Abstract
Temperate Earth-sized exoplanets around late-M dwarfs offer a rare opportunity to explore under which conditions planets can develop hospitable climate conditions. The small stellar radius amplifies the atmospheric transit signature, making even compact secondary atmospheres dominated by N2 or CO2 amenable to characterization with existing instrumentation1. Yet, despite large planet search efforts2, detection of low-temperature Earth-sized planets around late-M dwarfs has remained rare and the TRAPPIST-1 system, a resonance chain of rocky planets with seemingly identical compositions, has not yet shown any evidence of volatiles in the system3. Here we report the discovery of a temperate Earth-sized planet orbiting the cool M6 dwarf LP 791-18. The newly discovered planet, LP 791-18d, has a radius of 1.03 ± 0.04 R⊕ and an equilibrium temperature of 300–400 K, with the permanent night side plausibly allowing for water condensation. LP 791-18d is part of a coplanar system4 and provides a so-far unique opportunity to investigate a temperate exo-Earth in a system with a sub-Neptune that retained its gas or volatile envelope. On the basis of observations of transit timing variations, we find a mass of 7.1 ± 0.7 M⊕ for the sub-Neptune LP 791-18c and a mass of \({0.9}_{-0.4}^{+0.5}{M}_{\oplus }\) for the exo-Earth LP 791-18d. The gravitational interaction with the sub-Neptune prevents the complete circularization of LP 791-18d’s orbit, resulting in continued tidal heating of LP 791-18d’s interior and probably strong volcanic activity at the surface5,6.
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Data availability
The Spitzer data used in this study are publicly available at the Spitzer Heritage Archive, https://sha.ipac.caltech.edu/applications/Spitzer/SHA. The ground-based telescope observations are uploaded to ExoFOP and are publicly available. Source data are provided with this paper.
Code availability
We fit the light-curve data using the open-source tools emcee and batman, available at https://github.com/dfm/emcee and https://github.com/lkreidberg/batman. The TTVs are modelled using the open-source tool TTVFast, available at https://github.com/kdeck/TTVFast, and are also fit using emcee. We obtain the constraints on planet compositions using the open-source tool smint, available at https://github.com/cpiaulet/smint. The tidal heating energy balance calculations are performed with the open-source tool melt, available at https://github.com/cpiaulet/melt.
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Acknowledgements
This work is based principally on observations made with the Spitzer Space Telescope (Spitzer DDT-14309, principal investigator, Benneke), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. The material presented here is based on work supported in part by NASA under contract no. NNX15AI75G. B.B. and M.S.P. acknowledge financial support by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Fonds de Recherche Québécois—Nature et Technologie (FRQNT; Québec). We acknowledge the use of public TESS data from pipelines at the TESS Science Office and at the TESS Science Processing Operations Center, obtained from the Mikulski Archive for Space Telescopes data archive at the Space Telescope Science Institute (STScI). Funding for the TESS mission is provided by the NASA Explorer Program. Resources supporting this work were provided by the NASA High-End Computing Program through the NASA Advanced Supercomputing Division at Ames Research Center for the production of the SPOC data products. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract no. NAS 5–26555. 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. The Mid-Scale Innovations Program is supported by the National Science Foundation. This paper includes data taken from the EDEN telescope network, and we acknowledge support from the Earths in Other Solar Systems Project and Alien Earths (grant nos. NNX15AD94G and 80NSSC21K0593) sponsored by NASA. We acknowledge funding from the European Research Council (ERC) under the grant agreement no. 337591-ExTrA. This research has made use of the ExoFOP, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration. R.C. is supported by a grant from the National Aeronautics and Space Administration in support of the TESS science mission. The MEarth Team gratefully acknowledges funding from the David and Lucile Packard Fellowship for Science and Engineering (awarded to D.C.). This material is based on work supported by the National Science Foundation under grant nos. AST-0807690, AST-1109468, AST-1004488 (Alan T. Waterman Award) and AST-1616624. This work is made possible by a grant from the John Templeton Foundation. The opinions expressed in this publication are those of the authors and do not necessarily reflect the views of the John Templeton Foundation. This material is based on work supported by the National Aeronautics and Space Administration under grant no. 80NSSC18K0476 issued through the XRP Program. M.S.P. and C.P. acknowledge support from FRQNT Master’s and PhD scholarships. We acknowledge support from the Earths in Other Solar Systems Project, grant no. 3013511 sponsored by NASA. The results reported herein benefited from collaborations and/or information exchange within NASA’s Nexus for Exoplanet System Science research coordination network sponsored by NASA’s Science Mission Directorate. The research leading to these results has received funding from the Australian Research Council (ARC) grant for Concerted Research Actions, financed by the Wallonia-Brussels Federation. TRAPPIST is supported by the Belgian Fund for Scientific Research (Fond National de la Recherche Scientifique, FNRS) under the grant no. FRFC 2.5.594.09.F. TRAPPIST-North is a project supported by the University of Liège (Belgium), in collaboration with Cadi Ayyad University of Marrakech (Morocco). M.G. is F.R.S.-FNRS Research Director and E.J. is F.R.S.-FNRS Senior Research Associate. Resources supporting this work were provided by the NASA High-End Computing Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center for the production of the SPOC data products. The VATT referenced herein refers to the Vatican Observatory’s Alice P. Lennon Telescope and Thomas J. Bannan Astrophysics Facility. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (grant no. FP/2007-2013) ERC grant agreement no 336480, from the ARC grant for Concerted Research Actions, financed by the Wallonia-Brussels Federation, from a research grant from the Balzan Prize Foundation, and funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 803193/BEBOP), from the Mobilising European Research in Astrophysics and Cosmology foundation, and from the Science and Technology Facilities Council (grant no. ST/S00193X/1). This work is partly supported by Japan Society for the Promotion of Science (JSPS) KAKENHI grant nos. JP17H04574, JP18H05439, Grant-in-Aid for JSPS Fellows, grant no. JP20J21872, Japan Science and Technolocy (JST) PRESTO grant no. JPMJPR1775, JST CREST grant no. JPMJCR1761 and the Astrobiology Center of National Institutes of Natural Sciences (grant no. AB031010). This article is based on observations made with the MuSCAT2 instrument, developed by the Astrobiology Center (ABC), at Telescopio Carlos Sánchez operated on the island of Tenerife by the Instituto de Astrofísica de Canarias in the Spanish Observatorio del Teide. F.J.P. acknowledges financial support from the grant no. CEX2021-001131-S supported by MCIN/AEI/10.13039/501100011033. R.C. acknowledges support from the Banting fellowship.
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B.B. and M.S.P. conceived the project and wrote the manuscript. M.S.P. and B.B. carried out the main data analysis. C.P. and B.B. performed the tidal heating analysis. M.A.-D. performed the N-body integrations. J.G. performed the stellar characterization with the IRTF–SpeX Prism spectrum observed by J.F. V.G., B.B., I.J.M.C., J.L.C., C.D., T.E., A.W.H., S.K., M.W.W. and D.D. planned and implemented the Spitzer observations. Ground-based transit observations were executed and analysed by K.C., M.S.P., B.B., D.C., F.M., M.C., J.M.A., X.B., A.S., F.J.P., Q.J.S, J.D., J.I., W.W., Z.B.-T., D.A., H.P., E.P., N.N., M.G., E.J., E.D., Z.B., A.F., M.M., T.N. and K.K. P.-A.R., C.P., B.B. and R.C. modelled the mass-loss timescales. E.K. informed the discussion section. TESS observations were made possible by G.R., D.W.L., J.N.W., S.S., H.I., A.B., A.G., J.M.J., J.C.S., J.P.C., B.V.R., T.H., P.G., W.-P.C., N.E., E.L.N.J., K.I.C., R.P.S., D.M.C., G.W., J.F.K., S.M., K.H., R.S., S.N.Q., D.M., M.F., G.F. and T.B. All coauthors provided comments and suggestions on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 All 72 transit observations of LP 791-18c (left) and LP 791-18d (right) lined up to their fitted mid-transit times.
Circles indicate the detrended normalized flux measurements from TESS (black), Spitzer (red), LCOGT (green), EDEN (brown), MuSCAT/MuSCAT2 (dark blue), TRAPPIST telescope (purple), MEarth (yellow), ExTrA (pink), and SPECULOOS (grey), compared to the best-fitting transit models based on the transit times from the TTV analysis (solid black curve). Vertical lines indicate the 68% uncertainties. Multiple transits from a given telescope or telescope network are directly overplotted, while observations from the different telescopes are vertical offset and in different colors for clarity (Extended Data Tables 1–2).
Extended Data Fig. 2 Transit timing variations of the Earth-sized planet LP 791-18 (top panel) and the sub-Neptune LP 791-18c (bottom panel).
Colored data points indicate the 72 transit timing measurements obtained with TESS (black), Spitzer (red), LCOGT (green), MEarth (yellow), TRAPPIST telescope (purple), EDEN (brown), ExTrA (pink), SPECULOOS (grey), MuSCAT/MuSCAT2 (dark blue), compared to the best-fitting TTVFast model (blue curve). The vertical axis represents the deviation from the best fitting linear ephemeris and the horizontal axis the Barycentric Julian Date (BJD) of the observation. Dark and light shaded regions illustrate the posterior population of models in the MCMC fit corresponding to 68% and 95% confidence, respectively.
Extended Data Fig. 3 Damping of free eccentricity in the LP 791-18d system.
a,b, Long-term integration for a three-planet system with the observed orbital periods of LP 791-18b, c, and d and initially starting with relatively high free eccentricities (0.02–0.05). The free eccentricities are rapidly damped over a 100,000-year timescale. However, a significant non-zero forced eccentricity circulating between approximately 0.0013 and 0.0025 is preserved for LP 791-18d. The particular simulation shown assumes a tidal quality factor Qp=100 for all three planets, but even Qp=1000 will result in a qualitatively similar evolution (see Methods).
Extended Data Fig. 4 Dynamical stability analysis of the LP 791-18 system.
a,b, Long-term integration for the observed LP 791-18 system, showing that the system is stable. c,d, Integration of the LP 791-18 system with an additional hypothetical 1 M⊕ planet at 0.015 AU. The system remains long term stable with this hypothetical planet. e,f, Integration of LP 791-18 system with an additional hypothetical 1 M⊕ planet at 0.035 AU. The system remains long term stable with this hypothetical planet. g,h, Integration of LP 791-18 system with an additional hypothetical 1 M⊕ planet between planet d and c. The system with this hypothetical planet quickly becomes unstable.
Extended Data Fig. 5 Mass-radius diagram of small exoplanets.
LP 791-18c and d (bold stars) are shown in comparison to other known small planets with measured masses and radii (circles). Horizontal and vertical error bars represent the 68% confidence intervals of the mass and radius measurements for each planet, and the color indicates the planet’s stellar insolation. Mass and radius measurements of LP 791-18c and d are reported in this work, while all other measurements are taken from the Exoplanet Archive (https://exoplanetarchive.ipac.caltech.edu). Modeled mass-radius curves are shown for a pure iron composition, an Earth-like composition91, a pure rocky composition, an Earth-like core with 10% and 50% of the planet’s mass composed of a water envelope at Teq=400K10 (dashed blue curves), as well as an Earth-like core with 0.01%, 0.5%, 1%, 2% and 5% of the planet’s mass in a H2/He envelope9 (dashed green curves). A best match to the mass and radius of LP 791-18c is obtained for Earth-like core with approximately 2% of the planet’s mass in a H2 envelope or, alternatively, approximately 50% of the planet’s mass in water. LP 791-18d’s composition is consistent with a rocky, potentially Earth-like composition. The TRAPPIST-1 planets and K2-18b are labeled for comparison to LP 791-18d and LP 791-18c, respectively.
Extended Data Fig. 6 Mass-loss timescale ratio between LP 791-18c and LP 791-18d under a, the model of photoevaporation16 and b, the model of core-powered mass loss16.
Black curves indicate the posterior probability distributions of the mass-loss timescales ratio tloss,c/tloss,d based on simplified scaling laws12,16, accounting for the observational uncertainties on planet bulk and orbital parameters. The mass-loss timescale of the sub-Neptune LP 791-18c is greater than the mass-loss timescale of the Earth-sized LP 791-18d for all planet masses, radii, and semi-major axes consistent with our observations, indicating that the planet pair is consistent with both the model of photoevaporation and the model of core-powered mass loss.
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Peterson, M.S., Benneke, B., Collins, K. et al. A temperate Earth-sized planet with tidal heating transiting an M6 star. Nature 617, 701–705 (2023). https://doi.org/10.1038/s41586-023-05934-8
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DOI: https://doi.org/10.1038/s41586-023-05934-8
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