'Puffy' super-Neptune emerges 383 light-years away with a density of just 0.4 g/cm³

June 12, 2026 report 'Puffy' super-Neptune emerges 383 light-years away with a density of just 0.4 g/cm³ Tomasz Nowakowski Author Stephanie Baum Scientific Editor Robert Egan Associate Editor Using the Subaru Telescope, astronomers have conducted follow-up observations of a recently discovered exoplanet known as TOI-1883 b. Results of the new observations, published June 5 on the arXiv preprint server, indicate that TOI-1883 b is a low-density super-Neptune. Newfound super-sized alien world TOI-1883, also known as TIC 348755728, is an M-dwarf star at a distance of some 383 light-years, about half the size and mass of the sun. In 2024, an exoplanet orbiting this star every 4.5 days was discovered and received the designation TOI-1883 b. The alien world is more than five times larger than our planet. However, its mass had remained undetermined. In order to shed more light on the properties of TOI-1883 b, a team of astronomers led by Izuru Fukuda of the University of Tokyo in Japan decided to revisit this system with the InfraRed Doppler (IRD) instrument on the Subaru Telescope. Their study was complemented by data from the Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT). "After TOI-1883 b was validated as a bona fide planet, we attempted to determine its mass through RV [radial velocity] observations using the IRD instrument mounted on the Subaru Telescope. (...) Furthermore, we carried out transit observations with the MuSCAT series and examined potential transit timing variations (TTVs) that could arise from gravitational perturbations induced by an additional outer planet," the researchers explained. On the Neptunian 'ridge' The observations found that TOI-1883 b has a mass of approximately 13.7 Earth masses, which—together with its radius of 5.65 Earth radii—indicates a mean density of 0.4 g/cm3. Based on these properties, Fukuda's team classified TOI-1883 b as a super-Neptune. Therefore, TOI-1883 b turns out to be the lowest-density planet so far among super-Neptunes around M dwarfs. It is also likely to be an extremely inflated, "puffy" planet. Additionally, it represents the so-called Neptunian desert—a depletion of alien worlds at short orbital periods (below 10 days) with radii between "super-Earth" and subjovian planets, despite their detection advantages. More specifically, TOI-1883 b belongs to the subclass of the Neptunian desert known as the ridge, characterized by orbital periods within the range of 3.2–5.7 days. The evolution of TOI-1883 b The authors of the study noted that the location of TOI-1883 b on the ridge and its low density suggest that the planet may have migrated inward via disk-driven migration and subsequently experienced photoevaporative mass loss driven by stellar extreme ultraviolet irradiation. They added that the metallicity of the host star, which is estimated to be 0.32 and is therefore super-solar, may have inhibited runaway gas accretion, preventing TOI-1883 b from forming as a gas giant while it migrated inward. However, atmospheric characterization and further high-precision radial velocity measurements are required in order to confirm the assumed formation and evolution scenario. Written for you by our author Tomasz Nowakowski, edited by Stephanie Baum, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details Izuru Fukuda et al, The mass of TOI-1883 b: A low density super-Neptune in the ridge regime transiting an early-M dwarf, arXiv (2026). DOI: 10.48550/arxiv.2606.06868 Journal information: arXiv © 2026 Science X Network