For years it has been claimed that we on Earth are not alone, but now Aberdeen scientists think there could be dozens more planets supporting life forms than was previously thought.
Research published in Planetary and Space Science challenges the traditional “habitable zone” – the space around a star or sun which can support life – by examining life living deep below the ground.
The work was carried out by a team of academics from Aberdeen and St Andrews universities.
They found that liquid water, which is needed for life to exist, can be found on planets.
Aberdeen Phd student Sean McMahon said: “Traditionally, there is a habitable or Goldilocks zone around a star, but if a planet is too close to a star it will be too hot and if it is too far away it will be too cold for life to exist.
“But planets have their own heat inside and, once you get down to a temperature where liquid water can exist, life can exist there too.”
The team created a computer model that estimates the temperature below the surface of a planet based on its size and distance from its star.
The current habitable zone for our solar system stretches as far as Mars but the revised zone could extend farther than Jupiter and Saturn.
The team’s findings also suggest that many “rogue” planets drifting around in compete darkness could actually be habitable.
Mr McMahon added: “Mars is a good example of a planet that is too cold to have liquid water on the surface but probably does have liquid water below it.
“The deepest known life on Earth is 5.3 kilometres (3.2 miles) below the surface, but there may well be life even 10kilometres (6.2 miles) deep in places that haven’t been drilled yet.
“If we go deeper, and consider the top 10km below the Earth’s surface, then the habitable zone for an Earth-like planet is 14 times wider.”
And although the research brings the human race a step closer to discovering if there really is extraterrestrial life out there, it is unlikely to be invading Earth any time soon.
Mr McMahon said: “We are talking about simple life forms such as bacteria and fungi, not little green men.”