Astronomers Have Spotted a Weirdo ‘Jupiter’ With a Four-Day Year

Astronomers Have Spotted a Weirdo ‘Jupiter’ With a Four-Day Year
A new study describes a cloudless, Jupiter-like exoplanet. (Illustration: M. Weiss/Centre for Astrophysics | Harvard & Smithsonian)

Nearly 600 light-years from Earth, the exoplanet known as WASP-62b whips around its host star at a breakneck pace. The planet is a hot Jupiter, and despite its gassy constitution, its atmosphere is completely cloudless, according to a study published this month in the Astrophysical Journal Letters.

WASP-62b was first detected in 2012 in a sweep by the Wide Angle Search for Planets South survey (hence the acronym in its name). The survey detects exoplanets by spotting them as they pass in front of their host stars, causing a dip in the brightness of the star’s shine.

“We can’t actually see these planets directly. It’s like looking at a firefly next to a streetlamp,” Munazza Alam, an astronomer at the Harvard-Smithsonian Centre for Astrophysics and lead author of the recent paper, said in a phone call. “We’re gleaning all this information about the planet’s atmosphere from what we call combined light observations, meaning we’re looking at the light from both the star and the planet.”

Hot Jupiters are a class of exoplanets, named because they are gas giants (like our local Jupiter) that orbit close to their host stars and thus are quite hot. They stand among super-Earths, mini-Neptunes, and a slew of other classifications that seek to describe exoplanets based on their archetypes in our local solar system. As a result of a hot Jupiter’s proximity to its host star, the exoplanets have extremely short orbital periods. If WASP-62b’s orbit began on a Monday morning for Earth, its year would be over before you clocked out for the weekend.

Within the Milky Way, Alam said, hot Jupiters are rarer than smaller planets, and among exoplanets, it’s more common to find cloudy atmospheres. That makes this hot Jupiter a bit of an oddball.

The team looked at spectroscopic data gathered by the Hubble Space Telescope that focused on quantities of potassium and sodium in the atmosphere. None of the former turned up, but sodium was detected in “whopping” amounts, Alam said, suggesting that the atmosphere of WASP-62b was clear at the pressures detected by Hubble. The results make the planet the first hot Jupiter with a cloud-free atmosphere and only the second exoplanet with such a clear atmosphere after a hot Saturn (WASP-96b) detected in 2018. Both planets have that significant sodium content, which appears in a tent-like peak in the data, that make for a cloud-free gas giant.

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Down the line, the team aims to probe different atmospheric layers of the hot Jupiter that are not detectable by Hubble. Future observations of the exoplanet will be done with the upcoming James Webb Space Telescope, which will be able to see in near-infrared.

“Kepler showed us that there are thousands of planets out there, and TESS is doing that as well in different parts of the sky,” Alam said. “We found thousands of smaller planets, which is really changing the demographics of the planet population as we knew it.”