Science Fact Finder | Space - What Is Planet X?

What is Planet X?

There are presently nine known planets in the solar system. It is possible that a tenth planet, referred to as "Planet X," exists and is waiting to be discovered.

Soon after German astronomer (a scientist specializing in the study of matter in outer space) William Herschel discovered Uranus in 1781, scientists noticed that Uranus was changing direction from its predicted orbit. This led them to hypothesize that there was another planet out there, the gravitational field of which was tugging at Uranus. The next planet, Neptune, was discovered in 1841 independently by English astronomer John Couch Adams and French astronomer Urbain Leverrier. The existence of Neptune, however, was not able to totally explain Uranus's orbital disturbances. Neptune showed movement away from its predicted orbit, as well. The search for Planet X continued.

The next planet to be discovered, by American astronomer Clyde Tombaugh...

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