Wickramasinghe and colleagues claim that the heat emitted from radioactivity warms initially frozen material of comets to produce subsurface oceans that persist in a liquid condition for a million years.

The formation of the solar system itself is thought to have been triggered by shock waves that emanated from the explosion of a nearby supernova.

The supernova injected radioactive material into the primordial solar system and some became incorporated in the comets.

Comets in recent times could also liquefy just below their surfaces as they approach the inner solar system in their orbits. Evidence of recent melting has been discovered in recent pictures of comet Tempel 1 taken by the 'Deep Impact' probe in 2005.

These findings were published in the International Journal of Astrobiology.