Webb Detects Exotic Helium-and-Carbon-Rich Atmosphere around Pulsar-Orbiting Exoplanet

PSR J2322-2650b, an enigmatic Jupiter-mass exoplanet orbiting the millisecond pulsar PSR J2322-2650, appears to have an exotic atmosphere dominated by helium and carbon like never before seen.

This artist’s concept shows what the PSR J2322-2650b could look like. Image credit: NASA/ESA/CSA/Ralf Crawford, STScI.

“It was an absolute surprise,” said Dr. Peter Gao, an astronomer at the Carnegie Earth and Planets Laboratory.

“I remember after we got the data back, our collective reaction was, ‘What the hell is that?’ This is extremely different from what we expected.

“This system is unique because we are able to see the planet illuminated by its host star, but not the host star at all,” said doctoral student Maya Beleznay. candidate at Stanford University.

“So we get a truly pristine spectrum. And we can study this system in more detail than normal exoplanets.”

“The planet orbits a completely bizarre star: the mass of the Sun, but the size of a city,” said Dr. Michael Zhang, an astronomer at the University of Chicago.

“This is a new type of planetary atmosphere that no one has ever seen before. Instead of finding the normal molecules we expect to see on an exoplanet – like water, methane and carbon dioxide – we saw molecular carbon, specifically carbon.₃ etc₂.”

Molecular carbon is very unusual because at these temperatures (over 2,000 degrees Celsius), if there are other types of atoms in the atmosphere, the carbon will attach to them.

Of the approximately 150 planets studied by astronomers inside and outside the solar system, no others have detectable molecular carbon.

“Did this thing form like a normal planet? No, because the composition is completely different,” Dr. Zhang said.

“Did it form by stripping away the exterior of a star, as ‘normal’ black widow systems form? Probably not, because nuclear physics does not produce pure carbon.”

“It is very difficult to imagine how to obtain this extremely carbon-enriched composition. This seems to rule out any known formation mechanism.”

The authors propose an evocative phenomenon that could occur in this unique atmosphere.

“As the companion cools, the mixture of carbon and oxygen inside begins to crystallize,” said Dr. Roger Romani, an astronomer at Stanford University and the Kavli Institute for Particle Astrophysics and Cosmology.

“The pure carbon crystals float up and mix with the helium, and that’s what we see.”

“But then something has to happen to push the oxygen and nitrogen apart. And that’s where the mystery comes in.”

“But it’s nice not to know everything. I can’t wait to learn more about the strangeness of this atmosphere. It’s great to have a puzzle to solve.”

The discovery is described in an article in Astrophysical journal letters.

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Michael Zhang and others. 2025. A carbon-rich atmosphere on a windy Pulsar planet. ApJL 995, L64; doi: 10.3847/2041-8213/ae157c