Pluto

Pluto is, on average, the most distant planet from the Sun. For the next few years, however, it is in fact closer to the Sun than its closest rival, Neptune. The reason for this is that the orbit of Pluto around the Sun is an ellipse with quite a large eccentricity. This means that it is more `oval-shaped' than circular. At present Pluto is near perihelion, its closest distance to the Sun, and is about 4440 million kilometres away. Almost all of the nine planets travel around the Sun in nearly circular paths. That means most planets have eccentricities close to zero. The more eccentric a planet's orbit is, the more it deviates from a circle.

Three planets have orbits that are significantly elliptical. Mars has an eccentricity of 0.09 - small but noticeable. Mercury's is larger, at 0.206. But Pluto is the champ, with an eccentricity of 0.25. Right now, Pluto is about 30 AU from the Sun (AU = astronomical units), which is roughly as close as it gets to the Sun. One hundred and twenty four years from now - half of a Pluto year - Pluto will be 50 AU from the Sun! No other planet goes through such wide excursions.

Pluto was discovered in 1930 by Clyde Tombaugh at the Lowell Observatory. It was discovered as a result of astronomers comparing the observed positions in the sky of the two planets, Uranus and Neptune, with positions predicted from their orbits about the Sun. Small departures from the predicted positions indicated that the paths of these two planets were being disturbed by the gravitational pull of another body. Pluto was originally thought to be much bigger than it is. Percival Lowell was looking for a planet about 10 Earth-masses, typical of Uranus and Neptune. Pluto's surface is very bright, so when it was discovered, and people realised how far Pluto was from the Sun (by watching how fast it was orbiting the Sun), they still thought Pluto was pretty big. After all, they had no way of knowing if Pluto were small and bright or large and dark, since Pluto would reflect the same amount of light in either case.

The best indications that Pluto is much smaller than the Earth come from "stellar occultations," which occur when Pluto moves in front of a star . Pluto doesn't block much area in the sky, so these occultation's are rare and don't last very long. Astronomers realised that if Pluto's diameter is small, then the occultation will be especially short. The 1988 occultation showed that Pluto's radius is less than 1200 km, making it about two-thirds the size of the Moon. Pluto is only visible in fairly large telescopes where it appears as a star-like object of 14th magnitude. Because of its great distance from the Sun, Pluto only moves very slowly across the sky. At present it lies close to the borders of the constellations Libra and Serpens Caput. Pluto's orbit has the highest eccentricity and largest inclination to the ecliptic of all the planets.

Pluto is so unique it almost defies classification. Though it orbits the Sun, Pluto neither qualifies as a terrestrial nor as a gas giant planet. Though it behaves like a comet by periodically warming and losing its atmosphere into space, Pluto is far too large for that category. Pluto may be the last survivor of a lost population of objects called ice dwarfs that inhabited the primeval solar system. Neptune's moon Triton might be a distant cousin, and other relatives may dwell in the Kuiper belt, a disk of ice debris left behind from the solar system's birth. Pluto and Triton survive because they have found gravitational niches in the solar system where they remain in stable orbits. Pluto is in a resonance orbit with Neptune, it circles the Sun three times for every two orbits of Pluto), which means that Pluto never gets close enough to Neptune to be thrown out the solar system. Triton was gravitationally captured by Neptune and was therefore prevented from being ejected from the planetary region. It is believed that all of the other ice dwarfs formed inside 50 AU were ejected by gravitational interactions due to the giant planets in the ancient past.

Pluto and its 'moon' Charon is the best example of a double planet, which occurs when two bodies are reasonably close in mass and so orbit around a common centre of gravity - or barycenter (analogous to two children balancing on a teeter-totter). Charon may have been born through a head-on collision between Pluto and another large ice body, in much the same way as the Earth-Moon system is believed to have formed. According to computer models, some of the debris from this giant impact on Pluto went into orbit around Pluto and coalesced to form Charon. Due to its great distance, very little is known about Pluto.