A remnant planetary core in the hot-Neptune desert

Abstract

The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, difficulties in observation lead to large uncertainties in the properties of planetary cores. Exoplanets that have undergone rare evolutionary processes provide a route to understanding planetary interiors. Planets found in and near the typically barren hot-Neptune ‘desert’1,2 (a region in mass–radius space that contains few planets) have proved to be particularly valuable in this regard. These planets include HD149026b3, which is thought to have an unusually massive core, and recent discoveries such as LTT9779b4 and NGTS-4b5, on which photoevaporation has removed a substantial part of their outer atmospheres. Here we report observations of the planet TOI-849b, which has a radius smaller than Neptune’s but an anomalously large mass of \(39.1{\,}_{-2.6}^{+2.7}\) Earth masses and a density of \(5.2{\,}_{-0.8}^{+0.7}\) grams per cubic centimetre, similar to Earth’s. Interior-structure models suggest that any gaseous envelope of pure hydrogen and helium consists of no more than \({3.9}_{-0.9}^{+0.8}\) per cent of the total planetary mass. The planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it could have avoided substantial gas accretion, perhaps through gap opening or late formation6. Although photoevaporation rates cannot account for the mass loss required to reduce a Jupiter-like gas giant, they can remove a small (a few Earth masses) hydrogen and helium envelope on timescales of several billion years, implying that any remaining atmosphere on TOI-849b is likely to be enriched by water or other volatiles from the planetary interior. We conclude that TOI-849b is the remnant core of a giant planet.

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Fig. 1: Best-fitting model to the TESS, HARPS and NGTS data.
Fig. 2: Mass–radius diagram of known exoplanets from the NASA exoplanet archive.
Fig. 3: TOI-849b in the context of the hot-Neptune desert.

Data availability

TESS data are publicly available at MAST (https://archive.stsci.edu/missions-and-data/transiting-exoplanet-survey-satellite-tess). The HARPS data used in this study are available within the paper or the Supplementary Information files and were collected under ESO programme ID 1102.C-0249. The NGTS (Data Tags 19249 and 19250), LCOGT (Data Tags 5106 and 5386) and specific detrended TESS light curve (Data Tag 19248) used in this work are available via the Exofop-TESS archive (https://exofop.ipac.caltech.edu/tess/).

Code availability

The PASTIS code has been published previously11,59. The latest version of the ARES code (ARES v2) is available at http://www.astro.up.pt/~sousasag/ares.

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Acknowledgements

This paper includes data collected by the TESS missions, which are publicly available from MAST. Funding for the TESS mission is provided by NASA’s Science Mission directorate. We acknowledge the use of public TESS Alert 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) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. This research made use of the Exoplanet Follow-up Observation Program website and the NASA Exoplanet Archive, which are operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This work makes use of observations from the LCOGT network and is based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme IDs 1102.C-0249 and P103.C-0449. Some of the observations presented in the paper used the High-Resolution Imaging instrument Zorro at Gemini South (programme ID GS-2019B-Q-111). Zorro was funded by the NASA Exoplanet Exploration Program and built at the NASA Ames Research Center by S.B.H., N. Scott, E. P. Horch and E. Quigley. D.J.A., D.V. and S.L.C. acknowledge support from the STFC via Ernest Rutherford Fellowships ST/R00384X/1, ST/P003850/1 and ST/R003726/1, respectively. M.B. and S.Gandhi acknowledge support from the STFC research grant ST/S000631/1. G.M.K. is supported by the Royal Society as a Royal Society University Research Fellow. F.M. acknowledges support from a Royal Society Dorothy Hodgkin Fellowship. K.G.S. acknowledges partial support from NASA grant 17-XRP17 2-0024. C.Z. is supported by a Dunlap Fellowship at the Dunlap Institute for Astronomy and Astrophysics, funded through an endowment established by the Dunlap family and the University of Toronto. A.W.M. was supported by NASA grant 80NSSC19K0097 to the University of North Carolina at Chapel Hill. D.J.A.B. acknowledges support from the UK Space Agency. C.X.H. and M.N.G. acknowledge support from the Juan Carlos Torres Fellowship. This work was financed by FEDER (Fundo Europeu de Desenvolvimento Regional) funds through the COMPETE 2020 Operational Programme for Competitiveness and Internationalisation (POCI) and by Portuguese funds through FCT (Fundação para a Ciência e a Tecnologia) in the framework of projects UID/FIS/04434/2019; PTDC/FIS-AST/32113/2017 and POCI-01-0145-FEDER-032113; PTDC/FIS-AST/28953/2017 and POCI-01-0145-FEDER-028953. S.G.S., V.A., S.C.C.B. and O.D.S.D. acknowledge support from FCT through Investigador FCT contracts IF/00028/2014/CP1215/CT0002, IF/00650/2015/CP1273/CT0001, IF/01312/2014/CP1215/CT0004 and DL 57/2016/CP1364/CT0004. S.H. acknowledges support from fellowships PD/BD/128119/2016 funded by FCT (Portugal). Work by J.N.W. was partly funded by the Heising-Simons Foundation. C.A.W. acknowledges support from UK Science Technology and Facility Council grant ST/P000312/1. J.L.-B. and D. Barrado are funded by the Spanish State Research Agency (AEI) Projects ESP2017-87676-C5-1-R and MDM-2017-0737 Unidad de Excelencia María de Maeztu – Centro de Astrobiología (INTA-CSIC). J.S.J. acknowledges funding by Fondecyt through grant 1161218 and partial support from CATA-Basal (PB06, Conicyt). J.I.V. acknowledges support from CONICYT-PFCHA/Doctorado Nacional-21191829, Chile. The French group acknowledges financial support from the French Programme National de Planétologie (PNP, INSU). F.M. acknowledges support from the Royal Society Dorothy Hodgkin Fellowship. C.M. and A.E. acknowledge support from the Swiss National Science Foundation under grant BSSGI0_155816 “PlanetsInTime”. Parts of this work have been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation.

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Contributions

D.J.A. is Principal Investigator of the NCORES HARPS programme, which measured the planet’s mass, and a member of the NGTS consortium; D.J.A. developed much of the text and main figures and coordinated all contributions. T.A.L. performed the joint PASTIS analysis. V.A., S.G.S. and N.C.S. performed stellar spectral analysis including chemical abundances. R.A.B. and F.M. provided text analysing potential formation scenarios. K.A.C. and E.L.N.J. coordinated the TFOP SG1 photometric follow-up of the system. K.I.C. and T.G. performed analysis of the LCOGT photometric follow-up of the system. A.E. and C.M. applied and analysed the Bern Population Synthesis Models. C.X.H. and L.S. developed and ran the MIT Quick Look Pipeline, which identified the candidate in the TESS data. G.W.K. performed the photoevaporation analysis. J.L.-B. obtained and analysed the Astralux data and synthesized all high-resolution imaging results. E.M. obtained the NaCo imaging data. H.O. contributed to the NCORES HARPS programme and the NGTS survey and helped to create the main figures. J.O., O.M., M.D., R.H., M. Lozovsky and C.D. performed the interior-structure calculations. D.V. performed analysis on the potential for tidal self-disruption. C.Z. obtained the SOAR data and provided text summarising the SOAR results. T.-G.T. obtained a further transit with the PEST telescope. J.J.L. contributed to the internal structure discussion. K.G.S. provided the independent check of stellar parameters. M.B. and S. Gandhi calculated estimates of the required telescope time for atmospheric characterization. D.R.A., M.M., E.M.B., C.A.W., J.S.J., J.I.V., J.S.A., D. Bayliss, C. Belardi, M.R.B., S.L.C., A.C., P.E., S. Gill, M.R.G., M.N.G., M. Lendl, J.M., D.P., D.Q., L.R., R.H.T. and R.G.W. contributed to the NGTS facility, in planning, management, data collection or detrending. D.J.A.B., S.H., D. Barrado, S.C.C.B., P.A.W., L.D.N., D. Bayliss, F.B., B.F.C., R.F.D., O.D.S.D., X.D., P.F., J.J., G.M.K., A.S., S.U., P.A.W., J.M.A. and A.O. contributed to the HARPS large programme under which the HARPS data were obtained. D.R.C., I.J.M.C., J.E.S. and S.B.H. contributed to the NaCo imaging data. C. Briceño, N.L. and A.W.M. contributed to the SOAR imaging data. K.D.C., M.F., J.S.J., E.L.N.J., G.R.R., P.R., S.S., E.T., R.V., J.N.W., J.N.V. and Z.Z. provided essential contributions to the TESS mission, which discovered the candidate. All authors read the manuscript and provided general comments.

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Correspondence to David J. Armstrong.

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Extended data figures and tables

Extended Data Fig. 1 Photometric data captured by the LCOGT network.

a, b, Data captured on the nights of 30 July 2019 ut (a) and 9 August 2019 ut (b). The best-fitting model is plotted in red and the binned data in orange. Error bars of individual points show one standard deviation. In the case of binned measurements, points and error bars show the weighted mean and its standard error, respectively.

Extended Data Fig. 2 HARPS activity correlation indicators.

a, HARPS radial velocities plotted against their bisector value. Colours represent the time of observation measured in BJD–2,400,000. b, As for a, for the FWHM of the CCF. No correlation is seen in either case. All error bars show one standard deviation.

Extended Data Fig. 3 Tests on the HARPS residuals.

a, CCF for the HARPS spectra computed using a G2V template. A Gaussian fit has been removed to leave the residual noise. No clear evidence of a contaminating star is seen. b, Periodogram of the HARPS RV residuals. No evidence of periodic structure is found. FAP represents false-alarm probability.

Extended Data Fig. 4 Collected high-resolution imaging results from AstraLux/CAHA, NaCo/VLT, HRCam/SOAR and Zorro/Gemini (562 nm).

ac, Images from AstraLux (a), NaCo (b) and HRCam (c). d, Sensitivity curves for ac and the Zorro 562-nm observation. Our simultaneous 832-nm Zorro observation provides a similar result. The 1% and 10% contrast curves are also plotted.

Extended Data Fig. 5 TOI-849 compared to field stars.

Abundance ratio [X/Fe] against stellar metallicity for TOI-849 (black) and for field stars from the HARPS sample (grey) with similar stellar parameters: Teff = 5,329 ± 200 K, logg = 4.28 ± 0.20 dex and [Fe/H] = +0.20 ± 0.20 dex. All error bars show one standard deviation.

Extended Data Fig. 6 Planet mass against time for three similar planets to TOI-849b in the Bern Population Synthesis models.

Grey shaded regions mark the parameters of TOI-849b. Stars mark the time of a giant impact. The inset shows the envelope mass, which is removed after collision.

Extended Data Table 1 List of stellar and planetary parameters used in the analysis
Extended Data Table 2 List of instrument parameters used in the analysis
Extended Data Table 3 Stellar properties of TOI-849
Extended Data Table 4 HARPS radial velocities

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Armstrong, D.J., Lopez, T.A., Adibekyan, V. et al. A remnant planetary core in the hot-Neptune desert. Nature 583, 39–42 (2020). https://doi.org/10.1038/s41586-020-2421-7

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