Mysterious rainbow ’glory’ lights observed on distant planet

For the first time, signs of the rainbow-like ’glory effect’ have been detected on a planet outside our solar system. Glory are colourful concentric rings of light that occur only under peculiar conditions.

Glory occurs when light is reflected off clouds made up of a perfectly uniform, but so far unknown, substance. The effect, similar to a rainbow, forms when light passes between a narrow opening, for example between water droplets in clouds or fog, causing it to diffract and create ring-like patterns.

Data, analysed by astronomers including those at the University of Warwick, suggests this phenomenon is beaming from the hellish atmosphere of ultra-hot gas giant WASP-76b some 637 light-years away.

Observations from the European Space Agency’s Characterising Exoplanet Satellite (CHEOPS) suggests that between the unbearable heat and light of exoplanet WASP-76b’s sunlit face, and the endless night of its dark side, there may be ’glory’.

Seen often on Earth, the effect has only been found once on another planet, Venus. If confirmed, this first glory outside of the solar system will reveal more about the nature of this puzzling exoplanet.

Co-Author Thomas Wilson from the University of Warwick, said: "Never before have we seen these colourful, concentric rings on an extrasolar body. So this first exoplanetary glory, if confirmed with future studies, would make WASP-76b a truly unique body, and give us a beautiful tool for understanding the atmospheres of distance exoplanets and how habitable they could be."

Lead Author Olivier Demangeon from the Institute of Astrophysics and Space Sciences, Portugal, said: "There’s a reason no glory has been seen before outside our solar system - it requires very peculiar conditions. First, you need atmospheric particles that are close-to-perfectly spherical, completely uniform and stable enough to be observed over a long time. The planet’s nearby star needs to shine directly at it, with the observer at just the right orientation."

WASP-76b is an ultra-hot Jupiter-like planet. While has less mass than our striped cousin, it is almost double its size. Tightly orbiting its host star twelve times closer than scorched Mercury orbits our Sun, the exoplanet’s large size is caused by it being ’puffed up’ by intense radiation.

Since its discovery in 2013, WASP-76b’s bizarrely hellish environment has emerged. One side of the planet always faces the Sun, reaching temperatures of 2400 degrees Celsius. Here, elements that would form rocks on Earth melt and evaporate, only to condense on the slightly cooler night side, creating iron clouds that drip molten iron rain.

But scientists have been puzzled by an apparent asymmetry, or wonkiness, in WASP-76b’s ’limbs’ - its outermost regions seen as it passes in front of its host star.

Cheops intensively monitored WASP-76b as it passed in front of and around its Sun-like star. After 23 observations over three years, the data showed a surprising increase in the amount of light coming from the planet’s eastern ’terminator’ - the boundary where night meets day. This allowed scientists to determine the origin of the signal.

Olivier Demangeon added: "This is the first time that such a sharp change has been detected in the brightness of an exoplanet. This discovery leads us to hypothesise that this unexpected glow could be caused by a strong, localised and directionally dependent reflection - the glory effect."

NASA’s James Webb Space Telescope (JWST) could now be used to officially confirm that this is glory, say scientists. Confirmation would imply that the temperature of WASP-76b’s atmosphere must be stable over time, enabling the presence of clouds made up of perfectly spherical water droplets crucial to glory formation.
While the glory effect creates rainbow-like patterns, the two aren’t the same. Rainbows form as sunlight passes through one medium with a certain density to a medium with a different density - for example from air to water - which causes its path to bend (refract). Different wavelengths are bent by different amounts, causing white light to split into its various colours and creating the familiar round arc of a rainbow.

Glory, however, are formed when light passes between a narrow opening, for example between water droplets in clouds or fog. Again, light’s path is bent (in this case diffracted), most often creating concentric rings of colour, with interference between light waves creating patterns of bright and dark rings.
CREDIT: ESA, work performed by ATG under contract for ESA. CC BY-SA 3.0 IGO


CAPTION: Artist’s impression of WASP-76b’s night side - hot, but cool enough for molten iron rain drops to fall.

CREDIT: ESO/Martin Kornmesser


CAPTION: Simulated views of glory on Venus and Earth