Until 2006 it was the last planet of the solar system. It belongs today to the family of the dwarf planets because its characteristics are too different from those of the other planets. Pluto moves in an inclined and very elongated orbit that varies between 4.3 and 7.4 billion km from the Sun, so that for 20 years it is in front of Neptune! From so far away the Sun looks almost like a star and shines 1500 times less than on Earth! Pluto is so distant that even the light that travels very fast, at 300000 km/s, takes more than 4 hours to reach it! Pluto is half smaller than Mercury with a diameter of only 2370 km and a mass 5 times smaller than that of the Moon. Pluto is surrounded by 5 satellites. The most brilliant is Charon, which was discovered in 1978. It measures 1208 km in diameter. Given its red color, it is probably a small asteroid that was captured by Pluto. This satellite has a synchronous rotation, that is to say that it always remains in the same place in the sky relative t...
From time to time we see comets in the sky. Like asteroids, they are large, dusty, icy rocks several hundred meters long and a few kilometers in diameter. There are billions of them. They formed at the same time as the solar system but they stayed away from the Sun. There are two large reservoirs of comets: the Kuiper Belt beyond Neptune and the huge cloud of Oort that surrounds the entire solar system and is far behind Pluto. Most of the “pebbles” making up these clouds are so small and located far from the Earth that they are invisible. In time some of these small icy bodies collide or are deviated from their path. When this happens, some of these rocks may come close to the Sun. When the ice that composes them begins to melt under the heat of the Sun, we observe behind them a long bright tail: the “pebbles” becomes visible, a comet is born! Comets always have two tails: a white or slightly yellowish one consisting of dust which is often thick and slightly curved; a so-called ionic b...