This distant world is what astronomers call a hot-Saturn—a planet with a mass comparable to our own Saturn (even though this one is 20 percent larger than Jupiter), but with considerably hotter surface temperatures (in this case, temperatures can exceed 1,000 degrees Celsius). The planet periodically transits a sun-like star 980 light years away in the southern constellation Phoenix, halfway between the southern jewels Fomalhaut (α Piscis Austrini) and Achernar (α Eridani). Instead, using the Very Large Telescope in Chile, a research team led by Nikolay Nikolov from the University of Exeter captured the planet’s spectral signature as it passed in front of its host star. Known as the world’s most powerful exoplanet-hunting telescope, the VLT enabled the team to gauge the composition of WASP-96b’s atmosphere as they observed the “hot Saturn” passing in front of its parent star. 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,” said lead author of the study Nikolay Nikolov from University of Exeter in Britain. 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. And it just so happens that the unique signature of WASP-96b contains the complete fingerprint for sodium, a chemical element that produces a tent-shaped spectra and can only be seen in atmospheres without clouds. “Until now, sodium was revealed either as a very narrow peak or found to be completely missing,” said Nikolov, who is a research fellow at the British university. “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.”

Speaking of its importance to our understanding of similar planets, de Mooij of DCU said: “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.” Hot exoplanets, especially those as incandescent as WASP-96b, are typically known to have thick atmospheres, with plenty of clouds and hazes.
Scientists discover that WASP-96b has an atmosphere that is free of clouds.