Aliens on Television

One of the earliest examples of this was My Favorite Martian, from 1963-1966. Featuring Ray Walston as a Martian trying to disguise himself as human in contemporary Los Angeles, this show was more focused on the marvels that 'Uncle Martin' could do, and less on how such a stranger in a strange land might try to fit into a strange society.

Mork from Ork was television's resident non-human from 1978 to 1982, when Mork and Mindy spun off from a character in the 'sixties-nostalgia sitcom Happy Days. Mork was an ET who was taken in by human Mindy, and humor was found in his inability to understand human cultural mores and norms.

ALF, from 1986 to 1990, featured a puppetry alien (or Alien Life Form, hence the acroynm), and from 1996 to 2001, 3rd Rock From the Sun featured a group of extra-terrestrials attempting to pass themselves off as a typical suburban family.

... or as Unknowable, Implacable Enemies ...

Extra-terrestrials are often used in television as dramatic devices; the unknowable enemy. Star Trek: The Next Generation often barely escaped the clutches of the Borg, Space: Above and Beyond fought the mysterious chigs, the StarGate franchise has its Goa'uld and Battlestar Galactica its Cylons.

Non-human enemies are useful dramatically because they allow you to posit a humanity more highly evolved than our own, in which differences would not be settled militarily. Thus, human/human conflicts play out in courts and congresses, a no-doubt wonderful development for humanity, but death to drama. Enter the unknowable inhuman race.

Unknowability is one of the key features of these conflicts. Once you start to know your enemy, understanding creeps in. If you understand someone, you might actually begin to accept their point of view as equally valid. Accommodation, compromise, negotiation ensue. And there goes your dramatic plot points.

Unknowability also makes it easier to despise the enemy. It would be bigotry to despise another human race, but not an alien one. Thus warring parties turn their enemies into 'the other', something less than ourselves, to make hatred easier.

... and as Regular Characters

The Star Trek franchise pioneered the use of aliens as continuing dramatic characters, with Mr. Spock in the original series. Subsequent series invariably had at least one non-human on the bridge crew.

It is worth noting that Mr. Spock was half-human, perhaps to make him more accessible and acceptable to a human audience in the 'sixties, perhaps to make dramatic hay out of his 'at home nowhere' situation. Later Star Treks were allowed to offer us full-blooded aliens, including Klingon Worf from Star Trek: Voyager and Star Trek: Deep Space Nine, and Vulcan T'vok in Voyager.

Aliens as dramatic characters are so expected a part of television science fiction now that if you were producing a series without them, you'd better have a very good plot-driven reason.