Jimmy Westlake: The late, great planet Pluto


What a wacky month in astronomy this has been! First, astronomers announced that they might add three new planets to the existing family of nine, and next, we learn they've axed Pluto instead and cut the number of planets from nine to eight. What the heck is going on in astronomy-land?

To understand this strange situation, we need to take a look back in history. Today's planet debate began about 2,500 years ago, when Greek astronomers recognized five wandering stars, or planets, in the sky that didn't remain anchored to a constellation like the other stars. They called the five wanderers Hermes, Aphrodite, Ares, Zeus, and Kronos after some of their mythological gods and goddesses. We know them better by their Roman names: Mercury, Venus, Mars, Jupiter, and Saturn.

The definition of the word "planet" was simple back then: if it wanders around the sky, then it's a planet.

That definition worked fairly well for 2,500 years, even after the telescopic wanderers Uranus, Neptune, and Pluto were added to the mix. However, almost immediately after it was discovered in 1930 by American astronomer Clyde Tombaugh, Pluto was found to be an oddball planet that is smaller than our Moon, crosses the orbit of the planet Neptune and is tilted at a steep angle to the other planets. Out of respect for Pluto's discoverer, no one wanted to challenge Pluto's planetary status while Clyde was alive.

Clyde died in 1997, and no sooner had the obituary ink dried than the demote-Pluto crowd made its move. Complicating the picture were the discoveries of hundreds of other small, Pluto-like bodies in the outer solar system in the past decade, including one object dubbed Xena that is slightly larger than Pluto. Were all of these to be considered planets, too?

Something had to give. The ancient definition of planets as "wandering stars" just didn't work any more.

The responsibility of formulating a new, scientifically valid definition for the term "planet" fell upon the International Astronomical Union. After several years of heated debate, the IAU sub-committee that was appointed to hammer out a new definition of the word "planet" unanimously approved this resolution: If an object orbiting the Sun is large enough to be spherical and is not a star or satellite, then it is a planet. That definition would have allowed Pluto to remain a planet and also would have embraced Ceres, the largest of the asteroids, Pluto's large satellite Charon, and the large icy world Xena into the planet family.

But, when this resolution was presented to the IAU members for approval at its August conference in Prague, the normally mild-mannered astronomers nearly came to blows! A very vocal group of dissenters demanded that the resolution be revised to demote Pluto and the other small bodies that orbit the Sun. When the smoke cleared, we found that Pluto had been culled from the planet herd.


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