Planets - the Smallest: Pluto

by Jeanette Cain

More articles in Solar System

Pluto, the solar system's smallest planet, is a small ball of ice and rock and the farthest planet from the Sun. In 1930, Clyde Tombaugh, U.S. astronomer, discovered this little world. It's orbit is titled and the least circular of all planetary orbits. Its does the tilt, spin, and orbit dance: tilts from the vertical by 122 degrees, spins on the axis once every 6.39 days and orbits the Sun every 247.68 years.

When Tombaugh discovered Pluto, it was a tiny dot that moved through photographs taken over six days at the Lowell Observatory in Arizona. Pluto's diameter is 2,274 km of which the Hubble Space Telescope distinguishes by bright markings considered to be ice patches on the dark reddish surface. Not yet known, but Pluto is believed to have craters caused by collisions from other objects in space.

The temperature of the planet is around -220 degrees Centigrade, but when it reaches its closet point to the Sun, the surface will warm just enough to melt a little ice that turns to gas and then makes a thin atmosphere. Once Pluto moves away from the Sun again, the gas will freeze again, making a new layer of surface ice.

Pluto has an unusual orbit that takes it from the Sun by a distance of 4.44 to 7.38 billion kilometers. On every 248 year orbit, Pluto is closer to the Sun than Neptune. It will visit this region close to twenty years, as it did from 1979 to 1999. Compared to Earth's orbit, Pluto's orbit is tilted at seventeen degrees, and has a larger slant than the other planets.

The Hubble Space Telescope has captured photographs of Pluto and Charon, its moon. Charon's is more than half of the diameter of Pluto, which makes it larger than its parent, Pluto, and unlike the other moons with their parents. With this similarity in size, they are often considered as a double planet.

Pluto's moon, Charon, was discovered in 1978, a fairly recent discovery. Charon's diameter is around 1,150 kilometers. It has less density than its parent, which means there may be less rock and more ice. One theory suggests that Pluto collided with another object knocking off the icy moon of Charon.

This parent and child keep the same sides facing one another as they turn in their orbits. If you were to stand on one side of Pluto, and someone else were to stand on the other side, each would see different things. You would see Charon, but the person on the other side would not see the moon. It moves every six days and nine hours around Pluto at an approximate distance of 19,400 kilometers.

NASA was preparing to send a mission probe to Pluto in 2004. Its mission was to fly beyond Pluto and Charon, make photographs of the surface, and continue into the Kuiper Belt region. The probe's name was Pluto-Kuiper Express and expected to launch in December 2004, but, on September 13, 2000, NASA stopped the work order on this mission. NASA turned it sights to developing a new mi8ssion that is expected to reach Pluto before the year 2020.


1. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing, Inc.: NY 1999

2. Editors. Secrets of the Universe. International Master Publishing: US. 1999