The KPF SLOPE Survey: Small, Compact Multiplanet Systems Appear Spin–Orbit Aligned
Abstract The angle between stellar spin axes and planetary orbits—stellar obliquity—probes the dynamics of planetary migration and evolution. The obliquities of giant planets have been extensively studied because they are the most easily measured. Smaller planets, while more difficult to measure, have the advantage of better reflecting the dynamics of planetary systems because they trigger negligible backreactions onto the host star. This paper introduces a new observational campaign called the Small, Low-mass Oblique Planets Experiment (SLOPE) survey with the Keck Planet Finder (KPF) spectrograph and presents four new obliquity measurements. The SLOPE survey focuses on planets smaller than Saturn across a variety of system architectures. The sky-projected obliquities of the four planets measured—TOI-1386 b, TOI-480 b, TOI-4596 b, and TOI-1823 b—are all consistent with spin–orbit alignment. We validate the planetary nature of TOI-4596 b with a significant detection of the Rossiter–McLaughlin effect. Including these measurements, we conducted a statistical analysis of the obliquities of sub-Saturn-mass planets in different planetary system architectures. Compared to other architectures, those in compact multiplanet systems reside in orbits that appear preferentially aligned with the stellar equator with 6 σ confidence.
Authors
- Jack Lubin (ORCID: https://orcid.org/0000-0001-8342-7736)
- Pranav Nagarajan (ORCID: https://orcid.org/0000-0002-1386-0603)
- Andrew W. Howard (ORCID: https://orcid.org/0000-0001-8638-0320)
- Theron W. Carmichael (ORCID: https://orcid.org/0000-0001-6416-1274)
- Steven Giacalone (ORCID: https://orcid.org/0000-0002-8965-3969)
- Benjamin J. Fulton (ORCID: https://orcid.org/0000-0003-3504-5316)
- Luke B. Handley (ORCID: https://orcid.org/0000-0002-9305-5101)
- Claire J. Rogers (ORCID: https://orcid.org/0009-0005-5520-1648)
- Steven R. Gibson (ORCID: https://orcid.org/0009-0004-4454-6053)
- Gregory J. Gilbert (ORCID: https://orcid.org/0000-0003-0742-1660)
- Ryan A. Rubenzahl (ORCID: https://orcid.org/0000-0003-3856-3143)
- Pranav H. Premnath (ORCID: https://orcid.org/0000-0001-5728-4735)
- Jerry Edelstein (ORCID: https://orcid.org/0009-0002-2419-8819)
- Fei Dai (ORCID: https://orcid.org/0000-0002-8958-0683)
- Arpita Roy (ORCID: https://orcid.org/0000-0001-8127-5775)
- Yaguang Li (ORCID: https://orcid.org/0000-0003-3020-4437)
- J. M. Joel Ong
- Christopher Jacob Smith
- Howard Isaacson (ORCID: https://orcid.org/0000-0002-0531-1073)
Institutions
- University of Hawaiʻi at Mānoa (US)
- California Institute of Technology (US)
- The University of Sydney (AU)
- University of Hawaii System (US)
- University of California, Los Angeles (US)
- University of California, Irvine (US)
- NASA Exoplanet Science Institute (US)
- University of California System (US)
- Flatiron Institute (US)
- University of California, Berkeley (US)
Publication Details
- Journal
- The Astrophysical Journal
- Published
- 2026-10-26
- DOI
- https://doi.org/10.3847/1538-4357/aea6ca
- Primary Topic
- Stellar, planetary, and galactic studies
- Type
- article
- Field-Weighted Citation Impact
- 0.00
Funders
- National Science Foundation
- National Aeronautics and Space Administration
- California Institute of Technology
- Ames Research Center
- Jet Propulsion Laboratory