Pluto's Planetary Plight

Anything around 5 billion years old shouldn't have problems. Pluto, however has a real identity crises these days and some astronomers want to strip it of its status as planet and demote it to minor planet. In the near future might we  be teaching our kids about the 8 planets in the solar system?

Pluto's problem's began with Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge Mass. Proposed that Pluto qualified as both planet and minor planet. That met with some indignation from other astronomers. Minor Planet is a term generally used to classify asteroids and other extraneous small chunks of rock which is debris left over from the formation of the solar system. Pluto, Marsden's critics chanted, is no asteroid!

The controversy soon became the "Pluto Debate." It soon had a life of its own as heated debates cropped up amongst astronomers and planetary scientists. The problem with Pluto is that it doesn't fit in with the other planets.

The first four planets, Mercury, Venus, Earth and Mars, are called terrestrial planets. They are smaller, rocky worlds. They are more dense than the outer planets are. These planets, Jupiter, Saturn, Uranus and Neptune, are called gas giants.  They are large worlds, much less dense than the inner planets, and they are composed mostly of frozen gas and ices.

Pluto is a small world, about 1500 miles in diameter making it smaller than Mercury and even our Moon. Pluto is larger than the largest known asteroid, Ceres which is 585 miles in diameter.  It is composed af about 60 percent rock and 40 percent ices of frozen nitrogen, carbon monoxide, methane and water.  Pluto is a strange cross between the inner and outer planets. Pluto's orbit is another strange thing. It is inclined 17 degrees to the plane of the solar  system and it is highly elliptical (the other planets are only very slightly elliptical.) This highly elliptical orbit sometimes places Pluto closer to the Sun than Neptune! It is indeed a strange world.

Pluto does have a moon orbiting around it, though the two are quite close in size when compared to other planet/moon ratios.  Because of Charon's large size, the two are locked in an orbit around each other in a lopsided dumbbell tango. But having something orbiting around it is not enough to make one a planet. Recently, an asteroid, Ida was found to have a smaller body (dubbed dactyl) orbiting around it.

So what is Pluto, if not a planet? Earlier in this century a Dutch astronomer  Gerard Kuiper theorized that the solar system did not just end beyond Neptune and Pluto. Rather, he said, the area was inhabited by a belt of icy objects left over from the formation of the solar system (similar to our asteroid belt.) In 1992, after Kupier's death in 1973, the first Kuiper belt object was discovered. Now, we know of at least 130 objects in the Kuiper belt. Pluto has a  a lot of neighbors and may in fact be a large Kuiper belt object.

So what is a planet to do when faced with  being demoted?  Pluto probably has nothing to be worried about. Most astronomers, and certainly the public, will continue to see Pluto as the ninth planet. The International Astronomical Union (of which Marsden is the director) issued a release  in February stating that Pluto was in no immediate danger of being demoted. The debate will likely continue and we will look for updates of Pluto's planetary plight.

Copyright © 1999 Kathy Miles and Charles F. Peters II