The last life on Earth will perish in 2.8 billion years, scorched by the dying sun as it swells to become a red giant. For about a billion years before that, the only living things will be single-celled organisms drifting in isolated pools of hot, salty water.
A grim outlook, sure, but there's a silver lining for today's alien-hunters. The model that predicts these pockets of life on future Earth also hints that the habitability of planets around other stars is more varied than previously believed, offering new hope for finding life in unlikely places.
Using, what we know about Earth and the sun, researchers in the United Kingdom calculated a timeline for the phases of life on our planet as the sun expands to become a red giant. Previous studies modelled this scenario for Earth as a whole, but Jack O'Malley-James at the University of St Andrews from United Kingdom and his colleagues wanted to consider the possibility, that life might survive in a few extreme habitats.
Sun-like stars of different sizes age at different rates, so the team also looked at how long simple and complex life might thrive around smaller and larger stars.
"Habitability is not so much a set attribute of a planet, but more something that has a lifetime of its own," says O'Malley-James.
The team started by modelling rising temperatures on Earth's surface at different latitudes, along with long-term changes to the planet's orbital characteristics. Their model shows that as the sun ages and heats Earth more, complex life withers - plants, mammals, fish and finally invertebrates disappear as temperatures soar. The oceans vaporise, and plate tectonics grind to a halt without this water as a lubricant. Eventually, pools of hot brine are all that exist in the less scorching higher altitudes, in sheltered caves or far underground. Microbes living in these pools could rule the Earth for about a billion years before they, too, dwindle to extinction.
Applying the model to stars of various sizes, life on an Earth-like planet would be only single-celled for about the first 3 billion years. Complex life could exist for comparatively short periods before the star begins to die and conditions once again become favourable to microbes alone. Statistically then, if alien life is out there, it is more likely to be microbial simply due to timing, the team says.
Proof of any type of life elsewhere would nevertheless be an incredible achievement, says O'Malley-James. He is now working out what the chemical signatures of microbial life on a far-future Earth might look like, and whether we could spot similar signs on exoplanets that currently appear lifeless. "Rather than just being a dead planet, it might be a planet just slightly closer to the end of its habitable lifetime," he says.
Euan Monaghan at the Open University in Milton Keynes from United Kingdom agrees that we should be thinking of life on a planet as a cycle from simple to complex and perhaps back down to simple again. This will aid the hunt for alien life, he says. "If life exists in many places, we just have to find that particular band in which it is multicellular."