Helium 2

Helium 2 was a substance similar to helium which would had been produced in the mind-boggling temperatures of the Leptonic Era had they been marginally protracted. (PROSE: Time and the Rani)

planned to produce helium 2 by recreating the conditions of the Leptonic Era by exploding a strange matter asteroid. The helium 2 would have fused with Lakertya's upper atmosphere to form a shell of chronons, thus allowing her to transform the planet into a Time Manipulator. (TV: Time and the Rani)

Behind the scenes
In the real world, helium-2 is an unstable isotope of helium, with an atomic mass of 2.015894.