The KPF-SLOPE Survey - Small, Compact Multi-Planet 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 back-reactions 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-1386b, TOI-480b, TOI-4596b, and TOI-1823b -- are all consistent with spin-orbit alignment. We validate the planetary nature of TOI-4596b with a significant obliquity detection. Including these measurements, we conducted a statistical analysis of the obliquities of sub-Saturn size planets in different planetary system architectures. Compared to other architectures, those in compact multi-planet systems reside in orbits that appear preferentially aligned with the stellar equator with 6 sigma confidence.

Figures (4)

• Figure 1: Our spline-corrected TESS light curves, phase-folded at the median period from the posterior distributions of the joint model (no targets showed evidence of TTVs, see Appendix \ref{['sec:ttvs']}). The median transit model is plotted in blue. A binned dataset is also shown (bins were calculated from the phase-folded data at 1/20 of the transit duration).
• Figure 2: KPF RV time-series during each transit event. Red lines indicate the RM model of the transiting planet using the median of the posterior distributions of the joint model, while the orange lines indicate random draws from those distributions. Although the model is fit to the raw RVs, binned data points (black) are also included for clarity. The curved out-of-transit baseline during the observations of TOI-4596 (bottom left) lead us to choose a quadratic model for that system alone.
• Figure 3: Left: absolute value of measurements of the sky-projected spin-orbit angle $\lambda$ for planets we considered in our analysis, differentiated by their status as a compact multi-planet system. Targets from the SLOPE survey are clearly labeled and have larger markers. Right: distribution of the number of misaligned systems in each population draw from the experiment in Section \ref{['sec:statisticalvalidation']}.
• Figure 4: Transit times for all targets after each transit was fit with an independent midpoint (black points). The times are plotted as the best fit observed time minus the computed time (O-C) assuming a linear solution given by the best fit parameters in Section \ref{['sec:results']}. Uncertainties were estimated using an MCMC approach. None of the targets demonstrate significant deviations from a linear model (red dashed line) which would suggest the presence of additional planets.