Our recent studies on isotope anomalies in planetary
atmospheres, especially in Titan's atmosphere show an indication
that our early Sun underwent during the first 600 million years
after its origin a very active period which is called Post-T-
Tauri phase. We investigate the particle and radiation
environment during such an early solar period, where the solar
wind particle density and soft x-ray flux is up to 1000 times
higher than at present. We cooperate in our study with
colleagues from the Sun in time project, and use a of nine solar
like G-type stars with an age between 70 million and 9 billion
years. The physical parameters from these stars were obtained by
observations with the ROSAT and ASCA x-ray satellites. Our study
investigates: - How much such a strong solar wind outflow will
affect non-or weakly magnetized planets and solar system bodies.
An increased x-ray luminosity of the young Sun and an enhanced
solar EUV radiation will also affect the photoionization rates
of the paleoatmospheres of terrest