Two Views of the Radius Gap and the Role of Light Curve Fitting

TL;DR

This paper compares two different analyses of the planetary radius gap, emphasizing the importance of precise light curve fitting and transit duration measurements in accurately characterizing the gap and planetary populations.

Contribution

It highlights how improved measurement techniques and data quality influence the observed properties of the radius gap, offering insights into planetary formation and evolution.

Findings

V18's analysis shows a wider, more devoid radius gap due to precise measurements.

Uncertainties in $R_p/R_\star$ dominate over stellar radius errors after Gaia.

Transit duration can effectively distinguish planetary populations.

Abstract

Recently, several groups have resolved a gap that bifurcates planets between the size of Earth and Neptune into two populations. The location and depth of this feature is an important signature of the physical processes that form and sculpt planets. In particular, planets residing in the radius gap are valuable probes of these processes as they may be undergoing the final stages of envelope loss. Here, we discuss two views of the radius gap by Fulton & Petigura (2018; F18) and Van Eylen et al. (2018; V18). In V18, the gap is wider and more devoid of planets. This is due, in part, to V18's more precise measurements of planet radius $R_p$. Thanks to Gaia, uncertainties in stellar radii Rstar are no longer the limiting uncertainties in determining $R_p$ for the majority of Kepler planets; instead, errors in $R_p/R_\star$ dominate. V18's analysis incorporated short-cadence photometry along with constraints on mean stelar density that enabled more accurate determinations of $R_p/R_\star$. In the F18 analysis, less accurate $R_p/R_\star$ blurs the boundary the radius gap. The differences in $R_p/R_\star$ are largest at high impact parameter ($b \gtrsim 0.8$) and often exceed 10%. This motivates excluding high-$b$ planets from demographic studies, but identifying such planets from long-cadence photometry alone is challenging. We show that transit duration can serve as an effective proxy, and we leverage this information to enhance the contrast between the super-Earth and sub-Neptune populations.