Scientists may not have found extra-terrestrial life yet, but they now claim there are far more aliens out there than first believed.
Researchers in Scotland believe that Earth-sized planets can support life at least ten times further away from stars than previously thought.
This means that cold rocky planets previously considered uninhabitable may be teeming with life beneath the surface.
Academics at the University of Aberdeen and University of St Andrews claim the definition of the ‘Goldilocks’ zone – the area of space around a star, or sun, which can support life – is flawed.
They argue this definition 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,’ said Aberdeen University PhD student Sean McMahon.
In a galaxy of 200 billion stars of various types, scientists estimate there are 11 billion potentially habitable planets in the Goldilocks zone as it is currently defined.
Factor smaller, dimmer red dwarf stars, and the number of planets with mild temperatures similar to Earth rises to 40 billion.
Researchers in Aberdeen claim the figure is much higher. Mr McMahon told MailOnline the number was ‘probably several times’ larger but added that the is difficult to quantify.
Mr McMahon’s team created a computer model that estimates the temperature below the surface of a planet of a given size, at a given distance from its star.
‘The deepest known life on Earth is 5.3 km below the surface, but there may well be life even 10 km deep in places on Earth that haven’t yet been drilled,’ he said.
‘Using our computer model we discovered that the habitable zone for an Earth-like planet orbiting a sun-like star is about three times bigger if we include the top five kilometres below the planet surface.’
The model shows that liquid water could survive 5km below the Earth’s surface even if the Earth was three times further away from the sun.
‘If we go deeper, and consider the top 10 km below the Earth’s surface, then the habitable zone for an Earth-like planet is 14 times wider,’ said Mr McMahon.
The current habitable zone for our solar system extends out as far as Mars, but this re-drawn habitable zone would see the zone extend out further than Jupiter and Saturn.
The findings also suggest that many of the so-called ‘rogue’ planets drifting around in complete darkness could be harbouring life..
‘Rocky planets a few times larger than the Earth could support liquid water at about 5 km below the surface even in interstellar space, even if they have no atmosphere because the larger the planet, the more heat they generate internally,’ said Mr McMahon.
‘It has been suggested that the planet Gliese 581 d, which is 20 light years away from Earth in the constellation Libra, may be too cold for liquid water at the surface.
‘However, our model suggests that it is very likely to be able to support liquid water less than 2 km below the surface, assuming it is Earth-like.’
Another paper published by the same team suggests there could be more life below the surface of the continents on Earth than there is below the seafloor.
Mr McMahon hopes the studies will encourage other researchers to consider how life on other planets might be detected.
‘The surfaces of rocky planets and moons that we know of are nothing like Earth,’ he said.
‘They’re typically cold and barren with no atmosphere or a very thin or even corrosive atmosphere.
‘Going below the surface protects you from a whole host of unpleasant conditions on the surface. So the subsurface habitable zone may turn out to be very important.
‘Earth might even be unusual in having life on the surface.’