view counter

ApocalypseWe worked out what it would take to wipe out all life on a planet – and it’s good news for alien hunters

By Rafael Alves Batista and David Sloan

Published 14 July 2017

Earth has undergone at least five mass extinctions in its history. It’s long been thought that an asteroid impact ended the dinosaurs. As a species, we are rightly concerned about events that could lead to our own elimination – climate change, nuclear war or disease could wipe us out. So it’s natural to wonder what it would take to eliminate all life on a planet. There are many events, both astrophysical and local, that could lead to the end of the human race. Life as a whole, however, is incredibly hardy. As we begin our search for life away from Earth, we should expect that if life had ever begun on a planet, some survivors might still be there.

The first exoplanet was spotted in 1988. Since then more than 3,000 planets have been found outside our solar system, and it’s thought that around 20 percent of Sun-like stars have an Earth-like planet in their habitable zones. We don’t yet know if any of these host life – and we don’t know how life begins. But even if life does begin, would it survive?

Earth has undergone at least five mass extinctions in its history. It’s long been thought that an asteroid impact ended the dinosaurs. As a species, we are rightly concerned about events that could lead to our own elimination – climate change, nuclear war or disease could wipe us out. So it’s natural to wonder what it would take to eliminate all life on a planet.

To establish a benchmark for this, we’ve been studying what is arguably the world’s hardiest species, the tardigrade, also known as the “water bear” for its appearance. Our latest research suggests these microscopic eight-legged creatures or their equivalents on other planets would be very hard to kill off on any planet that was like Earth. The only astrophysical catastrophes that could destroy them are so unlikely there’s an insignificant chance of them happening. This extreme survival ability adds weight to the idea that life is hardy enough to be found on other planets less hospitable than our own.

Last survivors
Tardigrades are known to survive incredible conditions. Drop the temperature briefly to -272℃or raise it to 150℃and they go on. Increase atmospheric pressure to more than 1,000 times that at the Earth’s surface, or drop it to the vacuum of space and they continue. They can survive for up to 30 years without food or water. They can even withstand thousands of grays (standard doses) of radiation. (Ten grays would be a lethal dose for most humans.)

They live all over the planet but can survive far below the ocean’s surface, around volcanic vents at the bottom of the Mariana Trench happily oblivious to the life and death of surface-dwelling mammals. Stripping the ozone layer or upper atmosphere would expose humans to lethal radiation but, at the bottom of the ocean, the water overhead would provide shielding.