If we find alien life, we're most likely to find it around a rocky, Earth-like planet orbiting the right distance from a Sun-like star, right?
It turns out life could potentially exist on some of the most desolate, dark and unlikely places in the Universe: namely, moons orbiting rogue planets wandering through space.
That's because the lone, rogue planets may be able to keep water on their moons in a liquid form, even without heat from a host star.
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Oceans on rogue moons
Liquid water is considered essential for life, and so it's a key ingredient that astronomers look for when searching for signs of life beyond Earth.
It's why some of the most promising places to find life in our Solar System are icy moons like Europa and Enceladus, which have liquid oceans beneath their frozen crusts.
A team from the Excellence Cluster ORIGINS at the Ludwig-Maximilians-Universität München, Germany and the Max Planck Institute for Extraterrestrial Physics, say moons around free-floating planets could keep their water oceans in liquid form for up to 4.3 billion years.
The science team say a mixture of a dense hydrogen atmosphere and the flexing caused by the planet's gravitational tug on the moon – known as 'tidal heating' – could help keep the water liquid.
What causes free-floating planets?
Not all planets orbit a host star.
Within the chaos of the dusty discs surrounding newborn stars, cosmic debris pulls together to form larger objects and, eventually, planets in orbit around the star.
But in those early stages of planet formation, the discs are chaotic places, with young, still-forming planets colliding and even booting one another out into space.
This can lead to free-floating planets wandering through space without a parent star.
Tidal heating and liquid oceans
Just because a planet has been ejected from its orbit around a star, however, doesn't mean its moon will be ejected from orbit around the planet.
However, astronomers say the orbit of the rogue planet's moon would become highly elliptical, or egg-shaped, causing it to swing out far from the planet then swing in close again.
This pushing and pulling generates tidal forces that squash and squeeze the moon's interior, generating heat through friction.
Astronomers say tidal heating can be enough to maintain oceans of liquid water on the moon's surface, even without the energy of a star.
The need for hydrogen
But generation of heat on a moon via tidal heating is no use if that heat can't be retained at the surface.
Astronomers say carbon dioxide could stabilise conditions for life on moons for up to 1.6 billion years.
But free-floating planets are extremely cold and, they say, carbon dioxide would condense, causing the heat to escape.
The team behind this study looked at how hydrogen-rich atmospheres might be better at trapping heat.
They say hydrogen remains stable even at very low temperatures, and could act like a blanket for keeping the moon's heat from escaping.
Life on Earth
"Our collaboration helped us recognise that the cradle of life does not necessarily require a sun,” says David Dahlbüdding, doctoral researcher at LMU and lead author of the study.
"We discovered a clear connection between these distant moons and the early Earth, where high concentrations of hydrogen through asteroid impacts could have created the conditions for life."
The team say tidal forces could not only supply heat, but also drive processes of chemical development.
As well as this, the periodic squashing and squeezing of the exomoons produces water cycles, where water evaporates and condensese again.
This, they say, is important for the formation of complex molecules and could therefore be crucial for the emergence of life.
The team say there could be as many rogue planets in the Milky Way as there are stars.
That could mean stable habitats for life to develop, even where no host star is present, which in turn could increase the chances of our finding life beyond Earth.
