Rainbow-like "glory effect" seen on distant planet for the first time

Glory have been seen on a planet outside the solar system for the first time, but what are they and why do they occur?

Rainbow-like ‘glory effect’ seen on distant planet
Artist impression of glory on exoplanet WASP-76b. Each glory is unique, depending on the composition of the planet's atmosphere and the colours in the starlight that shines onto it. WASP-76 is a yellow-white main sequence star like our own, but different stars create glory with differing colours and patterns. Credit: ESA.

Colourful concentric circles of light reminiscent of rainbows have been detected on a planet outside our solar system for the first time.

This ‘glory effect’ occurs only under certain conditions – when light is reflected off clouds consisting of a perfectly uniform, but currently unknown, substance. The effect forms when light passes a narrow opening, between water droplets in clouds of fog, for example, causing the light to diffract or bend, and create ring-like patterns.

Truly unique

Astronomers believe the phenomenon originates in the atmosphere of the ultra-hot gas giant WASP-76b, which sits around 637 light years away. The ultra-hot Jupiter-like planet has less mass than Jupiter, but is almost double its size as it is "puffed up" by intense radiation.

Observations taken by European Space Agency’s Characterising Exoplanet Satellite (CHEOPS) suggests that the ‘glory’ may occur between the extreme heat and light of the exoplanet’s sunlit face, and the infinite night of its dark side.

The ‘glory effect’ occurs often on Earth, but has only ever been seen once on another planet – Venus; if confirmed, it would reveal more about the nature of this mysterious planet.

“Never before have we seen these colourful, concentric rings on an extrasolar body,” explains Thomas Wilson from the University of Warwick, co-author on the Astronomy & Astrophysics study. “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.”

Peculiar conditions

“There's a reason no glory has been seen before outside our solar system – it requires very peculiar conditions,” said lead author Olivier Demangeon from the Institute of Astrophysics and Space Sciences, Portugal. “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, fist discovered in 2013, as a uniquely ‘hellish environment’. One side of the planet constantly faces the sun and reaches a temperature of 2,400°C – which would melt and evaporate any elements that would form rocks on Earth. The other side is cooler and perpetually dark; here these elements would condense, creating iron clouds that drip molten iron rain.

Rainbow-like ‘glory effect’ seen on distant planet
Simulated views of the glory phenomena on Venus (left) and Earth (right). Glories occur when sunlight shines on cloud droplets – water particles in the case of Earth, sulphuric acid particles for Venus. The main difference between the appearance of the glory on Venus and on Earth is because particle size; droplets on Venus are typically much small than on Earth. Because of this, the coloured fringes are further apart than they would appear on Earth. Credit: ESA/C. Wilson/P. Laven

The apparent asymmetry has puzzled scientists. Observations taken by Cheops over three years suggest a surprising increase in the amount of light emitted from the planet’s eastern terminator, the boundary where night meets day, allowing scientists to determine the signal’s origin.

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

Official confirmation of the glory could come from NASA’s James Webb Space Telescope (JWST), say scientists. This confirmation would suggest 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.

Glory vs rainbows

Although the glory effect creates rainbow-like patterns, they aren’t the same. Rainbows form when sunlight passes though mediums of a different density – from air to water. This bends or refracts the light, with different wavelengths bent by different amounts, hence white light being split into various colours and creating the familiar round arc of a rainbow.

When a glory is formed, light passes between a narrow opening, for example between fog, or water droplets in clouds. The light’s path is diffracted, often creating concentric rings of colour, with interference between light waves creating patterns of bright and dark rings.

News reference:

O.D.S. Demangeon et al, (2024) Asymmetry in the upper atmosphere of the ultra-hot Jupiter WASP-76 b, Astronomy & Astrophysics.