When light hits the atmosphere, all sorts of interesting things can happen. Water vapor can split sunlight into a rainbow arc of colors, corpuscular rays can stream through gaps in clouds like light from the sky, and halos and sundogs can be created by sunlight reflecting off ice crystals. And then there's the Glory effect, which can create a colorful, almost holy halo around objects.
Like rainbows, the glories can be seen when facing away from the light source. Because of their similarity, they are often confused with circular rainbows, but glories have a unique effect. Rainbows are created by the refraction of light by water droplets, while glories are created by the wave interference of light. For this reason, a magnificence is most clearly seen when the water droplets of a cloud or nebula are small and uniform in size. The appearance of a glory tells us about the atmosphere. We assumed that some distant exoplanets would experience a similar glow to Earth, but now astronomers have found the first evidence of this.
A splendor of sunshine seen from an airplane. Photo credit: Brocken Inaglory
The observations come from the Characterizing ExOplanet Satellite (Cheops) and other observatories of an exoplanet called WASP-76b. It is not an exoplanet where one would expect a star to appear. WASP-76b is not a temperate, Earth-like planet with a moist atmosphere, but a hellishly hot Jupiter with a surface temperature of about 2,500 Kelvin. For this reason, the team wasn't looking for extraterrestrial glories, but rather studying the strange asymmetry of the planet's atmosphere.
WASP-76b orbits its star at a tenth of Mercury's distance from the Sun. At such a close distance, the world is likely to be tidal, with one side constantly boiling under the heat of the sun and the other side always in shadow. There is no such planet in our solar system, so astronomers are eager to study how this would affect the atmosphere of such a world. Previous studies have shown that the atmosphere is not symmetrical. The side facing the star is bloated by the intense heat, while the atmosphere on the dark side is denser.
For three years, the team observed WASP-76b as it passed in front of and behind its star, collecting data about the intersection between the light and dark sides. They found that there was a surprising increase in light at the planet's eastern terminator (the boundary between the light and dark sides). This additional glow could be caused by a glory effect. More observations will be needed to confirm this effect, but if confirmed, it will be the first shine observed outside our solar system. Currently, glories have only been observed on Earth and Venus.
The presence of a glory on WASP-76b would mean that spherical droplets must have been present in the atmosphere for at least three years. This means that they are either stable in the atmosphere or are constantly being replenished. One possibility is that the glory is caused by iron droplets raining from the sky on the cooler side of the planet. Although this particular effect is not confirmed, the ability of modern telescopes to collect this data suggests that we will soon be able to study many subtle effects of exoplanets' atmospheres.
Reference: Demangeon, ODS, et al. “Asymmetry in the atmosphere of ultra-hot Jupiter WASP-76 b.” Astronomy and Astrophysics 684 (2024): A27.
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