Astronomers reveal 3D structure of an alien planet

20/2/2025 6:11

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

characterized 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 characterized 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 characterized

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 Côte 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 2.5% 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.