Extra Planets is an add-on for the Galacticraft mod.
It gives new planets/rockets/ores etc... to the game.
This information was gathered using:
ExtraPlanets version: 0.7.3
Minecraft version: 1.12.2
If there is no difference between the Galacticraft and ExtraPlanets stuff there will be a link to the Galacticraft Wiki.
If you are new to the mod I recommend you to watch these tutorials.
If you wear a Space Suit then you can't reach 100/100 Radiation so you don't need to worry about that.
The Pressure is never a problem if you wear a Space Suit.