This plot was inspired by this web page (History's Biggest Sunspots) and uses data from this database (DPD). It presents the area of the biggest sunspot of each month since January 1990. The sunspot area is given in millionth of the solar hemisphere, so 1000 means that 0.1 % of half of the Sun's surface is covered by the spot.
So why is this interesting for my exopanet blog? If we take a look at the radius ratio between Sun and Earth, which is about 110, we see that a transiting Earth will reduce the solar brightness by about 1/(110)² = 1/12100, which is approximately 100 millionth.
A comparison with the plot shows that there was almost no month in the last 25 years where the Sun did not have at least one spot with an area larger than 100 millionth of the solar hemisphere. So almost all of these spots cause brightness reductions larger than the transiting Earth. The Sun is not even a very active star, so chances are that many other stars have even more and larger spots. This might give you an impression why it is pretty hard to find small transiting planets around active stars.
However, the good thing is that spots change. The brightness variations they imprint on lightcurves are quite different from exoplanetary transits and they always change with time - although sometimes they change very slowly. A transiting planet usually does not change its transit (although there seem to be exceptions), so the same signal is coming again and again at a predictable time. If the transit is changing, either its shape or its transit time, you always have to be extra cautious because this takes you very close to what spots do.
By the way, this plot also shows the 11 year activity cycle of the Sun. Around 1997 and 2009 you can see periods with only very few and comparatively small spots. In between the biggest spots are much larger. The magnetic activity of the Sun changes: when it is high, it has a lot of spots and also many large spots; when it is low, you sometimes even have periods of days where not a single spot is seen. These magnetic cycle are also an issue when searching for exoplanets, although not so much for transit surveys. You can see the different levels of activity of a star in the radial velocity measurements, which might mimic the signal of a far-out planet with a long period. Again, stellar activity can mess up things quite a bit.
