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Pluto: Planet or Comet?

Recent visitors to the Rose Center for Earth and Space at the American Museum of Natural History in New York City have been surprised to discover that Pluto is no longer considered a planet in the solar system.


Pluto, with its tiny moon Charon (right).

Photograph courtesy of NASA

"Pluto, being half ice by volume, should assume its rightful status as the King of the Kuiper Belt of comets," says Neil Tyson, director of the Hayden Planetarium at the American Museum of Natural History.

Some members of the International Astronomical Union agree with Tyson and recommend that Pluto be demoted to "object."

A Planet No More?

The scientists' main reason for the switch is Pluto's tiny size; with a diameter of approximately 1,420 miles ( 2,280 kilometers) it is six times smaller than Earth, and smaller than seven of the solar system's moons.

Pluto's orbit is unlike any other planet. It is the only planet to travel an elliptical orbit, and uncharacteristically crosses the orbit of its closest planetary neighbor, Neptune. This means that for 20 years of its 248-year orbit around the sun it is not the most distant object in the solar system.

Despite its unusual characteristics, Pluto's status as a planet was solid until 1992, when David Jewitt and J. Luu of the University of Hawaii discovered a strange object called 1992 QB1. QB1 is a small icy body, similar in size to an asteroid, orbiting one and a half times further from the sun than Neptune.

QB1 was the first clue that more objects than Pluto might populate the distant reaches of the solar system. Since 1992 nearly one hundred objects like QB1 have been found. They are thought to be similar to Pluto in composition, consisting primarily of ice and rock.

This swarm of Pluto-like objects beyond Neptune is known as the Kuiper Belt, named after Gerard Kuiper, who first proposed that such a belt existed.

Astronomers estimate that there are at least 35,000 Kuiper Belt objects (KBOs) greater than 62 miles (100 kilometers) in diameter. Pluto is practically indistinguishable from other objects in the Kuiper Belt. Aside from its large size, the only real difference is Pluto's reflectivity, which makes it much brighter than other KBOs.

Pluto: Space Object

In the "Hall of the Universe" at the American Museum of Natural History, Tyson and his staff group together families of like-objects, rather than using the all-embracing term of planet.

They organize the objects of the solar system into five broad families: the terrestrial planets, which include Mercury, Venus, Earth and Mars; the asteroid belt, the Jovian planets, which include Jupiter, Saturn, Uranus, and Neptune; the Kuiper belt, and the Oort cloud (a ring of dust even further away from the sun than the Kuiper Belt).

With this approach, Tyson believes, "numbers do not matter and memorized facts about planets do not matter. What matters is an understanding of the structure and layout of the solar system."

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Pluto Facts
  • Pluto is the only planet in the solar system that has not been visited by a space probe.
  • On September 13, 2000, NASA suspended its Pluto-Kuiper Express (PKE) mission.
  • In December 2000 NASA announced new proposals for a mission to Pluto. The costs must stay under U.S. $500 million. The Johns Hopkins Applied Physics Lab, which just landed a probe on the asteroid Eros, is planning to make a proposal to NASA.
  • Because of its great distance from the Earth, the exact size of Pluto is not known. It is believed to have a radius of roughly 1150-1200 miles (1850-1930 km).
  • Since its discovery in 1930, Pluto has traveled on only about one-quarter of its orbital path around the Sun. As a consequence, its exact orbital parameters are still not known.
  • The surface temperature on Pluto is between -378 to -396°F (-228 to -235°C).