  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.
