San Helios

San Helios was a planet in the Scorpion Nebula, on the other side of the universe from Earth. It once had an advanced civilisation with a population of 100 billion, and cities like San Helios City, but in the early 21st century it was ravaged by a swarm of Alien Stingrays, and all life on it devoured, turning the planet into a desert planet in less then a year. After feeding off of San Helios, the stingrays attempted to open a wormhole to Earth, accidentally trapping a group of humans on the planet in the process. The Doctor eventually re-routed the swarm to an uninhabited planet instead.

Before its demise, San Helios traded in "waste products" with the Tritovore race. As a result, a Tritovore crew was the first to discover the fall of the planet. (DW: Planet of the Dead)

Behind the Scenes

 * Interestingly, the similarly sounding Sanghelios is another fictional planet in the Halo series of games, from which the Sangheili (Elite) race originates. Both planets allude to the Sun, which is the meaning of Helios. In the case of Sanghelios, it refers to "Blood Sun", the several suns surrounding the planet and the warrior nature of the species that inhabits it, where as San Helios likely refers to the desert state the planet is now in
 * Another interesting point is that the name "San Helios" can be translated to "Three Sun" -"San" is japanese for Three, while "Helios" is Greek for Sun- a possible reference to the planet's three suns.