Astronomers using the European Space Agency’s CHEOPS Space Telescope say that the presence of a phenomenon similar to a rainbow known as a ‘glory’ may explain the “disturbing light asymmetry” coming from the two opposing sides of the distant exoplanet WASP-76b.
The ultra-hot exoplanet had previously stumped researchers trying to explain the difference in light on the two planets’ sides. This is especially true since the space body is tidally locked, meaning one side constantly faces its host star. Typically, this type of orbit would result in a more or less uniform heat distribution.
Now, researchers say the mystery behind the difference in light may be caused by a large atmospheric phenomenon known as a glory. If correct, this would represent the first observation of a rainbow-like glory on a planet outside of our solar system.
WASP-76b Has Clouds that ‘Drip’ Molten Iron Rain
According to Monika Lendl, an assistant professor in the Department of Astronomy of the UNIGE Faculty of Science and co-author of the study, WASP-76 has an extremely tight orbit, which places it twelve times closer to its host star than our Sun’s closest satellite, Mercury. As a result, the hot giant, which receives over 4,000 times as much radiation energy as Mercury, is significantly larger than our largest cosmic neighbor, Jupiter, even though it has a relatively similar mass.
“The exoplanet is ‘inflated’ by the intense radiation from its star. So, although it is 10% less massive than our cousin Jupiter, it is almost twice as big,” Lendl explains.
The side of the planet facing its host star receives so much radiation that it measures a scorching 2,400 degrees Celsius. As a result, materials that typically form rocks and minerals on Earth remain in molten liquid form until they migrate to the side facing away from the star. According to the press release announcing the discovery, these materials condense into “clouds of iron that drip molten iron rain.”
Perhaps surprisingly, these extreme conditions are not even the most interesting aspects of WASP-76b. According to the researchers, “one of the most disturbing observations for astronomers” is the asymmetry in the amount of light measured between the planet’s two terminators. For reference, the terminator is the place on the planet’s surface where the “daylight” ends and the nighttime begins.
Observations Offer Potential Explanation for Disturbing Light Asymmetry
Hoping to rectify this mysterious and disturbing light asymmetry, the researchers employed the ESA’s powerful CHEOPS space telescope. This meant making “no fewer than 23” distinct observations of the planet as it orbited and at times passed behind its host star, as well as uninterrupted “phase curves” where the planet was never lost from sight.
After analyzing their data, the astronomers cross-checked their findings with previous readings of WASP-76b gathered by the Hubble, TESS, and Spitzer space telescopes. According to the researchers, this work led to the determination that the difference in light was likely caused by an atmospheric-wide version of a rainbow, a.k.a. a glory.
“This unexpected glow could be caused by a strong, localised and anisotropic reflection – i.e. one that depends on direction – what we call a glory,’’ explains Olivier Demangeon, researcher at the Instituto de Astrofísica e Ciências do Espaço in Portugal and lead author of the study.
In their published findings, which appear in the journal Astronomy and Astrophysics, the researchers behind the discovery say this glory would represent the first such phenomenon ever detected outside of Earth and Venus. That’s because a unique set of conditions needs to exist on the target planet for such a space rainbow to pop up, and the telescope making the observation needs to be in just the right place.
‘‘The reason why no such glory has ever been observed outside our solar system is that this phenomenon requires very specific conditions. First of all, the atmospheric particles must be almost perfectly spherical, completely uniform, and sufficiently stable to be observed throughout a long time. These droplets have to be directly illuminated by the planet’s host star, and the observer – in this case, CHEOPS – must be in the right position,’’ Demangeon explains.
Confirming the Cause of Disturning Light Asymmetry Could Aid Future Life-Hunting Missions
Next, the researchers say they are hoping that follow-up observations will confirm their hypothesis regarding the disturbing light asymmetry on WASP-76b. If successful, such a confirmation would mean that there are perfectly spherical droplets in the planet’s atmosphere that last for at least three years or are constantly renewing themselves and mean the planet’s atmospheric temperature is relatively stable. According to the researchers, confirming both things would offer “a fascinating and detailed insight into what could be happening on WASP-76b.”
Although this planet is likely too hot to support life, the researchers also note that their findings could help researchers search for more habitable space bodies with the right conditions.
“Detecting such tiny phenomena at such a great distance will enable scientists and engineers to identify others that are just as crucial,” they write. “For example, the reflection of starlight off liquid lakes and oceans – a necessary condition for habitability.”
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.