Habitable Zone Larger Than Believed
January 8, 2014

Habitable Zone For Exoplanets May Extend 10 Times Farther Than We Thought

[ Watch the Video: Life In The Habitable Zone ]

Lee Rannals for redOrbit.com - Your Universe Online

A new paper published in the journal Planetary and Space Science claims planets that were once thought uninhabitable may actually be able to contain life after all.

University of Aberdeen scientists say that earth-sized planets are able to support life at least ten times farther away from stars than previously thought. This idea challenges the traditional 'habitable zone' theory because it takes into consideration life living deep below the ground. The habitable zone is the estimated distance an exoplanet needs to be from its host star in order to support life.

“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,” PhD student Sean McMahon said in a statement. “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 researchers created a computer model to estimate the temperature below the surface of a planet by using its size and distance from its star. So far, scientists know that the deepest known life on Earth sits at 3.2 miles below the surface, but McMahon points out that there could be more, deeper in places that haven’t yet been drilled.

“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 kilometers below the planet surface,” explained McMahon. “The model shows that liquid water, and as such life, could survive 5km below the Earth's surface even if the Earth was three times further away from the sun than it is just now. If we go deeper, and consider the top 10 km (6-miles) below the Earth’s surface, then the habitable zone for an Earth-like planet is 14 times wider.”

The currently accepted habitable zone model for our solar system extends out as far as Mars, but taking into account the new considerations would mean that this zone would extend out past Jupiter and Saturn.

“Rocky planets a few times larger than the Earth could support liquid water at about 5 km 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,” McMahon said. “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.”

The PhD student said he hopes this study will encourage more scientists to consider how life on other planets could be detected. McMahon also believes that it might be worth looking for signs of life outside conventional habitable zones because there could be signs at the surface that life exists deep below.

“The surfaces of rocky planets and moons that we know of are nothing like Earth. 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,” the researcher concluded.