There could be hundreds of planets covered in oceans up to 1000km deep, scientists say

'We like to think Earth has a lot of water, but it's nothing on these planets'

More than 20 years ago, Kevin Costner dared to dream of a planet completely covered in water.

His big-budget film, Waterworld, was a flop.

But perhaps it was just ahead of its time, as new modelling suggests there are hundreds of alien exoplanets (or planets outside our solar system) out there that are dominated by water.

External Link: Was the 1995 film Waterworld onto something?

What's the discovery?

Since 1992 we've known of exoplanets orbiting other stars, and pondered what they might look like or offer us here on Earth.

What scientists now believe, thanks to data from the Kepler Space Telescope, is some of these may contain as much as 50 per cent water — well above Earth's 0.02 per cent by mass.

What's more, scientists think as many as a third of all known planets bigger than Earth could fit this "water world" description.

A team of scientists, led by Li Zeng from Harvard University, presented these findings to the Goldschmidt Conference in Boston recently.

They haven't yet been published in a peer-reviewed journal and it's important to note it's theoretical at this point — we haven't directly observed the planets.

Still, Jonathan Horner, professor at the Centre for Astrophysics at the University of Southern Queensland, said it was an exciting development.

"The novelty here is Kepler has revealed that there is a kind of planet that we don't have in the solar system, which is something between the size of Earth and Neptune," he said.

"It looks like what they've done here is modelling of the interior of the planet where you know the mass to some degree and the radius to some degree, and you try to figure out what their composition could be.

"And therefore that's where they've got the idea that they're probably water worlds."

What would these water worlds look like?

We're not really talking about a big rocky core with a layer of ocean on top in the same way the Earth is structured.

Instead, while these planets do have a rocky centre, they tend to be about 2.5 times the radius of Earth and 10 times the mass.

They also look different to Earth, Professor Horner said.

"You've probably got a thick water atmosphere that's a very, very effective greenhouse," Professor Horner said.

"And then eventually when you get far enough down the pressure is high enough that that becomes either liquid or even ice."

While the Kepler telescope has given us some data on these water worlds, Dr Zeng said the TESS (Transiting Exoplanet Survey Satellite) mission would hopefully find more planets closer to us.

"These water worlds likely formed in similar ways to the giant planet cores (Jupiter, Saturn, Uranus, Neptune), which we find in our own solar system," Dr Zeng said in a statement.

"The newly launched TESS mission will find many more of them.

"The next generation space telescope, the James Webb Space Telescope, will hopefully characterise the atmosphere of some of them."

What does this mean for finding life?

Not too much, unfortunately, says Professor Horner.

"You might actually think that, 'Hooray, water worlds are good for life' but they're probably not," he said.

"There are studies from years ago … that say if you've got a deep enough ocean all of the life would be at the bottom of the ocean because that's where the nutrients are.

"So even if there's life there, on a planet with a 1,000km-deep ocean, it would be ridiculously difficult to detect, because it'll all be at the bottom of the ocean."

So it's not to say life couldn't exist on these worlds, but our chances of finding them with current technology is pretty slim.

Could Earth become a water world too?

It's possible, but not through the same process that led to these water giants.

According to astrophysicist Alan Duffy, Earth won't accumulate water in the same way these alien planets have.

He said theoretically it could have gone down this path, but if it was ever going to happen it would have done so back when the solar system was first created.

However, the Earth could one day become completely covered in water through a different process.

"If tectonic activity shut down and new mountains weren't being pushed up, eventually the rain will wear down even the tallest of mountains and essentially grind us down until we end up below sea level," Professor Duffy said.

"So it's not the same water world. We don't know why the Earth hasn't followed the same path, perhaps it wasn't big enough as a rocky core, which meant it couldn't capture all of this water vapor in the early days of the solar system.

"But if it wasn't for tectonic activity we would actually be covered in water. So we have a lot to thank volcanoes for."

So maybe Costner's vision may yet come to pass.

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