Don the stars vary more than the change caused by a transit?
Yes, the stars do vary in brightness all the time. In fact it is almost impossible to make a perfectly constant source of light. Fortunately, the stars we are most interested in are stars like our Sun. They vary less than the change in brightness caused by an Earth-size planetary transit on the same time scale as a transit (a few hours). Our Sun varies over many time scales: There are Maunder minimums, which do not occur for many centuries or longer and have caused “mini ice ages” even as recently as during the 17th century. There is an eleven-year “solar cycle” of minimum and maximum activity. The largest short-term variations are caused by “sun spots” that appear and fade, and rise and set as the Sun rotates with a period of four weeks. Planetary transits have durations of a few hours to less than a day. The measured solar variability on this time scale is 1 part 100,000 (10 ppm) as compared to an Earth-size transit of 1 part in 12,000 (80 ppm). Even then, most of the variability is
