New Delhi, July 14: Scientists investigating the resilience of life have labelled a tiny eight-legged aquatic creature as the Earth's likely last species, capable of surviving destructive extra-terrestrial extinction events until the end of the Sun.

A three-member team of astrophysicists at Oxford University and Harvard University has shown through pen-and-paper calculations that the tardigrade - already known as the Earth's hardiest species - would survive large asteroid impacts, supernovae and gamma ray bursts, events that could wipe out all other species.

Tardigrades, also called water bears, can survive for a few minutes at temperatures as low as minus 272 degrees Celsius or as high as 150 degrees Celsius and at minus 20 degrees Celsius for several decades. In May 2014, a team of Japanese scientists had revived tardigrades retrieved from Antarctic moss and preserved in a frozen state since November 1983.

The new study, published today in the journal Scientific Reports, examined what it would take for heat from supernovae - the end-stage explosions of stars - radiation from gamma ray bursts, or an asteroid impact to boil the Earth's oceans and extinguish the tardigrades.

"The question we want to answer is, what would it take to kill all life on a planet?" Rafael Alves Batista, a physicist and co-author of the study told The Telegraph by email. "In order to achieve that (on the Earth), we need something that can kill tardigrades."

Tardigrades, best seen under a microscope, grow to about 0.5mm and are found in some of the most hostile places on the Earth, from the bottom of oceans to rain forests to the Himalayan mountains. They mainly eat plant cells and algae.

The study has indicated that while a supernova or a gamma ray burst close enough or a large asteroid impact could wipe out all other species on the Earth, tardigrades would remain "unaffected", said David Sloan, a post-doctoral researcher at Oxford University and co-author. "It seems that life, once it gets going, is hard to wipe out entirely," Sloan said in a release.

The Earth has witnessed several mass extinctions in the past. The last one 65 million years ago, which wiped out nearly 75 per cent of all species on the Earth, including the dinosaurs, was believed to have been triggered by a collision with an asteroid.

The new calculations show that an exploding star would need to be 0.14 light years away to boil the Earth's oceans. The closest star to the Sun is four light years away and the probability then of a massive star exploding close enough to kill all life on the Earth is negligible.

The scientists point out that there are only 19 known asteroids in the Solar System large enough to cause the oceans to boil after a direct collision with the Earth, but none are in orbits that will hit the Earth. Their analysis also shows that a gamma ray burst would need to be within 40 light years for the oceans to boil. Tardigrades have also earlier been shown to be capable of surviving extremely heavy doses of radiation.

Biologists have over the past several decades already catalogued multiple organisms, mainly microbes, that can survive extreme cold and heat. The new study suggests that once life has emerged and evolved on a planet, it is hard to eliminate it. "We do not fully understand the mechanisms by which life started, but once it exists on an Earth-like planet, the complete removal of all life (other than the evolution of the host star) is a very unlikely event," Sloan and his co-authors said in their paper.

Astronomers have over the past two decades discovered hundreds of exoplanets, planets orbiting distant stars, most extremely hot or cold but some within habitable zones of stars. Many of these planets are likely candidates for hosting life forms.

The study broadens the scope for finding life beyond the Earth, within and outside the solar system. Abraham Loeb, co-author and chair of Harvard's astronomy department, said: "The subsurface oceans that are believed to exist on Europa and Enceladus (Jupiter's moons) would have conditions similar to the deep oceans of Earth where tardigrades are found." But time will run out for tardigrades, too, as the Sun approaches its end of life. As the Sun grows older and older, a few billion years from now, it will expand and send enough heat to boil the oceans and turn the Earth into a sterile planet. "We haven't considered this because it is not a potential threat, but a certain threat," Batista said.