The Gap-Giant Association: Are Planets Hiding in the Gaps?

TL;DR

This study investigates whether unseen planets could exist in the gaps of systems with outer giant planets, finding that such gaps could host small planets but are unlikely to contain larger ones, challenging previous theories.

Contribution

The paper provides a dynamical analysis of known systems with gaps and outer giants, assessing the likelihood of hidden planets in these gaps and testing theories of their association.

Findings

Small planets could exist in the gaps without causing instability.

Outer giants are unlikely to prevent transits of potential gap planets.

Gaps may be empty or contain smaller, undetected planets.

Abstract

A handful of stars are known to host both an inner system of multiple transiting planets and an outer giant planet. These systems all feature a prominent gap between the orbits of two of the transiting planets, distinguishing them from typical multiplanet systems with more uniform orbital spacings. The reason for the association between inner gaps and outer giants is unknown. In this paper, we assess whether undiscovered planets might occupy these gaps in systems with outer giants. For each of the four relevant systems - Kepler-48, Kepler-65, Kepler-90, and Kepler-139 - we found that a typical small planet ($\sim 1 - 20 M_\oplus$) could reside in the gap without inducing dynamical instability. However, in each case, the gravitational influence of the outer giant planet is insufficient to tilt the orbit of the hypothetical planet by enough to prevent transits, strongly disfavoring a proposed theory for the observed gap-giant association. The gaps might instead contain smaller, undetected planets ($\lesssim 1 R_\oplus$), or be entirely devoid of planets.