Not everyone is familiar with the tardigrade (also known as the water bear, also known as the moss piglet, previously known as "animalcules"), and that's a damn shame. They're less than a millimeter long, sure, but they're almost certainly the most indestructible animals on the planet. You can expose them to the unforgiving vacuum of space, starve them for decades, dehydrate them for literally-who-knows-how-long, boil them, mash them, stick them in a stew, whatever, and as soon as you return them to normal conditions they'll perk right back up and go on their merry way. Their abilities are so obscenely awesome that some scientists are convinced they contain an unprecedented ratio of DNA that's "stolen" from other organisms by way of horizontal gene transfer, though these results proved controversial.
Even if you love tardigrades as much as we do, a study on their post-apocalyptic fates might seem a little wasteful. But it makes a lot of sense if you stop to think about it: we know that tardigrades can survive basically anything that we can throw at them, to an extent we don't see with any other animal. So for researchers who want to understand how hard it would be to wipe out all life on Earth in one go, the water bear makes a great mile marker: if these eight-legged swimmers don't make it through a natural disaster, nothing else stands a chance—in the animal kingdom, anyway. Microbes are a different question entirely.
Asking that question is about more than figuring out the odds of Earth's survival. What the researchers are really trying to figure out is how likely it would be for life to evolve on another planet but get wiped out before we had the chance to find it. Space is big and time is long, so it wouldn't be surprising if our first encounter with a planet with the evolutionary potential to host life happened at a less-than-ideal point in that world's history. Take Mars, for example: some scientists hold out hope that this relatively Earth-y rock teemed with microbial life billions of years ago, before its atmosphere slipped away into space. But how likely is that sort of thing? When life does evolve (assuming it's happened more than once, which is unfortunately still an open question), does it tend to stick around?
Based on the humble moss piglet, scientists from Oxford and Harvard say it does. They calculated the risks posed to tardigrades' marine habitats by asteroids (conclusion: nothing big enough to totally boil up our oceans should ever intersect with Earth's orbit), supernovas (conclusion: the probability of a star explosion massive and close enough to totally boil up our oceans happening during our Sun's lifetime is "negligible"), and gamma-ray bursts (conclusion: again, the probability of this even-more-powerful-but-even-more-rare stellar explosion happening in close enough proximity to boil up our oceans is "minor").
In other words, water bears are in it for the long haul. Like, until-the-Sun-sterilizes-the-entire-planet long. Unfortunately, even these resilient critters won't survive once our host star ages into a bigger, hotter thing than it is now. The Sun should be around for about another 5 billion years, but most estimate that it will turn ocean-boiling hot in about 1 billion years. Until then, though, we know the animal kingdom's foothold on the planet is secured—by eight very tiny feet.
"A lot of previous work has focused on 'doomsday' scenarios on Earth—astrophysical events like supernovae that could wipe out the human race," David Sloan, co-author and physics researcher at Oxford University, said in a statement. "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."
Of course, with a sample size of one, scientists can't be sure that every planet capable of hosting life will create a creature as utterly indestructible as the tardigrade. They could be just as special in the galaxy as they are on our own planet.