The alien's natural form was a glowing white light that lived in space. It was able to penetrate a ship's hull in order to observe the inside. However, it could also take humanoid form through impregnating one and being born. When the alien did this, it had a very short gestation period and would grow extremely rapidly. Even as a humanoid child, it retained its memory. The alien also emitted Eichner radiation naturally.
In 2365, it wished to experience Human existence after happening to pass by the USS Enterprise-D. It decided to impregnate Deanna Troi and became a half-Human, half-Betazoid male baby after a thirty-six hour term. Deanna named him after her father, Ian Andrew Troi. He grew from an infant to an eight-year old in one day. During this time, he enjoyed his tactile senses, even when burning his fingers.
Unfortunately, Ian's presence turned out to be harmful to the crew. The radiation he gave off was causing an on-board container of a plasma plague to fail, putting everyone on the ship in danger. He then decided to leave the Enterprise by dying. In doing so, he returned to his natural form. (TNG: "The Child")
Background information Edit
Influenced by the character of Ian Troi, a similar alien was briefly considered as a main character of Star Trek: Voyager, which was said to be born and then "matures rapidly." (A Vision of the Future - Star Trek: Voyager, p. 178)
In the Star Trek: Destiny trilogy, Troi's attempts to conceive with Captain Riker after the two transfer to the Titan are hampered because carrying Ian caused previously-undetected damage to her reproductive system due to the unique radiation he generated. However, this damage is treated by the Caeliar, a highly-advanced race, resulting in Troi successfully carrying her daughter to full term.