TOI-1883 b is remarkably, almost impossibly puffy—a super-Neptune 383 light-years away with a density so low it would float in water, if water existed on that scale. Using the Subaru Telescope, an international team led by Izuru Fukuda of the University of Tokyo has just revealed the true identity of this strange alien world, confirming what earlier observations had hinted at: a planet that challenges our understanding of how worlds form and survive near their stars.
When TOI-1883 b was first discovered in 2024, orbiting an M-dwarf star called TOI-1883 every 4.5 days, astronomers knew it was unusual. The planet was more than five times larger than Earth, but its mass remained a mystery. To solve that puzzle, Fukuda's team turned to the InfraRed Doppler instrument on the Subaru Telescope and complemented their work with transit observations using the Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets, or MuSCAT. What they discovered is a world that sits at the extreme edge of planetary possibility.
TOI-1883 b has a mass of approximately 13.7 Earth masses, which combined with its radius of 5.65 Earth radii yields a mean density of just 0.4 grams per cubic centimeter. To put that in perspective, that makes it the lowest-density planet yet found among super-Neptunes orbiting M dwarfs—cooler, smaller red dwarf stars. The planet is so inflated it strains belief. The research, published June 5 on the arXiv preprint server, classifies TOI-1883 b as belonging to an exclusive and rare category called the Neptunian ridge, a narrow orbital sweet spot between 3.2 and 5.7 days where few planets are found, despite how easy they should be to detect.
This rarity hints at a dramatic story. Fukuda's team proposes that TOI-1883 b likely migrated inward through its original protoplanetary disk, then suffered catastrophic atmospheric loss due to intense ultraviolet radiation from its host star. Yet the planet didn't become a barren rock. The host star's relatively high metallicity—about 0.32, higher than our own sun—may have prevented the planet from accruing enough gas to become a true gas giant in the first place. Instead, it ended up inflated with what remains of its atmosphere, a lightweight world orbiting close to its star in just 4.5 days.
The discovery matters because TOI-1883 b sits squarely in the Neptunian desert, a mysterious depletion of short-period planets with intermediate sizes. For years, astronomers have puzzled over why such worlds are so rare, when their detection should be easier than distant giants. TOI-1883 b offers a clue: planets in this size range may be common enough, but few survive the brutal process of migration and stellar irradiation intact. This world did—barely—becoming a ghost of what it once was, puffy and strange.
To truly confirm how TOI-1883 b came to be, Fukuda's team notes that detailed atmospheric characterization and even more precise radial velocity measurements will be needed. For now, this puffy super-Neptune stands as a reminder that the cosmos contains worlds stranger than our theories often predict, and 383 light-years away, it orbits silently, challenging everything we thought we knew.