Today’s forecast: cloudy with a 100% chance of iron rain in the evening

The exoplanet, WASP-76b, about 640 light-years from Earth, orbits its host star, WASP-76, once every approximately 1.8 days at the relatively small distance of only 0.03 astronomical units (AU). The Earth, in contrast, is at a distance of 1 AU from the Sun. The star, WASP-76, is somewhat larger and hotter than the Sun, but that doesn’t make much of a difference for the planet orbiting around it. At such a small distance, the planet would be damn hot no matter how big the star was. The planet, almost as massive as our Jupiter, is therefore classified as a “Hot Jupiter.” The huge amount of heat has greatly inflated this alien world; WASP-76b has a diameter significantly larger than our Jupiter.

But that isn’t the planet’s most exciting feature. WASP-76b moves around its star in a tidally locked orbit, so the same side is always facing its star. It is so hot that molecules split into atoms and metals, such as iron, are vaporized into the atmosphere. If an observer were to move from the always dark night side (average temperature: 1500 °C [2730 °F]) to the much hotter day side (2400 °C [4350 °F]), he or she would be subjected to a rather unpleasant phenomenon: a storm front spanning the entire planet with rain of hot, molten iron. Astronomers of the European Southern Observatory (ESO) have now published an explanation for this phenomenon in an article in Nature.

“Observations have shown that iron vapor is abundant in the atmosphere of the hot day side of WASP-76b,” explains María Rosa Zapatero Osorio, an astrophysicist at the Centre for Astrobiology in Madrid, Spain, and chair of the ESPRESSO science team. “A fraction of this iron is carried into the night side due to the planet’s rotation and atmospheric winds. There, the iron encounters much cooler environments, condenses and rains down.” Therefore, “we don’t see iron vapor in the morning,” explains David Ehrenreich, professor at the University of Geneva in Switzerland and the lead scientist on the study of this unusual planet.

This illustration shows a night-side view of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets. To the left of the image, we see the evening border of the exoplanet, where it transitions from day to night. (illustration: ESO/M. Kornmesser)
This comic-book-style illustration by Swiss graphic novelist Frederik Peeters shows a close-up view of the evening border of the exoplanet WASP-76b. The ultra-hot giant exoplanet has a day side where temperatures climb above 2400 degrees Celsius, high enough to vaporise metals. Strong winds carry iron vapour to the cooler night side where it condenses into iron droplets. Theoretical studies show that a planet, like WASP-76b, with an extremely hot day side and colder night side would have a gigantic condensation front in the form of a cloud cascade at its evening border, the transition from day to night, as depicted here.

 

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