Alien Atmospheres
Planets classified as M class are capable of supporting oxygen breathers such as humans. Other planets are totally unsuitable; some are briefly survivable. Elba II is shrouded by a poisonous atmosphere and a penal colony for the criminally insane was once sited there. Although Galorndon Core possesses breathable gases, its atmosphere is an eletromagnetic soup. Prolonged exposure causes the breakdown of the brain's synaptic pathways. Unsurprisingly Starfleet concentrates most of its exploration on class M worlds.

Tachyon Core Planet
First encountered by the Starship U.S.S. Voyager, this hoop-shaped planet exhibits gravimetric readings similar to that of a collapsed dwarf star, and is also similar to a Quasar in that it has a high rate of rotation at 58 revolutions per minute. The planet derives its unusual shape from the existence of a tachyon core which maintains a subspace particle field between the poles. An effect of the particle field is to create a space-time differential between the planet and surrounding space, meaning that events on the planet happen hundreds of times faster than in the space around it.

Demon Class Planets
To ease exploration of the myriad planets which Starfleet vessels encounter on regular mission, a system of categorisation has been adopted for certain types of planet. The most inhospitable type of planet known to Starfleet is a Class Y planet, also commonly referred to as a 'Demon Class' planet. Demon Class planets have toxic atmospheres, with the existence of deadly thermionic radiation being a common occurrence. Surface conditions are often extreme, with temperatures in excess of 500 degrees Kelvin having been recorded in some instances. Because of the hostile conditions, even entering a normal orbit around a Demon Class planet can be considered suicidal, however orbit, exploration, and even landing on a Demon planet has been known. A common reason for this is to access the very rich sources of minerals, including the vital propellant deuterium, which can be found below the planet's surface.