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 originally had a varied topography, with oceans, forests, deserts and massive cities but after the attack by the Stingrays, the whole planet was covered in a sand-like substance and rendered inhospitable. (TV: Planet of the Dead)
Prior to the Stingrays‘ attack, San Helios was a heavily populated planet with a technologically advanced native race and varied wildlife. The population’s number had reached one hundred billion by 2009.
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 Stingrays, and all life on it was devoured, turning the planet into a desert in less than a year.
After feeding off San Helios, the Stingrays 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 moments before the Stingrays could follow. The Doctor stated that the Stingrays would just begin anew and generate a new wormhole to leave San Helios but promised to try to "nudge" the wormholes onto uninhabited planets so that further planets would be spared San Helios' fate. (TV: Planet of the Dead)