What kind of planet wears a diamond heart and a soot-filled sky? Astronomers now have an answer but it’s stranger than they ever imagined. PSR J2322-2650 b is a Jupiter-mass exoplanet about 750 light-years from Earth and has left researchers amazed with its atmospheric composition unlike anything seen so far. The atmosphere of this planet, detected and analyzed by NASA’s James Webb Space Telescope, JWST, is dominated by helium and molecular carbon-particularly C₂ and C₃-but does not detect oxygen or nitrogen. This is highly unusual because, at the 1,200°F nightside and 3,700°F dayside temperatures, carbon should easily bond with those elements. Instead, carbon clouds like soot float in the upper atmosphere, and deeper inside, carbon might crystallize into diamonds at very high pressure.
The shape of the planet is also unusually weird. It is stretched into an ellipsoid often referred to as “lemon-shaped” by tidal forces coming from its host star, a millisecond pulsar. Pulsars are the cores of massive stars that exploded as supernovae and have compressed as much as twice the mass of the Sun into a sphere about 12 miles across. This pulsar rotates particularly fast, emitting beams of gamma rays and high-energy particles but very little infrared light. That turned out to be good news for JWST’s infrared equipment, which captured a remarkably clean spectrum of the planet with minimal interference from the star’s glow. According to Maya Beleznay from Stanford University, “We get a really pristine spectrum. And we can better study this system in more detail than normal exoplanets.”
The orbital dynamics of PSR J2322-2650 b are extreme. The planet orbits its pulsar at a distance of only 1 million miles-about 1% of the Earth–Sun separation-completing a “year” in a scant 7.8 hours. This close proximity creates interior gravitational stresses that are severe enough to distort the planet’s shape and may also promote atmospheric loss. The system resembles a rare “black widow” configuration, in which a pulsar strips material from a companion. Usually, that companion is a low-mass star, but here it is a planet. That stripping process, along with the pulsar’s wind and radiation, might account for some of the atmospheric anomalies, though no existing model accounts for the planet’s extreme carbon enrichment.
The possibility of diamond formation inside PSR J2322-2650 b is supported by laboratory experiments on the interiors of carbon-rich exoplanets. Researchers have used diamond-anvil cells to model the extreme pressures and temperatures deep within such worlds, squeezing silicon carbide with water and heating it with lasers. The tests indicate that, under such conditions, silicon carbide becomes diamond and silica-a process which may occur naturally in many carbon-heavy planets. In the case of PSR J2322-2650 b, Roger Romani of Stanford University proposes a scenario in which, as the planet cools, the carbon and oxygen in its interior start to crystallize. “Pure carbon crystals float to the top and get mixed into the helium, and that’s what we see,” he said. The mystery is how oxygen and nitrogen are kept away, making molecular carbon dominant.
The finding also cuts across state-of-the-art high-pressure physics: recent experiments have squeezed solid carbon to 2,000 gigapascals five times the pressure in Earth’s core showing that diamond’s crystal structure is stable well beyond what theories predict. That robustness indicates the possibility that diamond phases might exist well inside big carbon-rich planets under conditions far more extreme than had been previously considered, further bolstering the case for a diamond-rich interior for PSR J2322-2650 b. From the perspective of observation, JWST’s role was a crucial one. Located a million miles away from Earth, cloaked behind a huge sunshield, the telescope works at cryogenic temperatures, thus enabling sensitive infrared measurements.
In this case, the deficiency in infrared emission allowed scientists to track the brightness variations of the planet across its orbit, model its distorted geometry, and probe its atmosphere with unprecedented detail. “This is a new type of planet atmosphere that nobody has ever seen before,” Michael Zhang of the University of Chicago, who led the study, said. PSR J2322-2650 b also challenges theories about planetary formation. It does not conform to the mold of a regular planet born from a protoplanetary disk, and neither does it conform to the stripped cores seen in typical black widow systems. Its composition seems to “rule out every known formation mechanism,” says Zhang. The very survival of this planet in such a hostile environment, its exotically composed atmosphere, and its distorted shape make it a very appealing target for future observations, not only to understand its origins but also to refine the models of planetary evolution under extreme astrophysical conditions.
