An Exoplanet Reaches 2400 C in One Hemisphere. Does it Really Rain Iron?

WASP-76b is an ultra-hot Jupiter about 640 light-years away from Earth in the constellation Pisces. A few years ago it gained notoriety for being so hot that iron falls as rain. It’s tidally locked to its star, and the planet’s star-facing hemisphere can reach temperatures as high as 2400 Celsius, well above iron’s 1538 C melting point.

Scientists have been studying the planet since its discovery in 2013, and new evidence suggests that it’s even hotter than thought. But, almost disappointingly, there might be no iron rain after all.

The discovery that WASP-76b might be even hotter than thought is based on multi-year observations of the exoplanet. A new paper based on some of those observations is published in The Astrophysical Journal Letters. Its title is “Detection of Ionized Calcium in the Atmosphere of the Ultra-hot Jupiter WASP-76b.” Emily Deibert, a University of Toronto doctoral student, is the paper’s first author.

The study of exoplanets is in an interesting place. Astronomers are developing new ways to find more exoplanets and to begin to study their atmospheres. The new research is based on a project called ExoGemS—Exoplanets with Gemini Spectroscopy. It uses the Gemini North Telescope on Mauna Kea and a high-resolution spectrograph to explore the diversity of exoplanet atmospheres. The ExoGemS survey is intended to study at least 30 exoplanets of interest to astronomers, with WASP-76b serving as a benchmark for the survey.

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

An illustration of a Hot Jupiter orbiting close to its star. Image Credit: ESA/ATG medialab, CC BY-SA 3.0 IGO

“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,” Jayawardhana said in a press release.

WASP-76b gained some attention because of a previous study that found it may rain iron. The dayside temperature is hot enough to vaporize iron, while the nightside temperature is low enough for the iron to condense into rain. The idea was that somewhere near the terminator of the tidally-locked planet, iron would condense into liquid and fall to the surface.

That may not be true, but we’ll get to that. This new research shows that WASP-76b might actually be hotter than thought. It stems from the discovery of a rare trio of spectroscopic lines of ionized calcium in the atmosphere.

The exoplanet has a complex atmosphere, and astronomers are studying it from hundreds of light-years away, so any conclusions are likely not final. In their paper, the authors say that WASP-76b likely has an escaping atmosphere and that the hydrodynamics of the atmosphere are affecting the ionized calcium lines in the spectrometry. “The higher temperature would then lead to enhanced production of ionized calcium and thus to strong absorption features,” they write.

“We’re seeing so much calcium; it’s a really strong feature,” said first author Emily Deibert, a University of Toronto doctoral student, whose adviser is Jayawardhana. “This spectral signature of ionized calcium could indicate that the exoplanet has very strong upper atmosphere winds,” Deibert said. “Or the atmospheric temperature on the exoplanet is much higher than we thought.”

There's a dazzling amount of detail in this figure from the study, but the dips represent the three spectroscopic lines of ionized calcium. Image Credit: Deibert et al 2021.
There’s a dazzling amount of detail in this figure from the study, but the dips represent the three spectroscopic lines of ionized calcium. Image Credit: Deibert et al 2021.

The iron that’s attracted so much attention could be partly responsible for the extreme heat, in one of its forms. Another form of iron, along with Mg, could also make it difficult for the atmosphere to cool. “Identified mechanisms leading to increases in temperature in the atmospheres of ultra-hot Jupiters are metal photoionization and NLTE <non-local thermodynamic equilibrium> effects in the form of overpopulation of species responsible for heating (e.g., Fe ii) together with underpopulation of species responsible for cooling (e.g., Fe i and Mg).”

WASP-76b is likely the only planet in the system, and it has about 92% of Jupiter’s mass. It has an orbital period of fewer than two days. Its frequent transits make it a good target for study. The authors of this research intend on studying the exoplanet more extensively. “In a forthcoming paper, we will analyze the full spectral range covered by these observations and search for absorption due to a suite of atomic, ionic, and molecular species modelled at high resolution…” they write.

What About the Iron Rain?

It’s being widely reported that WASP-76b rains molten iron. But according to a separate study, that may not be the case.

In a paper titled “No umbrella needed: Confronting the hypothesis of iron rain on WASP-76b with post-processed
general circulation models
,” a team of researchers say that iron rain is unlikely and that something else can explain the observations.

That team, with first author Arjun Savel, points out that WASP-76b is the first time that any kind of iron rain has been postulated in an exoplanet atmosphere. “The aforementioned iron chemistry gradient interpretation would be the first of its kind among exoplanet atmospheres,” they write. According to them, there’s another explanation, and it may be more likely.

They say that the data showing iron rain could be explained by temperature. “However, the necessity of a chemical gradient to explain the extant observations has been questioned by the forward models of Wardenier et al. (2021), who self-consistently calculate transmission spectra from a 3-D model of this planet and find that either iron condensation or a significant temperature asymmetry could match the Ehrenreich et al. (2020) data.”

The temperature asymmetry they mention is between the leading and trailing edges of the atmosphere. Unfortunately, existing data can’t differentiate between actual iron rain and temperature asymmetry. There’s another problem with the data. It turns out that the star WASP-76 may have a stellar companion about 85 AU away. The light from that star may have been present in some of the original Hubble spectroscopy of WASP-76b, fouling the data.

This artist's illustration shows a planet being heated enough to melt metals. Image credit: NASA, ESA, and G. Bacon (STSci)
This artist’s illustration shows a planet being heated enough to melt metals. Image credit: NASA, ESA, and G. Bacon (STSci)

They also point out that ultra-hot Jupiter atmospheres aren’t conducive to clouds of Fe. “Furthermore, Fe clouds are not necessarily favoured in hot Jupiter atmospheres; microphysics models indicate that the nucleation rate of Fe is low, causing Fe clouds to be sequestered deep in the atmosphere.”

Another paper, from 2021, titled “Decomposing the Iron Cross-Correlation Signal of the Ultra-Hot Jupiter WASP-76b in Transmission using 3D Monte-Carlo Radiative Transfer,” also argues against the iron rain conclusion. In that paper, the authors write “We also show that the previously published iron signal of WASP-76b can be reproduced by a model featuring iron condensation on the leading limb. Alternatively, the signal may be explained by a substantial temperature asymmetry between the trailing and leading limb, where iron condensation is not strictly required to match the data.”

To be fair to the researchers who initially suggested the presence of iron rain on WASP-76b, they only said it was a possibility because of the data they had. Headlines suggested it was a done deal, but that’s how headlines work. David Ehrenreich, a professor in the Department of Astronomy in the Faculty of Science at the University of Geneva, Switzerland, was the lead author of that paper.

In their paper, they said that their results can be explained two ways, and only one of them involves iron rain. The results “… can be explained by a combination of planetary rotation and wind blowing from the hot dayside,” on the one hand. On the other hand, “… no signal arises from the nightside close to the morning terminator, showing that atomic iron is not absorbing starlight there.” But as we’ve seen, the lack of an iron signal could be due to temperature asymmetry.

So for now, nobody’s sure that there’s a planet so hot it rains iron. But it sure would be interesting. More powerful instruments, like the upcoming James Webb Space Telescope, should help us find out for sure.