In 100,000,000,000,000, Malcassairo had no sun. Despite being a sunless world, it was still able to maintain both a gaseous atmosphere and sufficient heat to support life. (Jack Harkness thought it had an atmospheric shell) Malcassairo was mostly covered in dead scrub land and hills.
The only known artificial structures by 100,000,000,000,000 were Silo 16, a rocket/missile launch silo where the last remnants of humanity took refuge in preparation for their final quest for the planet Utopia, and the Malmooth Conglomeration, once the home of the nearly-extinct Malmooth. The Futurekind, a spin-off race of humanoids, also had at least one camp on the planet. Besides the humans and Futurekind, the population of Malcassairo at the end of time included the last surviving member of the Malmooth race, Chantho.
Shortly before its destruction by heat death, the Tenth Doctor, Martha Jones and Jack Harkness arrived on the planet. After an encounter with the Futurekind, the three of them took refuge with the humans. They assisted Professor Yana in his efforts to send the humans to Utopia. Soon after, however, it was discovered that Yana was in fact an incarnation of the Master. Subsequently, the humans were still sent to Utopia, but Chantho was killed by the Master and the Doctor, Martha, and Jack were, for a time, marooned on Malcassairo after the Master hijacked the Doctor's TARDIS and left the planet. (TV: Utopia) The time travellers were eventually able to depart Malcassairo and follow the Master to 21st century Earth using Jack's vortex manipulator. (TV: The Sound of Drums)