Planet X
Ever since the discovery of the ninth planet, Pluto, astronomers have speculated about whether a still more distant, tenth planet may exist. This thinking was initially based on two lines of reasoning. First, astronomers had beensuccessful at discovering Uranus, and then Neptune, and then Pluto. This led them to believe that additional planets might await discovery farther out, if they only searched hard enough for them. Second, the search for Pluto, like the search for Neptune before it, had been based on the apparent tug of a giant, unseen world affecting the orbit of its predecessor. Yet after Pluto's discovery, it became clear that the newly found planet was very small, smaller than the United States. Such a small world could not have tugged significantly on Neptune.
Not long after the discovery of Pluto, however, it was discovered that the discrepancies in the orbit of Neptune that led astronomers to search for Pluto had been fictitious—they were simply measurement errors made by old telescopes. Pluto's discovery had been a lucky accident. With this finding, most astronomers concluded in the early 1980s that the train of logic leading to suspicions of a "Planet X" was faulty, and that it was unlikely a large Planet X existed beyond Pluto.
More recently, however, the tide has begun to swing back to a general consensus that there may indeed be planets orbiting the Sun beyond Pluto. Indeed, there may be not just one (i.e., Planet X), but many. Why this change? For one thing, astronomers discovered the Kuiper belt, a teeming ensemble of miniature worlds within which Pluto orbits. Objects half as large as Pluto have already been discovered among the hundreds of Kuiper belt objects found since 1992, and most astronomers expect that still larger objects, probably including some larger than Pluto itself, will eventually be identified.
Moreover, it has become clear from computer-generated solar system formation models that during the final stage of the formation of giant planets, a significant number of larger, "runner-up" objects to the giant planets (some perhaps even larger than Earth) may have been ejected to orbits in the Oort cloud of comets lying far beyond Pluto.
Do such objects actually exist? We will not know until observational searches either find them or rule them out. Such a search is difficult. Because such objects will be farther out than Pluto, and therefore dimmer, locating them will be rather akin to finding a needle in a haystack. Searches now underway and planned for the first decade of the twenty-first century may well settle the question. Until then, however, the subject of Planet X (and planets Y, Z, and so forth) will remain a subject of ongoing scientific debate.
Extrasolar Planets (Volume 2);; Kuiper Belt (Volume 2);; Oort Cloud (Volume 2).
Bibliography
Stern, S. Alan, and Jacqueline Mitton. Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System. New York: John Wiley & Sons, 1998.
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