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Study: seemingly cool planets could be warm enough to host life underground


The parameters we use to judge whether or not a planet has the potential to host life, have just been smashed. 

A team from the University of Aberdeen and the University of St Andrews created a computer model that took life beneath the Earth’s surface into account in order to reassess how close to the Sun a planet needs to be in order to sustain life. It found that a planet or moon the size of Earth could feasibly host life at least ten times further away from its star than previously thought. The results have been published in the journal of Planetary and Space Science.

Typically, a planet too far from the Sun would be considered too cool to support life and not within the habitable zone. The zone was first defined in the 50s as a kind of Goldilocks region where conditions are not too hot and not too cold, but just right so that water can exist on the surface rather than boil off into the atmosphere or freeze. To calculate habitable zones across the galaxy we take into account a host star’s luminosity, along with the planet’s distance from it and that planet’s size relative to the star. 

The well-established Goldilocks theory, however, fails to take life beneath the surface into account, where temperatures dramatically change.

“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,” PhD student at the University of Aberdeen Sean McMahon said. “The deepest known life on Earth is 5.3km below the surface, but there may well be life even 10km deep in places on Earth that haven’t yet been drilled.”

The computer model was used to estimate what the temperature beneath the surface would be of any given planet it had the necessary parameters for. It found that the habitable zone would be around three times bigger than previously thought if it included the first 5km beneath an Earth-like planet’s surface. When depths of up to 10km below the Earth’s surface were included, the model found the habitable zone was 14 times wider. Applied to our own Solar System, it means the habitable zone extends beyond Saturn. 

“The results suggest life may occur much more commonly deep within planets and moons than on their surfaces.”

“Rocky planets a few times larger than the Earth could support liquid water at about 5km below the surface even in interstellar space (i.e. very far away from a star), even if they have no atmosphere because the larger the planet, the more heat they generate internally.”

A huge number of the exoplanets spotted by the Kepler spacecraft have been quite a bit larger than Earth, so could be ready for reassessment with this new model in mind.

Of course, all the other atmospheric conditions will have to be there in order for a planet to be habitable. One planet widely described this week as “Earth-like” since it has a similar mass to ours, sees surface temperatures reach an estimated and blistering 104C. Further investigation also showed it is 60 times bigger than Earth, which means it’s incredibly light and therefore likely to be a large gas planet that cannot sustain life. The same goes for one planet pointed out by McMahon.

His model suggests that Gliese 581 d, located 20 light years from Earth and suspected as such to be far too cold to support life, could host life 2km beneath its surface — he adds, however, “assuming it is Earth-like”.

Distance from the star is just one of many factors that will adjust the habitable zone. Last year one study reported that one in five Sun-like stars spotted by the Kepler spacecraft have an Earth-size planet in the habitable zone. What this study means, is that we need to potentially cast that already massive net even wider.


A study published in 2011 similarly suggested there could be life lurking below a planet’s surface that we are overlooking, but took a different tact. It looked at whether liquid oceans could survive beneath the surface in planets that have been flung from their solar system after coming too close to gas giants. The team behind the study found a planet with a mass 3.5 times that of Earth’s could be warm enough at its core to host an ocean beneath several kilometres of ice for five billion years.