The world's most robust animals may very well survive until the sun stops shining.
Also known as water bears, tardigrades are tiny water-dwelling creatures famed for their resiliency. The eight-legged invertebrates can survive for up to 30 years without food or water and can endure wild temperature extremes, radiation exposure, and even the vacuum of space.
"Tardigrades are extremely hardy animals," says Thomas Boothby, a tardigrade researcher at the University of North Carolina. "Scientists are still trying to work out how they survive these extremes."
At a minimum, all of Earth’s oceans would have to boil away to completely wipe out all life on the planet. Although Boothby said tardigrades are only known to survive high temperatures when dry — and those species living in the sea would likely die before the waters boiled — tardigrades are still expected to avoid extinction until our sun swells up and becomes a red giant roughly six billion years from now, according to researchers who investigated the effects of various doomsday scenarios.
Astrophysical events such as asteroid strikes and supernova explosions have been fingered as the causes of past mass extinctions on Earth. Such violent cataclysms could easily wipe out humans: We belong to a sensitive species, and subtle changes in the environment impact us dramatically, notes study co-author Rafael Alves Batista of the University of Oxford.
Intrigued by the resilience of tardigrades, Alves Batista and his colleagues wanted to explore the effects of potential astrophysical catastrophes on more than just human life. (Read "What the World’s Toughest Animal Is Really Made Of.")
“It's an exciting time to be asking questions about life in the rest of the galaxy or universe,” says study co-author David Sloan, also at Oxford.
In the past few years, for instance, astronomers have found thousands of planets beyond our solar system, including some that may be habitable. (Read "Seven Alien 'Earths' Found Orbiting Nearby Star.")
Closer to home, Jupiter’s moon Europa and Saturn’s moon Enceladus likely have subsurface oceans with volcanic vents that may have the right conditions to host life-forms not unlike tardigrades.
“We don't know how life starts on a planet, but since we've seen mass extinctions on Earth, we wanted to know if there are any astrophysical factors that could completely kill off all life on a planet once it gets started.”
According to their research, here are all the ways not to kill a tardigrade.
A large asteroid is the likely cause of the extinction event 66 million years ago that wiped out approximately 75 percent of species on the planet, including non-avian dinosaurs. (Read "Here's What Happened the Day the Dinosaurs Died.")
Today, astronomers know of only a dozen asteroids and dwarf planets with enough mass to boil Earth’s oceans if they collided with our planet. And none of these objects are expected to ever intersect Earth's orbit.
There are asteroids out there that do pose collision risks and are large enough to trigger an “impact winter,” blotting out sunlight and causing temperatures to drop. This would be catastrophic for many life-forms on the surface, but tardigrades would have a refuge.
“Tardigrades can live around volcanic vents at the bottom of the ocean, which means they have a huge shield against the kind of events that would be catastrophic for humans,” Sloan says.
The explosive deaths of massive stars, known as supernovae, can send debris and damaging radiation flying outward at high speeds. But to boil our oceans, a supernova would need to happen a mere 0.14 light-years away from Earth, the researchers found.
Luckily, the closest star to the sun—Proxima Centauri—is more than four light-years away. And it’s not even the right kind of star to go supernova, Sloan says. (Read "How to See a Star Explode in 2022.")
This puts Earth in a fortunate position, the researchers determined, where it’s highly unlikely a massive star will explode close enough to us to kill all forms of life within the sun's lifetime.
Gamma-ray bursts are even more powerful than supernovae, but they also happen too far away from Earth to be considered a practical threat, the researchers say.
To boil the world's oceans, a burst would need to happen less than 40 light-years away. Also, the rate of gamma-ray bursts is so low that it's highly unlikely the beam from one would hit Earth in the next few billion years.
In essence, the researchers say, only the death of the sun will ultimately lead to the total extinction of life on Earth, including tardigrades.
“It seems that life, once it gets going, is hard to wipe out entirely,” Sloan says. “Huge numbers of species, or even entire genera, may become extinct, but life as a whole will go on.”
And that’s an encouraging message for scientists seeking signs of life beyond our planetary shores.
“Tardigrades are as close to indestructible as it gets on Earth,” Alves Batista says, “but it’s possible that there are other resilient species examples elsewhere in the universe.”
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