Planetary eccentricity enhances inward comet transport but suppresses gentle impacts

Abstract

Comets can supply volatile and prebiotic material to rocky planets, but survival of fragile molecules depends on atmospheric entry and impact. Compact planet chains can transfer comets inward at low velocity. We test whether modest planetary eccentricity preserves this route. We follow 64,000 comets from the region between the two outermost planets of an eight-planet chain to an Earth-mass planet on a temperate orbit around a $0.1 M_\odot$ star. A second suite of simulations isolates the last transfer to the innermost planet across initial planet eccentricities $e_{p,0}=0$--$0.05$ and separations of 10 and 30 mutual Hill radii; a $\Delta=12$ comparison tests the influence of the near-3:2 period ratio at $\Delta=10$. At $e_{p,0}=0.05$, the fraction of comets intercepted by the outermost planet falls by 11 per cent and 6 per cent more reach the inner region. The innermost planet's impact probability changes by less than 3 per cent, while the fraction of impacts below 15 km s$^{-1}$ falls from 25 to 18 per cent. The fraction of all injected comets producing impacts below 15 km s$^{-1}$ consequently decreases by 30 per cent. Measurements of the comet-planet speed before the final close encounter show that eccentricity preferentially removes the slowest approaches. Applying an impact-chemistry model gives a 31 per cent lower HCN survival-equivalent yield. Modest planetary eccentricity can therefore increase inward transport while reducing the prebiotic material delivered under chemically favourable impact conditions.