Image of Pluto and its satellite Charon, taken by Hubble Space Telescope in 1994. (HST) 

Pluto is the ninth and last planet of the Solar System, and the smallest of them all. It is even smaller than the Moon. It was named after the god of the underworld, and its only satellite is called Charon; Greek mythology, Charon was the person who accompanied the souls of the deceased across the river Stiges, to the underworld. 

Pluto was discovered on February 18th 1930 by C.W. Tombaugh, on the basis of the prevision of a ninth planet beyond the orbit of Neptune. The orbits of Uranus and Neptune, in fact, have some anomalies that can be explained only by the gravitational perturbation of Pluto.
Pluto has a mass of 12.9 billion billion tons (12.9 1025 g), which is equal to two thousandths of that of the Earth. Its mean density is 2.05.
The diameter of Pluto is not yet precisely known, but it ranges between 2,300 and 2,375 Km.

The mass of Charon is approximately thirteen times smaller than that of Pluto. The satellite has a diameter that measures 1,270 Km approximately and rotates around Pluto at a distance of only 19,640 Km; therefore it would be better to call it a binary system of planets rather than a planet and a satellite. 
Some scientists even doubt about the fact that Pluto is a real planet; instead, they speculate that it could be an object which once belonged to the Kuiper belt. This is a region with many small rocky bodies situated beyond the orbit of Neptune and from which it is thought that the short period comets come.

Structure and composition 

The surface of Pluto, never seen before, was observed by Hubble Space Telescope, showing that its surface is full of irregularities and large scale contrasts, as that of the Earth. The small pictures are those taken by HST, the large ones are computerized pictures. (HST) 

The chemical composition of Pluto is not known, but it is probably composed by rock (80%) and frozen water (10%). 
The planet is covered by frozen Nitrogen, Methane and Carbon monoxide. Its temperature, in fact, although it is not exactly known, must range between -228 and -238 °C.  
The average albedo of Pluto is 0.3, but it varies from point to point; in particular, it is possible to distinguish on its surface darker equatorial zones, while the polar regions are of a lighter colour. These differences are probably due to the variation of the distribution of the ice on the planet during its seasonal cycle. The images of HST showed that the planet has a complex surface, that has many contrasts. 
The planet has also a very thin atmosphere, that can freeze when it moves away from the Sun. It seems that it is composed by Nitrogen, Methane and Carbon monoxide. 

Map of the surface of Pluto,
which covers 85% of the total.
Pluto has a dark equatorial zone,
that can be noticed in this picture. (HST)

The orbit 

A big improvement of the quality of the images of Pluto was obtained with HST. Left, a picture taken with a telescope on the Earth; right, an image taken by HST. The orbit of Charon is also shown. (HST) 

The orbit of Pluto is the most anomalous of the Solar System, because it has an inclination of 17 degrees on the plane of the ecliptic, and the most eccentric (with an ellipticity of 0.246), and finally it is the one with the biggest deviation from the Titius-Bode law, that describes the distances of the various planets from the Sun. 

The distance between Pluto and the Sun at the aphelion is 7,900 million Km; the mean distance is 5,913 million Km. Pluto completes its orbit in 248.54 years.
Its orbit is so eccentric that it intersects the orbit of Neptune, so that for a time Neptune is the farthest planet from the Sun. In 1979, Pluto travelled within the orbit of Neptune, and so it will remain until 1999. Therefore, at present it is in the vicinity of the perihelion, but in a few years it will be so far from the Sun that it will be impossible to observe it from the Earth.
Pluto is in orbital resonance with Neptune: its orbital period is exactly one and a half times that of Neptune. The rotation axis of Pluto has an inclination on its orbital plane of 122.5 degrees; as in the case of Uranus, this must cause anomalous seasonal cycles on the planet.

Artistic image of the Pluto Express mission.  (Pat Rawlings/NASA/JPL) 

Pluto is the only planet that was never approached by space probes. The only images from space are those of Hubble Space Telescope (HST). At the present time, NASA is preparing the Pluto Express probe, which will probably be launched in 2001. It should study the surface and the atmosphere of the planet before it moves too far away from us.  



HST image of Pluto and Charon. The satellite rotates around Pluto always showing the same face. (HST) 

Charon, the satellite of Pluto, was discovered in 1978 by J. Christy. It is geosynchronous with respect to the planet, in other words its rotation and its revolution have the same length of the rotation of Pluto, that is 6.39 days. As a consequence, Charon is visible only from a certain area of the planet, without ever rising or setting, and it always shows the same face to the planet. 

The mass of Charon is 1.77 billion billion tons (1,77 10 24 g), the diameter is not exactly known, but it ranges between 1,200 and 1,270 Km. The mean density of the planet is 1.83, its composition is not known. Scientists think that Charon originated from a gigantic impact, like the one that, maybe, originated the Moon.
It rotates around Pluto at an average distance of 19,640 Km. The surface of Charon differs from that of Pluto because it seems covered by frozen water, instead of frozen methane. It probably does not have an atmosphere.  
Its albedo is 0.5. 



Rotation of Pluto,  MPEG, 630 Kb (STScI)
Pluto, MPEG, 615 Kb (NASA/ESA/ESO)



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