Can Rocky Exoplanets with Rings Pose as Sub-Neptunes?

TL;DR

Rings on rocky exoplanets could cause their measured radii to appear larger, potentially mimicking sub-Neptune planets, but explaining all such cases with rings alone seems unlikely.

Contribution

This study explores how rings around rocky exoplanets can affect radius measurements and suggests focusing on sub-Neptune-sized planets to identify potential rocky planets with rings.

Findings

Rings can increase inferred radii by 2-3 times for rocky exoplanets.

A large fraction of rocky planets would need rings to explain all sub-Neptunes.

Focusing on sub-Neptune planets may help find rocky planets with rings.

Abstract

In our solar system, the presence of rings is exclusive to the gas giants, but is this the case for all planetary systems? In principle, it seems that rocky exoplanets could also have rings, which could be searched for by studying their subtle imprint on the ingress and egress of transits. Unfortunately, such effects are difficult to measure and require high precision photometric and/or spectroscopic observations. At the most basic level though, the presence of rings would result in an increased transit depth that could be mistaken as an anonymously large radius. Motivated by this, I consider how a population of exoplanets with rings would impact radius measurements, focusing on Earth-like exoplanets. It is found that this population introduces an enhancement of inferred radii in the range of $\sim2-3R_\oplus$, not unlike the sub-Neptunes that have been identified in recent transit surveys. Whether rings can explain all or most sub-Neptunes seems difficult, since it would require a large fraction of rocky planets to have rings ($\gtrsim40\%$) and/or a factor of $\sim2-3$ increase in the number of planets with radii $\lesssim1.2R_\oplus$. Even if rings cannot explain all sub-Neptunes, this work suggests that focusing on those planets currently classified as sub-Neptunes may be a good starting place for finding rocky planets with rings.