WASHINGTON: Astron­omers for the first time have deciphered the three-dimensional structure of the atmosphere of a planet beyond our solar system, revealing three layers like a wedding cake on a ferociously hot gas planet that orbits close to a star bigger and hotter than our sun.

The researchers peered through the atmosphere of WASP-121b, a planet also called Tylos, by combining all four telescope units of the European Southern Obser­vatory’s Chile-based Very Large Telescope, discerning a stratification of layers with different chemical compositions and intense winds.

Until now, researchers have been able to determine the atmospheric chemical composition for some planets outside our solar system — called exoplanets — but without mapping the vertical structure or how the chemical elements were distributed.

WASP-121b is an “ultra-hot Jupiter,” a class of large gas planets that orbit close to their host star, making them extremely hot. Its atmosphere is mainly composed of hydrogen and helium, like that of Jupiter, our solar system’s largest planet. But WASP-121b’s atmosphere is not like anything ever seen before.

The researchers differentiated three layers by looking for the presence of specific elements. WASP-121b’s bottom layer was characterised by the presence of iron — a metal in gaseous form because of the incredible heat of the atmosphere. Winds move gas from the planet’s eternal hot side to its cooler side.

The middle layer was characterised by the presence of sodium, with a jet stream blowing circularly around the planet at about 43,500 miles (70,000 km) per hour — stronger than any winds in our solar system. The upper layer was characterised based on its hydrogen, with some of this layer being lost into space.

“This structure has never been observed before and defies current predictions as to how atmospheres should behave,” said astronomer Julia Victoria Seidel of the European Southern Obser­vatory and the Lagrange Laboratory at the Obser­vatoire de la Cte d’Azur in France, lead author of the study published this week in the journal Nature.

The researchers also detected titanium in gaseous form in WASP-121b’s atmosphere. On Earth, neither iron nor titanium exist in the atmosphere because they are solid metal owing to our planet’s lower temperatures, relative to WASP-121b. Earth does have a sodium layer in the upper atmosphere.

“For me, the most exciting part of this study is that it operates at the very limits of what is possible with current telescopes and instruments,” said study co-author Bibiana Prinoth, a doctoral student in astronomy at Lund University in Sweden.

WASP-121b has roughly the same mass as Jupiter but twice the diameter, making it puffier. It is located about 900 light-years from Earth in the direction of the constellation Puppis. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

WASP-121b is tidally locked, meaning that one side of it perpetually faces its star and the other side faces away, like the moon is to Earth. The side facing the star has a temperature around 4,900 degrees Fahrenheit (2,700 degrees Celsius/3,000 degrees Kelvin). The other side is at about 2,200 degrees Fahrenheit (1,250 degrees Celsius/1,500 degrees Kelvin).

The planet orbits its star at about 2.5pc of the distance of Earth to the sun. It is about a third closer to its star than our solar system’s innermost planet Mercury is to the sun — so close that it completes an orbit in 1.3 days.

Its host star, called WASP-121, is roughly 1-1/2 times the mass and diameter of the sun, and hotter. Being able to make out the structure of an exoplanet’s atmosphere could be helpful as astronomers search for smaller rocky planets capable of harboring life.

“In the future, we will likely be able to provide similar observations for smaller and cooler planets and thus more similar to Earth,” Prinoth said, especially with the European Southern Obser­vatory’s Extremely Large Telescope due to be completed in Chile by the end of the decade as the world’s largest optical telescope.

“These detailed studies are necessary to provide context for our place in the universe,” Seidel said. “Is Earth’s climate unique? Can theories we derive from our one data point — Earth — actually explain the whole population of exoplanets?” “With our study we have shown that climates can behave vastly differently that predicted. There is much more diversity out there than what we have at home,” Seidel added.

Published in Dawn, February 20th, 2025