冥王星在发现之初曾被认为是一颗位于海王星轨道外的行星，但后来的事实证明并非完全如此。在1979年1月21日～1999年3月14日这段时间，冥王星就比海王星更靠近太阳。这是由于冥王星轨道的偏心率、轨道面对黄道面的倾角都比其它行星大。冥王星在近日点附近时比海王星离太阳还近，这时海王星成了离太阳最远的行星。每隔一段时间，冥王星和海王星会彼此接近，但它们不会碰撞，因为它们的轨道平面并不重合，即使在交叉点附近，它们之间的距离仍然是很大的。冥王星的卫星被命名为卡戎（Charon），它的公转周期与冥王星的自转周期一样，都是6.39日。由于冥王星太暗太小，发现后很长时间不能确定它的大小。近年一些天文学家观测指出，冥王星的直径约为2400千米，比月球（3475千米）还小，
參考答案:Pluto at the beginning of the discovery was once considered was is located outside the Neptune orbit the planet, but afterwards fact proof by no means completely so. ~ 1,999 is broad on 4th in 1,979 broad on 1st this period of time, pluto on approaches the sun compared to Neptune. This is because the pluto orbit eccentricity, the track are all bigger than facing the ecliptic surface inclination angle other planets. Neighbor pluto when recently was selecting was also nearer than Neptune to the sun, by now Neptune became has left the solar farthest planet. Each period of time, pluto and Neptune meet each other to approach, but they cannot collide, because their track plane certainly does not superpose, even if nearby the intersection, between them the distance still were very big. Pluto's satellite is named as card Rung (Charon), its revolution cycle and pluto's period of rotation is same, all is 6.39 dates. Because pluto too dark too is small, after discovered the very long time cannot determine its size. Recent years some astronomers observed pointed out that, pluto's diameter approximately was 2,400 kilometers, the Moon _ 475 kilometers) was also smaller than,
