# Transit Probabilities in Secularly Evolving Planetary Systems

## TL;DR

This paper develops a semi-analytical method to estimate transit probabilities in planetary systems influenced by inclined outer planets, helping to explain the Kepler Dichotomy and constrain outer planet inclinations.

## Contribution

It introduces a new semi-analytical approach to assess how outer inclined planets affect inner planet transit probabilities and applies this to Kepler systems and the Kepler Dichotomy.

## Key findings

- Outer inclined planets can significantly increase inner planet transit probabilities.
- The Kepler Dichotomy can be explained by a combination of system-intrinsic and dynamical inclination effects.
- Distribution of mutual inclinations constrains the fraction of systems with outer perturbers.

## Abstract

This paper considers whether the population of known transiting exoplanets provides evidence for additional outer planets on inclined orbits, due to the perturbing effect of such planets on the orbits of inner planets. As such, we develop a semi-analytical method for calculating the probability that two mutually inclined planets are observed to transit. We subsequently derive a simplified analytical form to describe how the mutual inclination between two planets evolves due to secular interactions with a wide orbit inclined planet and use this to determine the mean probability that the two inner planets are observed to transit. From application to Kepler-48 and HD-106315 we constrain the inclinations of the outer planets in these systems (known from RV). We also apply this work to the so called Kepler Dichotomy, which describes the excess of single transiting systems observed by Kepler. We find 3 different ways of explaining this dichotomy: some systems could be inherently single, some multi-planet systems could have inherently large mutual inclinations, while some multi-planet systems could cyclically attain large mutual inclinations through interaction with an inclined outer planet. We show how the different mechanisms can be combined to fit the observed populations of Kepler systems with one and two transiting planets. We also show how the distribution of mutual inclinations of transiting two planet systems constrains the fraction of two planet systems that have perturbing outer planets, since such systems should be preferentially discovered by Kepler when the inner planets are coplanar due to an increased transit probability.