Pluto is a small world with a diameter of 2294 km (about 85 percent the size of our moon.) It takes Pluto 248 years to go once around the Sun in its strange orbit. The orbit is inclined 17 degrees to the plane of the solar system. In addition to that, sometimes Pluto is closer to the Sun than Neptune (it happens to be right now.)
Being so far from the Sun, Pluto is cold enough (at -230 C) to freeze most compounds that would normally be gasses. What we know of Pluto comes from spectroscopic studies. They show that Pluto has methan on its surface, either as solid ice or frost. Astronomers also discovered that Pluto has a thin atmosphere of methane gas and another gas probably nitrogen or carbon monoxide. Many astronomers also believe that Pluto has water ice in its interior.
Pluto has one moon, named Charon which is about 1200 km in diameter, roughly half Pluto's size. Charon oribts Pluto at a distance of 17,000 km in 6.4 days. It was only in 1978 when Charon was discovered that the mass of Pluto could be determined, and therefore the density.
Pluto has a low density of about 1.8 g/cm3. This indicates the Pluto is an icy-rocky mixture. It does not fit into the category of gas planets , nor does it fit into terrestrial worlds either. Pluto is somewhere in between, and atill a mystery.Perhaps some of the answers will come from a spacecraft known as Pluto Express which NASA plans to send to this distant cold world.
Copyright © 1997 Kathy A. Miles and Charles F. Peters II