In a recent paper published in Scientific Reports, David Sloan of the University of Oxford and colleagues analyzed three different scenarios in which all life on Earth could be extinguished: large asteroid impacts, supernova explosions, and the cataclysm resulting from a gamma-ray burst. The most likely killing mechanism in all three cases is heat and radiation—particularly the boiling away of the planet’s oceans, which would exterminate all surface and underwater life. For an Earth-like planet, the authors calculate an overall chance of complete sterilization as being less than one in a million over a time period of a billion years. In other words, it’s very unlikely for life to be completely extinguished on a habitable planet, at least in these three scenarios.
Their conclusions are too optimistic. First, their analysis is based on the resilience of unusually hardy creatures known as tardigrades, or “water bears.” Tardigrades can withstand extremes of radiation and temperature, especially in their dormant “tun” state. But they can’t live forever in this state, and the authors should have considered whether organisms can grow and reproduce in extreme conditions. Also, tardigrades feed on algae and other organisms less hardy than themselves, so even if they survived a supernova blast, all their food sources would be gone. The point is that a minimal sustainable biosphere has to be able to survive, not just a single organism or species.
In addition, global catastrophes would have secondary effects such as greenhouse warming. There is a good chance that this happened on our neighboring planet, Venus, which had an early ocean, and possibly life within it. Then a cataclysmic event happened, such as a large asteroid impact or global lava outpourings (or both together), and the planet experienced a runaway greenhouse effect resulting in surface temperatures equal to those of an oven on broil. Thus, the planet next door may very well have experienced what the authors say is an “unlikely” end to all life.
In addition, there are more than three ways for life to be snuffed out. Mars used to have surface oceans, too, but not anymore, and it is not clear whether life, if it existed early in the planet’s history, survived. Our own Sun could even cause our demise. Even though we depend on it now, any major upheaval in its radiation output could spell doom for us, and much, if not all, of the life around us.
There may be reasons for Sloan’s optimism, however. Even if all oceans on a planet boiled away, enough microbes may still survive deep in the rock to resettle the surface once it cools down again. That may be exactly what happened early in Earth’s history, after a period called the Late Heavy Bombardment.
Sloan and coauthors deserve credit for putting some lower and upper bounds on the risk from specific astronomical doomsday scenarios. And it’s valuable to know that even planets in the center of a galaxy might be inhabited, and that some might survive nearby supernovas. But their analysis doesn’t really answer how long a terrestrial planet stays habitable, or how much danger we face that life on Earth could suddenly go extinct.