Halcyon, also referred to by Tamsin Drew as "Mars 2", was a planet located 90 light-years from Earth.

Tamsin Drew thought it looked like "something from a fairy tale". The planet had colour-changing flowers and trees like coral. Its native species included the Halcyon and shimmerwings.

The Monk took Tamsin Drew to Halcyon around the 33rd century to show her the culture of the Halcyon species. At that point, 20 billion Halcyon lived there. The Monk called it "a nirvana, the likes of which the universe will never see again". The people of Halycon gave thanks to nature, which was the closest thing they had to a religion. They weren't ignorant of war, but they saw no reason to become warlike. The Monk took Tamsin one Halcyon year later to the same location, where it had become hostile to human and Halcyon life. He explained the Ice Warriors had attacked the planet to make it their new home after Mars was no longer sustainable. They killed all life in the attack. The Eighth Doctor told the Monk that the destruction of the planet was "a part of the Web of Time". (AUDIO: The Resurrection of Mars)

Other accounts showed differing origins of Mars' successor, or having time scales inconsistent with what the Monk had showed Tamsin. (PROSE: GodEngine, et al)

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