In the book series, the teen aliens come from an unnamed planet and are stranded on Earth when their parents' ship crashes. In the television series, the teen aliens are reincarnations of the Royal Family of Antar. After a battle, their essences were mixed with human DNA, and the teen aliens are actually hybrids who were sent to Earth to survive the war.
Roswell High by Melinda Metz
The home planet, which is never called by name, is home to a race of beings that exist within a collective consciousness. At adolescence, each alien must go through an akino, a painful experience that only ends when they join the collective. Since the collective consciousness takes away an alien's privacy and individuality, a rebellion has formed on the home planet called the Kindred. Members of the Kindred, led by Elsevan DuPris want to destroy the collective and outlaw unfair laws, such as only being allowed to have one child. When DuPris is taken prisoner by the collective consciousness, he is placed on a spaceship for transport. The ship crashes on Earth in the human year 1947, in a town called Roswell. DuPris escapes, but he stays on Earth hoping to find the Stone of Midnight, an energy source that was misplaced during the crash. The only other adult survivor of the crash, Ray Iburg, manages to hide four incubation pods in the desert. He tries to go back for the fifth pod, but the ship and pod are already in the custody of Project Clean Slate. On the run from being captured himself, Ray is not around when the pods open--allowing Michael, Isabel, Max, and Nikolas--who look like regular human children--to wander out into the desert. Nikolas and Michael are placed in foster care, but Nikolas grows up outside of Roswell. Max and Isabel are adopted by the Evans, and grow up in Roswell with Michael as one of their best friends. The last pod, in the hands of Clean Slate, hatches a boy named Adam, who grows up underground in the Clean Slate compound.
Jason Katims' Roswell
On planet Antar, Zan and his wife Ava ruled over a system of five planets. Rath, Zan's second in command was betrothed to Zan's sister, Vilandra. As a group, they made the "Royal Four." An alien named Kivar fell in love with Vilandra, and she helped him to betray Zan. As a result, Kivar took over Antar and betrayed Vilandra (either accidently or on purpose) by killing the "Royal Four". Zan's people, unaware of Vilandra's part in Kivar's take-over, mixed the essences of Zan, Ava, Vilandra, and Rath with human DNA and sent the "Royal Four" to Earth where they would be safe until they could return home. As the ship neared Earth, something went wrong and the ship crashed. Two protectors of the "Royal Four", who would take the names Nasedo and Langley, survived. On the run from the government, Langley was forced to run and Nasedo just barely managed to put the incubation pods of the "Royal Four" in a safe cave before being captured himself. Years later, after escaping from the FBI Special Unit, Nasedo came back to the pods, but only found Ava, who he would later give the name Tess. The other three had hatched earlier and, with no memories of their past lives, took on the names Max, Isabel, and Michael. The three aliens grew up, living semi-normal human lives in Roswell, New Mexico, with no idea where they were from or why they were on Earth.