There are only two tiers of Planet Exploration, hence 50% and 100%.

I believe at 50% exploration you find planet bonuses like Megafauna, Penal Colony, Space Ponies, ect. At 100% you find artifacts.

Artifacts give your whole empire a bonus. The Kenetic Intensifier increases all weapon damage by 12%. Resiliant Metaloids increase hull points and hull regeneration by a certain amount. When you find an artifact, go to the research window, and it is the third tab, right above Fleet Logistics. It will show you what artifacts you have.

I dont think there are ever two of the same artifacts in a map.

There are two levels of exploration because a planet can have 2 different discoveries: the standard bonuses, and artifacts. You can only find one or the other (if both exist on a planet) for each exploration rank, but artifacts aren't limited to the tier 2 exploration. You can find them on the first hit, and a planet bonus on the second

Once you find it though, you don't need to research another rank, you'll get the benefit of an artifact at 50% exploration.

I wish they would randomize the numbers of times it takes to get to 100%.  That would shake things up a bit and make "larger" worlds all the more valuable...if they could have more than one bonus.