Monday, January 30, 2006

The Real Story of Pluto

by Elizabeth Bauman (Class of '05)

Recently, there has been fiery debate amongst astronomers about whether or not Pluto, one of the nine titles that students memorize in grade school, is indeed a planet. New discoveries and found statistics have led astronomers to contemplate whether Pluto should remain considered a planet or be “demoted” to a lesser status.

Pluto was named the ninth planet in our solar system in 1930 by Clyde Tombaugh in the Lowell Conservatory. According to some astronomers, however, Pluto, if found today, would not be named a planet. The definition of a planet, unfortunately for astronomers, is arbitrary.

"It's something of an embarrassment that we have no definition of what a planet is," wrote a professor in the Berkeley News. "People like to classify things. We live on a planet; it would be nice to know what that was."

Some claim Pluto is too distant from the Sun to be considered a planet, but is more similar to Trans Neptunian Objects. Trans Neptunian Objects, called TNOs and discovered in 1992, are small objects composed of rock and ice and are considerably distant from the Sun. These TNOs have characteristics quite similar to Pluto’s and even have a subset of TNOs called Plutinos, or little Plutos.

Size is also a gray area for Pluto. Pluto is twenty-five times smaller than Mercury and its mass is one-fifth of that of the moon. However, Pluto is also nine times larger than Ceres, the largest object in the asteroid belt, and its mass is over ten times that of Ceres. Pluto, while one hundred times more massive any other TNO, excluding its satellite Charon, has diameters closer to those of TNOs than of other classified planets.

Some persist that since Pluto is spherical and not egg shaped like many smaller objects, it would be automatically considered a planet if it were simply closer to the Sun. Such beliefs, however, would entitle the asteroid Ceres and Kuiper-belt objects of Varuna and Quaoar to also be named planets.

Some astronomers disregard these objections to Pluto’s status, waiting for a more concrete distinction between planets and TNOs. "Until there is a consensus that one of the physical definitions is clearly the most useful approach in thinking about the solar system, the IAU will not 'demote' Pluto or 'promote' Ceres," says the International Astronomical Union (IAU).

While the IAU currently denies any plans of change, a commercial opportunity is about to take place. Astronomers will soon name their ten thousandth asteroid discovery, and there are rumors that Pluto will be entitled number ten thousand and a TNO.

Astronomers at Caltech suggest that a planet is a body whose mass is greater than the total mass of its surrounding bodies. By this definition, Mercury is a planet because few asteroids are in its region. Jupiter, too, is a planet because its mass is sufficiently larger than the sum of the Trojan asteroids. Pluto, inevitably, is not considered a planet in this theory because its mass is less than half of the total of the Kuiper Belt Objects.

Currently, without further definitions of and distinctions between planets and TNOs, Pluto will remain a planet. "There is no plan to 'downgrade' or 'demote' Pluto. It will stay as a planet," Brian Marsden, head of IAU’s Minor Planet Center.


Cauchi, Stephen. "Pluto may Lose Planet Status." Sydney Morning Herald. 11 Jan. 2006.

Cuk, Matija. "Is Pluto a Planet?" 11 Jan. 2006.

Hajian, Allie, John Cannizzo, and Laura Whitlock. "Ask an Astrophysicist." NASA. 11 Jan. 2006.

Whitehouse, David. "Pluto will have 'dual citizenship'." BBC News. 11 Jan. 2006.