San Helios was in the Scorpion Nebula, on the other side of the universe from Earth. It had three suns, two blue and one red. The Tenth Doctor believed that they had the ability to enhance psychic powers, increasing Carmen's precognition. It appeared to have gravity and atmosphere roughly equivalent to Earth.
San Helios once had an advanced civilisation with a population of a hundred billion, and cities like San Helios City. San Helios traded in "waste products" with the Tritovore race. Around 2009 it was ravaged by the Swarm, and all life on it was devoured, turning the planet into a desert in less than a year.
After feeding off San Helios, the Swarm attempted to open a wormhole to Earth, accidentally trapping a group of humans on the planet. The humans escaped to Earth through the still-open wormhole. Doctor Malcolm Taylor closed the wormhole. (TV: Planet of the Dead)