Space Topics
Even while New Horizons is on its way, Pluto and its Kuiper belt companions continue to make news here on Earth.
Is Pluto the tiniest planet or the second largest member of the Kuiper belt? Is Charon just a really large moon, or are Pluto and Charon a double planet? What does this distant pair of icy bodies look like up close? There are far more questions than there are answers about a pair of objects that mark the outer edge of the orderly Solar System, and the inner edge of a vast region of frigid worlds, the Kuiper belt and Oort cloud, home of comets and, possibly, of many more planet-sized bodies.
Pluto is small relative to the other planets. In fact, there are a total of 15 bodies in the solar system larger than Pluto -- all eight of the other planets, plus the moons Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton. But Pluto makes up for its small size by having the most gargantuan moon -- compared to its planet -- in the solar system. Charon is a whopping half the size of Pluto. (The next largest duo is the Earth-Moon pairing, but our Moon is only a quarter the size of Earth, and even that is a standout among moon-planet pairs in the solar system.) Tidal effects that Pluto and Charon exert on each other have locked the two bodies into mutual spin-orbit resonance, meaning that one face of Charon always looks at Pluto while one face of Pluto always looks at Charon; it's as though the two bodies form a dumbbell spinning in space. Recently, two more small moons, Nix and Hydra, were discovered orbiting Pluto and Charon in their own (likely) resonant orbits.
Unlike the other eight planets, Pluto's orbit is quite eccentric and significantly inclined, at an angle of 17 degrees from the plane of Earth's orbit. Since the discovery of Pluto, many other objects have been observed in the Kuiper belt with orbital properties similar to Pluto's. Spectral measurements of Pluto suggest that its surface is similar to that of Neptune's moon Triton and to other objects in the Kuiper Belt. Because of the size, orbital, and spectral similarities between Pluto and Kuiper belt objects, Pluto is scientifically classified as a Kuiper belt object.
No spacecraft has yet visited Pluto. The best images come from the Hubble Space Telescope and from adaptive optics-equipped telescopes on Earth. NASA's New Horizons mission launched on January 19, 2006 for a 2015 Pluto flyby.
Pluto and Charon Numbers
Pluto Size: 9th largest planet - 2,306 kilometers - 0.181 Earths
Charon Size: 22nd largest body in the solar system - 1,205 kilometers - 0.094 Earths
Calendar: 1 Pluto/Charon year = 248.02 Earth years; 1 Pluto/Charon day = 6.387 Earth days
Orbit: 5,906,380,000 kilometers - 39.482 Earth orbits
Axial tilt: 122.53 degrees (retrograde, and tilted 9.08 degrees more than Earth's)
Number of moons: 2 or 3, depending on whether Pluto-Charon is a double planet or Charon is Pluto's moon