Rogue planets are the drifters of the galaxy, wandering interstellar space alone. Now it turns out they could have company in the form of moons — and perhaps even sustain life that hitched a ride on them.
New simulations show giant planets kicked out of their solar system could hang onto nearly half their moons during the ejection process, and potentially maintain conditions for life for billions of years.
The work, which will be published in the Monthly Notices of the Royal Astronomical Society, suggests that life might be more widespread through the galaxy than we thought, said astrophysicist and study co-author Jason Steffen from the University of Nevada, Las Vegas.
"Say there's one out of every million stars that ejects a planet that's like Jupiter, with its conditions.
"That's 300,000 rogue planets [in our galaxy] that could have life on their moons."
That's a conservative estimate, he added: "We suspect it's closer to 1 per cent, rather than one in a million."
Looking for life in likely places
When it comes to looking for life as we know it, it's hard to go past a solar system's "Goldilocks zone", a region around a star where it's not too hot and not too cold for liquid water to exist.
But in recent years, astrobiologists have turned their gaze to less intuitive targets: giant planets way out in the frozen reaches of a solar system — or, rather, their moons.
There, far from the warming rays of the sun, water stays liquid thanks to heat generated from friction, when a moon is warped by its planet's gravitational pull, as well as that of other moons.
We see this "tidal flexing" in our solar system with Jupiter's suite of moons, Dr Steffen said.
The three innermost moons, Io, Europa and Ganymede, traipse around Jupiter in what's called a resonant frequency: in the time it takes Ganymede to orbit once, Europa orbits twice, and Io makes it around four times.
When Europa is directly between Jupiter and Ganymede, for instance, it's pulled in two directions.
But when Ganymede is around the other side of Jupiter, Europa's tugged more strongly in one direction.
All this regular stretching and squashing causes enough heat to build up inside Europa and, planetary scientists suspect, sustain a liquid ocean beneath its icy crust — an ingredient for life.
And with the relatively recent discovery of rogue planets — huge, gas Jupiter-like objects that wander the galaxy, untethered to a star — Dr Steffen and his student Ian Rabago asked: could they also host and sustain life on their moons?
The moons that stick around when planets are ejected
To find out if moons might survive a planetary ejection — or if they'd be stripped off their planet — Mr Rabago and Dr Steffen ran 77 simulations of a planetary system with three Jupiter-like gas giants, each with moons.
To their surprise, they saw that almost half the moons survived.
"My gut feeling was that it would be less than that, maybe 10 per cent," Dr Steffen said.
"I certainly didn't expect to have that many systems that could keep any moons."
Around 22 per cent of moons were left behind in the solar system, while the remainder were slung out into space, free from any star or planet.
Simulations show around a quarter of ejected planets retain most or all of their moons.
The most gratifying result, Dr Steffen said, was that simulated moon systems with the same orbital resonance as Io, Europa and Ganymede were among those that survived.
"You could take Jupiter, you can kick it out of the solar system through natural processes, and those resonance conditions, and potential for life, can survive."
Aditya Chopra, an astrobiologist at the Australian National University who was not involved with the study, said life on a rogue planet's moon would probably find it a lot tougher to survive than if it remained in a solar system.
"Even though Europa is so far away, UV radiation from the sun is breaking down chemicals on its surface, which might be useful for life," he said.
"Rogue planets don't get a whole lot of photons.
"And in the solar system you have this constant influx of material, like comets, but you don't have that so much with rogue planets."
And while detecting rogue planets is no easy feat at the moment — let alone any life that may linger on their moons — there is an upside.
"The great thing about rogue planets and their moons is they may visit us," Dr Chopra said.
He pointed to 'Oumuamua, a comet from interstellar space that zipped through our solar system last year.
"Perhaps in the future we'll find one of these rogue planets come near us and we'll be able to test some of these ideas."
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