Mor-Tax is the name of the home planet of the race of aliens that invaded Earth in sometime in the 1980's, as revealed in the TV series. The inhabitants were mistaken for Martians, but they are close relatives.
According to the series, Mor-Tax is 40 light-years away from Earth in the Pleiades star cluster in the constellation Taurus. This is only as specific as the information in the show and supportive material gives the location. In "The Prodigal Son", Quinn points to a rough location on a map. However, this highlighted star seems to be unnamed, but is positioned between the brighter stars Alcyone and Maia.
Other than being a garden planet, rich with vegetation, there are no physical descriptions of the planet provided in the series.
According to a copy of Greg Strangis's writer's bible dated January 1, 1988, the planet was at one point named Kor'Tax. The spelling was first made public in "The Resurrection" novelisation, supported by its usage in the episode script. This was also the first utterance of the name, as the name is not heard in the series until halfway into the first season. Fans often misspell the name by using an apostrophe instead of a hyphen, or neither, and sometimes not capitalising the 'T'.