By BlueShift and AFP | 2025-03-18

The James Webb Space Telescope has directly observed the key chemical of carbon dioxide (CO2) in planets outside our solar system for the first time, scientists announced on March 17.

The gas giants are not capable of hosting extraterrestrial life but do offer clues in a lingering mystery about how distant planets form, according to a study in The Astrophysical Journal.

The HR 8799 system, 130 light-years from Earth, is only 30 million years old -- just a baby compared to our solar system's 4.6 billion years.

A US-led team of researchers used Webb to directly detect CO2 in the atmosphere of all four of the system's known planets, according to the study.

They used Webb's coronagraph instruments, which block the light from bright stars to get a better view of the planets revolving around them.

"It's like putting your thumb up in front of the Sun when you're looking up at the sky," lead study author William Balmer, an astrophysicist at Johns Hopkins University, told AFP.

Normally, the Webb telescope detects exoplanets only by glimpsing them when they cross in front of their host star.

This "transiting method" was how Webb indirectly detected CO2 in the atmosphere of the gas giant WASP-39 b in 2022.

However, for the latest discovery, "we're actually seeing the light that is emitted from the planet itself, as opposed to the fingerprint of that light from the host star," Balmer said.

This process is not easy -- Balmer compared it to using a torch to spot fireflies next to a lighthouse.

Learning from exoplanetary systems

While these gas giants may not be able to host life, it is possible that they had moons that could, Balmer added.

"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 contextualize our existence," he said in a statement released by NASA.

"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."

CO2, which is essential for life on Earth, is a key target in the search for life elsewhere.

Because CO2 condenses into little ice particles in the deep cold of space, its presence can shed light on planetary formation.

"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," Balmer said.

"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."

'Key piece of proof'

Jupiter and Saturn are believed to have first formed from a "bottom up" process in which a bunch of tiny, icy particles came together into a solid core that then sucked in gas to grow into giants, Balmer told AFP.

So the new discovery is a "key piece of proof" that far-off planets can form in a similar way to those in our celestial backyard, he said.

But how common this pattern is throughout the universe remains unclear.

"We have other lines of evidence that hint at these four HR 8799 planets forming using this bottom-up approach," said Laurent Pueyo, an astronomer at the Space Telescope Science Institute (STScI) 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."

Astronomers have now discovered almost 6,000 exoplanets, many of them massive -- and none of them known to be habitable.

The "huge leap forward we need to make" is to focus on smaller Earth-sized worlds, Balmer said.

NASA's Nancy Grace Roman space telescope will use a coronagraph to do that just after its planned launch in 2027.

"We knew Webb could measure colors of the outer planets in directly imaged systems," said 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."