More than fifty years since the discovery of Pluto, not too much additional information has been obtained about the planet: because of the almost star-like appearance it is very difficult to calculate its diameter and mass with any degree of precision. It is believed to be covered with a layer of frozen methane, but astronomers know nothing about its interior. It has a very unusual orbit: inclined at 17 degrees with respect to the ecliptic, it is so elliptical that sometimes it is closer to the Sun than Neptune; in fact, it takes 248 years to complete a full orbit. The only new information dates back to 1978, when an American astronomer, James Christy, who had been examining several plates of Pluto, noted that the disk did not appear to be circular, but slightly elongated. He then looked at other photographs to make sure that this wasn't a random distortion of the image and he discovered that, in fact, the disk was elongated on all of them, even if not so marked. The strange swelling could be nothing other than a satellite, very large and very close to the planet that it is difficult to distinguish one from the other. Immediately given the name Charon, the mythical ferryman who conveyed the souls of the dead into the kingdom of the Underworld, the satellite soon exhibited unusual features: with a diameter estimated to be 1 300 kilometers, about 40% of that of Pluto (which makes it the largest moon with respect to the "mother planet" of the entire solar system), it is only 20 000 kilometers from Pluto, i.e. it is 19 times closer than the Moon is to the Earth. But the strangest phenomenon is Charon's period of rotation around Pluto which is the same (six and a half days) time that Pluto takes to revolve around itself. This means that the satellite is always above the same point on the planet, like an enormous and implacable "eye" which, for a hypothetical inhabitant of that place, never rises or sets. The considerations about the strange behavior of Neptune's moons and some details concerning Pluto make astronomers raise the question about whether Pluto is really a planet or a moon that escaped from Neptune's attraction. But how? In the theory developed by some scientists, a long time ago a large planet passed very close to Neptune. Its gravitational attraction might have captured Pluto, launching it into its strange orbit around the Sun and would also have detached another part, transforming it into a satellite, i.e. Charon. But what happened to the intruder planet? Could that mysterious world be the tenth planet in the solar system? Some astronomers believe that having barely "missed" Neptune, it was thrown into an orbit very far from the Sun and that it is there, somewhere else, too dark and remote to be observed from the Earth. But many others do not agree: the most probable hypothesis is that Pluto, Charon and Chiron (a mysterious "asteroid" discovered in 1977 beyond the orbit of Saturn and for a brief period believed to be the elusive planet X) are nothing other than the remains of the long process of formation of the solar system which for some unknown reason were never attracted by their neighbors the giant planets or were not able to "coalesce" to form another giant.
More than fifty years since the discovery of Pluto, not too much additional information has been obtained about the planet: because of the almost star-like appearance it is very difficult to calculate its diameter and mass with any degree of precision. It is believed to be covered with a layer of frozen methane, but astronomers know nothing about its interior. It has a very unusual orbit: inclined at 17 degrees with respect to the ecliptic, it is so elliptical that sometimes it is closer to the Sun than Neptune; in fact, it takes 248 years to complete a full orbit. The only new information dates back to 1978, when an American astronomer, James Christy, who had been examining several plates of Pluto, noted that the disk did not appear to be circular, but slightly elongated. He then looked at other photographs to make sure that this wasn't a random distortion of the image and he discovered that, in fact, the disk was elongated on all of them, even if not so marked. The strange swelling could be nothing other than a satellite, very large and very close to the planet that it is difficult to distinguish one from the other. Immediately given the name Charon, the mythical ferryman who conveyed the souls of the dead into the kingdom of the Underworld, the satellite soon exhibited unusual features: with a diameter estimated to be 1 300 kilometers, about 40% of that of Pluto (which makes it the largest moon with respect to the "mother planet" of the entire solar system), it is only 20 000 kilometers from Pluto, i.e. it is 19 times closer than the Moon is to the Earth. But the strangest phenomenon is Charon's period of rotation around Pluto which is the same (six and a half days) time that Pluto takes to revolve around itself. This means that the satellite is always above the same point on the planet, like an enormous and implacable "eye" which, for a hypothetical inhabitant of that place, never rises or sets. The considerations about the strange behavior of Neptune's moons and some details concerning Pluto make astronomers raise the question about whether Pluto is really a planet or a moon that escaped from Neptune's attraction. But how? In the theory developed by some scientists, a long time ago a large planet passed very close to Neptune. Its gravitational attraction might have captured Pluto, launching it into its strange orbit around the Sun and would also have detached another part, transforming it into a satellite, i.e. Charon. But what happened to the intruder planet? Could that mysterious world be the tenth planet in the solar system? Some astronomers believe that having barely "missed" Neptune, it was thrown into an orbit very far from the Sun and that it is there, somewhere else, too dark and remote to be observed from the Earth. But many others do not agree: the most probable hypothesis is that Pluto, Charon and Chiron (a mysterious "asteroid" discovered in 1977 beyond the orbit of Saturn and for a brief period believed to be the elusive planet X) are nothing other than the remains of the long process of formation of the solar system which for some unknown reason were never attracted by their neighbors the giant planets or were not able to "coalesce" to form another giant.
