Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus. Orbit: Diameter: Mass: 5,913,520,000 km (39.5 AU) from the Sun (average) 2274 km 1.27e22 kg

Astronomy

Pluto is the farthest planet from the Sun (usually) and by far the smallest. It is smaller than 7 of the solar system's moons (the Earth's Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona, USA, did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. This also gave rise to the notion that Pluto may not, in fact, be a planet. The search for Planet X continued, but nothing was found. Nor is it likely that there is anything to find: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no tenth planet.

Some astronomers think that Pluto would be better classified as a large asteroid or comet, rather than as a planet. They consider it to be the largest of the Kuiper Belt objects (also known as Trans-Neptunian Objects). The Kuiper Belt is a band of small icy comets that orbit beyond Pluto. There is considerable merit to the latter position, but historically Pluto has been classified as a planet and it is very likely to remain so.

Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 through to February 11 1999). Also, Pluto rotates in the opposite direction from most of the other planets.

Pluto is locked in a 3:2 resonance with Neptune. That is, Pluto's orbital period is exactly 1.5 times longer than that of Neptune. Its orbital inclination is also much higher than those of the other planets. Although it appears that Pluto's orbit crosses the orbit of Neptune, this is not in fact the case and there is no danger of a collision.

Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface.

Pluto has a satellite, Charon, named after the ferryman who carried the dead across the River Styx to Pluto's kingdom. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. This made it possible for astronomers to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.

The unusual nature of the orbits of Pluto and of Triton, and the similarity in 'bulk' between the two, suggest some historical connection between them. Earlier speculation had Pluto as a satellite of Neptune's, but this now seems unlikely. A more feasible idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later 'captured' by Neptune. There is further speculation that Triton, Pluto and Charon are the only remaining members of a large class of similar objects, the rest of which were ejected into the Oort cloud. The Oort cloud, which is named after the Dutch astronomer who discovered the spiral structure of the Milky Way, is a swarm of millions of comets that extends towards the nearest star. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body.

Pluto's radius is hard to calculate. The sum of the masses of Pluto and Charon is known with reasonable accuracy, determined from careful measurements of the period and radius of Charon's orbit, and basic physics. However, the individual masses of Pluto and Charon are much more difficult to determine. This is because it requires determining their mutual motions around the center of mass of the system, and this requires much finer measurements -- they are so small and far away that even the Hubble Space Telescope has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157, and although more observations are underway, reliable data will not be available until a spacecraft is sent to investigate.

The frosty nitrogen and methane surface of Pluto is about minus 238 degrees Centigrade (-396F). The frost-free regions are a few degrees warmer and roughly correspond to the regions that appear darker in optical wavelengths. The difference in temperature is sufficient to stir up cold winds in the thin nitrogen-methane air.

Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions induced by cosmic rays.

Little is known with certainty about Pluto's atmosphere. It is thought to consist primarily of nitrogen with some carbon monoxide and methane, and the surface pressure is only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion, or point of orbit nearest to the Sun; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near the perihelion, it is likely that some of the atmosphere escapes into space perhaps even interacting with Charon. The planned 2013 Pluto-Kuiper Express spacecraft mission planners want the craft to arrive at Pluto while the atmosphere is unfrozen.

Pluto cannot be seen with binoculars and even a very large telescope has difficulty seeing anything other than a tiny disk.

If you would like to know more about the solar system, Starscine suggests the following websites:

www.nineplanets.org
www.nasa.gov