Planet Detection Methods
Planets in other solar systems are extremely difficult to observe directly. In addition to being relatively small, planets are often lost in the glare of the stars they orbit. Astronomers have devised innovative ways to hunt these furtive objects. Only in the past couple of decades have our technology and techniques been up to the task of finding extrasolar planets. In the coming decade, direct observations of exoplanets will become more feasible as the technologies for suppressing the glare from stars reach higher levels of performance.
The gravity of a planet, however weak it might be, is enough to pull its parent star just a little bit out of place as the planet orbits.
While the radial velocity method studies a star’s spectrum to measure the star’s “wobble” toward and away from Earth, there is another trick to measure this tell-tale motion: simply look for the star to move back and forth in the sky.
With exceptionally sharp vision, a telescope can actually see a star move relative to other stars. Astrometry is the science of precisely measuring the position of objects in the sky. If astronomers use this method to monitor a star and see that it sways with a periodic rhythm, like slow dancers at a prom, they can tell that an unseen companion — perhaps a planet — is orbiting the star and pulling it to and fro.
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.
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
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.
If a planet’s orbit around its star is angled just right, the planet will regularly pass in front of or “transit” its star from our point of view. When it does, the planet will block a tiny bit of the star’s light.
Astronomers can search for planets by watching for slight, periodic changes in the brightness of a star. If the brightness of a star changes regularly and consistently, it could mean that a planet is repeatedly transiting the star and blocking a little bit of starlight each time around.