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OCR: NEPTUNE'S IRREGULAR ORBIT WE KNOW OF NINE PLANETS in the Solar System, but are there any more? Some astronomers suggest that irregularities that have been observed in Neptune's orbit were caused by a massive tenth planet that lies beyond Pluto's orbit. This hypothetical planet is known as Planet X. Arguments against this theory include the possibility that there was not enough matter in the early Solar System to form Planet X along with the other nine planets, and that the Solar System is not old enough for a planet to have formed so far from the Sun.