Five ways to find a hidden world

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.

Each method sees some planets and misses others, so each has a bias.

• Transits favour planets that are close to their star (a better chance of lining up) and large (a deeper dip).
• Radial velocity favours heavy planets on short orbits, which tug the star hardest.
• Imaging favours young, hot, giant planets far from their star.
• Microlensing is good at planets far from their star and very far from us, and it can find cold planets that other methods miss. But the lensing alignment never repeats, so each planet is seen only once.
• Astrometry favours heavy planets on wide orbits around nearby stars.

The great majority of the planets known so far were found by transits, followed by radial velocity. The other three methods add up to a small fraction of the total.