A tale of dynamical instabilities and giant impacts in the radius valley

TL;DR

This paper investigates the origin of the radius valley in exoplanet size distribution, proposing that late dynamical instabilities and giant impacts explain the observed eccentricities and the placement of planets within the valley.

Contribution

It introduces a model where late dynamical instabilities after disk dispersal account for the radius valley and associated eccentricity signatures, supported by simulations.

Findings

Rocky planets in the valley are excited by instabilities and impacts.

Water-rich planets beyond the valley experience weaker dynamical effects.

Eccentricity peaks are linked to the number of rocky planets and external perturbers.

Abstract

The size distribution of planets with radii between 1 and $4 R_\oplus$ peaks near 1.4 and $2.2R_\oplus$, with a dip around $1.8 R_\oplus$ -- the so-called "radius valley." Recent statistical analyses suggest that planets within this valley ($1.5 < R < 2R_\oplus$) tend to have slightly higher orbital eccentricities than those outside it. The origin of this dynamical signature remains unclear. We revisit the "breaking the chains" formation model and propose that late dynamical instabilities -- occurring after disk dispersal -- may account for the elevated eccentricities observed in the radius valley. Our simulations show that sub-valley planets ($R < 2 R_\oplus$) are generally rocky, while those beyond the valley ($R > 2 R_\oplus$) are typically water-rich. Rocky planets that undergo strong dynamical instabilities and numerous late giant impacts have their orbits excited and their radii increased, ultimately placing them into the radius valley. In contrast, the larger, water-rich planets just beyond the valley experience weaker instabilities and fewer impacts, resulting in lower eccentricities. This contrast leads to a peak in the eccentricity distribution within the valley. The extent to which planets in the radius valley are dynamically excited depends sensitively on the orbital architecture before the orbital instability. Elevated eccentricities among radius valley planets arise primarily in scenarios that form a sufficiently large number of rocky planets within 100 days (typically $\gtrsim 5$) prior to instability, and that also host external perturbers ($P > 100$ days), which further amplify the strength of dynamical instabilities.