Astronomers find an exoplanet atmosphere free of clouds

Thanks to sodium abundance in the ‘hot Saturn’.
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Scientists have detected an exoplanet atmosphere that is free of clouds. Above an artist rendition of ‘hot Saturn’ WASP-96b. A distant observer would see WASP-96b blueish in colour, because sodium would absorb the yellow-orange light from the planet’s full spectrum. 

Scientists have detected an exoplanet atmosphere that is free of clouds. Above an artist rendition of ‘hot Saturn’ WASP-96b. A distant observer would see WASP-96b blueish in colour, because sodium would absorb the yellow-orange light from the planet’s full spectrum. 

Scientists have detected an exoplanet atmosphere that is free of clouds, marking a pivotal breakthrough in the quest for greater understanding of the planets beyond our solar system.

An international team of astronomers, led by Dr Nikolay Nikolov from the University of Exeter in the UK, have found that the atmosphere of the ‘hot Saturn’ WASP-96b is cloud-free. Using the the European Southern Observatory’s Very Large Telescope in Chile, the team studied the atmosphere of WASP-96b when the planet passed in front of its host-star. This enabled the team to measure the decrease of starlight caused by the planet and its atmosphere, and thereby determine the planet’s atmospheric composition.

“We’ve been looking at more than twenty exoplanet transit spectra. 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 characterisation,” Nikolov who is also the lead author of the study said.

Just like an individual’s fingerprints are unique, atoms and molecules have a unique spectral characteristic that can be used to detect their presence in celestial objects. The spectrum of WASP-96b shows the complete fingerprint of sodium, which can only be observed for an atmosphere free of clouds, according to the study published in the journal Nature.

It has long been predicted that sodium exists in the atmospheres of hot gas-giant exoplanets, and in a cloud-free atmosphere it would produce spectra that are similar in shape to the profile of a camping tent.

“Until now, sodium was revealed either as a very narrow peak or found to be completely missing. This is because the characteristic ‘tent-shaped’ profile can only be produced deep in the atmosphere of the planet and for most planet clouds appear to get in the way,” Nikolov added.

Clouds and hazes are known to exist in some of the hottest and coldest solar system planets and exoplanets. The presence or absence of clouds and their ability to block light plays an important role in the overall energy budget of planetary atmospheres.

The sodium signature seen in WASP-96b suggests an atmosphere free of clouds. The observation allowed the team to measure how abundant sodium is in the atmosphere of the planet, finding levels similar to those found in our own Solar System.