Transit of exoplanet and exomoon

This animation demonstrates how the measured light curve from the star Kepler-1625b led to the conclusion that the system may consist of not only a planet, but also at least one moon.

When the planet moves in front of its parent star a tiny portion of its light is blocked by the disc of the planet, so we observe a dimming of the light from the star. Right after the exoplanet has finished its transit the starlight is seen to dim slightly again, suggesting the presence of a moon trailing the planet.