In this blog post, we’ll examine the unique shape and origin of PSR J2322-2650b, discovered orbiting a pulsar, and explore the possibility of whether this celestial object is simply a planet or the remnants left behind after a star collapsed.
Planets have generally been thought to be round. Not only the planets in our solar system, including Earth, but also nearly all exoplanets discovered to date exhibit a shape that is close to spherical. But what if a planet existed that was elongated and flattened, resembling a lemon? Discovered approximately 2,000 light-years from Earth, PSR J2322-2650b challenges our conventional understanding of planets. Its atmosphere is rich in carbon, and there is even speculation that diamonds may have formed under the extreme pressure conditions within its core. These findings, based on an analysis of data from NASA’s James Webb Space Telescope conducted by researchers at the University of Chicago, were published in *The Astrophysical Journal Letters*.
Planets are round because their own gravity acts almost uniformly from all directions, pulling matter toward the center. This gravitational force gradually flattens irregularities on the surface, shaping the celestial body into a sphere. Earth, too, exhibits a slightly bulging equator due to its rotation, but the difference between its equatorial and polar diameters is only about 0.3%. However, PSR J2322-2650b deviates from this general rule. According to the research team’s theoretical model, the planet is estimated to be elongated by about 38% in the equatorial direction compared to its polar diameter, giving it a shape similar to a lemon or a rugby ball. The cause of this extreme deformation is that the planet orbits a pulsar rather than a typical star. A pulsar is a neutron star left over after a supernova explosion—an extremely dense celestial object with a mass similar to the Sun, compressed to an extreme degree. As it rotates rapidly, it emits powerful radio beams in the direction of its magnetic poles; whenever these beams pass by Earth, signals arrive at regular intervals. Because the period of these signals is extremely stable, pulsars are called “the most accurate clocks in the universe.”
The existence of PSR J2322-2650b was also first revealed through subtle fluctuations in this “cosmic clock.” During a detailed analysis of observational data from the Parkes Radio Telescope in Australia in 2017, researchers discovered that the arrival time of the radio signals was gradually advancing and then delaying in a cycle of approximately 7.75 hours. The research team interpreted this as a sign that the gravity of an invisible companion object was causing the pulsar to wobble, and as a result, the existence of this planet-sized object was revealed. The planet orbits in a very close orbit, approximately 1.6 million km from the pulsar, which is only about 1% of the distance between Earth and the Sun. At such a close distance, the pulsar’s gravity cannot act uniformly across the entire planet; instead, the difference in gravitational force between the side facing the pulsar and the opposite side—known as tidal forces—exerts a significant influence. As a result, the planet appears to have been deformed into its current asymmetrical shape due to continuous stretching forces acting in one direction.
Furthermore, the planet completes one orbit every approximately 7.75 hours, and its rotation period is identical, meaning it is in a state of tidal locking where the same face always faces the pulsar. This phenomenon is similar to how the Moon always shows the same face to Earth; as a result, one side of the planet is continuously pulled by strong gravity, while the opposite side remains stretched outward.
The nature of its atmosphere is also highly unusual. According to observational data, signals of molecular carbon—composed of two or three carbon atoms—have been detected, rather than the hydrogen and helium typically found in exoplanet atmospheres. The research team explains that in such an environment, minute carbon particles could aggregate to form soot-like substances, and under the extreme pressure conditions at the core, carbon might even transform into a diamond structure. The physical environment is also extreme. The temperature on the side facing the pulsar is estimated to reach thousands of degrees, and the opposite side is also likely to maintain temperatures of several hundred degrees or higher. These extreme temperature differences and asymmetric energy influx create powerful atmospheric flows within the planet, potentially forming strong westerly jets flowing in the opposite direction of the planet’s rotation.
Although the origin of this planet has not yet been clearly established, the most plausible explanation is the “black widow pulsar” hypothesis. This theory posits that the pulsar gradually absorbed a nearby companion star, stripping it of its mass. It is possible that PSR J2322-2650b was originally a small star composed of helium, but lost most of its mass due to the pulsar’s intense radiation and particle winds, shrinking to its current planetary size. If this hypothesis is true, this celestial body would not be a simple planet but would fall under a new classification: “remnant of a star.”
There have been other cases in the past that have challenged the very definition of a planet. In 1992, astronomers Alexander Wolszczan and Dale Frail discovered the first exoplanet orbiting the pulsar PSR B1257+12, located about 2,300 light-years from Earth. At the time, it was almost unimaginable that a planet could exist in the extreme environment following a supernova explosion, but over the past 30 years, the number of discovered exoplanets has surpassed 5,000. Nevertheless, planets discovered around pulsars remain rare cases, as most existing planetary systems are destroyed during the supernova explosion process. Although a 2022 study using the Hubble Space Telescope and the CHEOPS telescope revealed that WASP-103b has a rugby ball-like, flattened shape, the deformation of PSR J2322-2650b is far more extreme. This demonstrates that the unique environment of a pulsar can even shatter the long-held common sense that “planets are round.”
Many questions remain unanswered. Future observations and research will need to determine how this planet’s unique atmosphere formed, and whether it resulted from deformation by the pulsar or if it was an object with unusual properties from the very beginning. In an interview, Michael Zhang, a professor in the Department of Astronomy at the University of Chicago who led the study, stated, “PSR J2322-2650b is the most extremely flattened planet confirmed to date,” adding, “It may even be a new type of celestial body that hasn’t even been named yet.” Ultimately, this planet goes beyond a simple discovery; it provides crucial clues that force us to rethink the very concept of a “planet” as we know it.
