I’m assuming the question was intended to ask about how many planets are in our solar system, not plants. If you count Pluto, it was the last planet discovered in 1930. Unfortunately it was demoted to a “dwarf planet” in 2006. So if you don’t count Pluto, Uranus was discovered in 1781. The other planets were all known by the 1500s.
It depends on whether you think Pluto is a planet, really.
Pluto was discovered in 1930, and was the last planet to be discovered in our solar-system. So, one could say it was then that we discovered the total amount of planets in our system. However, in 2006, Pluto was no longer considered a planet by astronomy authorities, and is now considered the largest body in the Keiper Belt. So you could also argue that 2006 was the year we found out how many planets were in our solar-system.
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