In terms of supernovae, the researchers found that in order for an exploding star to boil the oceans, it would need to be 0.14 light-years away. The threat posed from these exploding stars to Earth is negligible, they determined, since the closest star to the sun is four light years away. The probability of a massive star exploding close enough to Earth to kill all forms of life on it, within the sun’s lifetime, is therefore very low.
If this phenomenon were to happen, Sloan said, “a nearby supernova would be a disastrous event for most life on a planet, but species like the tardigrade could well live on.”
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Gamma-ray bursts are even brighter than supernovae, but the scientists believe that they are too far away from Earth to be considered a viable threat. To boil our planet’s oceans, they would need to be no more than 40 light-years away, so the likelihood of a burst occurring at such proximity to Earth is minor.
“Mainly we’ve found that, once life starts, it seems very unlikely that it will be killed off by any of the usual suspects for annihilating humans,” Sloan said. “If we find planets fit for life and search them, if life ever was there, it probably still should be in some form.”
The researchers speculate that tardigrades, or animals similar to them, could be living on planets other than Earth, so long as there is a source of water.