Astronomers discover exoplanet where rain is definitely not a problem

7 May 2018

A blue planet, which is how WASP-96b would appear to a distant observer. Image: Makstorm/Shutterstock

Out in the cold, dark cosmos, a team of astronomers has discovered an exoplanet that is completely cloud-free.

The Kepler Space Telescope has contributed a great deal to astronomy, revealing thousands of strange exoplanets against the backdrop of the blackness of space.

And, sometimes, it helps astronomers find a particular planet that, to those of us on Earth, sounds truly alien.

The latest one to join that list is a ‘hot Saturn’ dubbed WASP-96b, analysed in detail by a team of international astronomers led by Dr Nikolay Nikolov from the University of Exeter and including Dr Ernst de Mooij from Dublin City University’s School of Physical Sciences.

An atmosphere’s fingerprint

Publishing their findings to Nature, the team revealed that a unique set of circumstances mean that WASP-96b has an atmosphere completely free of clouds.

Using 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 allowed them to measure the decrease of starlight caused by the planet and its atmosphere, thereby determining the planet’s atmospheric composition.

In the same way that a person’s fingerprints can reveal their identity, atoms and molecules can be identified through spectral analysis. In the case of WASP-96b, the results showed sodium.

This was important in determining the planet’s chemical composition because sodium can only be detected on a cloud-free planet.

Artist's drawing of WASP-96b

An artist’s interpretation of ‘hot Saturn’ WASP-96b. A distant observer would see WASP-96b as blueish in colour, because sodium would absorb the yellow-orange light from the planet’s full spectrum. Image: Engine House VFX

Bigger than Jupiter

WASP-96b has a sweltering average temperature of more than 1,000C and a mass similar to Saturn, but it is about 20pc larger than Jupiter.

It was the only planet out of 20 analysed during the study to show a cloud-free atmosphere.

“Until now, sodium was revealed either as a very narrow peak or found to be completely missing,” Nikolov said.

“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.”

Colm Gorey was a senior journalist with Silicon Republic