Hydrogen planets are found orbiting Gas Stars and contain primary resources in order to progress in the lore of Elemental Correstialism and in terms of armor. Sadly, they are not the perfect destination due to it's insane planetary hazard and lack of Oxygen material on top in order to craft Oxygen Capsules, but you can find some underground.
The underground of Hydrogen planets is the same as any other Gas planets. The shallow underground layer is either a Hydrogen-like or an Oxygen-like biome. Deeper on the mid layer is the Helium-like biome and the deep underground layers are either like Nitrogen, either like Neon biomes. The Core of all Gas planets, just like this one, is completely solid and frozen, due to it not being geologically active.
Hydrogen planets, like all other Gas planets feature 4 microdungeons that may spawn, including 2 that have a chest with a bit of treasure inside of it. Compared to other Gas planets, the Hydrogen gas planet is the most active. It has the most duststorms of any other gas planet. The ambience sound of Hydrogen planets is a NASA recording of Jupiter. The unique monster Gasterbina featured on a Gas Planet such as this is dangerous, as many Starbound vanilla healing items heal slower than the bleeding effect that Gasterbina damages you with. Gasterbinas take a while to kill, but also attack with a small delay, that give a chance to escape. Gassenite can be found on the planet's surface, then gets rarer in the middle layers of the underground, but gets even more likely right at the core. Terminite can be found deep in the planet.