In considering the relative brevity of human existence, astronomer Carl Sagan once wrote, "We are like butterflies who flutter for a day and think it is forever." The same could not be said of the microscopic animals known as tardigrades, which, scientists are now predicting in the journal Nature, could stick around on Earth until the Sun dies. 

The tardigrade, also known as the moss piglet or water bear, is an admirably durable little monster. It superpower is a sort of half-death state called cryptobiosis. When hard times come around, the tardigrade simply curls up, dries up, and mostly stops living, only to re-inflate and rejoin the world when conditions are more comfortable. Studies have shown that tardigrades can survive scorching heat, blistering cold, starvation, desiccation, radiation, and even the vacuum of space.

That's just on the individual level. There are more than 1000 different tardigrade species [PDF], and all have already been around for a long, long time. Scientists estimate that the first tardigrades appeared on the planet around 600 million years ago—about 370 million years before the first dinosaurs. The dinosaurs disappeared. The tardigrades kept on trucking. 

And according to the authors of the new paper, the moss piglets could just keep on trucking for another 10 billion years.

"A lot of previous work has focused on 'doomsday' scenarios on Earth—astrophysical events like supernovae that could wipe out the human race," co-author David Sloan of Oxford University said in a statement.

But humans are far from the toughest kids on the block. ("Human life is somewhat fragile to nearby events," as the researchers diplomatically put it.) Why not try to find out how sturdier species would fare in those same scenarios? To do so, the researchers calculated the effects of three doomsday events—an asteroid strike, a nearby supernova, and a gamma ray burst (another type of stellar explosion)—on tardigrades' environment and physiology.

You can probably guess what they discovered.

"Although nearby supernovae or large asteroid impacts would be catastrophic for people, tardigrades could be unaffected," Sloan said. "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."

Co-author Rafael Alves Batista, also of Oxford, said his team's findings should expand the scope of what we might consider a "habitable planet."

"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. 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."