Life in the dark

In the dark corners of our galaxy, there are billions of rogue planets roaming around, starless – can they support life?

Sean Raymond

Image courtesy NASA/JPL/Caltech

Sean Raymond

is an American astrophysicist with the Laboratoire d’Astrophysique de Bordeaux in France.

2,000 words

Edited by Ross Andersen

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It’s 10,000 years in the future. You are a space explorer, preparing to land on the surface of a newly discovered world that might support life. This planet is dark, so dark that you can’t identify any of its surface features. All you can see is an ominous black circle blocking the stars. You enter its atmosphere and descend through a thick layer of clouds detectable only by the spaceship’s sensors. There is no light outside your ship. No sunlight. No stars. You turn to your commander, perplexed, and shout: ‘Wait a minute! This planet has no Sun! What the hell are we doing here?’

The Sun gets a lot of good press. Nearly everyone likes sunny days and rainbows. Solar panels are virtuous. Sunlight drives photosynthesis, which produces the oxygen we breathe. Our bodies make such mood-improving substances as vitamin D from sun exposure. Sun worship and solar deities appear throughout recorded history. We love our Sun.

But, does the Sun live up to the hype? Do we really need it? Yes, we do. If the Sun were to suddenly turn off, Earth would freeze over into an ice ball. Our planet’s geological thermostat – the carbonate-silicate cycle – is useless without the Sun. Lakes, rivers and ponds would freeze first. It would take decades but the ocean would eventually freeze solid. Some heat would continue to leak out of Earth’s interior at volcanoes and mid-ocean ridges. Eventually, Earth would look like Hoth, the ice-covered planet from the film The Empire Strikes Back. Most of Earth’s life would vanish.

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Earth was born and grew up with the Sun. It’s not playing fair to just make the Sun disappear. Let’s consider a different type of planet, an Earth that never had a Sun, a ‘rogue’ or ‘free-floating’ planet. These planets don’t orbit stars. They wander the stars. They are free citizens of the galaxy. It might seem like the stuff of science fiction but several free-floating gas giants have been found in recent years. Our own gas giants, Jupiter and Saturn, are leashed to the Sun on well-behaved orbits, but this might not be the norm in our galaxy. One study, published in Nature in 2011, suggests that the Milky Way contains two rogue gas giants for every star. That particular study remains controversial, but most astronomers agree that rogue planets are common in our galactic neighbourhood. And for every rogue gas giant there are likely to be several rogue Earth-sized rocky worlds. There are likely tens to hundreds of billions of these planets in our galaxy.

A free-floating Earth would miss out on many of the things we enjoy on our actual Earth. There would be no seasons or sunsets. And with no Sun to revolve around, no birthdays. But could a rogue planet support life, let alone a vibrant biosphere like Earth’s?

To have any chance of life – at least life like our own – a free-floating Earth would need liquid water. And to have liquid water, a planet needs to keep warm. But space is ridiculously cold, just a few degrees above absolute zero. How could a rogue planet stay warm with no Sun? All planets generate heat in their interiors. Most of Earth’s internal heat was delivered by the giant collisions that built the planet, a large portion of which remains locked inside its crust. This heat slowly trickles to the surface, providing a source of internal energy that has endured since the Earth formed. This interior heat will last for billions of years to come, but it’s a puny amount of energy, 3,000 times smaller than the sunlight that blasts the Earth daily. A free-floating planet with no Sun cannot afford to lose any internal heat. Like a person suffering from hypothermia, a rogue planet needs a really warm blanket.

A layer of ice on a planet’s surface can act as a strong insulator, locking in a planet’s heat. If the ice layer is thick enough, then a planet can maintain an ocean of liquid water beneath the ice. But to prevent the ocean from freezing for billions of years, the ice layer needs to be at least 10 km (6 miles) thick. Two of Jupiter’s large moons – Europa and Ganymede – have oceans lurking under miles-deep ice layers and might be analogs for these icy rogue planets. Could Earth, if frozen, transition into this type of planet and still maintain an abode for life in the deep ocean? Unfortunately, no. Earth is too dry; a global ice layer would only be a few kilometres thick, too thin to act as a strong insulator. At best, Earth could possibly maintain local liquid water – for example, near a strong heat source such as a volcano, but not a global ocean.

A thick atmosphere can also act to retain a planet’s internal heat and allow an ice-free rogue planet to maintain liquid water at its surface. The best atmospheric gas for the job is hydrogen. Hydrogen is a very efficient thermal blanket, and as a bonus it doesn’t condense but rather remains gaseous even at the ridiculously low temperatures of space. A free-floating Earth with a thick hydrogen atmosphere could keep its surface temperature above the freezing point of water. The planet could have lakes and oceans (and possibly life) on its surface. But its atmosphere would need to be at least 10 to 100 times thicker than Earth’s.

Free-floating planets are the popcorn kernels that escape from the pot

Lit only by distant stars, a rogue-blanketed planet would be invisible to the human eye. Like Uranus and Neptune, it would likely have many different cloud layers. No starlight would touch its surface. The atmosphere would be so smothering that if you stood on its surface and looked up at the sky, all you’d see would be darkness.

Free-floating planets probably formed in orbit around stars. Planets grow within disks of gas and dust orbiting young stars. Starting from dust grains, a series of collisions grows ever-larger bodies. Some capture gas and can become giants, like Jupiter and Saturn. Others, like Earth, are smaller and rockier, and take millions of years to reach their final sizes, and settle into well-ordered planetary systems. But these systems are often unstable. Gas giants are so massive that their gravitational kicks can launch planets into interstellar space. Free-floating planets are the popcorn kernels that escape from the pot. Many of them will have thick layers of ice, or hydrogen blankets, and some will have a buddy, if they keep their moons. Tidal heating on an Earth-like rogue planet from interactions with either its moon or its gas giant companion could provide an extra source of internal heat.

Even if it has liquid water, could life exist on a planet without a Sun? Organisms on Earth’s surface don’t all rely on the Sun in the same way. Some forms of life, such as plants and simple microorganisms, are directly dependent on the Sun’s energy. These beings are primary producers. They transform sunlight into chemical energy, directly. Higher orders of life, including animals, rely mostly indirectly on the Sun by eating primary producers. But scientists have found some organisms on Earth that do not need the Sun. They are called chemoautotrophs and they live on the ocean floor. They make their own organic carbon using energy leaking out from inside the Earth. These organisms form the basis of the thriving ecosystems found around deep-sea hydrothermal vents.

Photosynthetic organisms convert light energy into chemical energy. In contrast, chemoautotrophs rely on pre-existing conditions – such as the strong changes in temperature that exist at hydrothermal vents – to drive chemical reactions. Photosynthesis is much more efficient. Its conversion of solar energy into biomass is more than 1,000 times more efficient than the conversion of internal heat done by chemoautotrophs. But that’s on Earth. On a rogue planet with no Sun as an energy source, life would have a strong incentive to more efficiently harvest the planet’s internal heat.

A free-floating planet’s biosphere must be built on chemoautotrophs. Only chemoautotrophs can provide the organic carbon required by other, more complex organisms. But because of their low biological efficiency, a biosphere built on chemoautotrophs would need a large amount of biomass to be productive. Instead of having one ‘photosynthetic’ tree growing 1,000 apples, it would need 1,000 ‘chemoautotrophic’ trees growing one apple each.

Could life arise on a free-floating planet, or would it need to be seeded from somewhere else? We can’t be sure, at least not yet. After all, we still don’t know how life originated on Earth. But there are several theories for the origin of biological metabolism, including the ‘deep sea vent hypothesis’, which propose that hydrothermal vents provided a cradle for Earth’s earliest life, and rogue planets are likely to be littered with hydrothermal vents. So life on free-floating planets could well be homegrown rather than implanted.

What might a ‘dark biosphere’ look like? Consider analogous biological communities on Earth. Earth’s internal heat does not escape uniformly across the planet’s surface. Rather, certain areas are hotter than others. Hydrothermal vents on the ocean floor host vibrant local ecosystems containing an assortment of exotic plants and otherworldly creatures such as two-metre-long tube worms, scaly-foot snails and eyeless shrimps, among others. The producers in these ecosystems are our friends the chemoautotrophs. They form thick bacterial mats that serve as the basic foodstuff of complex food chains. A free-floating planet could be speckled with biospheres, each clustered around a local heat source. They might even host giant plants such as tube worms. Each oasis would start off isolated and probably host its own unique species, but on some rogue planets these ecosystems could merge into a global biosphere.

we could convert a rogue planet into a jumping-off point, a waystation in our larger effort to spread out into the galaxy

We might not want to colonise these worlds, especially the icy rogues. It would be difficult to penetrate a miles-deep ice sheet and even harder to live underneath one. But rogue blanketed planets might prove tantalising. The lakes or oceans on their surfaces are comparatively easy to access. Unfortunately, the planet’s atmosphere would never be breathable for humans, as oxygen and hydrogen cannot happily co-exist. Before setting out for one, we would want to know whether its biomass was easy to access, or restricted to the bottom of deep oceans. But even in the best of circumstances, food production would be difficult given the lack of solar energy, and we don’t have a clue whether the native flora and fauna would be edible. Plus, no rainbows. So rogue planets probably wouldn’t be top priority targets if and when humans start to colonise the galaxy.

But free-floating planets might have the advantage of proximity. The census of faint objects in the Sun’s vicinity is far from complete. A rogue Earthlike planet could be among our closest galactic neighbours, and in that case colonisation could be worth the effort, because we could convert a rogue planet into a jumping-off point, a waystation in our larger effort to spread out into the galaxy.

Imagine being among the first to set foot on a rogue blanketed planet. The photons from your spaceship’s floodlights would represent the first visible light to strike the planet’s surface in billions of years. Your colony would be isolated from the elements, especially from the poisonous atmosphere. Your children would grow up without ever seeing the stars or playing outside (without spacesuits, at least). But as a consolation you’d know that your sacrifices were necessary, for humanity to take its first step toward the stars.

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