When were the planets discovered? Uranus and Neptune were definitely not found until modern science began, since a telescope was required to see them.  The other five planets were not actually discovered per se.  Since they are visible to human eyes, they have been observed since prehistoric times, and we see examples of cultures across the ancient world who observed them.  The five visible planets are Mercury, Venus, Mars, Jupiter, and Saturn.  Starting this weekend, you can see them all across one part of the sky, a visual alignment that hasn’t occurred since 2005. Planets are so-named because of the Greek...