Because planets are so hard to see, astronomers mostly find them by the effect they have on a star’s light. Five methods do almost all the work.

• Transit. A planet crossing in front of its star blocks a little of the light, so the star dims briefly and repeatedly. This only works when the orbit is seen nearly edge-on.
• Radial velocity (wobble). A planet’s gravity makes the star move in a small circle. When the star moves toward us its spectrum shifts slightly blue; when it moves away the spectrum shifts slightly red. The repeating shift reveals the planet.
• Direct imaging. Block the star and photograph the planet itself, as in the last lesson.
• Microlensing. When a star drifts exactly in front of a more distant star, its gravity bends and focuses the background light and the distant star brightens for days. A planet around the foreground star adds a short extra blip.
• Astrometry. Measure the star’s position on the sky very precisely and look for a small repeating wobble. The Gaia space telescope is built for this.