The hunt for habitable ocean worlds beyond our solar system

Astronomers think that planets covered in water, with oceans hundreds of metres deep, could be relatively common in our galaxy. Now the race is on to find one

Space 24 October 2022

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Luongdoo

IT LOOKS like there is nothing below the thick, hot clouds we are falling through. But descend a little further and the vista opens up to show an ocean as far as the eye can see. Its surface is steaming, feeding water vapour into the sky. Plunging into the waves, it is hot at first, but cools as we travel further into the darkness, the pressure becoming immense. We dive down hundreds of thousands of metres until we reach the bottom where, instead of rock, the sea floor is made of ice. We have landed on a water world.

When astronomers began discovering planets outside our solar system in the 1990s, they realised that alien worlds come in an array of orbits, masses, sizes and compositions. These exoplanets couldn’t be placed in simple rocky or gassy groups like those in our solar system. Some, it has even been suggested, could be 50 per cent rock and a whopping 50 per cent water. Send a spacecraft to a planet like this and the scene it would encounter may be similar to our imagined journey: a thick water vapour atmosphere shielding a vast ocean hundreds of kilometres deep, with an icy mantle below.

It was long thought these water planets would be rare, if they existed at all. But fresh insights have led many astronomers to think they may, in fact, be relatively common – and could be promising places to search for alien life. The race is now on to find one.

Life – as we know it, at least – is inextricably tied to liquid water. Our best guesses about the conditions from …

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