Iron Rain May Fall on This Scorching-Hot Exoplanet Where Temperatures Reach More Than 4,000 Degrees

An international team of researchers has shed new light on an extreme exoplanet located around 640 light-years away from Earth they believe may rain iron, according to a study.

The planet—known as WASP-76b—belongs to a category of giant planets called "ultra-hot Jupiters," which are quite unlike anything in our own solar system. These planets are large, gaseous and orbit very close to their host stars.

"WASP-76b is slightly less massive than Jupiter—the largest planet of the solar system—but almost a factor of two bigger in size," María Rosa Zapatero Osorio, an author on the latest Nature study from the Centro de Astrobiología, Spain, told Newsweek. "The orbit of WASP-76b around its parent star is very tight, i.e., the orbital size is 10 times smaller than the orbit of Mercury—the closest planet to the sun—and 'one year' on WASP-76b is equivalent to 1.8 terrestrial days."

Like other hot Jupiters, the planet is "tidally locked," meaning that one side perpetually faces its star, while the other is always turned away.

As a result, temperatures on the day side are scorching, reaching more than 2,400 C (over 4350 F)—hot enough to vaporize metals like iron, which then evaporate into the atmosphere. Meanwhile, on the night side—which experiences perpetual darkness—temperatures are much lower, hovering around 1,500 C.

For the latest study, Osorio and her colleagues observed the planet with the ESPRESSO instrument on the European Southern Observatory's (ESO) Very Large Telescope in Chile.

The data collected indicates that iron vapor is abundant in the atmosphere on the hot-day side of the planet. And some of this iron vapor is transferred to the night side by strong winds—driven by the huge temperature difference on both sides—where it potentially produces "iron rain."

"For the first time, we have evidence of the different chemistry taking place in the atmosphere of the day and night sides of a giant planet," Osorio said. "The hot side of WASP-76b—which is the part that is always receiving the heat from the parent star—has iron vapor. At the same time, the planet is rotating and has strong atmospheric winds."

"As a result, iron vapor travels from the hot side to the cold atmosphere. The drop in temperature is huge, probably higher than 1,000 Kelvin. At low temperatures, the iron condenses into other species, one of which is liquid iron," she said. "This is how we think 'iron rain' is occurring in the atmosphere of WASP-76b."

According to Osorio, this is the first "clear evidence" this phenomenon may be occurring on an exoplanet, although scientists have predicted that it takes place on other large, hot worlds similar to WASP-76b—which was discovered in 2016.

Christiane Helling—an astronomer with the Dutch Space Research Institut (SRON) and Director of the Centre for Exoplanet Science at the University of St. Andrews, U.K., who was not involved in the Nature study—described the research as a "very thorough investigation of high-resolution data from the ESPRESSO instrument."

"The main conclusion of this study is that the abundance of atomic iron changes from the day to the night side of the planet, which is a strong indication for a global wind system on this planet," she told Newsweek.

"It is now understood that the nights on ultra-hot Jupiters are so cold that clouds can condense from the chemically very rich gas of such giant gas planets. We know that from WASP-18b and HAT-P-7b, for example—which are ultra-hot Jupiter siblings of WASP-76b," she said. "Based on our own cloud formation simulations, we have demonstrated that the nightside of WASP-18b and HAT-P-7b are covered with clouds, while the day side is cloud-free because it is too hot."

However, Helling cautioned that more evidence was needed to back up the claim that the planet had rain made of iron droplets.

"Cloud particle on exoplanets have been shown to be composed of a mix of minerals as the consequence of the enormous chemical richness of these atmospheres," she said. "Such minerals lock up elements like magnesium, silicate, oxygen, titanium, et cetera, and a lot of iron, too. I therefore would consider the conclusion that WASP-76b has rain made of iron droplets a little premature and would be very happy to apply our detailed, kinetic cloud model to the study of WASP-76b to gain more insight."

WASP-76b
This illustration shows a night-side view of the exoplanet WASP-76b. ESO/M. Kornmesser

The latest results could have several implications for our understanding of these types of planets, the authors of the Nature study say.

"We may start to understand the properties of these inflated planets, of which we do not have any example in our solar system," Osorio said. "The open questions are how they form where they are and how they evolve. The study of their atmospheres—temperature, chemical composition, et cetera—may be critical to assess these questions."

For Helling, the results—assuming they can be verified by other researchers—provide "observational proof" that atomic iron is present in ultra-hot Jupiters, and that its abundance changes across these extreme worlds. This is important, she notes, as it is further proof that these gas giant planets have "very dynamic atmospheres."

Furthermore, "this observational finding demonstrates that our understanding of these atmospheres has progressed to a level that our models can make reliable predictions," she said. "The presence of atomic iron in ultra-hot Jupiters has been predicted for WASP-18b and seeing it measured for its sibling WASP-76b is a bit mind-blowing."

The latest findings were obtained from ESPRESSO's first scientific observations conducted in September 2018. The instrument—whose name is short for "Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations"—was initially designed to identify Earth-like planets around stars like our sun. However, the scientists found that it could also help provide insights into the climate of the most extreme exoplanets.

"We soon realized that the remarkable collecting power of the VLT and the extreme stability of ESPRESSO made it a prime machine to study exoplanet atmospheres," Pedro Figueira, another author of the study from the ESO in Chile, said in a statement.

The VLT is one of the world's most advanced visible-light astronomical observatories, located in Chile's Atacama Desert. It consists of four Unit Telescopes with main mirrors measuring more than eight meters [26 feet] in diameter.

Correction 04/26 5.20 p.m.: This name of the planet has been corrected to WASP-76b.