THIS may come as a shock. There are planets out there that are like nothing we have seen before. We know about rocky worlds like our own. We know about planets that are huge balls of gas. But now there is a new kid on the planetary block.
“We call them ‘ocean planets’ – giant worlds with oceans hundreds of times bigger than our own,” says Christophe Sotin of the University of Nantes in France. Ocean planets are more than wild speculation. They are an unavoidable conclusion from astronomical observations.
No one has seen one yet, but what astronomers observing some of the 100-plus known extrasolar planetary systems have found indicates that ocean planets are almost certainly out there. What’s more, Sotin and his colleagues believe they might be the best place to search for extraterrestrial life.
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All this has come about because of another recent surprise for astronomers. They have discovered “gas giant” planets – balls of rock and ice wrapped, like Jupiter, in a thick mantle of gas – orbiting incredibly close to their parent stars. In the most extreme case, a huge planet called 51 Pegasi B orbits eight times closer to its parent star than Mercury orbits our sun. And the conditions that led to this bizarre situation could also have created ocean planets.
Gas giants like 51 Pegasi B are surprising because they could not have formed so close to a star. The heat and the stellar wind there would have vaporised ice grains and dispersed gas long before they could congeal into a planet. Our own gas giants – Jupiter, Saturn, Uranus and Neptune – all formed in the cool outer regions of our solar system. So how come 51 Pegasi B is breaking all the rules?
The answer, planetary physicists believe, lies in the planet’s formative years. A solar system is born from a spherical nebula of gas and dust that collapses under its own gravity. Shrinking more easily along its spin axis than in any other direction, it forms a “protoplanetary” disc of material, with the central concentration heating up enough to become a star and the outer regions clumping into planets. Physicists believe that, like Jupiter and Saturn, 51 Pegasi B formed far out from its star. But the gravitational pull from the remnants of the protoplanetary disc then braked its motion, causing it to spiral inward over a period of about 10 million years.
No one is sure why this didn’t happen in our solar system. Possibly our protoplanetary disc was sparser and got gobbled up more quickly by embryonic planets. Once the material in the disc is used up, the planets stop spiralling in and their orbits become fixed. But longer-range planetary migration could take place around stars with denser protoplanetary discs.
Wet and wild
And if gas giants can spiral in towards their stars, others might do the same. “There is no reason why less massive outer planets should not migrate too,” says Sotin. “That raises the tantalising possibility of ocean planets.”
Because our planet detection techniques are still fairly insensitive, we have so far spotted only so-called hot jupiters – gas giants that have migrated far enough inward to have a visible gravitational effect on their star. But Sotin believes that smaller ice-shrouded planets could also have migrated towards their stars. And that means some will have reached a point where the outer ice crust has melted, forming vast oceans.
It has taken Sotin a while to come round to the idea, though. Ocean planets were first suggested by David Stevenson of the California Institute of Technology, a man not afraid of bold ideas – earlier this year he published a proposal in Nature for an experiment to tunnel to the Earth’s core. In 1998 he speculated that an icy, rocky planet could be kicked out of a developing solar system into the frozen wastes of interstellar space – but not before it gathered an atmosphere. Even though such a planet would be far from the warmth of a star, Stevenson claimed that its atmosphere could lock in enough residual heat to melt the ice and turn it into oceans. This made him speculate further about what types of planet could form around stars in the first place, and he came up with the idea of planets completely cloaked in water.
Sotin grew interested when his colleagues started speculating about the depth of the oceans that would form if Stevenson was right. He is an expert on astronomical ice, having modelled the high-pressure ices of Jupiter’s moon Callisto. Sotin’s colleagues were talking about ocean depths of thousands of kilometres, but he knew this was impossible. His calculations showed that ice would simply not melt under the crushing pressures that exist at such depths: there is a limit to how deep liquid water can be. So he set out to calculate what the real answer might be.
Though not thousands of kilometres, it is still pretty impressive. Sotin has now worked out that ocean planets could have waters more than 100 kilometres deep – 10 times the depth of the deepest ocean trench on Earth and 40 times the average depth of our oceans. With the ocean covering the whole surface, we are therefore talking about an astronomical volume of water. “By comparison, the Earth’s ocean is a mere puddle,” he says.
Of course, the depth of any planetary ocean would depend on a complex mixture of factors. One is the planet’s size. The biggest ocean planets could be 10 times the mass of Earth – but no bigger. The gravitational pull of any planet bigger than this means it would hold onto a shroud of hydrogen and helium, which would turn it into a gas giant. So Sotin’s team has done its calculations for ocean planets six to eight times the mass of Earth. These would have a radius about twice Earth’s, and a surface gravity about 50 per cent higher (see Diagram).
Another factor is the planet’s surface temperature. This depends on the internal radioactivity which keeps a planet’s interior warm long after the fires of its birth have died out, its proximity to its parent star and the composition of its atmosphere, which may contain heat-trapping greenhouse gases like carbon dioxide and water vapour.
Since the atmospheric details and the precise internal composition of such a planet are not well known, Sotin has calculated the ocean depth for various scenarios. If the surface temperature is 7 °C, the ocean will be 72 kilometres deep and the seabed temperature and pressure 35 °C and 11,000 atmospheres respectively – though even at this temperature, the enormous pressure of water above will ensure a layer of ice on the ocean bed. For a higher surface temperature, the ocean will be deeper, and vice versa. For instance, a surface temperature of 30 °C leads to a depth of 133 kilometres (arxiv.org/astro-ph/0308324).
With the ocean covering an area almost six times that of Earth’s oceans, we could therefore be talking about a volume of water 240 times greater than Earth’s. So what would it be like on such a water world?
For a start, there could be no dry land. Even if the ice layer on the ocean bed were thin enough to allow rock to poke through in places – and the depth of the ice layer depends on how much ice the planet had when it formed – a mass of rock poking above the surface of a 100-kilometre-deep ocean would in effect be a 100-kilometre-high mountain. Even Earth’s gravity prevents the formation of mountains bigger than Mauna Kea in Hawaii. Though only 4245 metres above sea level, the mountain’s summit is 10 kilometres from the floor of the Pacific Ocean which makes it taller than Mount Everest. And on an ocean planet with stronger gravity, not to mention a huge weight of water bearing down, nothing even as high as Mauna Kea could exist.
And that means that ocean circulation would be simpler than on Earth, though still dependent on the spin rate and inclination of the planet. It would also allow the build-up of enormous swells. “The circulation might resemble the South Pacific with huge rolling waves,” says Sotin. Hurricanes would persist longer than on Earth, so the weather could be a lot more severe. On Earth, hurricanes form when air circulates around a region of low pressure: water vapour drawn up into the sky condenses, releasing its latent heat, and it is this that powers the hurricane. Consequently, the hurricane dies when it hits land. Since ocean planets have no land, they might spawn “super hurricanes” that just keep on going, according to Alain Léger of the University of Paris South, the leader of Sotin’s team.
Hunting watery worlds
Léger, Sotin and Stevenson are not the only ones who believe in ocean planets. Sean Raymond and his colleagues at the University of Washington in Seattle also predict their existence – although the watery worlds they envisage are born much closer to their stars. Although this didn’t happen in our solar system, Raymond’s results show that it could happen elsewhere. His group made the discovery by simulating random collisions between icy rocks in protoplanetary discs around 42 sun-like stars. In a few rare cases, they found planets with oceans containing 100 times more water than those on Earth. However, these were much smaller than Sotin’s watery worlds, with a mass between a quarter and four times that of Earth. This would make them much harder to spot.
So what would these ocean planets look like? Earth shines brightly in space because of its water and clouds. One of Sotin’s ocean worlds, so much bigger than Earth and totally covered in water, would be as brilliant as a polished mirror floating in space. Not that we would actually be able to see this from Earth: so far, no extrasolar planet has ever been imaged directly. All our discoveries have come from indirect evidence, such as the light from a star being blocked out during “transit”, as a planet’s orbit takes it in front of its star. And the planets found this way have been about the size of Jupiter, too big to be ocean planets.
Several upcoming space missions might be able to detect ocean planets in this way. The French orbiting telescope COROT, due for launch in 2005, will look for the dimming of stars when planets pass in front of them and should be able to detect planets as small as five times the mass of Earth. The transit method will also be used by the Eddington and Kepler missions planned for the next few years.
An orbiting infrared telescope like the one that flew on the Infrared Space Observatory could go further and detect water on a planet if the planet was illuminated from behind by its star. The “bites” taken out of the infrared part of the stellar spectrum by water vapour in the planet’s atmosphere would be visible from the telescope’s vantage point beyond Earth’s own atmosphere. Best of all, we may eventually image such planets directly: NASA’s Terrestrial Planet Finder – an orbiting 8-metre visible-light telescope – might just about be able to see Earth-mass planets in the next decade.
Of course, it has not escaped anyone’s notice that these planets might provide the perfect breeding ground for life. “It seems plausible that these giant Earths, with their giant oceans, are the one of the best places in the galaxy for life to arise,” says Paul Shuch, director of the SETI League in New Jersey.
On Earth, many scientists believe life got started on the seabed at “black smokers”, volcanic vents that spew minerals into the ocean at high temperature. “This could also happen on an ocean world, but with one proviso,” says Sotin. “The layer of ice at the bottom of the ocean must be thin or else silicate material could never come into contact with water.” Judging whether the conditions for life are right on any particular ocean planet we find would be impossible without knowing the depth of the ice layer, which depends on how much ice the planet was born with.
There is another possibility: an ocean planet could be “seeded” by micro-organisms that have arisen elsewhere and been carried to the planet inside meteorites or comets. This “panspermia” theory has been championed for many years by the late Fred Hoyle and Chandra Wickramasinghe at Cardiff University in the UK. Once life got going, its spread throughout the ocean world would depend on there being enough nutrients, which could come from volcanism or meteorite impacts. “The main problem is the origin of life, because once it got going its survival would not be that different from Earth,” says Léger.
So, might the telescopes reveal signs of life? Perhaps – if we are lucky. The researchers think we would have to see signs of ozone to be sure. “The only way to have a significant amount of ozone in the atmospheric spectrum is if oxygen is produced at low altitude – that is, by biological photosynthesis,” says Léger. The telltale signature of ozone, like that of water vapour, could be read when an ocean planet is in transit, with its star illuminating its atmosphere from behind.
It’s a tough call, but not impossible. One day soon we might just spot aquatic ET’s home: a raging sea buffeted by everlasting hurricanes on a distant planet. It’s enough to make you grateful that you’re living on the dry land of planet Earth.