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> Pluto and Charon, border of the Solar System. Or not? Issue: 2003-2 Section: Astronomy



After Neptune’s discovery in the XIX century, astronomers began studying all the available information about it. However, during these studies, they found some strange results: the orbit of the eighth planet was under the influence of another celestial body. An American scientist, Percival Lowell, began searching for this X-planet. He founded Flagstaff observatory in Arizona and his staff studied the sky for many years, until 1930, when Clyde William Tombaugh found it. This new planet was called Pluto in honour of the Latin God of hell because he lived at a far distance from the Sun. Lowell’s initials (PL) were also given as the planet’s initials.


Pluto’s identikit
Distance from Sun 5.900.000.000 km
Time of revolution 90465 days (247,7 years)
Time of rotation 153,28 hours (6,375 days)
Diameter 2290 km
Ground temperature -230 °C (43 K)
Magnitude 15
Satellites 1 (Charon)
Charon’s identikit
Distance from Pluto 20.000 km
Diameter 1190 km
Magnitude 18


Unfortunately the planet was too far away to be studied accurately. The first datum given about Pluto was the magnitude of 15. The visibility of a normal star can be seen with a magnitude of 6 on a clear night. Another problem was that the influence on Neptune could not be explained. Pluto’s mass wasn’t enough. Astronomers continued to study it to find the problem. Little by little, they discovered another of the planet’s characteristics. The orbit was very elliptic. The distance from the Sun changed between 4,4 (perielium, the nearest point) and 7,3 billion kilometres (aphelium, the farthest point) on Pluto’s orbit around the Sun which means that for twenty years, it draws nearer to the Sun than Neptune. The last time this occurred was between 1979 and 1999. Astronomers calculated the orbit and they discovered that it was inclined on ecliptic of 17°. The axis was very inclined as well (57,5°). On the Earth it looked as if the North Pole had the same latitude as Milan. Other observers found an interest in this new planet and more data were collected within a short time. Gerard Kuiper tried to calculate the diameter of Pluto, but some strange phenomenon created many problems in the results.

Ten years before the perielium, in 1978, James Christy saw that in some pictures taken from Washington Naval Observatory the planet looked like a pear while in others it had a normal spherical form. The American astronomer guessed that Pluto had a satellite. He was correct and Charon, the soul ferryman of hell, was discovered. This discovery clarified many things concerning the previous studies. The influence on Neptune’s orbit was explained and Pluto’s mass was calculated exactly at 0,0026 (Earth being the equivalent of 1).

From 1985 to 1990 a new and different relationship between Pluto and Charon was discovered. They function more like a system rather than planet and satellite. They are two celestial body which rotate on a single center of gravity. This states another characteristic of the ninth planet: the double is always stopped in the same point of his sky and show always the same face. During this same period many other discoveries were revealed thanks to the perfect position of Earth in respect with the other planets. Vision was optimal. In fact, in 1988 Kuiper Airborne Observatory’s staff discovered that Pluto had a light atmosphere of nitrogen and methane.

Other observations produced new information about the surface. New advanced technologies (like the spectral analysis of reflected light coming from planets) helped to discover the composition of the two celestial bodies. Pluto has an irregular surface without a defined landscape, caused by small consistence of methane, nitrogen and carbon monoxide ices. Charon has a completely iced surface, which becomes as hard as stone at some degrees up to the absolute zero (about -273°C) and creates a rough landscape full of cliffs and gorges.

Scientists are now debating the colour of plutonian ground. There are white areas (good light reflexing) and black areas (bad light reflexing). Maybe, this colour difference is caused by nitrogen crystallization (some places being more or less crystallized than others) or by organic materials (like methane or ammonia) which don’t allow the light to reflect.


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