Planets with atmospheres of steam, oceans of liquid water and cores of rock surrounded by solid ice may be abundant around distant stars. An analysis of the 4000 or so known exoplanets estimates that around 1400 are water-rich worlds, potentially raising the stakes that some may harbour life.
Although a few individual water world exoplanets have been identified, the new data suggests they may be abundant.
鈥淟ife could develop in certain near-surface layers on these water worlds, if the pressures, temperatures and chemical conditions are appropriate,鈥� says Li Zeng of Harvard University in Cambridge, Massachusetts, who presented his results today in Boston at the Goldschmidt geochemistry conference. On many of them, water accounts for more than half the weight of the planet, compared with just 0.02 per cent on Earth, he says.
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Exoplanet estimate
Zeng and his colleagues worked out the likely compositions of exoplanets by analysing measurements of the radius and mass of each, and modelling how they might have evolved from their host stars.
They relied on a well-established theory of how planets evolve from the gases and material that form discs around newly formed stars. Small rocky planets like Mercury, Venus, Earth and Mars form in the hot, 鈥渢errestrial鈥� zone closest to the star.
Further out from the star, everything changes beyond a 鈥渇rost-line鈥�, where temperatures are low enough for water vapour to condense into ice grains and clump together into icy planets. Some then become enshrouded by huge quantities of gases, mostly hydrogen, and end up as giant planets like Jupiter, Saturn, Uranus and Neptune.
Others 鈥� around 35 per cent of the known exoplanets 鈥� don鈥檛 evolve fast enough and rebound closer their host star to form water worlds. Oceans form from the melting core, with atmospheres of steam billowing from the oceans.
鈥淭hese results point to the existence of dozens of billions of water worlds of a few Earth masses in our galaxy,鈥� says Micha毛l Gillon of the University of Liege in Belgium, who has previously identified individual water world exoplanets.
But he warns that despite the abundance of water, life may struggle to evolve on water worlds. 鈥淭hey could paradoxically be too rich in water,鈥� he says, because the seafloor would be sterile ice, rather than nutrient-rich rocks thought to be necessary for life.
