News-Letter

A decision last month by a group of astronomers to revoke Pluto's status as a planet has sparked intense reactions among many in the scientific community as well as the public. The ongoing debate provides unique insights into the scientific process, which often centers around classification as much as discovery.

On Aug. 24, the final day of a meeting of the International Astronomical Union in Prague, 424 astronomers voted and determined that Pluto is no longer a planet. The decision hinged on a refined definition of a planet.

At the IAU conference two of the four proposed resolutions were approved, assigning new definitions for planets, designating Pluto as a dwarf planet and leaving all those who had memorized a mnemonic device for remembering the original nine planets scrambling to keep up with the times.

According to the new IAU standards there are three primary qualifications for a celestial body to be considered a planet. First, it must be in orbit around the sun or another star. Second, it needs to have sufficient mass for gravity to pull it into a nearly round shape. Finally, it must have cleared the neighborhood around its orbit of all other orbital debris.

Although Pluto meets the first two qualifications, it is located within a ring of icy bodies and debris known as the Kuiper Belt and therefore fails to comply with the third criterion. While some scientists argue that this is an imprecise determining factor, as many larger planets frequently have asteroids cross their paths, there were enough voices against Pluto the planet for it to be demoted.

Richard Conn Henry, professor of Physics and Astronomy at Hopkins, agreed with the IAU's decision. "I think the notion that Pluto is a planet is absurd. When it was initially discovered, it was thought to be vastly more massive than it turned out to be. Its orbit is radically different from that of all the other planets," he said.

Pluto was initially discovered in 1930 by the American astronomer Clyde Tombaugh. For the past seven decades it has been considered the ninth planet in the solar system, although most astronomers have recognized its scientific quirks.

The orbit of Pluto varies widely, at some points taking it inside the orbit of the nearest planet, the gas giant Neptune. Pluto's orbit is also at an angle slightly outside the plane of the rest of the solar system, called the ecliptic.

Pluto is also unusually small for a planet, approximately one-fifth the size of Earth's moon. Unlike the other planets beyond Mars, tremendous balls of swirling gas many times larger than Earth, Pluto is rocky and lacks an atmosphere.

Those opposed to the IAU's decision include Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who is in charge of NASA's upcoming New Horizons mission to Pluto. He recently spoke to the astronomy interest Web site www.Space.com. "I'm embarrassed for astronomy. Less than five percent of the world's astronomers voted."

Although 424 astronomers voted on the status of Pluto, Stern argues that there are about 10,000 astronomers around the world who should have had a more direct say in the decision.

New Horizons is the first mission to the dwarf planet. It was launched in January of this year in order to unlock the secrets of Pluto. The probe onboard contains seven science instruments designed to tell astronomers about the surface, geology, interior and atmosphere of Pluto. New Horizons is not expected to reach Pluto until July 2015.

The debate over whether to demote Pluto has raged on for years. The continued discoveries of large Kuiper objects, which occupy a region of space beyond Neptune called the Kuiper Belt, have made Pluto seem less planet-worthy.

Hopkins professor of Earth and Planetary Science Bruce Marsh believes the formal change will do little to affect most people's views of the solar system. "I think everyone will still call it a planet, because nothing has changed in the solar system. And since it takes Pluto about 250 years to make one orbit around the sun, it has gone from a hero planet to an unknown without even making a single trip around the sun!"

Scientific knowledge progresses through both new discoveries, and, occasionally, definitions. Perhaps by the time Pluto completes is solar orbit its new name of "dwarf planet" will have been changed again and scientists will have finally settled on the celestial body's status.

As Marsh noted, "The bottom line is that Pluto is outclassed by the rest of the planets, so it is losing its rank. The real question to me is that, since we all know and love Pluto -- even Mickey Mouse had Pluto as a best friend -- what do we do with it?"

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