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 atmospheres1,2 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 desert6, 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.
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TESS photometric time series can be freely obtained from the Mikulski Archive for Space Telescopes (MAST) archive at https://exo.mast.stsci.edu/. All follow-up light-curve data are available on the ExoFOP-TESS website (https://exofop.ipac.caltech.edu/tess/target.php?id=73540072). RVs are presented in Extended Data Table 1. The simulation dataset of Methods section ‘Detailed impact simulations’ is available on Zenodo (https://doi.org/10.5281/zenodo.8033965)124. Source data are provided with this paper.
The juliet Python code is open source and available at https://github.com/nespinoza/juliet. The PYRAT BAY modelling framework is open source and available at https://github.com/pcubillos/pyratbay. astropy is a common core package for astronomy in Python and EXOFASTv2 is a well-known public exoplanet fitting software. swift symba5 is available at https://github.com/silburt/swifter. SWIFT is available at www.swiftsim.com. WoMa is available at https://github.com/srbonilla/WoMa. The repack package is available at https://github.com/pcubillos/repack. PandExo is available at https://github.com/natashabatalha/PandExo.
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We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center. Resources supporting this work were provided by the NASA High-End Computing (HEC) programme through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. The work is based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This work has also made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). This work makes use of observations from the LCOGT network. Part of the LCOGT telescope time was granted by NOIRLab through the Mid-Scale Innovations Program (MSIP). MSIP is funded by the National Science Foundation (NSF). This research has made use of the Exoplanet Follow-up Observing Program (ExoFOP; https://doi.org/10.26134/ExoFOP5) website, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This paper makes use of observations made with the MuSCAT2 instrument, developed by the Astrobiology Center at Telescopio Carlos Sánchez operated on the island of Tenerife by the IAC in the Spanish Observatorio del Teide and is also based in part on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia e Inovações do Brasil (MCTI/LNA), the US NSF’s NOIRLab, the University of North Carolina at Chapel Hill (UNC) and Michigan State University (MSU). This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grants 51NF40-182901 and 51NF40-205606. This paper made use of observations from the high-resolution imaging instrument ‘Alopeke, which were obtained under Gemini LLP proposal number GN/S-2021A-LP-105. ‘Alopeke was funded by the NASA Exoplanet Exploration Program and built at the NASA Ames Research Center by S. B. Howell, N. Scott, E. P. Horch and E. Quigley. ‘Alopeke was mounted on the Gemini North telescope of the international Gemini Observatory, a programme of the NSF’s OIR Lab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF. On behalf of the Gemini partnership: the NSF (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil) and Korea Astronomy and Space Science Institute (Republic of Korea). The giant impact simulations were carried out using the computational facilities of the Advanced Computing Research Centre, University of Bristol. Funding for the DPAC has been provided by national institutions, in particular, the institutions participating in the Gaia Multilateral Agreement. L.N. and D.L. acknowledge the support of the ARIEL ASI-INAF agreement 2021-5-HH.0. L. Mancini acknowledges support from the ‘Fondi di Ricerca Scientifica d’Ateneo 2021’ of the University of Rome “Tor Vergata”. A. Maggio and A.S.B. acknowledge support from the ASI-INAF agreement no. 2018-16-HH.0 (THE StellaR PAth project) and from PRIN INAF 2019. P.E.C. is funded by the Austrian Science Fund (FWF) Erwin Schroedinger Fellowship programme J4595-N. Funding for the TESS mission is provided by NASA’s Science Mission Directorate. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement SCORE no. 851555). S.B.H. acknowledges funding from the NASA Exoplanet Program Office. K.A.C. acknowledges support from the TESS mission through sub-award s3449 from MIT. T.Z. acknowledges support from CHEOPS ASI-INAF agreement no. 2019-29-HH.0. J.D. acknowledges funding support from the Chinese Scholarship Council (no. 202008610218,). This work is partly supported by JSPS KAKENHI grant numbers JP17H04574 and JP18H05439 and JST CREST grant number JPMJCR1761. We acknowledge DOE-NNSA grant number DE-NA0004084 to Harvard University: Z Fundamental Science Program.
The authors declare no competing interests.
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Extended data figures and tables
The light curves of MuSCAT2, LCOGT and ULMT have been shifted on the y axis for clarity and their respective filter band is indicated in the legend. The superimposed points represent approximately 10-min bins, whereas the error bars represent one standard deviation. The global fit from this work is depicted in black.
Final sensitivity of Keck (a), Gemini (b) and SOAR (c), plotted as a function of angular separation from the host star. The images reach a contrast of about 7.6 (a), about 5.2 and 6.3 (b) and about 4.7 (c) magnitudes fainter than TOI-1853 within 0.5″ in each respective band. Images of the central portion of the data are presented as insets in the relative panels.
The error bars represent one standard deviation. The best-fit model is shown as a solid black line.
The periodograms of the RVs, its residuals from the global fit and several activity indexes are plotted consecutively. The window function is on top as reference. The main peak of the RV GLS periodogram, at 1.24 days and its 1-day aliases, are highlighted by a red and green vertical bar, respectively. The horizontal dashed lines mark the 10% and 1% confidence levels (evaluated with the bootstrap method), respectively.
The blue lines indicate the average value of every parameter and the dashed vertical lines indicate the confidence levels at one standard deviation.
The colour scale expresses the detection function (for example, the detection probability) and the red circle marks the position of TOI-1853 b.
Extended Data Fig. 7 Transmission spectroscopy metric, emission spectroscopy metric and simulated spectra for JWST.
a,c Transit and emission spectroscopic metrics for TOI-1853 b (golden hexagon marker) in comparison with the population of transiting exoplanets (grey markers) and those selected for JWST cycles 1 and 2 observations (purple and green markers). TOI-1853 b has a transmission spectroscopy metric of 2.6 and an emission spectroscopy metric of 10.9. b, Synthetic transmission spectra for an H2-dominated atmosphere (solid orange line) and an H2O-dominated atmosphere (solid blue line). The markers with 1σ error bars show simulated JWST observations for selected detectors when combining three transits each. d, Same as b but for synthetic emission spectra.
The file includes a numerical description of the dynamical simulations carried out in Methods section ‘Formation simulations’ and a brief description of TOI-1853 b transit-time-variations analysis (that is, observed minus calculated transit times from the light curves), along with a figure, as proof that there is no measurable deviation from the expected orbital period of TOI-1853 b.
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Naponiello, L., Mancini, L., Sozzetti, A. et al. A super-massive Neptune-sized planet. Nature 622, 255–260 (2023). https://doi.org/10.1038/s41586-023-06499-2