Not a literal question, but a balance one. Explore planet. Or as my friend and I call it, the Gamble Button. Maybe you'll find an awesome artifact or hidden planet bonus. Or maybe you'll just waste a fair chunk of resources on nothing, or useless bonuses. The main problem with this is the cost. Specifically, no one can afford to do it as there are far more important things to get. The only TEC or Visari can afford to do this is late game, when the minor planet bonuses are almost meaningless (+culture rate? Gee that sure would have been nice to know BEFORE I used up all the logistic slots). Artifacts can be more game changing, but there are some pretty lame ones. Not to mention on your average medium sized map you'll be lucky to see more than 3 artifiacts total in the game. Advent can make more use of Explore Planet due to their research to drastically lower the cost, but even then this is a low priority to establishing your fleet, infastructure, economy, and tech.
Some of these may be better to put into Diplomacy, but I think it would really help - and make the game a little more interesting - if:
-Minor planet bonuses should be MUCH more common, and a few more added. There are only a small handful of searchable ones to be found. Most come on initial colonization.
-Why does 90% of the bonuses only turn up on the first search? This makes doing more than one search a true gamble, as you can really only hope for an artifact on the second try.
-Re-evaluate artifact usefulness. Some are awesome; Kinetic Intensifier, Relativistic Factories, Data Archive? These offer massive advantages over your opponent. Jump Drive Relic, Resilient Metalloids, Manifest Domination? These are pretty small time advantages, with Jump Drive Relic possibly doing nothing at all.
-The high cost makes doing this extremely unlikely for any player early-mid game. Generally it's reserved for Advent players after buying their special research, then doing a huge explore blitz on all their planets at once. Consider a cost reduction so this becomes a viable option early game.