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Neptune-sized planets exhibit a wide range of compositions and densities, depending on factors related to their formation and evolution history, such as the distance from their host stars and atmospheric escape processes. They can vary from relatively low-density planets with thick hydrogen-helium atmospheres 1,2 to higher-density planets with a substantial amount of water or a rocky interior with a thinner atmosphere, such as HD 95338 b (ref.  3 ), TOI-849 b (ref.  4 ) and TOI-2196 b (ref.  5 ). The discovery of exoplanets in the hot-Neptune desert 6 , a region close to the host stars with a deficit of Neptune-sized planets, provides insights into the formation and evolution of planetary systems, including the existence of this region itself. Here we show observations of the transiting planet TOI-1853 b, which has a radius of 3.46 ± 0.08 Earth radii and orbits a dwarf star every 1.24 days. This planet has a mass of 73.2 ± 2.7 Earth masses, almost twice that of any other Neptune-sized planet known so far, and a density of 9.7 ± 0.8 grams per cubic centimetre. These values place TOI-1853 b in the middle of the Neptunian desert and imply that heavy elements dominate its mass. The properties of TOI-1853 b present a puzzle for conventional theories of planetary formation and evolution, and could be the result of several proto-planet collisions or the final state of an initially high-eccentricity planet that migrated closer to its parent star., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

The atmospheres of gaseous giant exoplanets orbiting close to their parent stars (hot Jupiters) have been probed for nearly two decades 1,2 . They allow us to investigate the chemical and physical properties of planetary atmospheres under extreme irradiation conditions 3 . Previous observations of hot Jupiters as they transit in front of their host stars have revealed the frequent presence of water vapour 4 and carbon monoxide 5 in their atmospheres; this has been studied in terms of scaled solar composition 6 under the usual assumption of chemical equilibrium. Both molecules as well as hydrogen cyanide were found in the atmosphere of HD 209458b 5,7,8 , a well studied hot Jupiter (with equilibrium temperature around 1,500 kelvin), whereas ammonia was tentatively detected there 9 and subsequently refuted 10 . Here we report observations of HD 209458b that indicate the presence of water (H 2 O), carbon monoxide (CO), hydrogen cyanide (HCN), methane (CH 4 ), ammonia (NH 3 ) and acetylene (C 2 H 2 ), with statistical significance of 5.3 to 9.9 standard deviations per molecule. Atmospheric models in radiative and chemical equilibrium that account for the detected species indicate a carbon-rich chemistry with a carbon-to-oxygen ratio close to or greater than 1, higher than the solar value (0.55). According to existing models relating the atmospheric chemistry to planet formation and migration scenarios 3,11,12 , this would suggest that HD 209458b formed far from its present location and subsequently migrated inwards 11,13 . Other hot Jupiters may also show a richer chemistry than has been previously found, which would bring into question the frequently made assumption that they have solar-like and oxygen-rich compositions.