Stellar versus Galactic: The intensity of energetic particles at the evolving Earth and young exoplanets

TL;DR

This paper models the historical intensity of stellar and Galactic cosmic rays reaching Earth and young exoplanets, highlighting the dominance of stellar cosmic rays in early Earth conditions and their potential atmospheric effects on exoplanets.

Contribution

It introduces a combined stellar wind and cosmic ray model to estimate cosmic ray fluxes over stellar evolution, revealing the dominance of stellar cosmic rays in early planetary environments.

Findings

Stellar cosmic ray fluxes exceeded Galactic cosmic rays up to ~4 GeV 3.8 billion years ago.

Stellar cosmic rays may not have been continuous over time.

Stellar cosmic rays dominate over Galactic cosmic rays at 20 au from young stars.

Abstract

Energetic particles may have been important for the origin of life on Earth by driving the formation of prebiotic molecules. We calculate the intensity of energetic particles, in the form of stellar and Galactic cosmic rays, that reach Earth at the time when life is thought to have begun ($\sim$3.8Gyr ago), using a combined 1.5D stellar wind model and 1D cosmic ray model. We formulate the evolution of a stellar cosmic ray spectrum with stellar age, based on the Hillas criterion. We find that stellar cosmic ray fluxes are larger than Galactic cosmic ray fluxes up to $\sim$4 GeV cosmic ray energies $\sim$3.8Gyr ago. However, the effect of stellar cosmic rays may not be continuous. We apply our model to HR 2562b, a young warm Jupiter-like planet orbiting at 20au from its host star where the effect of Galactic cosmic rays may be observable in its atmosphere. Even at 20au, stellar cosmic rays dominate over Galactic cosmic rays.