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The Question

(Submitted May 07, 1997)

When were all of the planets discovered?

The Answer

Mercury, Venus, Mars, Jupiter, and Saturn have been known for a very long time...but no one knows when they were first seen. Being such bright objects in the sky, they have been known and observed by ancient civilizations.

Uranus was first recorded in 1690, but thought to be a star. William Herschel discovered it as a planet in 1781.

Neptune was discovered in James Challis (who recorded it first, but did not realize what it was) and Johann Galle (who knew exactly what he was observing).

Pluto was discovered by Clyde Tombaugh in 1930.

Laura Whitlock and Tim Kallman
for the Ask an Astrophysicist team

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