by There are many words that could be used to describe WASP-76b – hellish, scorching, turbulent, chaotic, and even violent. This is an extrasolar planet located so close to its star that it becomes hot enough to vaporize lead. So, as you can imagine, until now, “glorious” was not one of those words.
This more positive descriptor was recently added to the list, as astronomers have detected hints of something called “glow” in the atmosphere of the ultra-hot exoplanet Jupiter. The halo effect, revealed in data from the European Space Agency’s exoplanet-hunting mission Characterizing an Exoplanet Satellite (CHEOPS), is a rainbow-like arrangement of concentric rings of colorful light that only occurs under strange conditions.
This effect is often seen beyond our own planet, as well as the atmosphere of our violent neighbor Venus, but this is the first time scientists have seen it occur outside our cosmic neighborhood; WASP-76b is located 637 light years away.
If the effect is confirmed to be occurring beyond WASP-76b, it could reveal a lot about this strange and large exoplanet – life unlike anything seen in our stellar domain.
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“There’s a reason that no glory has been seen before outside our Solar System – it requires very specific conditions,” 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 close to perfectly spherical, completely uniform and stable enough to be noticed over a long time. The star of the nearby planet must shine directly on it, with the observer – here CHEOPS – at just the right orientation.”
WASP-76b is more than a shower of molten iron
Discovered in 2013, WASP-76b is located just 30 million miles from its yellow parent star, which is about 1.5 times the mass and 1.75 times the width of the Sun. This distance is only 12th of the distance between the sun and Mercury, the closest planet to our star.
As a result, the planet, which is about 1.8 times larger than Jupiter despite having only 92% of the gas giant’s mass, whips around its star in just 1.8 Earth days. This proximity also means that one side of WASP-76b, the “day,” is tidally locked to face its star, WASP-76. The other side of the planet, the “night side,” faces eternally out into space.
As radiation from the dayside host star WASP-76b erupts, the temperature then rises above 4,350 degrees Fahrenheit (2,400 degrees Celsius). That’s hot enough to vaporize iron. Strong, fast winds on WASP-76b then carry this iron vapor to the night side of the planet, where it condenses into droplets and falls as iron rain.
A great achievement for CHEOPS, which was launched in December 2019, is the hint of the effect of glory on this blistering exoplanet. It demonstrates the mission’s ability to detect never-before-seen calm phenomena on a distant world.
CHEOPS observed WASP-76b nearly two dozen times over a three-year period as scientists tried to understand a strange light asymmetry found in the planet’s outer limbs, seen when it crosses, or “transits,” the face of its parent star.
These observations showed an increase in light coming from WASP-76b’s “eastern terminal line,” the separation where the exoplanet’s night side becomes dayside. The team concluded that this sharp change in light output is due to strong, local and direction-dependent reflection. They call it the glory effect.
“What’s important to keep in mind is the incredible scale of what we’re seeing,” Matthew Standing, an ESA Research Fellow who studies exoplanets, said in the statement. “Hundreds of light years away is WASP-76b – a gas giant planet that is extremely hot and likely raining molten iron.
“Despite the chaos, it looks like we’ve found the signs of potential glory. It’s a very faint sign.”
What does the glory mean for WASP-76b?
The glory effect may look like a rainbow and a colorful striped pattern, but it is quite different from a literal rainbow.
A rainbow is formed when sunlight passes from a medium of one density to another medium of a different density, usually from air to water. This causes the path of light to bend, or “refract,” and different wavelengths are refracted to different degrees. Therefore, the white light of the sun is divided into consistent colors, leaving the familiar ordered and colorful arc of the rainbow.
On the other hand, the glow effect occurs when light passes through a narrow gap. On Earth, this gap could be the space between water droplets in clouds, for example. This creates a different form of refraction, called “diffraction,” which occurs when light passes through a barrier or aperture.
As the light waves split and then come together again, when peaks collide with troughs, destructive interference occurs. But, when a peak hits a peak, there is constructive interference. This results in dark and light bands, respectively, and concentric rings of color.
So what does the glory mean for WASP-76b?
The presence of this phenomenon in Jupiter’s ultra-hot atmosphere indicates the presence of clouds composed of perfectly spherical water droplets that have lasted for at least three years or clouds that are constantly being replenished.
If the clouds are persistent, this indicates that the temperature of WASP-76b’s atmosphere, although alarming, must be stable over time. This is a wonderful insight that suggests a sense of stability around a world that has long been thought to be a world of endless restlessness.
The findings also suggest that exoplanet experts could probe distant worlds for similar light phenomena, including starlight reflecting off liquid lakes and oceans. This is something that could be crucial in humanity’s ongoing search for life outside the solar system.
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“Further proof is needed to say definitively that this interesting ‘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 the job. Or ESA’s Ariel mission could prove it.
For Demangeon, this possible observation validates this ongoing interest in investigating the underworld of Wasp-76b.
“I was involved in the first detection of asymmetric light from this strange planet – and since then, I have been so curious about the reason,” said the ESA scientist. “It took some time to get here, with moments when I asked myself, ‘Why are you insisting on this? Maybe it would be better to do something else with your time.’
“But when this feature came out of the details, it was such a special feeling – a special pleasure that doesn’t happen every day.”
The team’s research has been published in the journal Astronomy & Astrophysics.
