Astronomers have discovered an exoplanet that has a clear atmosphere with no clouds, only the second-ever exoplanet like this discovered. It's also the first-ever cloudless “hot Jupiter” discovered, giving astronomy an incredible opportunity to study these unusual worlds in more detail.

The findings, published in The Astrophysical Journal Letter, report the observations of WASP-62b by the Hubble Space Telescope. First discovered in 2012, the planet is located 575 light-years from Earth and is a "hot Jupiter", a gas giant that orbits very close to its star, making it very hot. It has a mass of just half our Jupiter or about 180 Earths.

The researchers followed the planet for three orbits around its sun, which are relatively quick – just 4.5 days compared to our Jupiter's 12 years. As the planet passes in front of the star, the starlight is filtered through the planet's atmosphere, which is how astronomers discover what chemical elements are present around a planet.

"For my thesis, I have been working on exoplanet characterization. I take discovered planets and I follow up on them to characterize their atmospheres," lead author Munazza Alam, a graduate researcher at the Center for Astrophysics, said in a statement. "I'll admit that at first I wasn’t too excited about this planet. But once I started to take a look at the data, I got excited."

The researchers were looking for potassium and sodium in the planet's atmosphere. They did not find potassium, but what was incredible was how clear the chemical signature for sodium was. "This is smoking-gun evidence that we are seeing a clear atmosphere," Alam said.

The finding is very important for several reasons. Cloud-free planets beyond the Solar System are a rarity. It's possible to study the atmosphere of only a small fraction of all the discovered exoplanets, so finding a few that are cloudless is an incredible discovery.

There is only one other known cloudless planet currently, WASP-96b, discovered in 2018. It has a mass similar to Saturn but orbits very close to its star so is puffed up from the heat to 20 percent larger than Jupiter. Based on these two exoplanet examples, astronomers estimate that about 7 percent of all exoplanets in the galaxy have a clear atmosphere.

Without the hurdle of clouds or haze, we can study these worlds in more detail. We can work out what they are made of and learn how they came to be, which may be different from other planets with clouds. Upcoming instruments such as the James Webb Space Telescope will help with these observations, studying the atmospheres of this and many more worlds.