[P-AE18] Exoplanets

[PAE18-16] Impact erosion mechanism explaining the radius gap in sub-Neptunes through high-speed collisions with planetary embryos

*Yaxing He¹, Masahiro Ogihara¹, Kangrou Guo¹ (1.Tsung-Dao Lee Institute, Shanghai Jiao Tong University)

Keywords:Sub-Neptunes, Radius gap, Atmospheric loss, N-body simulation, Impact erosion

The radius gap in sub-Neptunes, which indicates a deficit in planet occurrence between about 1.5 and 2 Earth radii, has attracted a lot of attention over the past five years in exoplanet studies. Some previous studies consider atmospheric escape from sub-Neptunes through photoevaporation or core-powered mass loss to explain the observed bimodal distribution in the size of planets. According to theoretical models, as a byproduct of sub-Neptune formation residing at about 0.1–1 au from the star, planetary embryos with high eccentricity can remain in outer orbits about 1 au. These outer planetary embryos can collide with sub-Neptunes, leading to atmospheric loss through impact erosion. In this study, we examine the long-term evolution of systems consisting of close-in sub-Neptunes and outer high-eccentricity embryos. Specifically, we aim to quantify the impact velocities between these two populations and the resulting atmospheric loss through such collisions. Additionally, we explore how initial conditions, including the eccentricity and number of embryos, influence the final radius distribution of sub-Neptunes, particularly in explaining the observed radius gap. We utilize N-body methods to model the 50 Myr dynamics of the planetary system consisting of three close-in sub-Neptunes at 0.15–0.45 au, separated by a distance of 30 times their mutual Hill radius, and planetary embryos uniformly distributed in a two-dimensional space between 1–2 au. We perform a series of simulations with varying initial numbers of planetary embryos, ranging from 60 to 100, and different initial eccentricities, ranging from 0.7 to 0.9. Our results show that sub-Neptunes can experience high-speed collisions, with impact velocities 2–5 times the escape velocity of the sub-Neptunes, leading to substantial atmospheric loss. On average, each collision results in the loss of 15–30% of a sub-Neptune's atmosphere, and after 3–4 collisions, most of the atmosphere is dissipated. The figure shows the final radius distribution of sub-Neptunes (black histogram) for cases with different initial numbers and eccentricities of embryos, along with the initial radius distribution of sub-Neptunes (red histogram). The black and red vertical dashed lines indicate the observationally inferred peak positions of super-Earths (1.5 Earth radii) and sub-Neptunes (2.7 Earth radii), respectively, while the blue dashed line represents the position of the radius gap. A clear bimodal distribution can be seen under the initial conditions with a number of embryos ranging from 80 to 100 and eccentricities between 0.8 and 0.9. Our study demonstrates that impact erosion may contribute to the origin of the radius gap. This mechanism has advantages over the photoevaporation model. For instance, photoevaporation can only account for atmospheric loss in sub-Neptunes close to their stars with periods of less than 30 days, whereas we find that impact erosion can also have an effect on the atmospheres of longer-period sub-Neptunes. This could potentially be confirmed through future observations of the radius gap in longer-period planets.