Astronomers find a planet like Jupiter, but it has no clouds

Can you imagine Jupiter without any observable clouds or haze? This is not easy since Jupiter’s latitudinal cloud bands and its large red spot are iconic visual features of our solar system. These characteristics are caused by rising and falling gas, mainly ammonia. After the rings of Saturn, the cloudy forms of Jupiter are probably the most recognizable feature in the solar system.

Now astronomers with the Center for Astrophysics | Harvard & Smithsonian (CfA) found a planet similar in mass to Jupiter, but with a cloudless atmosphere.

These planets are rare and astronomers believe that only about 7% of exoplanets are like this. The discovery allows scientists to study how they are formed. With no clouds on the way, a clearer view awaits.

The team of astronomers behind this discovery published their results in The Astrophysical Journal Letters. The title is “Evidence of Clear Atmosphere for WASP-62b: The Only Gaseous Giant in Transit Known in the JWST Continuous Viewing Area.” The lead author of the study is CfA graduate student Munazza Alam.

WASP-62b is the closest planet to WASP-62, a main sequence star nearly 600 light years from Earth. 62b is the only planet in the system. It’s barely half the massive size of Jupiter and orbiting WASP-62 in about 4.5 days. It is about 1.4 times the size of Jupiter. It falls squarely into the category of hot Jupiters, with an average temperature of around 1330 K (1057 C; 1934 F.)

The temperature, size and density properties of the planet are not uncommon. What is rare is the absence of clouds from its atmosphere. And the atmosphere of the exoplanet is of particular interest to lead author Alam. In a press release, Alam said, “For my thesis, I worked on the characterization of exoplanets. I take discovered planets, and I follow them to characterize their atmospheres.

The name WASP comes from the Wide Angle Search for Southern Planets (WASP). The planet was first discovered in 2012 and was one of seven hot Jupiters found at the same time.

WASP-62b was discovered with WASP, but Alam and his colleagues used the Hubble to study it more closely. “I admit that at first I was not too excited about this planet,” Alam said. “But once I started taking a look at the data, I got excited.”

Using spectroscopy, they closely observed the planet passing in front of its star on three occasions, looking for potassium and sodium. As starlight passed through the planet’s atmosphere, they identified the full spectroscopic signature for sodium, but not potassium. The sodium signature told them the atmosphere was clear.

“It’s smoking proof that we see a clear atmosphere,” Alam said.

In an email exchange with Universe Today, Alam explained the team’s spectroscopic results and their significance.

The emphasis on potassium and sodium is based on several factors. First of all, their spectra are easily observable in optical light. “Sodium and potassium are two species that are readily observable in atmospheric observations of exoplanets taken at optical wavelengths, and their presence or absence can help us infer whether there are clouds. or mists in the atmosphere of an exoplanet, ”Alam said.

Sodium and potassium also play a role in the atmospheres of exoplanets, although the details are unclear. “Sodium and potassium are two elements that play an interesting – but not well understood – role in atmospheric physics and the chemistry of exoplanets,” Alam explained. She also mentioned that sodium was the first absorption characteristic identified in the atmosphere of an exoplanet.

Detecting the full spectroscopic signature of sodium tells astronomers that the atmosphere is clear, even though there is no way to see the atmosphere. “Clouds in a planet’s atmosphere will mask or obscure parts of the absorption line,” Alam said. “In the absence of clouds, we can solve the complete sodium signature – which is tent-shaped with a peak at the heart of the absorption function and wide wings. For our observations of WASP-62b, this is the second time that we have observed the full sodium function (i.e. with its linear wings) in an exoplanet and the first time that we have done so from space.

But the full sodium signature does more than tell us that the exoplanet’s atmosphere is cloud-free. This can help explain how much sodium is present and indicate what other elements are in the atmosphere.

“Not only does this tell us that the atmosphere is clear, but it can also help us limit really precise abundances (amounts) of sodium – as well as other elements that are present in the planet’s atmosphere,” said Alam said. “These abundances are useful for measuring key quantities that can help us trace the origins and evolution of this planet.”

Clearly, something different is happening when a cloudless planet forms. Since there are so few, astronomers are only just beginning their study. The only other cloudless exoplanet we know of is the hot Saturn named WASP-96b, found in 2018.

It will be up to the James Webb Space Telescope to take a closer look at the atmosphere of this exoplanet. And its clear sky makes the prospect even more exciting. Webb’s advanced observational capabilities mean it should be able to identify even more chemical constituents in the atmosphere of WASP-62b.

“In preparation for JWST, identifying cloudless / haze-free targets is important to mobilize community efforts to observe the best planets for detailed atmospheric monitoring.”

From “Evidence of a Clear Atmosphere for WASP-62b: The Only Gaseous Giant in Transit Known in the JWST Continuous Viewing Area.”

Due to the orientation and position of JWST in space, it will have two small Continuous Viewing Areas (CVZs). They are centered on each pole of the ecliptic. Fortune smiles on Alam and other scientists on exoplanets because WASP-62b is in one of Webb’s CVZs.

The team of researchers even predicted what JWST might find in 62b’s atmosphere. In their paper, they write: “We predict that JWST observations of WASP-62b, under the ERS program, can conclusively detect Na (12.1?), H₂O (35.6?), FeH (22.5?), SiH (6.3?), NH₃ (11.1?), CO (8.1?), CO₂ (9.7?) and CH₄ (3.6?). They also say that the JWST can offer precise constraints on the abundance of chemicals in the atmosphere.

In their conclusion, the authors argue for follow-up observations of WASP-62b with the JWST.

“In preparation for JWST, identifying cloudless / haze-free targets is important to mobilize community efforts to observe the best planets for detailed atmospheric monitoring. Although alternative targets have since been proposed, WASP-62 is the only star in JWST CVZ with a known transit giant planet that is bright enough for high-quality atmospheric characterization by transit spectroscopy.

The launch of the James Webb Space Telescope is scheduled for late October 2021.

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