An Exoplanet That Is So Hot It Rains Iron Could Be Even Hotter Than We Thought

An Exoplanet That Is So Hot It Rains Iron Could Be Even Hotter Than We Thought
An artist's impression of WASP-76b. (M. Kornmesser/ESO)
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Exoplanets, or planets outside our Solar System, continue to offer astronomers fascinating glimpses of other worlds, including WASP-76b. The daytime surface temperatures on this inferno-like planet, almost the size of Jupiter, are hot enough to vaporize iron, which could fall as rain on the slightly cooler night side.

WASP-76b has been re-examined by researchers, who have concluded that it may be hotter than previously thought. The discovery of ionized calcium, which would require “significantly hotter” conditions to form than previously outlined in studies, is key to that conclusion.


Temperatures on the surface of WASP-76b are thought to reach around 4,400 degrees Fahrenheit (2,246 degrees Celsius) during the day, according to previous research – but that may be an underestimation if the new and updated temperature profile proves to be more accurate.

“We’re seeing so much calcium; it’s a really strong feature,” says Emily Deibert, an astrophysicist at the University of Toronto in Canada. “This spectral signature of ionized calcium could indicate that the exoplanet has very strong upper atmosphere winds, or the atmospheric temperature on the exoplanet is much higher than we thought.”

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WASP-76b, discovered in 2016, is known as a ‘hot Jupiter’ exoplanet due to its close proximity to its star – an orbit takes only 1.8 Earth days. It’s about 640 light-years away from where we are in the Universe. It is also tidally locked, which means that the same side of the planet always faces its star, which is slightly hotter than our Sun.


The researchers looked at the planet’s moderate temperature zone, the border between day and night, using data from the Gemini North Telescope in Hawaii. They used transit spectroscopy, which involves shining the light of an exoplanet’s star through its atmosphere all the way back to Earth.

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Because of the quality and composition of that light, we can make calculations about the atmosphere at various depths. The team was able to identify a rare trio of spectral lines in this case, readings that indicate the presence of ionized calcium.

“It’s remarkable that with today’s telescopes and instruments, we can already learn so much about the atmospheres – their constituents, physical properties, presence of clouds and even large-scale wind patterns – of planets that are orbiting stars hundreds of light-years away,” says Cornell University astronomer Ray Jayawardhana.


Spectroscopy techniques like the one used here allow astronomers to learn all sorts of secrets about exoplanets that are hundreds of light-years (or more) away: everything from the details of the planet’s rotation to the wind patterns on its surface.

That means that as more exoplanets are discovered and cataloged, researchers will be able to group them for easier reference. We eventually learn more about our place in the Universe and where other forms of life might exist.

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This research is part of a multi-year project called Exoplanets with Gemini Spectroscopy, which will look at at least 30 exoplanets (ExoGemS). When the project is finished, experts will have a much better understanding of the variety of atmospheres that exist on these distant and exotic worlds.


“As we do remote sensing of dozens of exoplanets, spanning a range of masses and temperatures, we will develop a more complete picture of the true diversity of alien worlds – from those hot enough to harbor iron rain to others with more moderate climates, from those heftier than Jupiter to others not much bigger than the Earth,” Jayawardhana says.

The findings were published in the Astrophysical Journal Letters.


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