Can you imagine Jupiter with no clouds or haze visible? It’s not easy, as Jupiter’s latitudinal cloud bands and its large red spot are iconic visual features in our solar system. These characteristics are caused by ascending and descending gas, mainly ammonia. After Saturn’s rings, Jupiter’s cloud shapes are probably the most recognizable feature in the solar system.
Now astronomers at the Center for Astrophysics | Harvard & Smithsonian (CfA) have found a planet with a mass similar to Jupiter, but with a cloud-free atmosphere.
These planets are rare and astronomers believe only about 7% of exoplanets are like that. The discovery enables scientists to study how they form. With no clouds in the way, a clearer view awaits you.
The team of astronomers behind the finding published their findings in The Astrophysical Journal Letters. The title is “Evidence of a Clear Atmosphere for WASP-62b: The Only Known Transit Gas Giant in the JWST Zone for Continuous Viewing”. The study’s lead author is Munazza Alam, a student at the CfA.
WASP-62b is the closest planet to WASP-62, a main sequence star that is nearly 600 light years from Earth. 62b is the only planet in the system. It’s just over half as massive as Jupiter and orbits WASP-62 in about 4.5 days. It’s about 1.4 times the size of Jupiter. It falls exactly into the hot Jupiter category with an average temperature of about 1330 K (1057 C; 1934 F.)
Can you imagine Jupiter without clouds? We can’t either. Photo credit: Hubble / NASA / ESA
The temperature, size and density properties of the planet are not uncommon. What is rare is the cloudlessness of 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 characterizing exoplanets. I take discovered planets and trace them to characterize their atmosphere. “
The WASP name comes from the Wide Angle Search for Planets (WASP) South. 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 her colleagues used Hubble to investigate it more closely. “I admit I wasn’t too excited about this planet at first,” said Alam. “But when I started looking at the data, I was excited.”
With the help of spectroscopy, they observed exactly how the planet moved three times in front of its star, looking for potassium and sodium. As the starlight passed through the planet’s atmosphere, they identified the full spectroscopic signature of sodium, but not potassium. The sodium signature told them the atmosphere was clear.
“This is proof that we are seeing a clear atmosphere,” said Alam.
A screenshot of WASP-62b from NASA’s Eyes on Exoplanets website. Photo credit: NASA
In an email exchange with Universe Today, Alam explained the team’s spectroscopic findings and what they mean.
The focus on potassium and sodium is based on a couple of things. First, their spectra are easy to observe in optical light. “Sodium and potassium are two types that are easily seen when observing the exoplanet’s atmosphere at optical wavelengths, and their presence or absence can help us infer whether there are clouds or opacities in an exoplanet’s atmosphere,” Alam said.
Sodium and potassium also play a role in exoplanet atmospheres, although the details are not clear. “Sodium and potassium are two elements that play interesting – but not well-understood – roles in the atmospheric physics and chemistry of exoplanets,” Alam said. She also mentioned that sodium was the first absorption feature identified in an exoplanet’s atmosphere.
Evidence of sodium’s full spectroscopic signature shows astronomers that the atmosphere is clear even when there is no way to see the atmosphere. “Clouds in a planet’s atmosphere will mask or obscure parts of the absorption line,” Alam explained. “In the absence of clouds, we can resolve the full sodium signature, which has a tent-like shape with a tip at the core of the absorption feature and broad line wings. For our observations of WASP-62b, this is the second time we have observed the full sodium feature (i.e., with its line wings) in an exoplanet, and the first time we have done so from space. “
This illustration from the study shows the Hubble Space Telescope Imaging Spectrograph data for WASP-62b and the only other known exoplanet with a clear atmosphere, WASP-96b. Both exoplanets show the “… distinctive pressure-broadened wings of the Na-D lines at 0.59 μm”. Seeing the sodium spectrum with wings showing that both planets have clear atmospheres. WASP-96b also shows the presence of lithium and potassium. Photo credit: Alam et al., 2021.
However, the full sodium signature not only tells us that the exoplanet’s atmosphere is cloud-free. It can help explain how much sodium there is and what other elements are in the atmosphere.
“Not only does it tell us that the atmosphere is clear, it can also help us limit really accurate amounts (amounts) of sodium – as well as other elements that are present in the planet’s atmosphere,” Alam said. “These frequencies are useful for measuring key quantities that can help us trace the origins and evolution of this planet.”
There is clearly something else going on when a cloud-free planet forms. Since there are so few of them, astronomers are only just beginning their studies. The only other cloud-free exoplanet we know is hot Saturn named WASP-96b, which was found in 2018.
It will be up to the James Webb Space Telescope to study the atmosphere of this exoplanet more closely. And the clear sky makes this view even more exciting. Because of the Webb’s advanced observation capabilities, it should be able to identify even more chemical constituents in the WASP-62b’s atmosphere.
“In preparation for JWST, it is important to identify targets that are cloud-free / haze-free in order to mobilize community efforts to observe the best planets for detailed atmospheric tracking.”
From “Evidence of Clear Atmosphere for WASP-62b: The Only Known Transit Gas Giant in the JWST Zone for Continuous Viewing”.
Because of the orientation and position of JWST in space, it has two small continuous viewing zones (CVZ). They are centered on each pole of the ecliptic. Fortune smiles at Alam and other exoplanet scientists because WASP-62b is in one of Webb’s CVZs.
James Webb’s field of view contains two continuous viewing zones, which are indicated by ovals in the picture. The rest of the JWST’s field of view drifts through the sky over time. Fortunately, WASP-62b is in one of JWST’s CVZs. Photo credit: NASA / JWST.
The research team even predicted what the JWST might find in the atmosphere of 62b. In their work they write: “We predict that JWST observations from WASP-62b in the context of the ERS program Na (12.1?), H2O (35.6?), FeH (22.5?), SiH (6.3?) Can finally prove. NH 3 (11.1 µm), CO (8.1 µm), CO2 (9.7 µm) and CH 4 (3.6 α). “They also say the JWST can provide precise restrictions on the abundance of chemicals in the atmosphere.
As part of their work and to warrant the follow-up with the Webb, the team predicted what the Webb might find. Photo credit: Alam et al., 2021.
In their conclusion, the authors advocate follow-up observations of WASP-62b with the JWST.
“In preparation for JWST, it is important to identify targets that are cloud-free / haze-free in order to mobilize community efforts to observe the best planets for detailed atmospheric tracking. Although alternative targets have since been suggested, WASP-62 is the only star in JWST CVZ with a known traversing giant planet bright enough to allow high quality atmospheric characterization using transit spectroscopy. “
The James Webb Space Telescope is scheduled to launch in late October 2021.
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