Exoplanet atmosphere free of clouds discovered

Scientists have detected an exoplanet atmosphere that is free of clouds, a finding that may improve our understanding of the planets beyond our solar system.

Using 8.2m Very Large Telescope in Chile, researchers from the University of Exeter in the UK 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.

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.

WASP-96b is a typical hot gas giant similar to Saturn in mass and exceeding the size of Jupiter by 20 per cent.

The planet periodically transits a Sun-like star 980 light years away in the southern constellation Phoenix, researchers said.

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.

"We have been looking at more than twenty exoplanet transit spectra," said Nikolay Nikolov from the University of Exeter.

"WASP-96b is the only exoplanet that appears to be entirely cloud-free and shows such a clear sodium signature, making the planet a bench-mark for characterisation," said Nikolay, lead author of the study published in the journal Nature.

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, researchers said.

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.

"It is difficult to predict which of these hot atmospheres will have thick clouds," said Jonathan J Fortney, a professor at the University of California, Santa Cruz (UCSC).

"By seeing the full range of possible atmospheres, from very cloudy to nearly cloud-free like WASP-96b, we will gain a better understanding of what these clouds are made of," said Fortney.

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.

"WASP-96b will also provide us with a unique opportunity to determine the abundances of other molecules, such as water, carbon monoxide and carbon dioxide with future observations," said Ernst de Mooij from Dublin City University in Ireland.