Updated Mass, Eccentricity, and Tidal Heating Constraints for the Earth-sized Planet LP 791-18 d

Creators: Greklek-McKeon, Michael¹; Knutson, Heather A.¹; Levine, W. Garrett²; Hu, Renyu^{1, 3}; Saidel, Morgan¹; Gomez Barrientos, Jonathan¹; Batygin, Konstantin¹; Benneke, Björn^{4, 5}

1. California Institute of Technology
2. Yale University
3. Jet Propulsion Lab
4. University of Montreal
5. University of California, Los Angeles

Abstract

LP 791-18 d is a temperate Earth-sized planet (R P = 1.03 R ⊕, P = 2.76 days) orbiting a late M dwarf, with an interior super-Earth (LP 791-18 b, R P = 1.2 R ⊕, P = 0.95 days) and an exterior sub-Neptune (LP 791-18 c, R P = 2.5 R ⊕, P = 4.99 days). Dynamical interactions between LP 791-18 d and c produce transit timing variations (TTVs) that can be used to constrain the planet masses and eccentricities. These interactions can also force a non-zero eccentricity for LP 791-18 d, which raises its internal temperature through tidal heating and could drive volcanic outgassing. We present three new transit observations of LP 791-18 c with Palomar/WIRC, including the most precise TTV measurements (<6 s uncertainty) of this planet to date. We fit these times with a TTV model to obtain updated constraints on the mass, eccentricity, and tidal heat flux of LP 791-18 d. We reduce the mass uncertainty by more than a factor of two (M d = 0.91 ±0.19 M ⊕). We perform an updated fit assuming tidally damped free eccentricities and find e_d = 0.0011_(−0.0008)^(+0.0010) and e_c = 0.0001 ± 0.0001, consistent with circular orbits. We find that the observed TTVs are not sensitive to e ≤ ∼0.01. Without a tidally damped eccentricity prior, e_d = 0.056_(−0.014)^(+0.015), much higher than the eccentricity predicted by n-body simulations incorporating the effects of dynamical excitation and tidal damping. We predict the timing offset relative to the prediction for a circular orbit of upcoming JWST secondary eclipse observations for LP 791-18 d (Δt = −0.2_(−2.7)^(+2.0) minutes and Δt = −117_(−47)^(+41) minutes for the damped and undamped eccentricity cases, respectively), which could tightly constrain the eccentricity and tidal quality factor of this Earth-sized exoplanet.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We thank the Palomar Observatory telescope operators, support astronomers, hospitality, and administrative staff, without whom this research would not have been possible. We are especially grateful to Paul Nied, Carolyn Heffner, Kathleen Koviak, Diana Roderick, and Rigel Rafto who supported our observations of LP 791-18 c, and to Monastery keeper Jeff Paxton. We thank all of the Palomar Monastery chefs who made observing at Palomar a uniquely pleasant experience, and are sorely missed. Part of this program was supported by JPL Hale telescope time allocations. We are thankful to the PARVI team and Palomar Observatory directorate, especially Chas Beichman, Aurora Kesseli, and Andy Boden for their gracious support of the Palomar TTV survey program during periods requiring quick readjustment of the 200-inch observing schedule.

This research was supported from the Wilf Family Discovery Fund in Space and Planetary Science established by the Wilf Family Foundation. This research has made use of the NASA Exoplanet Archive (NASA Exoplanet Archive 2024b) and the Exoplanet Follow-up Observation Program website, which are operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. The research made use of the Swarthmore transit finder online tool (E. Jensen 2013).

Funding

ADS - , Exoplanet Archive - , TESS - , Hale - Palomar Observatory's 5.1m Hale Telescope (WIRC).

Software References

exoplanet (D. Foreman-Mackey et al. 2021a, 2021b) and its dependencies (Astropy Collaboration et al. 2013, 2018; J. Salvatier et al. 2016; Theano Development Team 2016; D. Foreman-Mackey et al. 2017, 2018; R. Kumar et al. 2019; R. Luger et al. 2019; E. Agol et al. 2020) astropy (Astropy Collaboration et al. 2018), scipy (SciPy 2001), numpy (C. R. Harris et al. 2020), matplotlib (J. D. Hunter 2007), rebound (D. Tamayo et al. 2020), reboundx (T. Lu et al. 2023), BATMAN (L. Kreidberg 2015), emcee (D. Foreman-Mackey et al. 2013), corner (D. Foreman-Mackey 2016), TTVFast (K. M. Deck et al. 2014), lightkurve (Lightkurve Collaboration et al. 2018), and Claude 3.5 Sonnet.

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