‘Rogue planets’, which orbit galaxies without a parent sun, could harbour life beneath their surface, a scientist claims - and extraterrestrial lifeforms could be more common than we have thought.
The ‘free floating’ planets, not bound to any star, are common in our galaxy - with one study suggesting that there are 100,000 times as many ‘rogue’ planets in the Milky Way than the galaxy’s 300 billion stars.
“You don't necessarily need a sun for the maintenance of life. Life can survive on a rogue planet if it is far enough below the surface and the planet is generating enough heat,” says Sean McMahon of the University of Aberdeen.
“Life doesn't need oxygen gas, sunlight or organic food. It can survive at high temperatures and pressures, miles below the Earth's surface, feeding off chemical reactions between water, minerals and carbon dioxide. That's probably a common environment elsewhere.”
Life might be more common in the universe beneath the surface of planets than on the surface, the researcher says - and could even still lurk beneath the surface on Mars.
[Size of universe measured with 1% accuracy]
“In a rocky "rogue planet" producing 10 times as much heat as the Earth, there could be liquid water less than four miles below the surface. The Earth itself would produce that much heat if it were a few times bigger, so it's not unrealistic.”
Extraterrestrials may in fact be more common under the surface of planets, he says.
“If these habitable environments are commonly inhabited, then deep biospheres may even be more common in the universe than surface biospheres,” McMahon says.
McMahon’s paper caused a storm of discussion online - ‘rogue planets’ and subsurface life being a favourite topic for the internet’s chattering classes.
“I've been amused by some of the discussion threads on the internet,” he says. “The fact is that nobody has all the answers. Our first job is to figure out what to look for. That's what we're trying to do. Yes, there are lots of possibilities we haven't considered yet. But we have to start somewhere.”
In 2012, the discovery of a ‘rogue planet’ 100 light years from Earth sparked furious discussion on internet message boards relating to the supposed Mayan apocalypse - with believers claiming that Earth’s doom would come at the hands of a mysterious planet Nibiru, or Planet X.
“What if Nibiru is an intelligently controlled planet inhabited by ETs?” asked one conspiracy website. “Maybe they can control the path of the planet and even travel through wormholes and instantaneously move from one point of the universe to another.”
Planet X, of course, failed to show up for its ‘date’ - but McMahon says that real extraterrestrial life could be much more common than we have thought, both on rogue planets, and on planets further out from stars.
“For an Earth-like planet in our solar system, the habitable zone for the top 10 km reaches further out than Saturn,” he says. “For planets that generate more heat - for example, a larger version of the Earth - or for greater depths below the surface, these distances increase.”
Projects such as NASA’s Kepler mission have detected hundreds of planets, but the search for life has focused on those where liquid water might be found on the surface - in the so-called ‘Goldilocks zone’.
McMahon suggests that this focus may ‘miss out’ planets that could harbour life - even if the surface lacks water.
“Water is fundamental to life as we know it and very common in the universe,” McMahon says. “Maybe other liquids could do the job in different circumstances, but it makes sense to focus on liquid water since it is so abundant and we know for sure that it works.”
If they're in the Goldilocks zone, Earth-like planets receive the right amount of energy to keep water in the liquid state on the surface (or in our extension, below the surface). It's a very useful tool for picking out planets where life might potentially survive.We are absolutely not dismissing the traditional idea of Goldilocks zones, as some have reported. We're just illustrating mathematically one of the ways it could be extended, which we think could be important.”
The problem may be finding such life forms, McMahon says - and that may require a greater understanding of the origins of life. Mars, for instance, had a very different atmosphere at one point, so life may have ‘retreated’ beneath the surface.
On other worlds, life may have started, and remained, underground.
“As for the conditions for the origin of life, that's a different question. There are some compelling theories, but in my opinion, we don't know enough yet about the origin of life to say with confidence what kind of environmental conditions it needs,” he says. “If life can only originate on the surface of a planet, then obviously the subsurface can only be inhabited following inoculation from the surface. Maybe that's what happened to Mars, which was probably habitable at the surface in the past.”
Telescopes may be able to detect traces of life that once existed - but if life began beneath the surface, then detection might be difficult, McMahon says.
“If life is restricted to the subsurface of a planet, we presumably won't be able to see it directly from a distance unless its remains make it to the surface in some kind of icy residue. We might still be able to detect mixtures of gases that result from biology, but it will be hard to rule out non-biological explanations.”
“The way I see it, if planets with subsurface life are much more common than planets with surface life, this might balance out the greater difficulty of identifying the former, so that we stand at least an equal chance of finding a planet in either category.”