Nucleogenic particles are particles of microscopic dust found in the atmospheres of most M-class planets. These particles are necessary for the nucleation of water droplets, a necessary step in the production of rain on a planet, and their absence would lead to a barren, desert-like world.
Such a planet was encountered in the Delta Quadrant by the crew of the USS Voyager in 2371. This planet's nucleogenic particles were eliminated by the alien entity known as "the Caretaker," who did not realize until it was too late that his technology had an extremely destructive effect on the atmosphere. As penance for the damage done to the planet's inhabitants, known as "the Ocampa," the Caretaker elected to stay and protect them for nearly a millennium, after which time he began pulling vessels from all over the galaxy in an attempt to find a lifeform with a compatible biomolecular pattern, with which to produce an offspring to carry on his responsibilities. (VOY: "Caretaker")