Contrary to popular belief, not all planets orbit stars – some drift freely through the cosmos on their own. These cold, dark worlds don’t make great candidates for hosting life, but a new study suggests that their moons could be more habitable than they might seem.

Since Earth is the only place we know for sure has life, it makes sense to focus the hunt for extraterrestrial life on exoplanets with the most Earth-like conditions. Liquid water and pleasant temperatures top the list, both of which require the planet to orbit its host star at just the right distance.

But what if a planet doesn’t orbit a star at all? So-called rogue planets have been discovered floating untethered to any stars, which may seem to immediately rule them out in the search for aliens. But maybe we’re being too hasty, say astrophysicists in Germany and Chile.

The team created simulations of a Jupiter-sized rogue planet orbited by an Earth-sized moon. The latter body was where they focused their attention, modeling the thermal structure of the atmosphere of this exomoon based on its composition, as well as outside forces from the planet and space beyond.

Surprisingly, the researchers found that conditions there could be comfortable enough to sustain enough water to allow life to thrive. This moon would be far drier than Earth, though – the amount of water would be just one 10,000th of that contained in our oceans, but that’s still 100 times more than can be found in Earth’s atmosphere.

While there may not be a star to drive vital chemical reactions, cosmic rays could fill the role instead. Tidal forces from the planet’s gravitational influence could generate heat, and if the atmosphere is 90 percent carbon dioxide, the greenhouse effect could be strong enough to retain that heat.

That’s an awful lot of “coulds” though, and just because it’s possible doesn’t mean it actually exists anywhere. And even if there is a watery moon orbiting a rogue planet somewhere out there, that doesn’t mean there’s life on it. But the fact that it could (there’s that word again) happen implies that we shouldn’t rule out more extreme environments just because we wouldn’t want to live there ourselves.

After all, even here on Earth life constantly turns up in places long presumed to be inhospitable.

The research was published in the International Journal of Astrobiology.

Source: Ludwig Maximilians-Universität München

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