Obliquity Constraints for the Extremely Eccentric Sub-Saturn Kepler-1656 b

Creators: Rubenzahl, Ryan A.¹; Howard, Andrew W.¹; Halverson, Samuel²; Petrovich, Cristobal^{3, 4}; Angelo, Isabel⁵; Stefánsson, Guđmundur⁶; Dai, Fei⁷; Householder, Aaron⁸; Fulton, Benjamin⁹; Gibson, Steven R.¹; Roy, Arpita¹⁰; Shaum, Abby P.¹; Isaacson, Howard¹¹; Brodheim, Max¹²; Deich, William¹³; Hill, Grant M.¹²; Holden, Bradford¹³; Huber, Daniel^{7, 14}; Laher, Russ R.⁹; Lanclos, Kyle¹²; Payne, Joel N.¹²; Petigura, Erik A.⁵; Schwab, Christian¹⁵; Walawender, Josh¹²; Wang, Sharon X.¹⁶; Weiss, Lauren M.¹⁷; Winn, Joshua N.¹⁸; Wright, Jason T.¹⁹

1. California Institute of Technology
2. Jet Propulsion Lab
3. Indiana University Bloomington
4. Millennium Institute of Astrophysics
5. University of California, Los Angeles
6. University of Amsterdam
7. University of Hawaii at Manoa
8. Massachusetts Institute of Technology
9. NASA Exoplanet Science Institute
10. Schmidt Sciences
11. University of California, Berkeley
12. W.M. Keck Observatory
13. University of California, Santa Cruz
14. University of Sydney
15. Macquarie University
16. Tsinghua University
17. University of Notre Dame
18. Princeton University
19. Pennsylvania State University

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Abstract

The orbits of close-in exoplanets provide clues to their formation and evolutionary history. Many close-in exoplanets likely formed far out in their protoplanetary disks and migrated to their current orbits, perhaps via high-eccentricity migration (HEM), a process that can also excite obliquities. A handful of known exoplanets are perhaps caught in the act of HEM, as they are observed on highly eccentric orbits with tidal circularization timescales shorter than their ages. One such exoplanet is Kepler-1656 b, which is also the only known nongiant exoplanet (<100 M_⊕) with an extreme eccentricity (e = 0.84). We measured the sky-projected obliquity of Kepler-1656 b by observing the Rossiter–McLaughlin effect during a transit with the Keck Planet Finder. Our data are consistent with an aligned orbit but are also consistent with moderate misalignment with λ < 50° at 95% confidence, with the most likely solution of 35.0_(−21.6)^(+14.9) deg. A low obliquity would be an unlikely outcome of most eccentricity-exciting scenarios, but we show that the properties of the outer companion in the system are consistent with the coplanar HEM mechanism. Alternatively, if the system is not relatively coplanar (≲20° mutual inclination), Kepler-1656 b may be presently at a rare snapshot of long-lived eccentricity oscillations that do not induce migration. Kepler-1656 b is only the fourth exoplanet with e > 0.8 to have its obliquity constrained; expanding this population will help establish the degree to which orbital misalignment accompanies migration. Future work that constrains the mutual inclinations of outer perturbers will be key for distinguishing plausible mechanisms.

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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

Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 nonprofit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Keck Observatory occupies the summit of Maunakea, a place of significant ecological, cultural, and spiritual importance within the indigenous Hawaiian community. We understand and embrace our accountability to Maunakea and the indigenous Hawaiian community, and commit to our role in long-term mutual stewardship. We are most fortunate to have the opportunity to conduct observations from Maunakea.

R.A.R. acknowledges support from the National Science Foundation through the Graduate Research Fellowship Program (DGE 1745301). A.W.H. acknowledges funding support from NASA award 80NSSC24K0161 and the JPL President's and Director's Research and Development Fund. C.P. acknowledges support from ANID BASAL project FB210003, FONDECYT Regular grant 1210425, CASSACA grant CCJRF2105, and ANID+REC Convocatoria Nacional subvencion a la instalacion en la Academia convocatoria 2020 PAI77200076. D.H. acknowledges support from the Alfred P. Sloan Foundation, the National Aeronautics and Space Administration (80NSSC21K0652), and the Australian Research Council (FT200100871).

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 and Development Fund Program.

Facilities

Keck:I - KECK I Telescope (KPF).

Software References

astropy (Astropy Collaboration et al. 2022), corner (Foreman-Mackey 2016), emcee (Foreman-Mackey et al. 2013), matplotlib (Hunter 2007), numpy (Harris et al. 2020), pandas (pandas development team 2020), PyDE (Parviainen 2016), rmfit (Stefànsson et al. 2022), scipy (Virtanen et al. 2020), spinspotter (Holcomb et al. 2022).

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Resource type: Journal Article
Publisher: American Astronomical Society
Published in: Astrophysical Journal Letters, 971(2), L40, ISSN: 2041-8205.
Languages: English