Two gas giants orbiting a distant star have been identified as among the least dense planets ever discovered, with densities so low they rival that of cotton candy, according to a study published in Nature on July 9, 2026. The findings challenge existing models of planetary formation and raise new questions about the stability and evolution of such extreme worlds.
What Happened
Astronomers using data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and ground-based observatories confirmed the existence of two “super-puff” exoplanets—WASP-193 b and TOI-4201 b—both of which exhibit densities far lower than any known gas giants in our solar system. WASP-193 b, located approximately 1,200 light-years from Earth, has a radius 1.5 times that of Jupiter but only 14% of its mass, resulting in a density of just 0.059 grams per cubic centimeter. For comparison, cotton candy has a density of about 0.05 grams per cubic centimeter. TOI-4201 b, while slightly denser at 0.089 grams per cubic centimeter, still falls into the super-puff category.
The planets were detected using the transit method, which measures the dimming of a star’s light as a planet passes in front of it. Follow-up observations from ground-based telescopes, including the High Accuracy Radial velocity Planet Searcher (HARPS) and the European Southern Observatory’s Very Large Telescope (VLT), confirmed their unusually low densities.
Why It Matters
The discovery of WASP-193 b and TOI-4201 b challenges long-held assumptions about planetary formation and stability. Most gas giants, such as Jupiter and Saturn, have dense cores surrounded by thick atmospheres, but these super-puffs appear to lack the gravitational pull needed to compress their atmospheres into a more compact form. Their existence suggests that current models of planetary evolution may be incomplete or that additional factors—such as extreme atmospheric heating, tidal forces, or residual heat from formation—play a role in shaping these worlds.
The findings also underscore the diversity of exoplanetary systems. While our solar system contains gas giants with well-defined structures, the universe appears to host planets with far more extreme and unexpected properties. Understanding how these super-puffs form and persist could provide new insights into the processes that govern planetary atmospheres and their long-term stability.
Background and Context
Super-puff exoplanets were first identified in the early 2010s, with a handful of such worlds detected in the following years. These planets are characterized by their large radii—often comparable to or exceeding that of Jupiter—but with masses far lower than expected, resulting in densities similar to that of styrofoam or, in this case, cotton candy. Prior to this discovery, the least dense known exoplanet was Kepler-51 b, which has a density of approximately 0.03 grams per cubic centimeter.
The mechanisms behind the formation of super-puffs remain poorly understood. One leading theory suggests that extreme heating from their host stars causes their atmospheres to expand, reducing their overall density. Another possibility is that these planets formed farther from their stars and migrated inward, retaining heat from their formation that keeps their atmospheres inflated. However, neither theory fully explains why these planets have not lost their atmospheres over time, given their proximity to their host stars.
Competing Claims and Uncertainty
While the discovery of WASP-193 b and TOI-4201 b is well-supported by observational data, the exact mechanisms driving their low densities remain a subject of debate. Some researchers argue that tidal forces—gravitational interactions between the planets and their host stars—could be heating their interiors and inflating their atmospheres. Others suggest that residual heat from the planets’ formation, combined with ongoing stellar radiation, may be sufficient to maintain their puffy states.
There is also uncertainty about the long-term stability of these worlds. Given their low densities, it is unclear how they avoid losing their atmospheres to space over time. Some models predict that super-puffs may be short-lived phenomena, with their atmospheres gradually dissipating as they age. However, the fact that these planets have been observed at all suggests that they may be more stable than previously thought, or that they are replenishing their atmospheres through some unknown process.
What to Watch Next
The discovery of WASP-193 b and TOI-4201 b opens several avenues for future research. Astronomers are likely to conduct follow-up observations using the James Webb Space Telescope (JWST) to analyze the atmospheric compositions of these planets. Spectroscopic data could reveal the presence of specific gases, such as hydrogen and helium, and provide clues about the processes driving their inflation.
Additionally, researchers may explore whether other super-puffs share similar characteristics or if WASP-193 b and TOI-4201 b represent a unique subclass of these planets. Further detections of super-puffs could help refine models of planetary formation and evolution, particularly for gas giants in close orbits around their host stars.
Finally, the discovery raises questions about the potential for super-puffs to host moons or rings. Given their low densities, these planets may have weak gravitational fields, making it difficult for them to retain large satellites. However, if such moons or rings exist, they could provide additional insights into the dynamics of these unusual systems.
Conclusion
The confirmation of WASP-193 b and TOI-4201 b as two of the least dense planets ever discovered marks a significant milestone in exoplanet research. Their existence challenges current understanding of planetary formation and stability, highlighting the need for new theories to explain how such extreme worlds can persist. As astronomers continue to study these super-puffs, their findings could reshape our knowledge of the universe’s vast and diverse planetary systems.
Story synopsis gathered from: [Nature](https://www.nature.com/articles/d41586-026-02114-2) — source.
Corrections
If you believe this article contains an error, contact Herald Express with the source URL and supporting evidence.
Story synopsis gathered from: Nature — source.
