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Nine Planets, or Eight?

Probing Pluto's place in the solar system

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10:58am, June 5, 2001
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Any fourth grader knows the names of the solar system's planets. There's Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Oh yeah, and Pluto, right?

Maybe not anymore.

An oddball compared with the other eight orbs, Pluto is smaller than Earth's moon. It orbits the sun at a 17-degree tilt relative to the plane in which the other planets move. Pluto doesn't belong to either of the two groups in which astronomers classify the other planets. It isn't rocky like the so-called terrestrials–Mercury, Venus, Earth, and Mars. Nor is it one of the gas giants–like Jupiter, Saturn, Uranus, and Neptune. Pluto's icy composition resembles that of a comet.

That's why some scientists have argued for years that Pluto isn't a planet at all. Rather, they say, Pluto is an unusually large comet whose true family lies within the Kuiper belt, an outpost of icy objects beyond the orbit of Neptune. Since 1992, when astronomers spied the first object in the belt (SN: 9/26/92, p. 142), 377 have been found. This includes a group of four dubbed Plutinos because, like Pluto, their orbit has a special synchrony with that of Neptune. Every time Neptune goes around the sun three times, Pluto and the Plutinos go around twice.

Two new reports add to the mounting evidence that Pluto may be the king of the Kuiper belt rather than the pip-squeak of planets.

Researchers wanting to preserve Pluto's planetary status had noted that the body was the only denizen of the outer solar system known to possess a moon. Astronomers now report, however, that a card-carrying member of the Kuiper belt, a large cometlike object dubbed 1998 W31, also has a moon.

Reviewing images of 1998 W31 that they had taken last December, astronomers led by Christian Veillet of the Canada-France-Hawaii Telescope in Mauna Kea, Hawaii, found that the icy object was accompanied by a dimmer companion. Images taken by another team nearly a year earlier also show evidence of a satellite, Veillet and his collaborators report in an April 15 circular of the International Astronomical Union (IAU). They estimate that the moon lies at least 40,000 kilometers from 1998 W31, about a tenth the distance between Earth and its moon.

In the IAU circular, Veillet and his colleagues pull no punches about how they classify Pluto. They say their finding "indicates that 1998 W31 is the second [member of the Kuiper belt] to have a satellite (after Pluto)."

In the May 24 Nature, astronomers trounce another possible objection to a Plutonian association with the Kuiper belt–the body's large size relative to known belt members. David Jewitt and Hervé Aussel of the University of Hawaii in Honolulu, with Aaron Evans of the State University of New York at Stony Brook, monitored a recently discovered Kuiper belt object, known as (20000) Varuna.

Observing Varuna in both visible light and in submillimeter wavelengths, Jewitt's team determined that Varuna has a diameter of about 900 km. That's roughly 40 percent the diameter of Pluto and only slightly less than that of Pluto's moon Charon.

"The results suggest that Pluto and Charon are not uniquely large objects [in the outer solar system] and that a continuum of sizes may exist," say Stephen C. Tegler and William Romanishin, both of the University of Oklahoma in Norman, in a commentary accompanying the Nature report.

"We can now imagine that bodies even larger and more distant than Pluto will be found," the commenters note.

"I think findings over the past few years put the last nails in the Pluto-the-planet coffin," declares Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Some astronomers argue that the whole debate is a waste of time, having to do with semantics and not science. Still, the controversy continues. In 1999, the IAU decided not to reclassify Pluto as a denizen of the Kuiper belt (SN: 2/27/99, p. 139). Yet, many astronomers concede that if Pluto had been discovered yesterday rather than in 1930, it probably wouldn't be called a planet.

Marsden notes that in 1801, when astronomers spied a large chunk of rock called Ceres, they called it a planet. As more and more such rocks turned up, scientists reclassified Ceres as a member of the asteroid belt occupying the region between the orbits of Mars and Jupiter. Ceres remains the largest asteroid known, but no one doubts it's an asteroid, Marsden adds.

When the American Museum of Natural History's Rose Center for Earth and Space opened last year in New York, it became clear that Neil deGrasse Tyson, director of the museum's planetarium, had taken a quiet but firm stance on the Pluto controversy. One of the displays has these words: "Beyond the outer planets is the Kuiper Belt of comets, a disk of small, icy worlds including Pluto." When the New York Times publicized that statement, it caused an uproar.

"More power to him!" says Marsden about deGrasse Tyson's decision. Giving Pluto a dual status as both the smallest planet and the largest Kuiper belt member, or even stripping it of its planethood, is not a demotion, insists Marsden.

"It's still an interesting object," he says. "It's just that it's an interesting member of the Kuiper belt."

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