The Artemis II crew is on its way to the moon as you read this – but scientists say if they’re hunting for aliens, they’re trying the wrong moon.
Extraterrestrial life may be hiding on the moons of rogue planets, cosmic drifters that don’t orbit a star.
Their moons, called exomoons, could be where stargazers should be pointing their telescopes instead, rather than Earth-like planets.
According to a paper recently published in Oxford Academic, this is because these lone planets could keep water on their moons liquid.
A team from the Excellence Cluster ORIGINS at the Ludwig-Maximilians-Universität München, Germany and the Max Planck Institute for Extraterrestrial Physics say the moons of certain rogue planets could hold water for 4.3 billion years – the Earth is about 4.5 billion years old.
Becky Ferreira, a New York-based science writer who was not involved in the study, knows life on these watery, dark worlds sounds out there.
What is a ‘rogue planet’
Free-floating planets weren’t always dark, lonely orbs. They were once planets orbiting a star that were kicked out of their cosmic neighbourhood, such as by a larger planet.
Some astronomers estimate there are trillions of these orphaned worlds in the universe.
Exoplanets (planets not in our solar system) that orbit a star are easy-ish to study, as their stars ‘backlight’ them, allowing scientists to take a peek.
The exomoons doing laps around rogue planets, on the other hand, have no backlight, making them harder (but not impossible) to detect.
But so did the idea that moons could harbour life, with experts suspecting life exists in the water sloshing under the ice of one of Saturn’s moons.
‘It’s practical to look for other life, like Earth life. That’s the only life we know exists,’ Ferreira says.
‘But that certainly does not mean that’s the most likely life out there. We could be really rare as surface creatures, out here exposed to space, that might not be the model for a lot of life.
‘Maybe moons are generally more habitable elsewhere, so that’s all really exciting stuff that is getting worked out through both.’
Why do scientists think rogue planet moons could harbour aliens?
Liquid water is one of the key ingredients for life. It’s part of the thinking behind the habitable zone, often known as the ‘Goldilocks zone’, where it is neither too hot nor too cold, so liquid water can form on the surface.
This is the case on Earth, where we’re just far enough away from the sun, our solar system’s heat source, for water to exist.
Ferreira explains that what the German experts are suggesting is that, rather than a star, this heat source could come from a rogue planet.
‘If a gas giant gets ejected from its native system, there’s no reason to think that it’s gonna lose its gas or anything like that,’ she says.
‘There are a lot of studies about the potential habitability of these planets if they could have some kind of internal heating source that could keep them going.
‘It’s possible that you could have them not need a star, andthat gets into the idea of, do all aliens really need sunlight?’
Researcher David Dahlbüdding, of the Max Planck Institute for Extraterrestrial Physics, focused on exomoons that orbit planets with thick, scorching hydrogen atmospheres.
The moon’s orbit would soon stretch, whipping tidal forces and generating enough heat to keep the water liquid, even in the abyss of space.
Ferreira says that these gassy free floaters could act as a ‘perpetual’ heat source, heating their moons in a way never seen before.
Lunar beings may have ‘no idea there’s a universe out there’, as they drift endlessly through the universe.
If we find aliens, what then?
Humans, according to Ferreira, have a few ways to find them.
This includes measuring how a rogue planet’s gravity warps and magnifies the light arriving from faraway stars behind it, a process called microlensing.
Figuring out an exoplanet’s atmosphere is the go-to way to determine a planet’s habitability at the moment, something that can’t be done with the exomoons of rogue planets just yet.
Ferreira says that, for an alien looking at Earth, it’s ‘pretty obvious’ life is on the blue marble, given its atmosphere
‘Especially now as there’s radio leakage and pollution – that’s a “technosignature”,’ she says. ‘An alien could look at that and think, not only is there life, but technology, too.’
If and when the day comes that humans discover there is life on other worlds, Ferreira has one question.
‘So what? So what if there’s an exoplanet with strong signs of life 400 light-years away?
‘I think humans are gonna have to learn to be a little unsatisfied. We could try to send them a message, but maybe that life is not communicative. Maybe they won’t have any interest.
‘It might very well be an anticlimax of, yeah, there’s aliens there, but that’s it. Even if they do communicate, it’s gonna take 800 years for a two-way conversation – well beyond our lifespan.’
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