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% 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.
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.
We wanted to consider what cataclysmic events might be able to finally kill off the hardy tardigrade. What would need to happen to destroy every living thing on the planet? The simplest answer is that the planet's entire oceans would have to boil. On Earth, this would require an incredible amount of energy -- 5.6 x 1026 joules (around a million years' of total human energy production at current rates). We therefore have to consider the astrophysical events that could provide such an enormous amount of energy.
There are three primary candidates that could do this: asteroid impacts, supernovae and gamma-ray bursts. Of these, asteroids are the most familiar. We've been hit by several over the course of Earth's history. But, in our solar system there are just 17 candidate objects (including dwarf planets like Pluto and Eris) large enough to provide this energy - and none with orbits coinciding with that of Earth.
By looking at the rate of asteroid impacts on Earth, we can extrapolate the rate at which doomsday events like this would likely occur. This turns out to be approximately once every 1017 years - far longer than the life of the universe. So it's very, very unlikely to ever happen.
Supernovae (massive explosions of stars) release huge amounts of energy -- 1044 joules, which is more than enough to boil our oceans. Fortunately, the energy delivered to a planet rapidly drops off the further away it is from a supernova. So for the Earth, sterilisation would require a supernova to occur within around 0.013 light-years. The nearest star apart from the Sun, Proxima Centauri, is 4.25 parsecs away(and is the wrong type to go supernova).
For Earth-like planets in our galaxy, the distance between stars depends on their distance from the galactic centre. The central bulge is more densely populated than our neighbourhood. But even closer in, given the rates at which supernovae occur, sterilisation is unlikely to happen more than once in 1015 years, again far beyond the age of the universe.
Finally there are gamma-ray bursts, mysterious explosions producing enormous amounts of energy focused into jets of radiation as narrow as a couple of degrees. Analysing these bursts as we did supernovae, we found that they could only kill off life on an Earth-like planet if their origin was within about 42 light-years and the planet lay within the beam. Again, the rate at which this would occur is sufficiently low that very few planets would ever be sterilised by a gamma-ray burst.
Given how tiny the chances are of any of these apocalyptic events actually happening, we're left with the conclusion that tardigrades will survive until the Sun expands about 1 billion years from now. One final, incredibly unlikely possibility is that a passing star could kick a planet out of its orbit. But, even then, volcanic vents that host some tardigrades could potentially provide heat for long enough for the planet to be captured by another star.
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.