The KPF SLOPE Survey: Small, Compact Multiplanet Systems Appear Spin–Orbit Aligned
10 affiliations
- 1California Institute of Technology
- 2University of Hawai‘i, 2680 Woodlawn Drive
- 3Flatiron Institute
- 4University of California Berkeley
- 5University of Sydney
- 6University of California Los Angeles
- 7University of California Irvine
- 8NASA Exoplanet Science Institute/Caltech-IPAC
- 9Astrophysics & Space Institute
- 10University of California
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.
Related
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BibTeX
@article{handley2026slope,
title = {{The KPF SLOPE Survey: Small, Compact Multiplanet Systems Appear Spin–Orbit Aligned}},
author = {Luke B. Handley and Andrew W. Howard and Fei Dai and Ryan A. Rubenzahl and Steven Giacalone and Howard Isaacson and J. M. Joel Ong and Theron W. Carmichael and Yaguang Li and Jack Lubin and Pranav H. Premnath and Claire J. Rogers and Pranav Nagarajan and Gregory J. Gilbert and Benjamin Fulton and Steven R. Gibson and Arpita Roy and Jerry Edelstein and Christopher Smith},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea6ca},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea6ca},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The KPF SLOPE Survey: Small, Compact Multiplanet Systems Appear Spin–Orbit Aligned
AU - Luke B. Handley
AU - Andrew W. Howard
AU - Fei Dai
AU - Ryan A. Rubenzahl
AU - Steven Giacalone
AU - Howard Isaacson
AU - J. M. Joel Ong
AU - Theron W. Carmichael
AU - Yaguang Li
AU - Jack Lubin
AU - Pranav H. Premnath
AU - Claire J. Rogers
AU - Pranav Nagarajan
AU - Gregory J. Gilbert
AU - Benjamin Fulton
AU - Steven R. Gibson
AU - Arpita Roy
AU - Jerry Edelstein
AU - Christopher Smith
PY - 2026
DA - 2026/10/26
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea6ca
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea6ca
AB - 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.
SN - 0004-637X
SN - 1538-4357
ER -