Astronomers discover curious signal on scorching planet where it rains iron

Calcium is more abundant on WASP-76b, a hot Jupiter, than scientists expected.

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Jackson Ryan
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This artist's impression shows a hot Jupiter orbiting nice and close to its parent star.

ESO/L. Calçada

If you haven't heard about WASP-76b then, sorry to tell you this but, you've been missing out. Last year, astronomers revealed the "hot Jupiter," which lies about 640 light-years from Earth, has a curious nighttime quirk. Every evening on the planet it rains iron. Yes, the stuff that goes into building our skyscrapers and apartments literally buckets down from the sky on WASP-76b.

The reason for the iron rain is intense heat. The exoplanet is tidally locked to its parent star, which means it only ever shows one side to its scorching hot companion, and its face is constantly on fire. In early 2020, researchers estimated that the planet probably reached around 3,800 degrees Fahrenheit (or around 2,100 degrees Celsius), enough to vaporize metals like iron. 

But, concerningly for anyone looking to move to WASP-76b, the estimates may have been a little low.

A new study, published Sept. 28 in the journal Astrophysical Journal Letters, used the Hawaiian Gemini telescopes to examine the planet from Earth. Through a technique known as spectroscopy, which allows scientists to detect elements based on a characteristic light signature, the researchers analyzed the upper atmosphere of WASP-76b and, unexpectedly, found it contained the strong spectral signal of ionized calcium

"The signal we see from calcium, which comes from the tenuous upper atmosphere of the planet, is much stronger than what we expect from models," said Ernst de Mooij, an astrophysicist at Queen's University Belfast and co-author of the study.

Because the planet is so far away, it's not possible at present to see exactly what is causing the calcium copiousness. We don't have telescopes powerful enough to visualize what's going on at WASP-76b, but the new information does begin to help astronomers unravel exactly what's going on a world 640 light-years away. 

Previous research had examined ionized calcium on two other giant, hot exoplanets known as KELT-9b and WASP-33b. That research team noted that models of these superhot Jupiters didn't match up with the calcium signal they were seeing, suggesting an unknown process might be bringing calcium higher into the atmosphere. The new study lines up with that research and could help astronomers study the atmospheric escape of gases and elements to space -- something that happens a lot faster if you're right next to your host star and hot enough to vaporize iron.

The results form part of a survey of exoplanet atmospheres known as ExoGemS, which uses the Hawaiian Gemini observatory. The program will use the same technique on other exoplanets and give a greater accounting of their diversity, with WASP-76b's curious calcium informing the next series of exoplanet analyses. 

"We are planning to observe a large sample of exoplanets over a range of masses and temperatures to explore the diversity of exoplanet atmospheres and help us better understand the underlying processes," said de Mooij.