There are five ba­sic types of ex­o­planet that we know about so far

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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.

Exoplanets, like those seen in the artist’s im­pres­sions be­low, are plan­ets that ex­ist out­side of our So­lar Sys­tem

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