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The mysterious yellow sky of WASP-79b



Large bright sphere with cloudy bands, with a partially bright white sphere filling the background.

The artist’s concept for WASP-79b, a hot Jupiter-like exoplanet 780 light-years from Earth. New research shows that the planet has a yellow sky instead of blue, as expected. Image by NASA / ESA / L. Hustak (STScI) / Hubblesite.

The earth is beautiful and sometimes we can take it for granted that we can look up and see a deep blue sky. But what about the other planets? The specific sky of the planet depends, of course, on its unique atmosphere. For example, we here on Earth have a blue sky because the blue component of our sunlight is scattered in all directions by air molecules in the Earth̵

7;s atmosphere. Mars, on the other hand, has a pinker sky due to the ever-present dust raised from the surface of Mars by winds. Scientists have now announced the results of a study of a distant exoplanet orbiting a star hotter and brighter than our sun. This planet – called WASP-79b – does not have a blue or pink sky, but instead a yellow one!

The finding was made by researchers combining data from the Hubble Space Telescope, the Transiting Satellite Research Exoplanet (TESS) and the Magellan Terrestrial Telescopes at the Las Campanas Observatory in Chile.

The intriguing results were first published on December 10, 2019 in The Astrophysical Journal and later published in the January 2020 issue of the magazine.

WASP-79b is a large planet for gas giants of the type known as hot Jupiter, which orbits very close to its star and completes its orbit in just 3 1/2 days. Its star, WASP-79, is about 780 light-years away in the constellation Eridanus.

Scientists expected the planet to undergo Rayleigh scattering, where certain colors of light are scattered by very fine dust particles in the upper atmosphere. This makes the Earth’s sky blue because the shorter (blueer) wavelengths of sunlight are scattered.

Diagram: Three partial circles with wavy lines, arrows and text explanations on a dark background.

Diagram of the atmosphere of an exoplanet in relation to the visible light passing through it. Above: A cloudless, diffused atmosphere (above) scatters mostly blue light through Rayleigh scattering, while longer waves such as red light pass through. Below: the cloudy atmosphere equally prevents the passage of all wavelengths of visible light. Medium: if the atmosphere is less extended and cloudless, all visible light passes through the same plane. Image through the National Astronomical Observatory of Japan / JHUAPL.

But that’s not what the researchers found. The lack of Rayleigh distraction is unexpected and “strange,” according to the scientists involved. This may be evidence of currently unknown atmospheric processes on the planet. As a result, the sky on the planet is probably yellow. Researchers have also found that the planet’s atmosphere is humid and hissing 3,000 degrees Fahrenheit (1,648 degrees Celsius). Hot! Its scattered manganese sulfide or silicate clouds can lead to a rain of molten iron.

WASP-79b is definitely a world very different from Earth, not a place you would like to go on vacation. As lead author of the study, Christine Scholter Sotzen of the Johns Hopkins University Laboratory for Applied Physics (JHUAPL) said in a statement:

This is a strong indication of an unknown atmospheric process that we simply do not take into account in our physical models. I have shown the WASP-79b spectrum to a number of colleagues and their consensus is “this is strange”.

Since this is the first time we have seen this, we are not really sure what the reason is. We need to watch for other planets like this, because this may be indicative of unknown atmospheric processes that we do not currently understand. Since we have only one planet as an example, we do not know if this is an atmospheric phenomenon related to the evolution of the planet.

Large sphere, medium sphere with cloud bands and much smaller blue and white sphere on a black background.

Comparison of the dimensions of WASP-79b, Jupiter and Earth. Image via WASP planets.

So how did researchers determine the color of the sky on WASP-79b?

They did it with the help of a spectrograph of the Magellan telescopes. Spectrographs analyze the different wavelengths of light and thus in this case can find clues about the chemical composition of the atmosphere of the exoplanet. It was expected that the atmosphere of WASP-79b would scatter on Rayleigh as in the Earth’s atmosphere, leading to a blue sky. But – surprise – they found the exact opposite. There was less absorption and dissipation in the atmosphere, which means that the planet probably has a yellow sky instead of a blue one. The findings from the Magellan telescopes were also confirmed by TESS.

WASP-79 is now one of the largest stars known to have a planet, making this study even more interesting. So far, most exoplanets have been found around red dwarf orbits, the most common stars in the galaxy, or around stars similar to our sun.

WASP-79b is huge, about 1.7 times the radius of Jupiter. Its deep, elongated atmosphere, because it is so hot, makes it an ideal exoplanet to study with the telescopes used in this study. It is also interesting in other ways.

Two bright glowing orange spheres on a black background, one almost twice the size of the other.

WASP-79 compared to the size of the sun. This is one of the largest stars known to have an exoplanet. Image via WASP planets.

Previous research on the planet by Hubble has shown that it has water vapor in its atmosphere. This in itself is not very surprising, despite the heat, but the discovery may also help scientists better understand what is happening in the exotic atmosphere of WASP-79b.

The latest unusual results will keep scientists busy and should help shed light on how hot Jupiters and other giant planets form and evolve. In another JHUAPL press release, Socen said:

In fact, we are not sure what is happening here. But if similar characteristics are found in other such worlds, it will raise questions about modern theories of atmospheric processes and evolution. We try to look at many different hot Jupiters because they are easier to analyze. What we learn from them will help us predict the atmosphere of other exoplanets, such as whether they have clouds or not.

A smiling young woman with blond hair and a black jacket.

Christine Scholter Sotzen of JHUAPL, lead author of the new study. Image via STARGATE.

Kathleen Mand, a planetary scientist at JHUAPL, also said:

In our own solar system, we do not know how much solid material contributed to the formation of giant planets, how quickly they formed and from what processes, and more importantly how they migrated after formation. As we obtain new results for exoplanets such as WASP-79b, we gather critical information about the formation of giant planets around other stars to better understand the basic processes of planet formation and evolution.

The upcoming James Webb Space Telescope – tentatively scheduled for launch in 2021 – will be able to look even better at WASP-79b and analyze its chemical composition in more detail. This can help solve the mysteries of this intriguing yellow world.

Bottom row: Using three different telescopes, scientists have discovered that a huge, hot exoplanet has a yellow sky instead of a blue one.

Source: WASP-79b transmission spectroscopy from 0.6 to 5.0 μm

Via Hubblesite

Through JHUAPL

Paul Scott Anderson

Paul Scott Anderson has a passion for space exploration that began as a child when he watched Carl Sagan’s Space. While in school, he was known for his passion for space exploration and astronomy. He started his blog The Meridiani Journal in 2005, which was a chronicle of planetary research. In 2015, the blog was renamed Planetaria. Although he is interested in all aspects of space exploration, his main passion is planetary science. In 2011, he began writing about freelance space, and is currently writing for AmericaSpace and Futurism (part of Vocal). He has also written for Universe Today and SpaceFlight Insider, and has also been published in The Mars Quarterly and has done additional writing for the well-known iOS Exoplanet app for iPhone and iPad.


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