matter of the albedo -- 18 percent, instead of the assumed 4 percent for previous calculations.
The measured object, designated 2002 AW197, is one of the largest nonplanet bodies in the solar system, and yet is only about 30 percent as large as the 1,400-mile-wide Pluto.
As more K.B.O. albedos are studied, astronomers should find out if the first object is typical, or an oddball, and if Pluto appears to be significantly different from these smaller objects.
A planned survey of 30 such objects, Dr. Stansberry said, ''will help us determine what their compositions are, how they evolve, how massive they are, what their real size distributions and dynamics are -- and how Pluto fits into the whole picture.''
Dr. Michael Shara, chairman of the astrophysics department at the Hayden Planetarium in New York, said the Spitzer telescope's infrared observations ''appear rock-solid and a terrific piece of work.''
The planetarium has had a leading role in reclassifying Pluto. In its exhibition hall, eight planets are prominently displayed, and the supposed ninth, Pluto, rates only a plaque, casting doubt on its status as a planet.
''We remain convinced that Pluto is not a planet, but one of the Kuiper Belt Objects,'' Dr. Shara said. But, he allowed, the new observations could eventually restore Pluto's planetary status.