Astronomers Discover First Exoplanet Atmosphere Without Any Clouds

Artist's impression of WASP-96b and its parent star. Engine House

It's a lovely day on WASP-96b – if you like temperatures that would melt silver and pressures equivalent to the depths of the oceans. It may be hellish, but at least the planet has no clouds. For astronomers, the discovery isn't about possible tourist locations but a first-of-its-kind opportunity to probe this sort of planet's atmosphere.

In the process, we have found confirmation of a pattern seen in the gas giants of our own Solar System.

WASP-96b is a gas giant with a mass similar to Saturn. It's puffed up by heat from its parent star WASP-96, which makes it 20 percent larger than Jupiter. It also has an average temperature that's 1,000°C (1,830°F) higher than Earth's. What makes it interesting, a paper in Nature notes, is that it frequently passes in front of WASP-96, allowing us to observe what happens when light passes through its atmosphere.

When light passes through a gas it produces an absorption spectrum, where the elements in the gas create a unique signal alerting us to their presence. The spectrum shifts subtly when under pressure, as in the lower part of a dense atmosphere. In theory, transiting planets – those that pass in front of their home star from our perspective – should provide us with the perfect opportunity to analyze their atmospheres, even at this enormous distance. However, clouds usually block any spectra produced deep within the atmosphere.

If our understanding of hot gas giants is correct, they should have plenty of sodium and potassium in their atmospheres, but we haven't been able to confirm this because clouds got in our way. So the identification of WASP-96b's clear skies using Chile's Very Large Telescope was an exciting discovery.

"We've been looking at more than 20 exoplanet transit spectra," said Dr Nikolay Nikolov of the University of Exeter in a statement. "WASP-96b is the only exoplanet that appears to be entirely cloud-free and shows such a clear sodium signature, making the planet a benchmark for characterization.” 

On two transits across WASP-96, Nikolov and his co-authors didn't just manage to determine the presence of sodium, they also saw the distinctive pattern produced by the metal when under a range of intense atmospheric pressures. The measurements suggest we are picking up sodium at a depth of 3,700 kilometers (2,300 miles), enabling us to estimate its abundance. Signs of lithium and potassium were ambiguous, leaving their presence undetermined.

Jupiter, Saturn, Neptune, and Uranus have a pattern where the more massive the planet, the lower the proportion of heavy elements in its outer atmosphere. Allowing for differences between WASP-96 – a star rich in heavy metals – and our Sun, this pattern holds for WASP-96b, suggesting it is a universal feature of gas giants.

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