Astronomers have found a naked hot Jupiter steaming up the neighbourhood

Ever wonder what Jupiter would look like naked, without its thick coat of turbulent clouds? An exoplanet 575 light-years away might be the answer to the question you didn't ask.

The exoplanet is a gas giant called WASP-62b, and, according to an analysis of starlight that passed through its atmosphere, the exoplanet is completely starkers. There's not even a whisper of cloud or haze.

Instead, the atmosphere is completely clear - something that is not only almost vanishingly rare in exoplanets, but gives us a unique opportunity to peer inside and see what the atmosphere is made of.

"I'll admit that at first I wasn't too excited about this planet," said astronomer Munazza Alam of the Harvard & Smithsonian Center for Astrophysics, who has been working to characterise the atmospheres of confirmed exoplanets. "But once I started to take a look at the data, I got excited."

When it was discovered in 2012 (and in follow-up studies) WASP-62b seemed to be pretty par for the exoplanet course. It's about 1.4 times the size and 0.57 times the mass of Jupiter, orbiting a young yellow-white dwarf star a little bigger than the Sun, called WASP-62.

It's also, as with most confirmed exoplanets, on a fairly close orbit, although WASP-62b is closer to its star than many exoplanets - its orbital period is just 4.41 days. This means it is extremely hot, with an average temperature of around 1,330 Kelvin, therefore falling into the category of "hot Jupiter".

At those temperatures, WASP-62b is more than likely evaporating, like a puddle on a hot day (but way more slowly).

Because it's relatively close to Earth, and because it passes between us and its star on such a short period, WASP-62b was a good candidate for trying to characterise its atmosphere. That's because when an exoplanet passes in front of its star, the starlight changes, as some wavelengths of the incoming light are absorbed by atoms in the atmosphere.

Different chemical elements absorb different wavelengths, so this spectral information can be used to see what the exoplanet's gaseous envelope is made of.

When Alam and her team took a look at the spectra from three transits of WASP-62b, they found something really unusual: a full set of absorption lines from sodium. In most exoplanet atmospheres, the sodium signature is usually partially or fully obscured by clouds or haze.

The strong sodium signature from WASP-62b also has a very broad base, caused by something called pressure broadening. This could only happen deep in the exoplanet's atmosphere, at higher pressures. For the signal to be emerging so clearly from so deep inside the atmosphere suggests that there are no clouds to prevent its emergence.

"This is smoking gun evidence that we are seeing a clear atmosphere," Alam said.

That makes WASP-62b a very rare beast indeed - less than seven percent of the exoplanets discovered to date are expected to have a clear atmosphere. And only one other has been identified - WASP-96b, a hot Saturn 1,160 light-years away.

Not only is WASP-62b closer and bigger, it is excellently positioned for follow-up study. It falls in a region that can be continuously observed by the James Webb Space Telescope (JWST), called the Continuous Viewing Zone. This makes it the only gas giant exoplanet, and the only exoplanet with a clear atmosphere, in this region. It is therefore an excellent candidate for follow-up observation when the telescope finally begins operations.

So what's inside that atmosphere? According to the data and simulations, the team expect that the JWST would be able to detect water, iron hydride, ammonia, carbon monoxide and dioxide, methane, and silylydine, all in addition to sodium.

Clearly identifying these should reveal both the metallicity and the carbon-to-oxygen ratio inside WASP-62b's atmosphere. The carbon-to-oxygen ratio is particularly exciting - it can be used to understand conditions within the planet-forming disc when the planet was being born, and help us better understand planetary formation processes.

"WASP-62 is the only star in the JWST CVZ with a known transiting giant planet that is bright enough for high-quality atmospheric characterisation via transit spectroscopy. JWST transit programs require many repeated visits, which ideally could be scheduled at any time of the year and executed quickly," the researchers wrote in their paper.

"WASP-62b is therefore one of the most readily accessible targets for atmospheric studies with JWST."

The research has been published in The Astrophysical Journal Letters.

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