Three Traditional Methods of Detecting Exoplanets

(a) A planet and its star both orbit around their common center of mass, always
staying on opposite sides of that point. The planet’s motion
around the center of mass often provides astronomers with
the information that a planet is present. (b) As a planet
moves toward or away from us, its star moves in the opposite
direction. Using spectroscopy, we can measure the
Doppler shift of the star’s spectrum, which reveals the effects
of the unseen planet or planets. (c) If a star and its planet are
moving across the sky, the motion of the planet causes the
star to orbit its center of mass. This motion appears as a
wobbling of the star across the celestial sphere. (d) If a
planet happens to move in a plane that takes it across its
star (that is, the planet transits the star), as seen from Earth,
then the planet will hide some of the starlight, causing the
star to dim. This change in brightness will occur periodically
and can reveal the presence of a planet.