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Astronomers Discover Cloud-Free Exoplanet Atmosphere

The atmosphere of the ‘hot Saturn’ WASP-96b is cloud-free according to an international team of astronomers, led by Dr. Nikolay Nikolov from the University of Exeter.

An artist rendition of ‘hot Saturn’ WASP-96b. A distant observer would see WASP-96b bluish in color because sodium would absorb the yellow-orange light from the planet’s full spectrum. (Image credit: Engine House)

With Europe’s 8.2 m Very Large Telescope in Chile, the team investigated the atmosphere of WASP-96b when the planet transited in front of its host star. This enabled the team to calculate the reduction of starlight caused by the planet and its atmosphere, and thus establish the planet’s atmospheric composition.

Similar to the fact that fingerprints of an individual are unique, atoms and molecules have a unique spectral feature that can be used to detect their existence in celestial objects. The spectrum of WASP-96b displays the whole fingerprint of sodium, which can only be seen in an atmosphere free of clouds.

The results have been published in the esteemed research journal Nature on May 7, 2018.

WASP-96b is a standard 1300 K hot gas giant similar to Saturn in mass and greater than the size of Jupiter by 20%. The planet occasionally transits a sun-like star 980 light years away in the southern constellation Phoenix, midway between the southern jewels Achernar (α Eridani) and Fomalhaut (α Piscis Austrini).

It has long been projected that sodium is present in the atmospheres of hot gas-giant exoplanets, and in a cloud-free atmosphere, it would form spectra that are similar in shape to the outline of a camping tent.

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 characterization.

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.

Nikolay Nikolov - Lead Author, University of Exeter

Hazes and clouds are said to be present in some of the coldest and hottest solar system planets and exoplanets. The absence or presence of clouds and their ability to obstruct light plays a key role in the total energy budget of planetary atmospheres.

"It is difficult to predict which of these hot atmospheres will have thick clouds.  By seeing the full range of possible atmospheres, from very cloudy to nearly cloud-free like WASP-96b, we'll gain a better understanding of what these clouds are made of", explains Professor Jonathan J. Fortney, study co-author, based at the Other Worlds Laboratory (OWL) at the University of California, Santa Cruz (UCSC).

The sodium signature observed in WASP-96b indicates an atmosphere free of clouds. The observation enabled the team to measure how copious sodium is in the atmosphere of the planet, finding levels akin to those found in Earth’s 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.

Ernst de Mooij - Co-Author, Dublin City University.

Sodium is the Universe’s seventh most common element. On Earth, sodium compounds such as salt give the white color of salt pans in deserts and seawater its salty taste. In animal life, sodium is known to control metabolism and heart activity. Sodium can also be found in technology, such as in sodium-vapor street lights, where it creates yellow-orange light.

The team is focused on studying the signature of other atmospheric species, such as water, carbon dioxide, and carbon monoxide with the James Webb Space Telescope and the Hubble Telescope as well as telescopes on the ground.

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