In the diagram the same planet is plotted four times at different positions of its way around the star. In the lightcurve these four positions are indicated on the time axis. I will explain these four situations in detail:
- The planet is not yet on the stellar disk. In the lightcurve the brightness of the system is at 100 % because all of the light coming from the star can be see.
- The planet just moved onto the stellar disk. During the time period when the planetary disk moves on the stellar disk, but is not yet complete on it, the brightness goes down steeply. This is the beginning of the transit, which is also called ingress.
- Now the planet is completely on the stellar disk and the brightness dropped down to a lower level. As long as the entire disk of the planet is on the star, the brightness stays down there. This flat bottom is again a simplification; in reality, the brightness will change, even during the transit, because the stellar disk is not equally bright in all parts. In particular, it is darker at the edges than in the center, which is referred to as limb-darkening. However, I will ignore this effect here. Important is that the brightness goes down to a constant value of 0.99, so the light we receive from the system is 1 % less than when there is no transit. It is easy to understand where this comes from: The planetary disk blocks 1 % of the stellar disk, so 1 % of the light cannot reach us anymore. You now know the size of the planet relative to the star. The relative disk size is 1 %, the relative radius is the square-root of ita: 10 %. In our example the radius of the planet is 10 % the radius of the star. This is the beauty of the transit method: Just by measuring the depth of the transit, which can be easily done by anybody, we know the radius of the planet.
- The planet moved on its orbit to the other side of the star. When it moved off the disk, which is called egress, the lightcurve went up steeply back to the 100 % brightness it had before the transit. Now the planet moves around the star and will in a certain period of time, depending on its orbital period, come back for another transit.
In the end I would like to mention that until about a year ago it was not considered to be good practice to announce a new planet just because transits in a lightcurve were found. There are some other problems I did not mention here, which make it possible that the "transits" you observe do not really come from a planet. So one always had to check the system with the RV technique; only if the planet was found there, too, it was accepted as a real exoplanet. However, this has changed lately and you do not always have to backup a transit detection anymore. Although this might be based on good reasons, some astrophysicists are not very happy with it - maybe I will talk about this controversy in some other post.