yes you can change them, not sure off the top of my head what modifiers there are but you can pull up the Distant Stars mod as we have tons of planetary bonuses that you can study

You have a set list of modifiers for planetary bonuses, all of which are listed in every bonus. You can mix and match (including negative values!) as you please though. More than ~5 modifiers on 1 bonus will clog up the screen though, especially if there are more bonuses/an artifact on the planet.

You can also give planets abilities, which allow you to use all buff modifiers, but these aren't tied to the exploration level. (You can tie them to researches though, as in the Mad Scientist mod)

In case you don't know, possible bonuses are set in each planet's .entity file, including a space for "required" bonuses that will always be present. (Like the pirate treasure)