Exoplanet likely to be a waterworld

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Like Earth in Kevin Costner’s 1995 epic Waterworld, HAT-P-26b contains a surprising amount of water.
Universal Pictures

Data from the Hubble and Spitzer space telescopes has enabled scientists to complete a detailed atmospheric study of a Neptune-sized exoplanet about 437 light years from Earth.

The study, led by Hannah Wakeford of NASA’s Goddard Space Flight Center in Maryland, affirms the exoplanet – HAT-P-26b, first discovered in 2010 – shows clear signs having water and clouds. 

Using the abundance of water calculated in the atmosphere as a proxy, the team have also been able to estimate the atmospheric prevalence of heavy elements – that is, gases heavier than hydrogen and helium – with the lower-than-expected result indicating the planet’s atmosphere is still primordial, unaffected by planetesimal debris.

Neptune-sized worlds are the most common planets that astronomers have identified in the galaxy, the space scientists note in their research, published in Science. While thousands of exoplanets have been discovered to date, so far little is still known about their atmospheres, especially for bodies smaller than Jupiter. 

Speculated possibilities include atmospheres rich in hydrogen and helium, in carbon dioxide, or in water, depending on their formation and evolutionary history. 

Using data from the Hubble telescope’s Imaging Spectrograph and Wide Field Camera as well as Spitzer’s Infrared Array Camera, the research team derived the transmission spectrum of HAT-P-26b, with the measurements best fitting a profile pointing to distinct water absorption and uniform cloud opacity.

The content of heavy elements in the atmosphere of HAT-P-26b – referred to as its metallicity – appears lower than the trend observed with the four giant planets of our Solar System. This suggests, the researchers say, that the exoplanet is the result of different formation and/or evolutionary processes.

With the composition of a planet’s atmosphere able to provide valuable clues as to how it formed, the research team are hopeful their findings will contribute to better understanding of how atmospheric composition varies between exoplanets with different masses, and narrow down hypothetical models of planet formation.

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