Lemon-shaped planet around a pulsar defies explanation, JWST reveals carbon-rich atmosphere

NASA's James Webb Space Telescope has revealed a Jupiter-sized exoplanet with a lemon-like shape that orbits a pulsar, PSR J2322–2650b, about 750 light-years from Earth. The discovery adds a startling twist to our understanding of where planets can form and survive, as this is the only known gas giant to encircle a neutron star and one of the most extreme examples of planetary chemistry ever observed.

The planet completes an orbit around its dense stellar host at roughly one million miles (about 1.6 million kilometers) and races through space on a blistering 7.8-hour year. The pulsar's immense gravity stretches the world into an elongated shape, while relentless gamma-ray bombardment from the star drives severe temperature differences between day and night sides, with days reaching up to about 2,030°C (3,700°F) and nights cooling to around 650°C (1,200°F). Astronomers describe the world as a rare laboratory for extreme physics and chemistry, where conventional planetary atmospheres do not apply.

The noteworthy findings, published in The Astrophysical Journal Letters, come from JWST spectra that reveal an atmosphere dominated not by the water, methane, or carbon dioxide commonly seen on exoplanets, but by molecular carbon—specifically C3 and C2. The detection of pure carbon-bearing species at the scorching temperatures present on PSR J2322–2650b challenges standard formation models, which typically predict oxygen- or nitrogen-bearing molecules at elevated temperatures. Co-author Dr. Michael Zhang of the University of Chicago notes that the atmosphere appears to be carbon-rich in a way that has “nobody has ever seen before.” He adds that the presence of molecular carbon under such conditions strongly implies an atmosphere with very little oxygen or nitrogen mixed in, a surprising departure from familiar exoplanet chemistries.

The team reports that the planet’s atmospheric composition cannot easily be reconciled with known formation pathways. “This is a new type of planet atmosphere that nobody has ever seen before,” Zhang said. “Instead of finding the normal molecules we expect to see on an exoplanet — like water, methane, and carbon dioxide — we saw molecular carbon, specifically C3 and C2.” At the same time, the extreme irradiation from the pulsar complicates the chemistry further: at those temperatures, carbon would normally bond with other elements, so the dominance of pure carbon species suggests unusual atmospheric processes at work.

Co-author Dr. Peter Gao of the Carnegie Earth and Planets Laboratory recalls the moment the data came down: “I remember after we got the data down, our collective reaction was ‘What the heck is this?’ It’s extremely different from what we expected.” The observation underscores a broader question about how such a planet could form in a system dominated by a compact, radiative neutron star rather than a sun-like star. The pulsar’s gravity and radiation fields are so extreme that most planets would be torn apart or evaporated, making PSR J2322–2650b an exceptionally rare survivor.

The working teams emphasize that the planet’s carbon-enriched atmosphere cannot be easily traced to ordinary planetary evolution. The researchers propose that carbon and oxygen may have crystallized within the planet’s interior as it cooled, allowing pure carbon crystals to rise to the surface and mix with helium. Yet this scenario still requires that oxygen and nitrogen be excluded from the atmosphere, a condition that Romani says remains difficult to achieve in any known formation pathway. “Something has to happen to keep the oxygen and nitrogen away,” Romani noted, calling the puzzle “a puzzle to go after.”

Despite the unresolved questions, the finding stands out even within the small cohort of exotic exoplanets studied in depth. Among roughly 6,000 confirmed exoplanets, PSR J2322–2650b remains the sole gas giant known to orbit a neutron star. Its intimate orbit and extreme irradiation offer a rare laboratory for testing how planetary interiors respond to intense gravity and high-energy radiation, as well as how unusual chemistries can emerge when conventional atmospheric inputs are absent.

To probe these atmospheric mysteries, researchers relied on transmission spectroscopy with JWST, a method that infers atmospheric composition from the way a planet’s atmosphere absorbs starlight at different wavelengths. The absence of light at specific wavelengths betrays the presence of particular molecules, while the relative strengths of absorption features help constrain the atmospheric inventory. The team cautions that interpreting a carbon-rich atmosphere under such extreme conditions is challenging, and the observational signal is difficult to disentangle from the pulsar’s intense flux. Still, the results mark a significant step forward in pushing the boundaries of what researchers can detect in the atmospheres of distant worlds.

The study also highlights how extreme environments can reveal new physics and chemistry that do not appear around more familiar stars. Neutron stars, formed when massive stars explode in supernovae, create some of the universe’s densest objects. The pulsar at the heart of PSR J2322–2650b represents a particularly harsh environment, with radiation and tidal forces that would be lethal to ordinary planets. Yet in this case, a gas giant not only persists but exhibits a chemically unusual atmosphere that defies easy classification. That paradox is exactly the kind of result scientists expect when studying worlds beyond the habitable zone, where the usual rules of planetary science can be rewritten.

Looking ahead, researchers say additional observations with JWST and other facilities will be crucial to confirming the carbon-dominated signature and refining models of how such planets could form and evolve in neutron-star systems. The discovery underscores the value of pushing instruments to their limits to explore planetary diversity in the cosmos, even in environments once thought too extreme to host planets. Scientists anticipate that PSR J2322–2650b will become a touchstone for future work on planetary atmospheres under intense irradiation and for testing theories of carbon-rich chemistry at high temperatures. The work also invites new questions about the resilience of planets and the potential for exotic chemistries to emerge in the universe’s most unlikely places.