有沒有人知道 Planet X ?

天文學家在觀察『天王星』的軌道時，發現實際軌道與計算的軌道有所偏差，相信這是由於另一棵（第八個）行星所導致。兩位數學家分別工作，計算出該行星的位置及質量，最後由一位年輕的天文學家在1846年尋找到這棵星並名為『海王星』。
找到了『海王星』之後，天文學家發現，『海王星』並不足以解釋，『天王星』軌道所受擾動的程度。而『海王星』的軌道，也像受另一個未知天體的影響。所以，很自然地引發了尋找太陽系第九個行星的工作。1930年，美國天文學家 湯苞(Clyde W. Tombaugh, 1906-1997)，發現了第九顆行星，後來命名為『冥王星』 (Pluto)。
發現了『冥王星』後不久，天文學家發現了『冥王星』質量太少了，不足以解釋『天王星』及『海王星』軌道所受擾動的程度，因此，科學家相信還有第十行星，稱為『X行星』，但卻沒有發現。
科學家推測X行星的體積與天王星相同，不過質量較少，它與太陽的距離，是海王星與太陽的距離的三倍，而且軌道接近垂直。除X行星外，可能還有更多的行星存在。
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In astronomy, Planet X is a large hypothetical planet beyond Neptune. It was postulated to explain apparent discrepancies in the orbits of the gas giants, especially those of Uranus and Neptune. Those discrepancies were mostly resolved by measurements made in the 1980s.[1]
The "X" in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for ten. At the time of its conception there were eight known planets in the solar system; its existence, first as a ninth planet, and then from 1930 until its demise as a tenth. Although Pluto was discovered as a result of the search for Planet X, it is not considered Planet X. Neither is Eris, even though it was at one point considered for reclassification as a planet under a proposal outlined by the International Astronomical Union (see 2006 redefinition of planet).
In popular culture, "Planet X" has become a stand-in term for an undiscovered planet in the solar system.