The Gas Giants (cont.)

Pluto

Pluto is a planetary oddball, a strange world that has baffled scientists ever since it was discovered in 1930. It is not the large gas giant that one might expect to find in the outer reaches of the solar system. Instead, it is a small world, much smaller than the Earth, and in fact roughly as large as our Moon. It is probably composed of a mixture of rock and ice. It even has been suggested that Pluto is not a genuine planet, but simply a moon that somehow escaped from Neptune.

Pluto is usually the farthest known planet from the Sun, its mean distance almost 6 billion kilometers (almost 4 billion miles) out. It takes 248 years for Pluto to complete one orbit around the Sun, but the orbit is so elongated that it actually spends about 20 years of this time inside the orbit of Nepture. (In fact, Pluto is inside Neptune's orbit now, and will be until 1999, so that Neptune is temporarily the furthest planet from the Sun.)

The last planet?
Tiny, mysterious Pluto is so far from the Sun that it appears only as a tiny speck of light (arrow) that moves slowly against the background of the fixed stars. So inconspicuous that it was not discovered until 1930, Pluto is not a gas giant planet like all the others in the outer solar system. Instead it is a small, rocky world about the size of Earth's Moon. Recent examinations of old photographs, combined with new observations, indicate that Pluto itself has a moon.

Despite Pluto's distance and the extreme difficulty of observing it, our view of the faraway planet has changed greatly in the last few years. As we have looked more carefully, Pluto has become an even smaller and brighter object than we thought it was. It seems to have a bright layer of frozen methane ("marsh gas," chemically CH₄) on its surface. Even more surprising, reexamination of old photographs revealed that Pluto is not alone; it has a moon. Pluto now seems to be about 3000 to 3500 kilometers (1900 to 2200 miles) in diameter. Pluto's moon, Charon, is large by comparison, about 1200 to 1500 kilometers (750 to 930 miles) in diameter, so that the two bodies form a kind of unique double planet.

Pluto will hold its secrets for a long time yet. It is simply too far away for our current spacecraft to reach it in a reasonable length of time. It will be many years before any machines or humans see Pluto up close, dimly lit by a Sun so distant that it seems like just a rather bright star in the blackness of space.