The HD 191939 Exoplanet System is Well Aligned and Flat

Creators: Lubin, Jack¹; Petigura, Erik A.¹; Van Zandt, Judah¹; Beard, Corey²; Dai, Fei^{3, 4}; Halverson, Samuel⁵; Holcomb, Rae²; Howard, Andrew W.⁴; Isaacson, Howard^{6, 7}; Luhn, Jacob²; Robertson, Paul²; Rubenzahl, Ryan A.⁴; Stefánsson, Guđmundur⁸; Winn, Joshua N.⁹; Brodheim, Max¹⁰; Deich, William¹¹; Hill, Grant M.¹⁰; Gibson, Steven R.⁴; Holden, Bradford¹¹; Householder, Aaron¹²; Laher, Russ R.¹³; Lanclos, Kyle¹⁰; Payne, Joel¹⁰; Roy, Arpita¹⁴; Smith, Roger⁴; Shaum, Abby P.⁴; Schwab, Christian¹⁵; Walawender, Josh¹⁰

1. University of California, Los Angeles
2. University of California, Irvine
3. University of Hawaii at Manoa
4. California Institute of Technology
5. Jet Propulsion Lab
6. University of California, Berkeley
7. University of Southern Queensland
8. University of Amsterdam
9. Princeton University
10. W.M. Keck Observatory
11. University of California, Santa Cruz
12. Massachusetts Institute of Technology
13. NASA Exoplanet Science Institute
14. Schmidt Sciences
15. Macquarie University

Abstract

We report the sky-projected spin–orbit angle λ for HD 191939 b, the innermost planet in a six-planet system, using Keck/KPF to detect the Rossiter–McLaughlin (RM) effect. Planet b is a sub-Neptune with radius 3.4 ± 0.8 R_⊕ and mass 10.0 ± 0.7 M_⊕ with an RM amplitude <1 m s⁻¹. We find the planet is consistent with a well-aligned orbit, measuring λ = 37 ± 50. Additionally, we place new constraints on the mass and period of the distant super-Jupiter, planet f, finding it to be 2.88 ± 0.26 M_J on a 2898 ± 152 days orbit. With these new orbital parameters, we perform a dynamical analysis of the system and constrain the mutual inclination of the nontransiting planet e to be smaller than 12° relative to the plane shared by the inner three transiting planets. Additionally, the further planet f is inclined off this shared plane, the greater the amplitude of precession for the entire inner system, making it increasingly unlikely to measure an aligned orbit for planet b. Through this analysis, we show that this system's wide variety of planets are all well-aligned with the star and nearly coplanar, suggesting that the system formed dynamically cold and flat out of a well-aligned protoplanetary disk, similar to our own solar system.

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 recognize and acknowledge the cultural role and reverence that the summit of Maunakea has within the indigenous Hawaiian community. We are deeply grateful to have the opportunity to conduct observations from this mountain.

J.L. and E.P recognize support from the Heising-Simons Foundation, grant No. 2022-3833. 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. This research made use of the open source Python package exoctk, the Exoplanet Characterization Toolkit (M. Bourque et al. 2021).

This research was carried out, in part, at the Jet Propulsion Laboratory and the California Institute of Technology under a contract with the National Aeronautics and Space Administration and funded through the President's and Director's Research & Development Fund Program.

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