Webb Finds Diamond-Rain Planet Around Neutron Star

A Lemon-Shaped Planet Around a Neutron Star

1. The Pulsar Host: PSR J2322-2650

   PSR J2322-2650 is a millisecond pulsar – the ultradense, city-sized remnant of a massive star that exploded as a supernova. Containing roughly the mass of the Sun compressed into a sphere only about 20 kilometers across, it spins hundreds of times per second and emits beams of high-energy radiation that sweep across space like a cosmic lighthouse. When those beams point toward Earth, astronomers see regular pulses in radio or gamma rays, allowing exquisitely precise timing of the pulsar’s rotation.[cite:471][cite:473] Pulsar timing surveys first revealed the presence of a planetary-mass companion in 2017, but the companion’s nature remained obscure. Only with JWST’s infrared spectroscopic capabilities could astronomers isolate the planet’s light and dissect its atmosphere, because the pulsar itself shines mainly in high-energy gamma rays that are effectively invisible to Webb.[cite:466][cite:472] This unique geometry provides a rare opportunity: a planet fully illuminated by its host but without stellar glare contaminating the spectrum, giving an unusually clean view of an exoplanet orbiting a neutron star.

2. Extreme Orbit and Tidal Distortion

   PSR J2322-2650b orbits just about 1 million miles from the pulsar – barely more than 1 percent of Earth’s distance from the Sun – completing a full orbit in roughly 7.8 hours.[cite:463][cite:471] At such proximity, the neutron star’s gravity exerts enormous tidal forces on the planet. Thermal phase-curve measurements and orbit modeling show that these tides have stretched the planet into a pronounced ellipsoidal, “lemon-like” shape, with its long axis always pointing toward the pulsar.[cite:471][cite:465][cite:472] This extreme distortion is similar in principle to the tidal bulges on hot Jupiters, but amplified by the pulsar’s far more intense gravity. The dayside of the planet roasts at thousands of degrees – with estimates ranging from roughly 1,200 to 3,700 degrees Fahrenheit depending on longitude – while the nightside is significantly cooler, creating strong atmospheric circulation and likely driving vertical mixing of carbon-rich material between deep layers and the upper atmosphere.[cite:465][cite:467] Such conditions make PSR J2322-2650b not only one of the physically weirdest planets known, but also an exceptional laboratory for studying how atmospheres behave under extreme tidal and radiative stress.

An Atmosphere of Helium, Carbon, Soot Clouds, and Diamond Rain

1. Helium–Carbon Chemistry and Missing Molecules

   JWST’s spectroscopy of PSR J2322-2650b delivered a shock. Instead of the usual molecular signatures – water (H₂O), methane (CH₄), carbon dioxide (CO₂) – the planet’s spectrum is dominated by helium and molecular carbon species such as C₂ and C₃.[cite:466][cite:463][cite:474] No oxygen- or nitrogen-bearing molecules are detected within the sensitivity of the observations, making this composition “unlike any ever seen before” among the roughly 150 exoplanet atmospheres studied in detail to date.[cite:466][cite:473] The strong carbon bands imply an extraordinarily high carbon-to-oxygen ratio and raise the possibility that the planet formed in, or was transformed into, a carbon-rich environment very different from the protoplanetary disks that birthed most known gas giants. The absence of conventional volatiles suggests either that oxygen- and nitrogen-bearing compounds were never present in significant amounts, or that they have been stripped or sequestered by processes not yet understood. Either way, PSR J2322-2650b stands out as a chemically exotic “carbon world” orbiting a dead star.

2. Soot Clouds and Possible Diamond Core

   The same spectra that reveal molecular carbon also indicate the presence of soot-like clouds high in the atmosphere. Webb’s infrared observations show broad absorption and scattering features consistent with carbonaceous particulates lofted into the upper layers, creating a dark, hazy shroud.[cite:466][cite:464] Deeper within the planet, where pressures reach millions of times Earth’s atmospheric pressure, theory predicts that carbon can crystallize into diamond phases – a process previously suggested for carbon-rich giant planets but never tied so directly to observed atmospheric chemistry.[cite:465][cite:470][cite:474] On PSR J2322-2650b, carbon soot likely condenses into diamond crystals as it sinks, generating “diamond rain” that falls toward a deep, possibly diamond-rich interior.[cite:463][cite:472] This scenario echoes earlier predictions for “diamond planets” but in a far more extreme environment: a carbon-heavy gas giant locked in a tight orbit around a neutron star, where both atmospheric and interior conditions favor the formation of crystalline carbon. While direct confirmation of a diamond core remains beyond current capabilities, the atmospheric composition and extreme pressures make this an increasingly plausible picture.

Formation Mysteries: How Do You Build a Planet Like This?

1. Surviving a Supernova and Pulsar “Black Widow” Scenarios

   PSR J2322-2650b orbits a star that has already died in a supernova, raising immediate questions about its origin. Did the planet somehow survive the supernova that created the neutron star, or did it form afterward from fallback debris or a disrupted companion? Some models draw parallels to “black widow” pulsar systems, where the pulsar ablates a close companion through intense radiation, gradually stripping it down to planetary mass.[cite:462][cite:465] In that picture, PSR J2322-2650b might be the remnant core of what was once a more massive star or brown dwarf, now boiled down to a dense, carbon-rich husk enveloped by an exotic atmosphere. However, the planet’s helium–carbon composition, lack of oxygen and nitrogen, and well-ordered orbit complicate this narrative. Surviving or forming in such a hostile environment requires finely tuned conditions that current population-synthesis models struggle to reproduce, leaving the system’s formation history an open and active area of research.[cite:460][cite:473]

2. Crystallization, Fractionation, and Diamond-World Hypotheses

   One speculative formation pathway centers on interior crystallization and chemical fractionation. In this scenario, a progenitor object – perhaps originally more star-like – cools and differentiates over time, with carbon and oxygen forming crystalline phases deep inside while helium and uncondensed carbon remain in the envelope. As crystallization proceeds, pure carbon crystals (diamonds) could float or migrate within the fluid interior, while oxygen and nitrogen become trapped or sequestered in deeper layers, leaving the observable atmosphere depleted in those elements but enriched in helium and molecular carbon.[cite:462][cite:474] This might explain why the atmosphere is so carbon-heavy and oxygen-poor, yet even this model does not fully account for all of the planet’s observed properties. Researchers emphasize that PSR J2322-2650b likely holds clues to exotic pathways of planetary evolution in post-supernova environments — pathways that blur the line between ablated stellar cores, failed stars, and bona fide planets.[cite:460][cite:472]

Implications for Exoplanetary Science and Extreme Worlds

1. Planets in Post-Supernova Systems

   PSR J2322-2650b joins a very small class of known pulsar planets, which includes the first exoplanets ever discovered around PSR B1257+12 in the early 1990s. Yet unlike those rocky, Earth-mass bodies, PSR J2322-2650b is a gas giant with a directly measured atmosphere, making it the first pulsar planet characterized in such detail.[cite:471][cite:472] Its existence confirms that planets (or planetary remnants) can survive, or form anew, in the chaotic aftermath of a supernova, orbiting neutron stars at extremely close separations. For the Exoplanet Discovery Program, this expands the catalog of viable planetary environments to include not just sunlike and red dwarf stars, but also compact remnants whose harsh radiation and gravity were once thought to preclude stable, observable planets. JWST’s ability to isolate planetary light in systems where the host emits primarily at inaccessible energies (gamma rays in this case) opens a new observational window on these exotic systems.

2. Using Pulsars and JWST as Precision Laboratories

   The PSR J2322-2650b system is also a precision laboratory for testing atmospheric physics under extreme conditions. Because JWST effectively “does not see” the pulsar, observers obtain a remarkably clean transmission and emission spectrum of the planet across its orbit, free from stellar contamination.[cite:473][cite:472] At the same time, pulsar timing provides exquisitely accurate measurements of the planet’s orbital period and mass function, anchoring dynamical models. Combining these tools enables detailed phase-curve analyses, shape modeling, and constraints on the planet’s temperature distribution and tidal distortion. Such synergy between high-energy pulsar astronomy and infrared exoplanet spectroscopy is rare, but PSR J2322-2650b demonstrates its power: by exploiting the unusual spectral mismatch between host and planet, scientists can probe atmospheric chemistry, dynamics, and energy transport in a regime far beyond the well-trodden territory of hot Jupiters around main-sequence stars.[cite:466][cite:471]

Conclusion: Redefining What a Planet Can Be