Deltan (Lost in Space)

The Deltans are a sapient race of alien humanoids from the planet Delta. They're notable for having a peculiar form of neoteny in which they can never grow into adulthood, and so spend their lives as teenagers, both physically and mentally.

History
A group of Deltans, led by Captain Bartholomew, wished to grow up like other species and disguised themselves as Human teenagers when they detected the approach of the Earth ship Jupiter II. By sending deceiving magnetic signals, they managed to trick the ship's navigational system and convince its occupants that they were approaching a Human colony in the Alpha Centauri System.

After the ship landed, the Deltans separated the younger crewmembers, Will and Penny Robinson, from the remaining group, and hoped to obtain their collaboration in teaching the Deltans how to grow. They also tried to obtain hormonal transfusions from them, in case that it could trigger their own biological maturation. Ultimately, all of their methods failed and the Jupiter II escaped.

Maureen Robinson wondered if there was some factor intrinsic to the Deltans' physiology that prevented them from becoming adults. Prof. John Robinson, on the other hand, speculated that the condition might be rooted in the Deltans' psychology, more than their metabolism, suggesting that if they could reach mental maturity, their bodies could then follow up into physical maturity as well.

Appearances

 * Lost in Space, s03e19, "The Promised Planet" (1968)