TOI-880 is an Aligned, Coplanar, Multiplanet System

Creators: Zhang, Elina Y.¹; Teng, Huan-Yu^{1, 2, 3}; Dai, Fei¹; Howard, Andrew W.⁴; Halverson, Samuel P.⁵; Isaacson, Howard⁶; Rubenzahl, Ryan A.⁷; Wang, Xian-Yu⁸; Wang, Songhu⁸; Fulton, Benjamin J.⁹; Nielsen, Louise D.¹⁰; Lubin, Jack¹¹; Giacalone, Steven⁴; Handley, Luke B.⁴; Petigura, Erik A.¹¹; Turtelboom, Emma V.⁶; Polanski, Alex S.¹²; Gibson, Steve R.⁴; Rider, Kodi⁶; Roy, Arpita¹³; Baker, Ashley⁴; Edelstein, Jerry⁶; Smith, Christopher L.⁶; Walawender, Josh¹⁴; Winn, Joshua N.¹⁵

1. University of Hawaii at Manoa
2. Korea Astronomy and Space Science Institute
3. National Astronomical Observatories
4. California Institute of Technology
5. Jet Propulsion Lab
6. University of California, Berkeley
7. Flatiron Institute
8. Indiana University Bloomington
9. NASA Exoplanet Science Institute
10. Ludwig-Maximilians-Universität München
11. University of California, Los Angeles
12. University of Kansas
13. Astrophysics & Space Institute, Schmidt Sciences, New York, NY 10011, USA
14. W.M. Keck Observatory
15. Princeton University

Abstract

Although many cases of stellar spin–orbit misalignment are known, it is usually unclear whether a single planet’s orbit was tilted or if the entire protoplanetary disk was misaligned. Measuring stellar obliquities in multitransiting planetary systems helps to distinguish these possibilities. Here, we present a measurement of the sky-projected spin–orbit angle for TOI-880 c (TOI-880.01), a member of a system of three transiting planets, using the Keck Planet Finder. We found that the host star is a K-type star (T_eff = 5050 ± 100 K). Planet b (TOI-880.02) has a radius of 2.19 ± 0.11R_⊕ and an orbital period of 2.6 days; planet c (TOI-880.01) is a Neptune-sized planet with 4.95 ± 0.20R_⊕ on a 6.4 days orbit; and planet d (TOI-880.03) has a radius of 3.40^(+0.22)_(-0.21)R_⊕ and a period of 14.3 days. By modeling the Rossiter–McLaughlin effect, we found the sky-projected obliquity to be ∣λ_c∣ = 7.4^(+6.8)_(-7.2)°, consistent with a prograde, well-aligned orbit. The lack of detectable rotational modulation of the flux of the host star and a low v sin i ⋆ (1.6 km s⁻¹) imply slow rotation and correspondingly slow nodal precession of the planetary orbits and the expectation that the system will remain in this coplanar configuration. TOI-880 joins a growing sample of well-aligned, coplanar, multitransiting systems. Additionally, TOI-880 c is a promising target for James Webb Space Telescope follow-up, with a transmission spectroscopy metric of ∼170. We could not detect clear signs of atmospheric erosion in the Hα line from TOI-880 c, as photoevaporation might have diminished for this mature planet.

Copyright and License

© 2025. The Author(s). Published by the American Astronomical Society. 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

The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

We acknowledge the use of public TESS 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 paper made use of data collected by the TESS mission and are publicly available from the Mikulski Archive for Space Telescopes (MAST) operated by the Space Telescope Science Institute (STScI).

This work is supported by National Key R&D Program of China, No. 2024YFA1611802. H.Y.T. appreciates the support by the EACOA/EAO Fellowship Program under the umbrella of the East Asia Core Observatories Association.

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