2023ApJ...956...90F

Short and ultrashort period planets are peculiar types of exoplanets with periods as short as a few days or less. Although it is challenging to detect them, already several have been observed, with many additional candidates. If these planets have formation pathways similar to their longer-period counterparts, they are predicted to reside in multiplanet systems. Thus, gravitational perturbation from potential planetary neighbors may affect their orbital configuration. However, due to their close proximity to their host star, they are also subject to general relativity precession and torques from the stellar spin quadrupole moment (J₂). Here we show that an evolving J₂ due to magnetic braking affects the magnitude and location of secular resonances of the short-period planet in a multiplanet system, thus driving the short-period planet into and out of a secular resonance, exciting the planet's eccentricity and inclination. The high inclination can hinder transit observation and, in some cases, the high eccentricity may result in an unstable configuration. We propose that evolving J₂ in a multiplanet system can be critical to understanding the detectability and stability of short-period planets.