When the sun dies, could life survive on the Jupiter ocean moon Europa?

Can life survive in the solar system once the sun dies and becomes a red giant star? New research suggests there may be a narrow window of possibility for life to persist on the icy moons of the outer solar system.

It's not exactly clear where the habitable zone of the red giant sun will be, but it could possibly reach the orbit of Jupiter. Although the planet itself won't be habitable because it will still be a giant ball of hydrogen and helium gas, Jupiter's moons might become promising homes for life.

That's according to researchers at the Carl Sagan Institute at Cornell University, who reported the theory in a paper accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

In about 4.5 billion years, the sun will enter the final phase of its life. Its core of hydrogen fusion will expand and, in doing so, inflate the outer atmosphere of the star into gross proportions. It will swell and become a red giant star that will engulf Mercury and Venus and incinerate Earth. In the best-case scenario, all that will remain of our planet will be a lump of smoldering iron and nickel. In the worst-case scenario, it will be obliterated.

The sun's habitable zone — the band where the influx of radiation is just right to support liquid water on the surface of a planet — will steadily march outward as the sun begins this new phase of life.

Jupiter's ice-covered moon Europa will get a lot of heat. Not only will the giant sun be bearing down on it, but Jupiter itself will become hotter and reflect more sunlight, which will provide its own source of heat to the little moon. The researchers found that the icy outer shell will sublimate and the oceans underneath will evaporate. The most sublimation will occur on the side of Europa facing Jupiter because it will receive the most heat. And because of circulation and convection, the equatorial bands that face opposite Jupiter will also suffer significant water loss.

However, northern and southern latitudes on the anti-Jupiter side of Europa will have a more modest rate of water loss. The researchers found that this could provide a tenuous atmosphere of water vapor that could persist for up to 200 million years. That's a blink of an eye compared with the opportunities life has had to thrive on Earth — but it's not nothing, and Europa may become the home for any life that remains in the solar system in that deep future.

The researchers also found that we might be able to find biosignatures on (formerly) icy moons around red giant stars. We have yet to have any confirmed detections of exomoons, but there are several promising candidates. Future observations with the James Webb Space Telescope or the planned Habitable Worlds Observatory might have the resolving power to examine the atmospheric features of these moons. Although it might be an unlikely scenario to find life, it does widen the possible locations for our search, as there may yet be refuges around stars that are nearly dead.