Planet-Planet Occultations in TRAPPIST-1 and Other Exoplanet Systems

TL;DR

This paper investigates the occurrence, detectability, and scientific potential of planet-planet occultations in exoplanet systems, especially TRAPPIST-1, proposing models and observational strategies for their detection and analysis.

Contribution

It introduces a photodynamical model for predicting PPOs, assesses their detectability with JWST and OST, and demonstrates how PPOs can reveal system dynamics and planetary surface properties.

Findings

PPOs in TRAPPIST-1 occur at about 1.4 per day.

10-20 PPOs per year are detectable with JWST at 12-15 microns.

Modeling PPOs can constrain planetary eccentricities, masses, and surface maps.

Abstract

We explore the occurrence and detectability of planet-planet occultations (PPOs) in exoplanet systems. These are events during which a planet occults the disk of another planet in the same system, imparting a small photometric signal as its thermal or reflected light is blocked. We focus on the planets in TRAPPIST-1, whose orbital planes we show are aligned to within 0.3 degrees at 90% confidence. We present a photodynamical model for predicting and computing PPOs in TRAPPIST-1 and other systems for various assumptions of the planets' atmospheric states. When marginalizing over the uncertainties on all orbital parameters, we find that the rate of PPOs in TRAPPIST-1 is about 1.4 per day. We investigate the prospects for detection of these events with the James Webb Space Telescope, finding that ~10-20 occultations per year of b and c should be above the noise level at 12-15 microns. Joint modeling of several of these PPOs could lead to a robust detection. Alternatively, observations with the proposed Origins Space Telescope should be able to detect individual PPOs at high signal-to-noise. We show how PPOs can be used to break transit timing variation degeneracies, imposing strong constraints on the eccentricities and masses of the planets, as well as to constrain the longitudes of nodes and thus the complete three-dimensional structure of the system. We further show how modeling of these events can be used to reveal a planet's day/night temperature contrast and construct crude surface maps. We make our photodynamical code available on github.