How to discover an exoplanet – let's take a look

How to find an exoplanet (the transit method):

There are a few different ways in which astronomers can deduce the existence of one or more planets in a neighboring planetary system. You just gotta keep in mind here that there are hundreds of billions of stars in the Milky Way galaxy alone. So, to find planets in a new solar system, we must hone in on one star, usually hundreds or sometimes thousands of light-years away from Earth, and study its light. Whenever a planet “transits” the star relative to Earth (like when we see objects in our solar system pass between Earth and the Sun), the object blocks out a minuscule amount of light, more or less depending on the planet’s size and composition. 

If we see one of these dips, we continue to study the starlight further until a pattern emerges. Like our own solar system, these planets should be in a fixed orbit around their star, so we should be able to determine exactly when these dimmings will occur. Interestingly, we can also glean further information about the planet’s composition and atmospheric makeup this way. How? Well, follow the light.

When a planet transits its star relative to Earth, some of the star’s light – a teensy, tiny fraction of a little bit – will filter through the planet’s atmosphere. How much light is obstructed gives us information about the exoplanet’s mass. Then, using a technique called the transit spectroscopy method, we can obtain the planetary transmission spectrum by breaking that light apart, much like a prism breaks white light into various colors of the rainbow, to study the composition of the planet’s atmosphere (if it doesn't have one, that can often be determined). We know that certain molecules absorb light in certain wavelengths. Breaking apart the light, we can correlate certain wavelengths of light to particular elements – each has its own atomic structure. Depending on what is present, the elements reflect or absorb light at certain wavelengths.

This is most certainly not the only way to detect an exoplanet, but the next doesn’t help us glean as much information about the planet as the transit method. But of course, it has its own advantages.

The radial velocity method: 

Unlike the transit method, the radial velocity method (sometimes called Doppler Spectroscopy) is extremely helpful in allowing us to discover exoplanets that are orbiting red dwarf stars, which are smaller, more volatile, and dimmer than Sun-like stars. They also tend to live much longer because they burn fuel at a slower rate. The closest star system to us, Alpha Centauri, hosts a red dwarf star known as Proxima Centauri. It’s located about 4.24 light-years from Earth (and has a few planets itself), or if you wanna warp your mind, it’s approximately 25 trillion miles (40.2 trillion km) away from us. Better buckle up because it’s a long, bumpy ride.

Essentially, the radial velocity method relies on studying the spectra of a star and seeing if there are any slight “wobbles” or movements when the host star moves slightly toward or away from our telescopes. These wobbles are caused by exoplanets exerting their gravitational influence on their home star. They are tiny, nearly indistinguishable tugs, but they nonetheless change the velocity of host stars; thus, their light shifts.

More specifically, we’re able to see these wobbles and glean information from their effects thanks to something known as the Doppler effect. This phenomenon is widely used across many subsets of astrophysics. It is a shift in the frequency of waves emitted from an object moving relative to the observer. With respect to the radial velocity method, from our vantage point, these very minute  “tugs” noticeably change the usual color signature of the star. Looking at the light using spectroscopy once again, if the star is moving farther from us, its spectra shifts toward the blue end of the electromagnetic spectrum. If it is moving closer to us, the spectra will appear to shift further on the red end of the spectrum. Blue represents shorter wavelengths, while red means longer wavelengths.

We can also gather information about the approximate size of the planet based on how much the gravitational interaction with the star and planet makes the star wobble, which results in a  change in velocity. Because this method relies on observing these small movements, the radial velocity method is best for discovering large planets, including those much larger than Jupiter – the largest planet in our solar system – that complete full revolutions around their stars every couple of days. The smaller and more distant the planet is, the more minute the wobble, and the more difficult to spot. It’s easy to see the logic there.

The direct imaging method: 

Thanks to the giant leaps we’ve made in technology, not only have we discovered a number of exoplanets using the transit and radial velocity methods, but we’re now able to image exoplanets directly. It’s not easy, but it can be done. One disadvantage is that it’s difficult to directly image a smaller body like Earth orbiting its star in the habitable zone. The larger the planet and the farther it orbits its star, the more likely we are able to capture a direct image. There’s also a little more to it, obviously.

Instead of being able to capture pictures of the stars and planets in visible light, we must typically look at them through infrared wavelengths (though it is sometimes possible to image them in visible light). This is because the enormous amount of light the star generates washes the planets out when viewing them, as they are in the deep, dark abyss. 

So, we use a tool called a coronagraph (sometimes called a starshade) to block out light from the star, keeping in mind that the star can be billions of times brighter than the planets in its system. That allows us to see planets that reflect a lot of starlight or, even better, “nearby” planets that are still so young they retain much of the energy and heat left over from their formation. 

Depending on whether the planet was imaged in visible light or at infrared (heat) wavelengths, we can once again look at the spectra of light and uncover information about the composition of the planet’s atmosphere. Of course, we can also try to look for signs of potential habitability, which could include the planet being a rocky, terrestrial body not much larger than Earth, it orbiting its star at a distance that would allow water to exist in liquid form, and the presence of certain elements like oxygen and methane in its atmosphere. All of those characteristics are promising in the search for life beyond Earth, but there are still many hurdles we need to jump through before we can truly deem any exoplanet habitable. There are even more obstacles when it comes to sending a probe to one of these planets or sending humans there one day. We can dream.