Humans are newcomers on Earth and it's almost certain that we won't be around, on this planet at least, when the solar system's star finally goes nova. But boffins have identified at least one animal that will be.
It is estimated that our star will die in about five billion years, after growing to a red giant that will most likely engulf earth and the other three rocky planets in the solar system. The seas will boil and the plasma from the sun will flay our planet down to its iron core, or even destroy it completely.
But there's one animal that will be around to see this happen – or at least sense that something has gone very wrong, according to scientists at Oxford University. It's the humble tardigrade – or water bear – which turns out to be one of the roughest, toughest animals out there.
Tardigrades (milnesium tardigradum) are a type of eight-legged animals that grow to about 0.5mm (0.02in) long and are found in every climate and pressure zone on the planet. They are capable of going without food and water for 30 years, can survive in temperatures ranging from -458°F (-272°C) to about 300°F (150°C), can endure massive doses of radiation, and can even exist and be revived after 10 days of being exposed to the vacuum of space.
The research, published today in Nature's Scientific Reports, looks at three scenarios that could extinguish life on earth before the sun dies, and found that none of them would wipe out the tardigrades. They examined an asteroid strike, supernovae, and gamma ray bursts – any one of which could easily kill off humanity.
"Tardigrades are as close to indestructible as it gets on Earth, but it is possible that there are other resilient species examples elsewhere in the universe," said Dr Rafael Alves Batista, co-author and research associate in the Department of Physics at Oxford University.
"In this context there is a real case for looking for life on Mars and in other areas of the solar system in general. If Tardigrades are Earth's most resilient species, who knows what else is out there?"
On the asteroid front, the only thing that's going to kill the water bears is a strike large enough to boil away our oceans. Even the 65 million-year-old impact that killed off the dinosaurs didn't come close to that, in fact the researchers noted that deep-sea animals were barely affected by that disaster.
There are only 17 known asteroids in the Solar system capable of coming close to sterilizing earth, and none are in orbits that would ever intersect with this planet. Dwarf planets like Pluto and Eris might be able to, but are too far away to be a threat.
Supernovae are also not a threat, considering the distances involved. In order to fry our planet we'd need a supernova to go off within 0.14 light year away and the nearest star is currently over four light years distant.
As for gamma ray bursts, the chances of a fatal event are mathematically slim. These bursts occur when stars collapse and are immensely powerful, but are also highly directionally focused and would need to be a maximum of 40 light years away, and no likely candidates have been found.
"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," said Dr David Sloan, co-author and also from the Department of Physics at Oxford University.
"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."
It's worth bearing in mind that even if humanity does extinguish itself, the hardiness of complex life like tardigrades could provide the basis for new forms of life to evolve after us. After all, humans have only been around for about a quarter of a million years and the planet has billions of years to play with before the sun goes night-night. ®