Unraveling The ‘Rainbow’ Mysteries Of WASP-76 b — An Ultra-Hot Jupiter Exoplanet

In the vast expanse of the cosmos, WASP-76 b has captured the attention of astronomers and planetary scientists alike. This ultra-hot Jupiter, a gas giant orbiting perilously close to its host star, has been the subject of intense scrutiny since the detection of a peculiar “rainbow” asymmetry between its two hemispheres during transit observations. Now, a team of researchers from the University of Geneva has compiled the most comprehensive dataset yet on this enigmatic world, shedding new light on the complex physical processes shaping its atmosphere.

The study, published in the journal Astronomy & Astrophysics, brings together an impressive array of observations from the CHEOPS and TESS space telescopes, as well as archival data from the Spitzer Space Telescope. With three phase curves, 20 occultations, and six transits in the visible light spectrum from CHEOPS, along with 34 phase curves from TESS, researchers have painted a detailed picture of WASP-76 b’s atmospheric dynamics.

The ultra-hot giant exoplanet WASP-76 b, depicted here, is an extremely hot world orbiting very close to its giant star.
The ultra-hot giant exoplanet WASP-76 b, depicted here, is an extremely hot world orbiting very close to its giant star. (Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani)

One of the key findings of the study is the confirmation of the planet’s occultation, or secondary eclipse, when it passes behind its host star. The team measured an occultation depth of 260 ± 11 parts per million (ppm) in the TESS bandpass and 152 ± 10 ppm in the CHEOPS bandpass. These measurements allowed them to estimate the planet’s geometric albedo, a measure of its reflectivity, which ranges from 0.05 to 0.189, depending on the assumed atmospheric composition and the data reduction techniques applied to the infrared observations.

While the infrared phase curves show no detectable asymmetry between the planet’s hemispheres, the visible light phase curves hint at a possible flux excess just before the occultation, intriguing researchers. This excess, with an amplitude of around 40 ppm, an orbital offset of approximately minus-30 degrees, and a width of about 20 degrees, could provide clues to the underlying atmospheric processes responsible for the asymmetry observed in previous high-resolution transit spectroscopy studies.

Researchers also constrained the orbital eccentricity of WASP-76 b to less than 0.0067 with 99.7 percent confidence, ruling out earlier scenarios that invoked a slight eccentricity to explain the observed asymmetry.

So, what could be causing the mysterious flux excess and the asymmetry between WASP-76 b’s hemispheres? The team proposes a fascinating hypothesis: the presence of night-side clouds that extend predominantly towards the planet’s eastern limb. These clouds might contain spherical droplets or aerosols made of an unknown substance, which could give rise to a glory effect — a bright spot or “rainbow” halo caused by backscattering of light — in the visible phase curves.

This hypothesis, while intriguing, is not without its challenges. Clouds on hot Jupiters are typically expected to form on the cooler night side and be carried to the day side by strong atmospheric winds. However, the lack of a clear hot spot offset in the infrared phase curves of WASP-76 b suggests that these winds might not be present, making it unclear why the clouds would preferentially form on one hemisphere over the other.

Each glory is unique, depending on the composition of the planet’s atmosphere and the colors of the light from the star that illuminates it
Each glory is unique, depending on the composition of the planet’s atmosphere and the colors of the light from the star that illuminates it. WASP-76 (the «Sun» of WASP-76b) is a yellow and white main sequence star like our Sun, but different stars create glories with different colors and patterns. (credit: ESA, work performed by ATG under contract for ESA. CC BY-SA 3.0 IGO)

‘‘The reason why no such glory has ever been observed outside our solar system is that this phenomenon requires very specific conditions,” says study lead authorOlivier Demangeon, researcher at the Instituto de Astrofísica e Ciências do Espaço in Portugal, in a media release. “First of all, the atmospheric particles must be almost perfectly spherical, completely uniform and sufficiently stable to be observed throughout a long time. These droplets have to be directly illuminated by the planet’s host star, and the observer — in this case CHEOPS — must be in the right position.”

Despite these challenges, the study represents a significant step forward in our understanding of WASP-76 b and highlights the power of combining observations from multiple instruments and wavelengths to probe the complexities of exoplanetary atmospheres. As astronomers continue to refine their techniques and gather ever more precise data, we can look forward to unraveling the many remaining mysteries of this captivating world and the countless others waiting to be discovered.


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