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What are other ways to find exoplanets?
Another way to find planets is the radial velocity method. The first method used to find planets, it observes how orbiting planets can cause their stars to wobble. By looking at the degree of star-wobbling, we can identify whether (or whether not) there are any orbiting exoplanets.
How does this work?
Every single object has a gravitational pull that attracts objects towards it. The greater the mass an object has, the greater of a gravitational pull it has as well. Just like the Sun has a gravitational pull on Earth, causing it to orbit, the Earth has a similar, much weaker gravitational pull on the Sun. Even us humans have miniscule gravitational pulls on the Sun. (Yes, you are pulling on the Sun!) So, imagine the relationship between a planet and its star to be like an unfair game of tug of war. The star is gravitationally pulling on its planets (and winning because of its greater mass), and the planets are pulling on the star*. Since the stars have much greater mass, their gravitational field is much larger and much more powerful than that of the planets, forcing the planets to orbit around them. However, the gravitational pull of planets does have some impact, with their pulls causing miniscule star movements as the planets orbit around them. By observing these wobbles, we can calculate the degree of pull and whether or not there are exoplanets orbiting a certain star.
*To view a formula representing this relationship, please click here or proceed to the page, "Laws Used" and scroll to "Newton's Universal Law of Gravitation."
The Doppler Effect: How do we observe this wobble?
To observe this wobble, we look at something called the doppler shift, a phenomenon that causes the compressing and stretch of electromagnetic waves. Energy from our stars moves in waves, also known as electromagnetic radiation. Electromagnetic radiation in the form of infrared radiation is what heats up planet Earth. However, these waves can get stretched and squeezed based on the distance of an object emitting such waves. The closer an object is to you, the more the waves get squished to travel to you. And, the further away an object is, the more such waves will stretch. This causes these electromagnetic waves to shift in frequency, oftentimes compressing or stretching waves in a process called blueshift (where observable visible light waves have been compressed to the frequency of a bluer light) or redshift (where observable visible lightwaves get stretched to a longer frequency of redder light.) Therefore, when stars move closer, the energy emitted will be of higher frequency, or blue shifted. When they get further, their energy will be redshifted. We can use specific formulas to determine the degree of the Doppler effect.*
By using sensitive spectrographs on telescopes, we can track the spectrum of a star's emitted electromagnetic waves. If there are orbiting exoplanets, we will see the following changes: At first, the spectrum appears slightly blue shifted as the star wobbles and moves closer; the waves are higher frequency than normal as the star is closer. Next, they appear slightly red shifted as the star moves further away; the waves have been compressed and are lower frequency. If these shifts are regular, and occurring at different intervals, it is likely that this is an observation of a star's slight wobble caused by the gravitational pull of orbiting planets.
*To see how the exact Doppler shifts are calculated, click here, or navigate to the "Laws Used" page and scroll down to "Doppler Shift."
Why is this important?
By observing the degree of wobble, we can compute the gravitational force of these exoplanets, which helps determine whether or not exoplanets exist. Then, by focussing different telescopes, we can find these planets.
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