Heavy ranging to very thin atmospheres consisting of various elements, mostly
comprised of helium or of hydrogen. Specific gravities can range from 0.2 to
8.0 of Earth normal. Climate patterns usually extremely violent with temps
ranging from -200 to 400 Celsius. Full life support required at all times. No
production can sustain itself on a Class U planet.  Some miners have hinted
at very valuable products that they have extracted from Class U worlds but
the Federation does not have them in its "Official Guide to Mining". Class U
planets are not recommended for colonizations as the environment is harsher
than being in space itself.