Pluto is the ninth and outermost planet of the solar system, except when its eccentric orbit takes it inside Neptune's orbit (for example, in the period 1979 to 1999). It is named after the god that rules the underworld. Pluto was discovered by Clyde Tombaugh in February of 1930. The history behind the discovery goes back to the second half of the 19th century, when perturbations in the motions of Uranus and Neptune suggested that another planet must be present to affect their orbits. Percival Lowell, who built the Flagstaff Observatory to study the (non-existent) canals in Mars, calculated an orbit for this hypothetical planet and launched methodical searches for it. Lowell died in 1916 and the search was stopped until April 1929, when a wide-field telescope was completed and Tombaugh was put in charge of the search. Tombaugh was originally from Kansas and ever since he was a young boy he loved observing the sky. After eleven months of a very tedious photographic search, Tombaugh discovered Pluto, in February of 1930. As it turns out, Pluto's mass is too small to affect the motions of Uranus and Neptune, and indeed the supposed perturbations are not real.

Charon (the ch is pronounced like in chaos) was discovered in 1978 when photographs of Pluto showed a "bump" on one side. Charon is appropriately named after the ferryman who carried the souls of the dead across the Styx, a river in the lower world.

Pluto and Charon are a system on their own. They belong to neither the terrestrial nor the Jovian planets. No spacecraft has visited the Pluto-Charon system yet, so all we know about them is from Earth. A great deal has been learned since Charon's discovery and the subsequent eclipses and occultations that occurred in the period 1985-1991. Also, the development of better detectors, the construction of bigger telescopes, and observations with the Hubble Space Telescope have made enormous contributions to our knowledge of this planet-moon system.

Pluto has the most eccentric orbit of all the planets; it is so eccentric that for about 20 years of its period it goes inside Neptune's orbit, and its distance from the Sun ranges from 29.7 to 49.3 astronomical units. Pluto also has the most inclined orbit of all planets with respect to the ecliptic.

The rotation of Pluto has to be considered along with the revolution and rotation of its moon Charon, because the gravitational and the tidal forces that they exert on each other affect the rotation of both, as well as Charon's orbit around Pluto. This is because of their close proximity and their relatively large planet/satellite mass ratio (roughly 5 to 10), the largest of any planet-satellite system. The fact that Charon was not discovered until 1978 (see below) means that the determination of the rotational and physical parameters of Pluto and its moon are still imprecise.

As Uranus, Pluto's axis of rotation is tilted and points nearly directly to the Sun. The seasons on Pluto are therefore similar to those of Uranus. Likewise, Pluto's rotation is retrograde, meaning that if you were to view Pluto from above the ecliptic (north direction from Earth), it rotates in the opposite direction as the other six planets (remember that Uranus and Venus are the other planets that have retrograde rotation).

Pluto is so distant that only the best ground-based pictures or the Hubble Space Telescope can resolve Charon.

As seen from Earth, the orientation of Charon's orbit changes as Pluto orbits the Sun. Twice during Pluto's sidereal period, Pluto and Charon occult each other.

The last eclipse season of Pluto and Charon, as seen from Earth, started in 1985 and ended in 1991, with the deepest eclipses happening in 1988. This orientation gave astronomers the opportunity to calculate the brightness, masses, sizes, and therefore densities of the two bodies. Still, these parameters are not determined with the same precision as for the rest of the planets.

The composition of Pluto must be similar to that of Triton, Neptune's largest moon, because the sizes and densities of both are similar. Pluto is composed of half ice and half rock.

Surface

Pluto's surface contains methane, carbon monoxide and nitrogen ices. Charon, on the other hand, is coated with water ice.

Surface map of Pluto. Imaged with the Hubble Space Telescope.

Pluto has a thin atmosphere, with nitrogen being the most abundant gas. Because the ices on the surface of Pluto and Triton are the same, we expect the atmospheric gases to also be the same. Due to Pluto's highly eccentric orbit, the intensity of sunlight between perihelion and aphelion varies by a factor of three. This means that when Pluto is in the far side of its orbit, the temperature drops enough to freeze out the atmosphere, and when it is near perihelion (which happened in 1989), an atmosphere can form. Astronomers were fortunate to be able to study the atmosphere at this time, in conjunction with the eclipses of Pluto and Charon.

Moons and Planets, W. Hartmann, 1993, 3rd edition, Wadsworth Publishing Co.
Astrophysical Data: Planets and Stars, 1992, K. R. Lang, Springer-Verlag New York, Inc.
The Planetary System, D. Morrison and T. Owen, 1996, 2nd edition, Addison-Wesley Publishing Co., Inc.

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