20th August 2018

Water worlds may be common in our galaxy

Research presented at the Goldschmidt conference in Boston concludes that water is likely to be a common feature of exoplanets between two and four times the size of Earth, with implications for the search of life in our galaxy.



Credit: ESO/M. Kornmesser

The 1992 discovery of exoplanets orbiting other stars has sparked interest in understanding the composition of these planets to determine, among other goals, whether they are suitable for the development of life. Now a new evaluation of data from the exoplanet-hunting Kepler Space Telescope and the Gaia mission indicates that many of the known planets may contain as much as 50% water. This is far more than the Earth's 0.02% (by weight).

"It was a huge surprise to realise that there must be so many water worlds," said lead researcher Dr Li Zeng, from Harvard University.

Scientists have found that many of the 4,000 confirmed or candidate exoplanets fall into two size categories: those with a planetary radius averaging around 1.5 that of Earth, and those averaging around 2.5 times the radius of Earth. Now a group of international scientists, after analysing the exoplanets with mass measurements and recent radius measurements from the Gaia satellite, have developed a model of their internal structure.

"We have looked at how mass relates to radius, and developed a model which might explain the relationship," said Li Zeng. "The model indicates that those exoplanets which have a radius of around x1.5 Earth radius tend to be rocky planets (of typically x5 the mass of the Earth), while those with a radius of x2.5 Earth radius (with a mass around x10 that of the Earth) are probably water worlds."



Credit: NASA

"This is water – but not as commonly found here on Earth," said Li Zeng. "Their surface temperature is expected to be in the 200 to 500°C (392 to 592°F) range. Their surface may be shrouded in a water-vapour-dominated atmosphere, with a liquid water layer underneath. Moving deeper, one would expect to find this water transforms into high-pressure ices before reaching the solid rocky core. The beauty of the model is that it explains just how composition relates to the known facts about these planets."

Li Zeng continued: "Our data indicate that about 35% of all known exoplanets which are bigger than Earth should be water-rich. 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. The newly-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb Space Telescope, will hopefully characterise the atmosphere of some of them. This is an exciting time for those interested in these remote worlds."

"It's amazing to think that the enigmatic intermediate-size exoplanets could be water worlds with vast amounts of water," said Sara Seager, Professor of Planetary Science at the Massachusetts Institute of Technology, and deputy science director of the recently-launched TESS. "Hopefully atmosphere observations in the future – of thick steam atmospheres – can support or refute the new findings."

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