Astronomers may have detected a rare atmospheric phenomenon in the skies of a distant exoplanet. New research analyzed atmospheric signals from a superhot gas giant, WASP-76 b. The researchers suggested that a peculiar difference between the planet’s morning and evening horizons could be caused by a circular rainbowlike phenomenon called the “glory effect.”
If confirmed, this would be the first detection of a glory on an exoplanet and could help unveil the chemical composition of WASP-76 b’s atmosphere.
Glory on the Horizon
Glories are common on Earth. They happen when light passes through a narrow gap between atmospheric droplets, which bend its path. The resulting effect looks like a circular rainbow with concentric rings of color.
“There’s a reason no glory has been seen before outside our solar system—it requires very peculiar conditions,” lead researcher Olivier Demangeon, an astronomer at the Instituto de Astrofísica e Ciências do Espaço in Portugal, said in a statement.
“First, you need atmospheric particles that are close-to-perfectly spherical, completely uniform and stable enough to be observed over a long time,” he said. On Earth, those are fine spherical water droplets, such as those in fog. Then, “the planet’s nearby star needs to shine directly at it, with the observer…at just the right orientation.” The first glory beyond Earth was seen on Venus in 2014 and was generated by sulfuric acid particles rather than by water.
WASP-76 b is about as unlike Earth as an exoplanet can be. It is a gas giant planet orbiting so close to its host star that its atmosphere is overinflated from the additional heat, or a so-called ultrahot Jupiter super-puff. Past observations from the Hubble and Spitzer space telescopes show that the planet has a permanently illuminated dayside and permanently shadowed nightside, as well as complex atmospheric chemistry in its superhot skies, some of which condenses into metallic rain on the nightside.
What’s strange about the planet is that its morning (western) and evening (eastern) horizons block differing amounts of light as the planet passes in front of its star. The Hubble and Spitzer space telescopes measured that asymmetry in several visible-light wavelengths, but the effect disappears in infrared light. Astronomers have suggested that different quantities or varieties of iron compounds in the morning and evening skies, nightside iron rain, or clouds could cause the asymmetry. Global atmospheric circulation models have not yet confirmed these theories.
Demangeon and his team sought to explore this mystery using more recent observations of WASP-76 b. Their team observed the planet’s passage in front of and behind its star several times from 2020 to 2022 using the European Space Agency’s (ESA) Characterising Exoplanet Satellite (CHEOPS). They also gathered archival data from NASA’s Transiting Exoplanet Survey Satellite (TESS) from 2020 to 2021 and older Hubble (2015–2017) and Spitzer (2016–2018) observations.
“This is the first time that such a sharp change has been detected in the brightness of an exoplanet.”
These visible-light and infrared data allowed the team to explore how the combined light from the hot planet and the star changed along the planet’s entire orbit. When combined with sophisticated computer modeling, this type of observation, called a phase curve, can give a holistic picture of how a planet’s atmosphere behaves.
All told, the researchers re-created 34 phase curves for WASP-76 b. Their analysis revealed that the planet’s atmosphere glinted ever so slightly just before the planet passed behind the star.
“This is the first time that such a sharp change has been detected in the brightness of an exoplanet,” Demangeon said in the statement.
The faint, unexpected glint of extra light could explain the horizon asymmetry and point to the presence of clouds on the evening, or eastern, horizon. “This discovery leads us to hypothesize that this unexpected glow could be caused by a strong, localized, and anisotropic (directionally dependent) reflection—the glory effect,” Demangeon said.
The researchers published these results in Astronomy and Astrophysics.
Glorious Skies Ahead?
Each glory is unique, depending on the composition of a planet’s atmosphere, the size of droplets suspended in that atmosphere, and the colors of starlight that shine on it. WASP-76 b’s star is different from our Sun, and its atmosphere is wildly different than both Earth’s and Venus’s. Up close, its glory would be unlike anything seen before.
“I don’t know if what they’re seeing is due to a glory, but the possibility of having detected such a rare phenomenon in an exoplanet atmosphere is intriguing to say the least,” said Kevin Stevenson, an exoplanet astronomer at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., who was not involved with the research.
“We could even find more gloriously revealing colors shining from other exoplanets.”
“It’s curious that the effect would appear on the planet’s eastern hemisphere,” Stevenson added. “Models often predict the presence of clouds on the western hemisphere, so there would have to be an unusual mechanism to have iron rain on the eastern hemisphere.”
The researchers hope that future observations could help explain why the planet might be cloudy in the east rather than the west and better constrain the atmospheric chemistry and circulation, all of which are key ingredients for possibly generating an exoplanet glory.
“Further proof is needed to say conclusively that this intriguing ‘extra light’ is a rare glory,” Theresa Rank-Lueftinger, an astrophysicist at ESA who was not involved with this research, said in a statement. Additional observations from the James Webb Space Telescope or the upcoming ESA Ariel mission, designed to characterize exoplanet atmospheres, could help confirm this glory.
“We could even find more gloriously revealing colors shining from other exoplanets,” Rank-Lueftinger added.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer