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 terrestrial planets.

• Our investigation is also very important for exobiology since the early radiation environment in the planetary nebular and planetary forming time period was very important for the formation of complex organic molecules and the interaction with comets and asteroids.