September
2025
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2025AJ....170..175Z
Authors
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Zhang, Elina Y.
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Teng, Huan-Yu
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Dai, Fei
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Howard, Andrew W.
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Halverson, Samuel P.
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Isaacson, Howard
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Rubenzahl, Ryan A.
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Wang, Xian-Yu
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Wang, Songhu
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Fulton, Benjamin J.
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Nielsen, Louise D.
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Lubin, Jack
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Giacalone, Steven
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Handley, Luke B.
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Petigura, Erik A.
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Turtelboom, Emma V.
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Polanski, Alex S.
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Gibson, Steve R.
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Rider, Kodi
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Roy, Arpita
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Baker, Ashley
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Edelstein, Jerry
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Smith, Christopher L.
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Walawender, Josh
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Winn, Joshua N.
Abstract
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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 (Teff = 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 3.40‑0.21+0.22R⊕ and a period of 14.3 days. By modeling the Rossiter–McLaughlin effect, we found the sky-projected obliquity to be ∣λc∣=7.4‑7.2+6.8 °, consistent with a prograde, well-aligned orbit. The lack of detectable rotational modulation of the flux of the host star and a low vsini⋆ (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.
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