Question

What was the first planet to be discovered in our solar system?

Answer 1

Earth.

Explanation:

I suppose you meant to ask what was the first planet after Earth was discovered.

If that was what you intended then unfortunately I cannot provide any more helpful answer either. There are 5 planets visible to the naked eye and have all been mentioned even prior to the Greeks mentioning them. These are Venus, Mars, Jupiter, Saturn and Mercury.

Of these Venus is the brightest so it is most likely the first one discovered, however Jupiter is a close second and Mars tends to stand out due to it's red tinge. Unfortunately, we have no way to determine which was discovered first.