  New discoveries of transiting extrasolar planets are reported weekly. Ground
based surveys as well as space borne observatories like CoRoT and Kepler are
responsible for filling the statistical voids of planets on distant stellar
systems.
  I want to discuss the stellar activity and its impact on the discovery of
extrasolar planets. Up to now the discovery of small rocky planets called
"Super-Earths" like CoRoT-7b and Kepler-10b are the only exceptions. The
question arises, why among over 500 detected and verified planets the amount of
smaller planets is strikingly low. An explanation besides that the verification
of small planets is an intriguing task, is the high level of stellar activity
that has been observed.
  Stellar activity can be observed at different time-scales from long term
irradiance variations similar to the well known solar cycle, over stellar
rotation in the regime of days, down to the observations of acoustic modes in
the domain of minutes. But also non periodic events like flares or the activity
signal of the granulation can prevent the detection of a transiting Earth sized
planet.
  I will describe methods to detect transit-like signals in stellar photometric
data, the influences introduced by the star, the observer and their impact on
the success. Finally different mathematical models and approximations of
transit signals will be examined on their sensibility of stellar activity.
  I present a statistical overview of stellar activity in the CoRoT dataset.
The influence of stellar activity will be analysed on different transiting
planets: CoRoT-2b, CoRoT-4b und CoRoT-6b.
  Stellar activity can prevent the successful detection of a transiting planet,
where CoRoT-7b marks the borderline. Future missions like Plato will be
required to provide long-term observations with mmag precision to overcome the
limitations set by active stars in our Galactic neighbourhood.
