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The same number as there are now, but they weren't all discovered by then.

Mercury, Venus, Earth Mars, Jupiter, and Saturn have been known since ancient times.

Uranus was first recorded in 1690

Neptune was discovered in 1846

Pluto was discovered in 1930, and later declassified as a planet.

So, in 1780, we knew of 7 planets,

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16y ago

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