Xenology is the scientific study of all aspects of extraterrestrial (alien) life, intelligence, and civilization.
The Enterprise's mission statement is: 'to explore strange new worlds, to seek out new life and new civilizations, to boldly go where no man has gone before'. Starfleet has discovered numerous alien species on its travels through space and many of their scientists are involved in studying alien species. For example, in The Next Generation, Dr.Kila Marr is a xenologist renowned as the foremost expert on the Crystalline Entity.
However, Starfleet's Prime Directive states that there can be no interference with the internal development of alien civilisations, especially those that haven't developed warp technology and don't yet know about life on other planets. Starfleet conducts a lot of its research in secret, using hidden observation posts or disguising themselves a local lifeforms.
The X Files
In the X Files, Mulder and Scully investigate marginalised unsolved cases that often involve extraterrestrial life. They attempt to uncover a government conspiracy to hide the existence of extraterrestrials on earth.