Malcassairo

The planet Malcassairo was one of the last surviving planets in the universe by the year 100,000,000,000,000, when it served as the last known outpost of humanity in the year.

Profile
Apparently a "rogue planet" with no sun as the universe was in heat death, but still able to maintain an atmosphere and generate sufficient heat to support life (believed by Jack Harkness to be an atmospheric shell), Malcassairo was mostly covered in dead scrub land and hills.

Among the only known artificial structures on the planet by the year 100000000000000 were Silo 16, a rocket/missile launch silo where the 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 race the Malmooth. The Futurekind, a spin-off race of near-humans, also had at least one basic 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. It is not known if the planet harbored any animal life by the year 100,000,000,000,000.

In 100,000,000,000,000, the Doctor, Martha Jones and Jack Harkness arrived on the planet. After taking shelter with the humans after an encounter with the Futurekind, the trio 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. (DW: Utopia) The time travellers were eventually able to depart Malcassairo and follow the Master to 21st century Earth using Jack's vortex manipulator. (DW: The Sound of Drums)

Afterwards, with the Futurekind the only known remaining inhabitants, Malcassairo presumably continued to exist for a time until the universe finally came to an end some point after the year 100,000,000,000,000.