Active stars and early Sun

Exoplanets are planets  orbiting around other stars, they do not belong to the solar system. Because of their relatively small distance from their host star, it is extremely difficult to detect them; the easiest way is to observe transits: in that case the exoplanet moves in front of its host star and a small decrease of the brightness of the star can be recorded.

The habitable zone around a star is the region where on a hypothetical planet water could exist in liquid form on its surface.  Water is assumed to be essential for life and it is commonly believed that without liquid water live is hardly to have evolved on a planet. We can simulate the irradiance - especially the contribution of UV-radiation -  on a planet by using artificial light sources. Such experiments can also be used to study the early Sun and test the influence of the expected high UV-contribution to the evolution of life on early Earth. We can also simulate stars that are more active than the Sun, this applies mostly to low massive stars which are more numerous. The influences of the UV radiation on habitable zones that become smaller in extension for low massive stars can then be analyzed.

The other questions arise: do close exoplanets have any influence on stellare activity. And how is the orbit of such close exoplanets influenced by strong stellar winds and eruptions.