In about 5 billion years' time, our Sun will use up its reserves of hydrogen and begin to cool down and expand, cooking the Earth in a miasma of heat and radiation. Given our current trajectory, humans will probably be long gone by then anyway, but at least one lifeform will likely still be plodding along: the utterly unkillable tardigrade. According to a new study from Harvard and Oxford, it'll take nothing short of the death of the Sun to finally do the species in – which bodes well for the resilience of life as a whole.

Tardigrades look a little goofy, and they often go by the unassuming nicknames of water bears or moss piglets. But don't let that fool you: these microscopic creatures may just be the hardiest lifeforms on the planet. By entering a state of suspended animation, they've been known to withstand temperatures as low as -272º C (-457.6º F) and as high as 150º C (302º F), they can live without food, water and oxygen for extended periods of time, and are fine with both the vacuum of space and the crushing pressures at the bottom of the deepest parts of the ocean.

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With this list of superpowers to their name, tardigrades are a good model for how tough life is overall. When scientists study large-scale threats to life on Earth, it's usually focused on our own survival, but in the grand scheme of things humans are a pretty fragile species. If, for example, a huge asteroid were to strike the planet, it might wipe out human civilization and a good chunk of other animals and plants on land and in the sea, but life would find a way to carry on without us.

In fact, life on Earth has already endured five mass extinction events, in some cases killing 90 percent of all species. Tardigrades, however, are one of the few animals to have survived all of them, so to come up with an idea of the circumstances and likelihood of an event completely sterilizing all life on Earth, the Oxford and Harvard scientists looked at what it would take to wipe out these little go-getters. In short: the oceans will have to boil before life can be fully extinguished, and that's no easy feat.

The researchers considered three astrophysical events that the universe could throw at us to achieve this: a large asteroid impact, a supernova or a gamma ray burst. A reasonably small space rock could wipe out land-based life, but according to the researchers, it would take something with the mass of Pluto before creatures in the Mariana Trench even noticed. Thankfully for all of us, nothing that big is zipping around anywhere near Earth – at least, as far as we know.

In theory, a supernova meets all the criteria for bubbling the oceans right off the planet, but again, our planet's position in the galaxy saves us from that threat. The team calculated that for a supernova to blast the Earth with enough radiation to strip away the protective ozone layer, it would need to be less than 0.14 light-years away. But the nearest star, Proxima Centauri, is four light-years away and isn't big enough to go supernova anyway.

Gamma ray bursts are the bigger, deadlier cousins of the supernova. It's thought that they occur when two neutron stars collide or when massive stars collapse into a black hole, and in doing so they unleash more energy into space than any other known phenomenon. As such, a burst could decimate Earth from a distance of 40 light-years, but again, we're pretty safe thanks to the fact that there aren't any candidates within that range.

That leaves just one event that will no doubt boil the oceans right off the planet: the death of the Sun. Humanity's "best" efforts might leave the Earth uninhabitable by our fragile standards, but the tardigrade – and by extension, life itself – will likely continue to plod along for billions of years yet.

By looking at the persistence of life in a wider, non-human-centric context, the researchers believe it paints a positive picture of the likelihood of life existing elsewhere in the universe, and thriving long enough for us to find it.

"A lot of previous work has focused on 'doomsday' scenarios on Earth – astrophysical events like supernovae that could wipe out the human race," says David Sloan, co-author of the study. "Our study instead considered the hardiest species - the tardigrade. As we are now entering a stage of astronomy where we have seen exoplanets and are hoping to soon perform spectroscopy, looking for signatures of life, we should try to see just how fragile this hardiest life is.

"To our surprise we found that although nearby supernovae or large asteroid impacts would be catastrophic for people, tardigrades could be unaffected. Therefore it seems that life, once it gets going, is hard to wipe out entirely. Huge numbers of species, or even entire genera may become extinct, but life as a whole will go on."

The study was published in the journal Nature.

Source: Oxford University