There are many words that can be used to describe WASP-76b: hellish, torrid, turbulent, chaotic and even violent. This is an extrasolar planet that is so close to its star that it gets hot enough to vaporize lead. So, as you can imagine, “glorious” wasn’t one of those words until now.
This more positive description was added to the list quite recently, as astronomers have discovered evidence of something called ‘glory’ in the atmosphere of the ultra-hot exoplanet Jupiter. The glory effect, hinted at in data from the European Space Agency’s exoplanet hunting mission Characterizing Exoplanet Satellite (CHEOPS), is a rainbow-like arrangement of colorful, concentric rings of light that only occur under special circumstances.
This effect is often seen above our own planet, as well as in the atmosphere of our violent neighbor Venus, but this is the first time scientists have seen this happen outside our cosmic neighborhood; WASP-76b is 637 light-years away from us.
If the impact is confirmed to occur above WASP-76b, it could reveal a lot about this strange and extreme exoplanet – a world unlike anything we’ve seen in our stellar domain.
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“There is a reason why glory has not yet been seen outside our solar system – it requires very special circumstances,” Olivier Demangeon, team leader and astronomer at the Institute of Astrophysics and Space Sciences in Portugal, said in a statement. ‘First, you need atmospheric particles that are almost perfectly spherical, completely uniform and stable enough to be observed for long periods of time. The planet’s nearby star should shine directly on it, with the observer – here CHEOPS – in exactly the right spot. correct orientation.”
WASP-76b has more to offer than molten iron rain
WASP-76b, discovered in 2013, is just 30 million miles (48 million kilometers) from its yellow parent star, which has about 1.5 times the mass and 1.75 times the width of the Sun. This distance is only one-twelfth of the distance between the Sun and Mercury, the planet closest to our star.
As a result, the planet, which is about 1.8 times the size of Jupiter despite possessing only 92% of the gas giant’s mass, orbits its star in just 1.8 Earth days. This proximity also causes one side of WASP-76b, the ‘day side’, to be tidally locked to its star, WASP-76. The other side of the planet, the ‘night side’, constantly looks into space.
As the dayside of WASP-76b is bombarded by radiation from its parent star, temperatures there rise above 2,400 degrees Celsius. That’s hot enough to evaporate iron. Strong and fast winds on WASP-76b then carry this iron vapor to the cooler night side of the planet, where it condenses into droplets and falls as iron rain.
The hint of the glory effect on this sizzling exoplanet is a remarkable achievement for CHEOPS, which launched in December 2019. It illustrates the mission’s ability to detect subtle, never-before-seen phenomena on distant worlds.
CHEOPS observed WASP-76b nearly 20 times over the course of three years as scientists tried to understand a strange light asymmetry found in the planet’s outer limbs, seen when it crosses or “crosses” the face of its parent star.
These observations revealed an increase in light coming from WASP-76b’s eastern ‘terminator line’, the gap where the exoplanet’s night side becomes the day side. The team concluded that this sharp change in light output is caused by a strong, local and direction-dependent reflection. They call it the glory effect.
“What is important to keep in mind is the incredible scale of what we are witnessing,” Matthew Standing, an ESA Research Fellow who studies exoplanets, said in the statement. ‘WASP-76b is several hundred light years away – an intensely hot gas giant planet likely raining molten iron.
“Despite the chaos, it seems we’ve spotted the potential signs of glory. It’s an incredibly weak signal.”
What does glory mean to WASP-76b?
The glory effect may have a rainbow-like appearance and a colorful striped pattern, but it is actually very different from a literal rainbow.
Rainbows are created when light from the sun passes from a medium of one density to another medium of a different density, usually from air to water. This causes the light’s path to bend or “break”, and different wavelengths are refracted to different degrees. This splits the sun’s white light into consistent colors, creating the well-known orderly and colorful arc of a rainbow.
On the other hand, the glory effect occurs when light passes through a narrow opening. On Earth, for example, this opening could be the space between water droplets in clouds. This causes another form of refraction called ‘diffraction’, which occurs when light passes over an obstacle or through an opening.
When the light waves split and then reunite, where peaks and valleys meet, there is destructive interference. But where a peak meets a peak, there is constructive interference. This results in dark and light bands and concentric color rings, respectively.
So what does glory mean for WASP-76b?
The presence of this phenomenon in the atmosphere of ultra-hot Jupiter indicates the presence of clouds composed of perfectly spherical water droplets that have either lasted for at least three years or clouds that are constantly being replenished.
If the clouds persist, it indicates that the temperature of WASP-76b’s atmosphere, while intimidating, should be stable over time. This is a fascinating insight that points to stability around what has long been considered an endlessly turbulent world.
The results also indicate that exoplanet experts could examine distant worlds for similar light phenomena, including starlight reflected from liquid lakes and oceans. This is something that could be crucial in humanity’s continued search for life beyond the solar system.
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“Further evidence is needed to say conclusively that this intriguing ‘extra light’ is a rare glory,” said Theresa Lüftinger, project scientist for ESA’s upcoming Ariel mission. ‘Follow-up observations from the NIRSPEC instrument aboard the James Webb Space Telescope could do just the job. Or ESA’s upcoming Ariel mission could prove its presence. We could find even more gloriously revealing colors coming from other exoplanets.”
For Demangeon, this potential observation confirms this continued interest in exploring the hellish world of Wasp-76b.
“I was involved in the first detection of asymmetric light coming from this strange planet – and ever since I have been so curious about the cause,” the ESA scientist concluded. “It took some time to get here, with moments where I asked myself, ‘Why do you keep insisting on this? It might be better to do something else with your time.’
“But when this feature emerged from the data, it was such a special feeling; a special satisfaction that doesn’t happen every day.”
The team’s research has been published in the journal Astronomy & Astrophysics.