David Sloan, Rafael Alves Batista & Abraham Loeb of the University of Oxford published a research paper, The Resilience of Life to Astrophysical Events, in Scientific Reports in July. In it, they attempted to answer a question of deep relevance to the search for extra-terrestrial life: what does it take to sterilise a planet entirely?
The famous Drake equation contains many of the parameters that need to be considered when attempting to work out the chances of finding intelligent aliens. One key parameter is how long life can endure on a planet. For us to detect aliens, it’s not just a question of being near enough to them in space; we’ll need to be close to them in time, as well. Researchers attempting to resolve the Fermi paradox – the question of why no alien life has yet been detected – have in the past suggested that astrophysical events may be responsible. Gamma ray bursts and supernovae deposit energy into the surrounding regions of space at incredible rates, and could potentially destroy or sterilise any unfortunate planet in their path. If these events are common enough, it could be that alien intelligent life is regularly scrubbed out by exploding stars, explaining why we’ve been unable to find any.
But this research suggests otherwise. Sloan, Alves-Batista and Loeb decided to work out how resilient life was by choosing the hardiest creatures known to us. There are microbes called extremophiles that positively relish living in the kind of conditions that would kill ordinary life, including high doses of radiation and acidic waters. The researchers chose the tardigrade, or water bear – not technically an extremophile, but hardy nonetheless.
“Even if the worst apocalyptic event happens, only intelligent life is doomed.”
These microscopic, eight-legged animals can survive temperatures as low as 1K – colder then liquid nitrogen, and close to absolute zero. Other species can survive at a roasting 420K or 150 Celsius – well above the boiling point of water. Even blasting a water bear into outer space won’t necessarily kill it.
The researchers simulated some of the most damaging astrophysical events – including supernovae, gamma ray bursts, and asteroid strikes like the one that wiped out the dinosaurs. It turns out to kill the tardigrades requires an asteroid up to a thousand times heavier than that. The events considered by the physicists that might have a prayer of killing the tardigrades would also be sufficient to strip away the atmosphere, and boil the oceans. Luckily for us, such events are rare: for a supernovae or gamma ray burst to do it, they’d have to be extremely close to our solar system, which is highly unlikely.
Dr David Sloan, Co-author and Post-Doctoral Research Associate in the Department of Physics at Oxford University, said: ‘To our surprise we found that although nearbysupernovae 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.
Dr Rafael Alves Batista, Co-author and Post-Doctoral Research Associate in the Department of Physics at Oxford University, said: Without our technology protecting us, humans are a very sensitive species. Subtle changes in our environment impact us dramatically. There are many more resilient species on earth. Life on this planet can continue long after humans are gone.
In some ways, it’s reassuring.
Regardless of what humans do, we are unlikely to sterilise the earth as thoroughly as a supernova, asteroid, or gamma-ray burst might: it seems that life, in some form, is here to stay. In some ways, the tardigrades are a superior species: they’re far more likely to still be around to watch the sun burn out than we are. Indeed, the researchers conclude in the paper that once life has been established and has had some time to evolve – allowing creatures like tardigrades to exist – only the evolution of its host star is likely to kill it. Tardigrades, then, are likely to survive for a billion years or more. Time, then, to turn our gaze to the stars. Alves-Batista and his colleagues are well aware of the possible implications of their research for the search for intelligent life elsewhere in the Universe:
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?
What this research shows is that if planets that look uninhabitable at the moment – such as Mars and Venus – once housed life, there’s no reason to believe that life doesn’t still exist there. The only concern you might have is that whatever life might exist elsewhere in the solar system has already proved itself very, very difficult to kill.
This story was written by Thomas Hornigold. Some content may be edited for length.
Image: Warner Brothers