All Planets Are Earth Like

VI. Consistent with the limitations of your vessel and equipment, you will confine your landings an contacts to Class "M" planets approximating Earth-Mars conditions.

Star Trek original series outline

Basic Information

All Planets Are Earth Like is a speculative fiction trope denoting certain types of science fiction universes. In them, all planets are either gas giants or Earth-like - solid, with gravity comparable to that of The Earth, and an atmosphere breathable by humans. This is sometimes explained as the result of terraforming, though often no further explanation is given.

Sources

TV Tropes Wiki entry

Game and Story Use

• Generally, this should be avoided if you are aiming for hard science fiction.
  • Indeed, it borders on You Fail Astronomy Forever, and Space Does Not Work That Way.
• But imagine a universe where all star systems where human-habitable planets could exist do have a planet that's perfectly livable by human standards. Have scientists remark at great length how extremely unlikely this is, and look for precursors who might have arranged it. This will set a nice, paranoid base for the setting.
• In the original Star Trek series they were forced to do this for budget reasons, but they justified it by saying that Starfleet deliberately limited their missions to planets capable of sustaining human life.
• This would seem a reasonable explanation generally - most planets may well not be earthlike, but all the ones we visit and do anything on will be - with the occasional foray to some airless or otherwise hostile planet where we never go outside.