Taking a picture of an extrasolar planet sounds like the easiest way to discover a planet, but in reality, it is probably the most difficult. Seeing the faint glow of a planet amidst the brilliant glare of its parent star is like spying a firefly buzzing around a huge spotlight from a mile away.

But it has been done. Astronomers use a dark “mask” called a coronagraph to cover up the star and most of its glare. If the planet is far enough away from its star so that it isn’t also covered by the coronagraph, a keen-eyed telescope might be able to pick out the dim reflected light of the planet shining through.

Even though a planet is so much smaller than its star, it still exerts a tiny gravitational tug on the star as it orbits. When a planet is behind its star (from our point of view), it pulls the star slightly away from us. When the planet is in front of its star, it pulls the star slightly toward us. This causes the star to wobble ever so slightly back and forth. Most planets have been discovered after astronomers caught this wobble.

Powerful telescopes equipped with a spectrograph can detect a star’s to-and-fro motion by examining the star’s light. A spectrograph, like a prism, splits a star’s light into its component colors, producing a spectrum. Some of the starlight gets absorbed as it passes through the star’s atmosphere, though, and this produces small, dark gaps or lines in the spectrum. As the star moves closer to us, these lines shift just a smidge toward the blue end of the spectrum. As the star moves away, the lines shift back toward the red end of the spectrum. Astronomers can look for orbiting planets by looking for these back-and-forth motions of the lines in a star’s spectrum.

Astronomers can discover a planet not by noticing the planet’s gravitational effects on its parent star but rather on a star much farther away.

When one celestial object passes in front of another from our point of view, the closer object’s gravity can bend and magnify the light of the more distant object, causing the distant object to appear brighter than normal for a short time. This phenomenon is called gravitational microlensing.

For example, if a closer star passes in front of a farther star, the farther star will appear temporarily brighter and then return to its normal brightness after the closer star has passed.

But if, by chance, the closer star is accompanied by an unseen planet, the farther star’s brightness will increase a second time as the planet passes in front of it.

By watching for a distant star’s brightness to intensify twice, astronomers can find out whether the closer star has a secret traveling companion.