On Jan. 1, 1801, astronomer Giuseppe Piazzi discovered an object in orbit around the sun between Mars and Jupiter in a location where other astronomers predicted there should be a planet. Piazzi at first thought he had found a comet, but astronomers quickly decided it was the predicted missing planet. They named it Ceres.
For half a century, textbooks listed it as a planet. With the subsequent discovery of similar objects in the same part of the solar system, astronomers decided that Ceres no longer deserved the “planet” designation and demoted it to asteroid status. It's still the largest known of that class of objects.
Jump ahead to the early 20th century. Astronomers once again predicted a Planet X, this time beyond Neptune, because there seemed to be a source of gravity affecting the orbits of other planets. Clyde Tombaugh, a young amateur astronomer at Lowell Observatory, was assigned the task of seeking the proposed trans-Neptunian planet. He was rewarded after many nights and days of hard work with the discovery of Pluto on Feb. 18, 1930.
At first, astronomers studying the new planet calculated its mass to be roughly equivalent to Earth's. But subsequent observations with better equipment eventually reduced Pluto's size to smaller than our moon. Like Ceres before it, some astronomers began to doubt the correctness of calling Pluto a planet. In 2005, astronomers at Palomar Observatory discovered Eris, another object in the same region of the solar system as Pluto, the first of several such discoveries. Even larger than Pluto, Eris sparked anew the debate in the astronomical community about what defines a planet. In August 2006, the International Astronomical Union defined “planet” in such a way that removed Pluto from that classification. Pluto became the first of a new class of objects called dwarf planets.
NASA launched the New Horizons space craft, one of the fastest-moving crafts of any kind, in the same year Pluto was demoted to explore that object, which has gone from Planet X to planet to dwarf planet. I suspect that if Tombaugh, whom I met on several occasions, were still alive, he'd have some really mixed feelings about all of this.
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