Webb maps carbon rich atmosphere on distorted pulsar planet

by Robert Schreiber
Berlin, Germany (SPX) Dec 17, 2025

Scientists using NASA's James Webb Space Telescope have identified an exoplanet with an atmosphere unlike any seen before, orbiting a rapidly spinning neutron star known as a pulsar. The object, officially designated PSR J2322-2650b, has a mass comparable to Jupiter but is stretched into a lemon-like shape by the strong gravitational pull of its compact host star. Its atmosphere appears dominated by helium and carbon, with soot-like carbon clouds that models suggest could condense into diamonds deep inside the planet under high pressure.

Lead author Michael Zhang of the University of Chicago described the host as "a star that's completely bizarre - the mass of the Sun, but the size of a city," noting that the team is seeing "a new type of planet atmosphere that nobody has ever seen before." Team member Peter Gao of the Carnegie Earth and Planets Laboratory recalled the moment the Webb data arrived, saying, "I remember after we got the data down, our collective reaction was 'What the heck is this?'" The work has been accepted for publication in The Astrophysical Journal Letters.

PSR J2322-2650b orbits its neutron star at a distance of only about 1 million miles, far closer than Earth's roughly 100 million mile separation from the Sun. Because the pulsar emits mainly gamma rays and other high-energy particles that do not overwhelm Webb's infrared detectors, astronomers can track the planet's spectrum throughout its orbit without the usual glare from a bright star. Graduate student Maya Beleznay of Stanford University, who modeled the planet's shape and orbital geometry, said this configuration provides an unusually "pristine" spectrum, allowing the system to be examined in finer detail than typical exoplanets.

Instead of the common molecules usually detected in exoplanet atmospheres, such as water, methane and carbon dioxide, the Webb data reveal molecular carbon species C3 and C2. At the planet's core, the team suggests that extreme pressures could compress carbon into diamond. Zhang emphasized that the composition poses a major challenge for formation theories, saying, "It's very hard to imagine how you get this extremely carbon-enriched composition. It seems to rule out every known formation mechanism."

The pulsar - planet pair likely belongs to the class of "black widow" systems, where a rapidly rotating neutron star is paired with a small companion that has lost material to the pulsar over time. As mass transferred onto the neutron star, it would have spun up and generated an intense particle wind that then erodes the companion. In classic black widow binaries the companion is a low-mass star, but in this case the International Astronomical Union classifies the object as an exoplanet, underscoring its unusual nature.

Zhang argued that PSR J2322-2650b does not fit standard scenarios for either planets or stripped stellar remnants. "Did this thing form like a normal planet? No, because the composition is entirely different," he said. "Did it form by stripping the outside of a star, like 'normal' black widow systems are formed? Probably not, because nuclear physics does not make pure carbon."

Co-author Roger Romani of Stanford University and the Kavli Institute for Particle Astrophysics and Cosmology suggested a possible process inside the companion that might explain its atmosphere. "As the companion cools down, the mixture of carbon and oxygen in the interior starts to crystallize," he proposed. "Pure carbon crystals float to the top and get mixed into the helium, and that's what we see. But then something has to happen to keep the oxygen and nitrogen away. And that's where there's controversy." He added, "I'm looking forward to learning more about the weirdness of this atmosphere. It's great to have a puzzle to go after."

The discovery relied on Webb's high sensitivity in the infrared and its thermally stable environment about a million miles from Earth, where a large sunshield keeps its instruments extremely cold. Zhang noted that similar measurements are effectively impossible from Earth's surface because thermal emission from the surroundings adds interfering photons to the signal. The team also included University of Chicago scientists Jacob Bean, graduate student Brandon Park Coy, and Rafael Luque, now at the Instituto de Astrofisica de Andalucia in Spain.

Research Report:A Carbon-rich Atmosphere on a Windy Pulsar Planet