We worked out what it would take to wipe out all life on a planet – and it’s good news for alien hunters

We wanted to consider what cataclysmic events might be able to finally kill off the hardy tardigrade. What would need to happen to destroy every living thing on the planet? The simplest answer is that the planet’s entire oceans would have to boil. On Earth, this would require an incredible amount of energy –- 5.6 x 10²⁶ joules (around a million years of total human energy production at current rates). We therefore have to consider the astrophysical events that could provide such an enormous amount of energy.

There are three primary candidates that could do this: asteroid impacts, supernovae and gamma-ray bursts. Of these, asteroids are the most familiar. We’ve been hit by several over the course of Earth’s history. But, in our solar system there are just 17 candidate objects (including dwarf planets like Pluto and Eris) large enough to provide this energy – and none with orbits coinciding with that of Earth.

By looking at the rate of asteroid impacts on Earth, we can extrapolate the rate at which doomsday events like this would likely occur. This turns out to be approximately once every 10¹⁷ years – far longer than the life of the universe. So it’s very, very unlikely to ever happen.

Supernovae (massive explosions of stars) release huge amounts of energy –- 10⁴⁴ joules, which is more than enough to boil our oceans. Fortunately, the energy delivered to a planet rapidly drops off the further away it is from a supernova. So for the Earth, sterilization would require a supernova to occur within around 0.013 light-years. The nearest star apart from the Sun, Proxima Centauri, is 4.25 parsecs away(and is the wrong type to go supernova).

For Earth-like planets in our galaxy, the distance between stars depends on their distance from the galactic center. The central bulge is more densely populated than our neighborhood. But even closer in, given the rates at which supernovae occur, sterilization is unlikely to happen more than once in 10¹⁵ years, again far beyond the age of the universe.

Finally there are gamma-ray bursts, mysterious explosions producing enormous amounts of energy focused into jets of radiation as narrow as a couple of degrees. Analyzing these bursts as we did supernovae, we found that they could only kill off life on an Earth-like planet if their origin was within about 42 light-years and the planet lay within the beam. Again, the rate at which this would occur is sufficiently low that very few planets would ever be sterilized by a gamma-ray burst.

Apocalypse never
Given how tiny the chances are of any of these apocalyptic events actually happening, we’re left with the conclusion that tardigrades will survive until the Sun expands about 1 billion years from now. One final, incredibly unlikely possibility is that a passing star could kick a planet out of its orbit. But, even then, volcanic vents that host some tardigrades could potentially provide heat for long enough for the planet to be captured by another star.

There are many events, both astrophysical and local, that could lead to the end of the human race. Life as a whole, however, is incredibly hardy. As we begin our search for life away from Earth, we should expect that if life had ever begun on a planet, some survivors might still be there.

Rafael Alves Batista is Postdoctoral research associate, University of Oxford. David Sloan is Postdoctoral research associate, University of Oxford. This article is published courtesy of The Conversation (under Creative Commons-Attribution / No derivative).