We haven't actually found any aliens yet - but already it looks like there are way more than we thought.
According to new research, life can survive on planets up to 10 times farther away from their host stars than previously imagined.
Most astronomers currently think that a planet has to be within a star system's "habitable zone" in order to host life. That zone is defined as a point where liquid water can persist without either boiling or freezing.
But new studies show that cold rocky planets could support life underneath their surface, even if the sun is too weak to support life above, according to a paper by astronomers at the University of St Andrews, published in Planetary and Space Science.
Above: Saturn's moon Titan
One of those worlds could even be located within our own solar system - both Titan and Enceladus (moons of Saturn) and Europa (a moon of Jupiter) could potentially host the conditions for life beneath their surface, where it is warmer.
The St Andrews study says that taking these theories into account dramatically increases the possibility of life on other planets by a factor of 10.
"The traditional habitable zone is also known as the Goldilocks zone," explains PhD student Sean McMahon. "A planet needs to be not too close to its sun but also not too far away for liquid water to persist, rather than boiling or freezing, on the surface.
"But that theory fails to take into account life that can exist beneath a planet's surface. As you get deeper below a planet's surface, the temperature increases, and once you get down to a temperature where liquid water can exist – life can exist there too."
The paper adds that even worlds without a host star could host life, if they generate enough internal heat to support liquid water beneath the surface.
They point out that life is possible on Earth up to 10km beneath the surface, and that - for instance - the planet Gliese 581 d (20 light years from Earth) might be able to support liquid water 2km underground.
McMahon urged researchers to widen their search for life based on the findings:
"The results suggest life may occur much more commonly deep within planets and moons than on their surfaces. This means it might be worth looking for signs of life outside conventional habitable zones. I hope people will study the ways in which life below the surface might reveal itself. Because it's not unimaginable that there might be signs at the surface that life exists deep below."
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