PSR J2322-2650b, a mysterious Jupiter-mass exoplanet orbiting the millisecond pulsar PSR J2322-2650, appears to have an exotic helium-carbon-dominated atmosphere unlike any ever seen before.
This artist's concept shows what the PSR J2322-2650b might look like. Image credit: NASA/ESA/CSA/Ralph Crawford, STScI.
“It was an absolute surprise,” said Dr. Peter Gao, an astronomer at the Carnegie Earth and Planetary Laboratory.
“I remember after we recorded the data, our collective reaction was, 'What the hell is this?' It's very different from what we expected.”
“This system is unique because we can see a planet illuminated by its host star, but not see the host star at all,” said Maya Beleznay, Ph.D. candidate at Stanford University.
“So we get a really pristine spectrum. And we can study this system in more detail than regular 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 would expect to see on an exoplanet – such as water, methane and carbon dioxide – we saw molecular carbon, specifically C3 and C2“
Molecular carbon is very unusual because at these temperatures (over 2000 degrees Celsius), if there are any other types of atoms in the atmosphere, carbon will bond with them.
Of the roughly 150 planets that astronomers have studied inside and outside the solar system, none of the others have detectable molecular carbon.
“Did this thing form like a normal planet? No, because its composition is completely different,” Dr. Zhang said.
“It formed by removing the outer part of the star, how do 'normal' black widow systems form? Probably not, because nuclear physics doesn't produce pure carbon.”
“It is very difficult to imagine how to obtain this extremely carbon-enriched composition. It seems to rule out all known formation mechanisms.”
The authors suggest one memorable phenomenon that could have occurred 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.
“Pure carbon crystals float to the top and mix with the helium, and that's what we see.”
“But then something has to happen to keep the oxygen and nitrogen out. And that's where the mystery arises.”
“But it's nice not to know everything. I'm looking forward to learning more about the strangeness of this atmosphere. It's great to have a puzzle to solve.”
The discovery is described in paper V Letters in an astrophysical journal.
_____
Michael Zhang etc.. 2025. Carbon-rich atmosphere on a windy pulsar planet. ApJL 995, L64; doi: 10.3847/2041-8213/ae157c
