Mutual Inclination of Ultra-Short-Period Planets with Time Varying Stellar J2-moment

Systems with ultra-short-period planets (USPs) tend to possess larger mutual inclinations compared to those with planets located farther from their host stars. This could be explained due to precession caused by stellar oblateness at early times when the host star was rapidly spinning. However, stellar oblateness reduces over time due to the decrease in the stellar rotation rate, and this may further shape the planetary mutual inclinations. In this work, we investigate in detail how the final mutual inclination varies under the effect of a decreasing J2. We find that different initial parameters (e.g., the magnitude of J2 and planetary inclinations) will contribute to different final mutual inclinations, providing a constraint on the formation mechanisms of USPs.

In general, mutual inclination decreases over time due to the decay in the J2 moment. If the inner planets start in the same plane as the stellar equator or if the planets start co-planar while misaligned with the stellar spin-axis, since the inner orbit typically possesses less orbital angular momentum. However, if the outer planet is initially aligned with the stellar spin while the inner one is misaligned, the mutual inclination nearly stays the same. Overall, our results suggest that either the USP planets formed early and acquired significant inclinations (e.g., >30 deg with its companion or >10 deg with its host star spin-axis for Kepler-653c) or they formed late (>Gyr) when their host stars rotate slower.