Located at the far end of the solar system like a sentinel guarding the infinite vastness, Pluto is no less than six billion kilometers from the Sun, about 40 times the distance of the Earth. From here, our star is nothing but a small bright spot, even if 400 times brighter than the full Moon. On this cosmic outpost, the temperature is only slightly higher than the so-called absolute zero, the point at which molecules stop moving. The discovery of this planet, which with a probable diameter of 3 000 kilometers is the smallest in the solar system, was even more incredible than that of Neptune. Following in the footsteps of Adams and Le Verrier, two American astronomers, William Henry Pickering, among other things the person who discovered Phoebe, the ninth satellite of Saturn, and Percival Lowen, a staunch supporter of the existence of artificial canals on Mars, had hypothesized the existence of a ninth planet. In fact, the orbit of Neptune and Uranus seemed to be "disturbed" by a body much farther away than Neptune. This was 1902, and three years later, they began the complicated calculations to determine the position of the new planet and to make telescopic observations. Lowell was the most tenacious researcher, who for ten years scanned the sky in vain from the American observatory in Flagstaff, Arizona: he died in 1916 without reaching that which he considered to be the most important objective of his life. Several years later, in 1919, another American astronomer, Milton Humason, pointed the famous telescope on Mt. Wilson to that area of the sky that had been studied in vain by Pickering and Lowell and took two historical photographs in which, however, he did not find the planet. Thus, the theory of a ninth planet seemed to fade away and for several years the search was abandoned. It was only in 1929 that an assistant at the Flagstaff observatory, Clyde Tombaugh, was assigned to continue the search and finally, after months of tedious studies of hundreds of plates of the sky, the young man found what he was looking for: a bright point had moved to the "right" distance with respect to the other background stars. Pluto had been discovered. How come the great astronomers could not find the planet? In the years that followed the discovery, all the old plates were "unearthed" and carefully examined: the results were surprising. Humason had been the victim of some strange coincidences: in the first photograph, the planet had been "hidden" by the very intense light of a star; in the second picture, Pluto had been in the exact same position as a defect on the photographic plate! But Lowell had been really unlucky: the planet had been photographed two times at his observatory while he was still alive, but both times, everyone had thought it was a star. The name of the planet was chosen based on two considerations: the Greek god of the Underworld seemed to be the most suitable to "live" in such a remote and desolate place in the solar system, but, above all, its two letters, P and L, were the initials of Percival Lowell, whose calculations and tenacity were the real basis of the discovery.
Located at the far end of the solar system like a sentinel guarding the infinite vastness, Pluto is no less than six billion kilometers from the Sun, about 40 times the distance of the Earth. From here, our star is nothing but a small bright spot, even if 400 times brighter than the full Moon. On this cosmic outpost, the temperature is only slightly higher than the so-called absolute zero, the point at which molecules stop moving. The discovery of this planet, which with a probable diameter of 3 000 kilometers is the smallest in the solar system, was even more incredible than that of Neptune. Following in the footsteps of Adams and Le Verrier, two American astronomers, William Henry Pickering, among other things the person who discovered Phoebe, the ninth satellite of Saturn, and Percival Lowen, a staunch supporter of the existence of artificial canals on Mars, had hypothesized the existence of a ninth planet. In fact, the orbit of Neptune and Uranus seemed to be "disturbed" by a body much farther away than Neptune. This was 1902, and three years later, they began the complicated calculations to determine the position of the new planet and to make telescopic observations. Lowell was the most tenacious researcher, who for ten years scanned the sky in vain from the American observatory in Flagstaff, Arizona: he died in 1916 without reaching that which he considered to be the most important objective of his life. Several years later, in 1919, another American astronomer, Milton Humason, pointed the famous telescope on Mt. Wilson to that area of the sky that had been studied in vain by Pickering and Lowell and took two historical photographs in which, however, he did not find the planet. Thus, the theory of a ninth planet seemed to fade away and for several years the search was abandoned. It was only in 1929 that an assistant at the Flagstaff observatory, Clyde Tombaugh, was assigned to continue the search and finally, after months of tedious studies of hundreds of plates of the sky, the young man found what he was looking for: a bright point had moved to the "right" distance with respect to the other background stars. Pluto had been discovered. How come the great astronomers could not find the planet? In the years that followed the discovery, all the old plates were "unearthed" and carefully examined: the results were surprising. Humason had been the victim of some strange coincidences: in the first photograph, the planet had been "hidden" by the very intense light of a star; in the second picture, Pluto had been in the exact same position as a defect on the photographic plate! But Lowell had been really unlucky: the planet had been photographed two times at his observatory while he was still alive, but both times, everyone had thought it was a star. The name of the planet was chosen based on two considerations: the Greek god of the Underworld seemed to be the most suitable to "live" in such a remote and desolate place in the solar system, but, above all, its two letters, P and L, were the initials of Percival Lowell, whose calculations and tenacity were the real basis of the discovery.
