The James Webb Space Telescope has captured images of gas giant planets similar to Jupiter and Saturn orbiting a distant star.
The star, known as HR 8799, is 130 lightyears away – a stone's throw in cosmic terms – and was already known to have planets orbiting it.
But Webb's observations of HR 8799 show the planets around the star are rich in carbon dioxide gas which, say astronomers, is strong evidence those planets formed like Jupiter and Saturn.
More on Webb and exoplanets
They began their lives – as most planets – within the dusty disc surrounding a newborn star.
Bits of that dusty material coalesced over time to form a solid core, which then attracted more and more gas from within the protoplanetary disc, in a planet-growing process known as accretion.
Webb and HR 8799
This study of HR 8799 shows the James Webb Space Telescope enables astronomers to see what chemicals are in the atmospheres of planets around distant stars (known as exoplanets).
"By spotting these strong carbon dioxide features, we have shown there is a sizable fraction of heavier elements, like carbon, oxygen, and iron, in these planets’ atmospheres," says William Balmer of Johns Hopkins University in Baltimore.
"Given what we know about the star they orbit, that likely indicates they formed via core accretion, which is an exciting conclusion for planets that we can directly see."
Balmer is the lead author of the study published in The Astrophysical Journal.
What we know about HR 8799 and its planets
HR 8799 is a young system, just 30 million years old. That may sound old, but our own Solar System, by comparison, is about 4.6 billion years old.
The planets themselves, as a result, are still very young and glowing hot in infrared.
Studying infrared light is the Webb Telescope's speciality, and in the case of exoplanets, this can provide astronomers with information about how the planets formed.
Astronomers say giant planets form by one of two known methods: either through solid cores that attract more and more gas over time, or by gas particles in the disk around the star rapidly coalescing into massive objects.
Knowing which is more common helps astronomers distinguish what sort of planet they're finding around distant stars.
“Our hope with this kind of research is to understand our own Solar System, life, and ourselves in the comparison to other exoplanetary systems, so we can contextualise our existence," Balmer said.
"We want to take pictures of other solar systems and see how they’re similar or different when compared to ours. From there, we can try to get a sense of how weird our Solar System really is — or how normal."
Webb's images of exoplanets
Direct images of exoplanets are still a relatively new phenomenon, but Webb is enabling astronomers to capture more.
In 2024, astronomers announced Webb had captured a direct image of Epsilon Indi Ab, a 'super Jupiter' 12 lightyears from Earth.
And the same team behind the study of HR 8799 have analysed Webb's direct image of 51 Eridani b, a young exoplanet 96 lightyears from Earth.
These images were made possible by Webb’s NIRCam (Near-Infrared Camera) coronagraph, an instrument that blocks light from bright stars to reveal their dimmer planets in orbit.
The technique is much the same as holding your hand up to cover the Sun on a bright day, in order to better see what's around you.
Using the coronagraph, astronomers are able to look for infrared light emitted by the planets in wavelengths absorbed by specific gases.
Webb found the four exoplanets around HR 8799 contain more heavy elements than previously thought.
"We have other lines of evidence that hint at these four HR 8799 planets forming using this bottom-up approach," says Laurent Pueyo, an astronomer at the Space Telescope Science Institute in Baltimore, who co-led the work.
"How common is this for planets we can directly image? We don't know yet, but we're proposing more Webb observations to answer that question."
"We knew Webb could measure colors of the outer planets in directly imaged systems," says Rémi Soummer, director of STScI’s Russell B. Makidon Optics Lab and former lead for Webb coronagraph operations.
"We have been waiting for 10 years to confirm that our finely tuned operations of the telescope would also allow us to access the inner planets. Now the results are in and we can do interesting science with it."
The NIRCam observations of HR 8799 and 51 Eridani were conducted as part of Guaranteed Time Observations programs 1194 and 1412 respectively.
