Exoplanet found to have cloud-free atmosphere

WASP-96b marks first detection of atmospheric sodium long predicted by scientists for hot gas giants.
By Laurel Kornfeld | May 09, 2018
A hot exoplanet of approximately Saturn's mass and a size about 20 percent larger than that of Jupiter has been found to have no clouds in its atmosphere.

WASP-96b, which orbits a Sun-like star 980 light years away in the constellation Phoenix, has a temperature of 1,300 degrees Kelvin.

An international team of astronomers successfully determined the planet's atmospheric composition when it passed in front of its star, allowing them to measure the decrease in starlight caused by the transiting planet and its atmosphere.

Led by Nikolay Nikolov of the University of Exeter, the researchers observed the transit using the 8.2-meter Very Large Telescope (VLT) in Chile.

Each atom and molecule in a planet's atmosphere has a unique spectral signature, which scientists use to confirm their presences. Significantly, WASP-96b shows the signature of sodium, long predicted by scientists to be present in the atmospheres of gas giant exoplanets but never actually detected until now.

Sodium in a planet's atmosphere can be detected only if that atmosphere has no clouds.

"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," Nikolov stated.

"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 planets, clouds appear to get in the way."

Nikolov's "tent-shaped profile" refers to the shape of the spectra produced by sodium in a cloud-free atmosphere.

Hazes and clouds are present in the atmospheres of many of the coldest and hottest planets. Predicting which ones will not have these atmospheric features is extremely difficult.

WASP-96b appears to have sodium levels similar to those found in our solar system. The seventh most common element in the universe, sodium is present in ocean water on Earth and also regulates animals' metabolism and heart activity.

"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 study team member Ernst de Mooij of Dublin City University (DCU).

A paper on the researchers' findings has been published in the journal Nature.



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