Finding exoplanets is hard:  They appear as extremely tiny signals in any data set.  Intrinsic stellar variability and variability induced by the spacecraft (or telescope or atmosphere) are both larger (in almost all data sets) than the exoplanet signals of interest.  The best information about these sources of variability comes from the scientific data themselves; that is, the most sensitive searches for exoplanets are self-calibrated.  I use Kepler and K2 data to illustrate these points, showing a set of discoveries in the K2 mission, where the spacecraft-induced variability is larger than most exoplanet signals by more than an order of magnitude.  (Come prepared to suffer through some linear algebra.)