A massive "rainbow" detected on an exoplanet

Published by Adrien - Sunday, April 14, 2024 - Other Languages: FR, DE, ES, PT
Source: University of Geneva

New observations from the CHEOPS space telescope point to the existence of a "glory" in the atmosphere of WASP-76b, a luminous phenomenon similar to a rainbow.

The CHEOPS space telescope, with its science operations center located at the University of Geneva (UNIGE), brings new information about the mysterious exoplanet WASP-76b. This ultra-hot giant is marked by an asymmetry in the amount of light observed on its eastern terminator - the imaginary line separating its night side from its day side - compared to its western terminator.


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

This particularity is attributed to a "glory," a luminous phenomenon similar to a rainbow, which occurs if the starlight - the "sun" around which the exoplanet orbits - is reflected by clouds made of a perfectly uniform substance. If this hypothesis is confirmed, this would be the first detection of such a phenomenon outside our solar system.

These findings, conducted in collaboration with the European Space Agency (ESA) and the University of Bern (UNIBE), can be explored in Astronomy & Astrophysics.

WASP-76b is an ultra-hot giant planet. Orbiting around its host star twelve times closer than Mercury orbits around our Sun, it receives over 4000 times the solar radiation Earth does. "The exoplanet is 'inflated' by the intense radiation from its star. Therefore, even though it is 10% less massive than our cousin Jupiter, it is almost twice as large," explains Monika Lendl, assistant professor in the Department of Astronomy at the Faculty of Science of the UNIGE and co-author of the study.

Since its discovery in 2013, astronomers have scrutinized WASP-76b thoroughly. An oddly infernal picture has emerged. One side of the planet always faces its star, reaching temperatures of 4372 degrees Fahrenheit. Elements that would form rocks on Earth melt and evaporate, before condensing on the slightly cooler night side, creating clouds of iron that rain down as iron rain.

The crucial contribution of CHEOPS

One of the most puzzling observations for astronomers is the asymmetry between the planet's two terminators. The terminator is that imaginary line separating the day side from the night side of a planet. For WASP-76b, observations show an increase in the amount of light for the terminator on the east side of the planet compared to the one on the west.

To solve this mystery, astronomers used no less than twenty-three observations with the CHEOPS space telescope, spread over three years. The ESA satellite, led by Switzerland and with its science operations center at the Department of Astronomy of the UNIGE, observed many secondary eclipses of the planet (when it passes behind its star) and several phase curves (continuous observation during a complete orbit of the planet).


Artistic representation of the CHEOPS space telescope.
© ESA / ATG medialab

By combining these new data with those from other telescopes (TESS, Hubble, and Spitzer), astronomers have been able to propose a surprising hypothesis to explain the surplus of light on the eastern side of the planet: "this unexpected glow could be caused by a strong, localized, and anisotropic reflection - that is, direction-dependent - what we call a 'glory'," explains Olivier Demangeon, researcher at the Instituto de Astrofísica e Ciências do Espaço in Portugal and lead author of the study.

A first outside our solar system

Glories are common phenomena on Earth. They have also been observed on Venus. The effect, similar to a rainbow, occurs when light is reflected by clouds made up of a perfectly uniform substance. On Earth, the cloud is made of water droplets, but for WASP-76b, the mystery remains. It might be iron since it has already been detected in the planet's extremely hot atmosphere. The detection of this phenomenon on WASP-76b is a first of its kind outside our solar system.

"If no glory has been observed outside our solar system before, it's because this phenomenon requires very specific conditions. First, atmospheric particles must be almost perfectly spherical, completely uniform, and stable enough to be observed over a long period. The planet's neighboring star must shine directly on it, and the observer - here CHEOPS - must be in the right position," explains Olivier Demangeon.

Results to be confirmed

Further data will be needed to state with certainty that this intriguing surplus of light on the eastern terminator of WASP-76b is indeed a glory. This confirmation would attest to the presence of clouds made of perfectly spherical droplets, existing for at least three years or constantly renewing themselves. For such clouds to persist, the atmosphere's temperature would also need to be stable over time - a fascinating and detailed glimpse into what might be happening on WASP-76b.

The detection of such minute phenomena from such a great distance will enable scientists and engineers to identify other phenomena just as crucial, such as the reflection of stellar light on liquid lakes and oceans - a condition necessary for habitability.
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