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Some alien planets thought to be locked in place around their parent star may actually be able to rotate, creating stable climates long enough for potential life to arise – as long as any inhabitants didn’t mind sudden disruption.

Many exoplanets that closely orbit red dwarf stars, such as in the TRAPPIST-1 system, are thought to be tidally locked by their star’s gravity, so the planets have permanent day and night sides that are extremely hot or cold. It has previously been thought that life could evolve on such a world, perhaps living near the boundary between the two sides.

Now, and Cody Shakespeare at the University of Nevada, Las Vegas, have found that some of these planets may actually be able to unlock and rotate before locking up again with day and night flipped, according to simulations of millions of orbits in similar systems to TRAPPIST-1.

The simulations looked at how long planets would take to transition between four different states: two tidally locked phases, or rotating clockwise or anticlockwise. The researchers found that, in about 30 per cent of the systems they looked at, the planets can sit in any of these phases in a seemingly stable configuration, some of which last for as long as 100,000 years, before changing to another.

They then fed the timings from the simulations into a simple climate model and found that, if a planet stayed in one of the rotating states for long enough, a stable climate could form, though more research is needed to figure out if this could be at all similar to Earth’s.

The duration of the stable periods could also be longer than their models suggest, says Steffen, as longer simulations led to longer stable periods. “There’s no real limit, it doesn’t truncate at any point. If we had simulated it for a trillion years, we would get some episodes that are hundreds of millions of years long.”

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A stable state that lasted this long might be enough time for life to develop, though any life forms would have to be highly adaptable for when the planets’ spin states change, says Shakespeare. There would be very little warning, he says. “They would have to keep their adaptations for the extreme cold and extreme heat almost indefinitely, even whenever they’re not using them, because they never know when it’s going to go back.”

Understanding how these systems spin is also useful for future exoplanet observations, says at the University of Southampton, UK. “We now understand that, for habitability, there’s this additional criterion for some of these systems where we also need to look at the spin,” says Wittig.