Medium-sized exoplanets may be mostly made of water

Interstellar explorers of the future might want to pack a pair of fins and a snorkel, because a new analysis of data from the Kepler Space Telescope and ESA’s Gaia mission suggests that many exoplanets may be mostly made of water.

To date, astronomers have confirmed the existence of 3,815 exoplanets, which can be broken down into five types: ‘hot Jupiters’; cold gas giants; water worlds; rocky planets; and lava worlds. Until recently, exoplanets measuring 1.5x or 2.5x the radius of Earth were thought to be rocky planets, while larger exoplanets at least 4x the radius of Earth identified as water worlds. But now, research led by Harvard University’s Dr Li Zeng suggests that exoplanets with 2.5x Earth’s radius may actually be up to 50 per cent water, compared to Earth’s 0.02 per cent.

“Our data indicated that about 35 per cent of all known exoplanets which are bigger than Earth should be water-rich,” said Zeng. “This is an exciting time for those interested in those remotes.”

Water worlds can be found in our own Solar System. Beyond Neptune there are several dwarf planets and satellites composed mostly of ice clustered around a rocky core. The composition of exoplanetary water worlds, however, is thought to be somewhat different: they’re likely to have atmospheres of steam, with a thin layer of water underneath and surface temperatures of up to 500°C.

It’s hoped that NASA’s recently launched TESS mission will discover more such planets, and that the James Webb Space Telescope, due to launch in 2021, will reveal more about their composition and atmospheres.

This is an extract from issue 327 of BBC Focus magazine.

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