How Do We Find Exoplanets?

Astronomers have found more than 3700 worlds outside of our solar system in the past 25 years. How do they find them?

Sabrina Stierwalt, PhD
5-minute read
Episode #276

Astronomers monitor the positions of stars using spectroscopy by looking for a Doppler shift in their emission lines which indicates forward or backward motion relative to us, the viewers. Between 600-700 exoplanets have been discovered this way and it is still the easiest technique to use from ground-based observatories. However, the more massive the planet and the closer that planet is to its host star, the bigger its gravitational influence on its host star, i.e. the bigger the star’s wobble. The Doppler method thus is better suited for finding massive planets very close to their host stars, a type of planet thus nicknamed “Hot Jupiters.”

Direct Imaging of Exoplanets

You may be wondering why astronomers use these indirect methods for finding exoplanets rather than simply taking pictures of them directly. Well, since planets do not emit their own (optical) light and instead only reflect light from their host star, and since they are far smaller in size than their host star, looking for the light of a planet is akin to trying to pick out a firefly hanging out next to a search light. From thousands of miles away.  

However, now that astronomers know there are thousands of exoplanets out there waiting to be discovered, they are developing techniques that enable direct imaging by somehow masking the light from the host star. This blocking can be done before or after the light enters the telescope. The direct imaging method is still in its beginnings but it shows substantial promise and around 40 exoplanets have already been found using this method. Direct images have the potential to tell us more about the planets, including information on their atmospheres and compositions, than we can learn from more indirect methods and is thus an important area of active exoplanet research.  

The search for exoplanets is driven by our desire to understand how common or unique our tiny corner of existence is and ultimately if we are alone in the universe.

Other Exoplanet Detection Methods

There are a few other, although less common, ways of finding exoplanets, including gravitational microlensing. Einstein’s theory of General Relativity tells us that light traveling through a gravitational field will appear to bend around the massive object producing that field, an effect known as lensing, or in the case of very low mass objects like planets, microlensing. Thus, it is possible to see light from a distant star bend around a planet in the foreground as that planet passes between us and the distant star. The star’s light also gets amplified in the process, so observing this brief spike in a star’s brightness reveals not only that an object has passed in front of the star, but also the mass of that traveling object. Unfortunately, these microlensing events usually are not repeated and so cannot be easily verified.

The first exoplanet ever discovered was found by an entirely different technique known as pulsar timing. Pulsars, or rapidly rotating neutron stars that send out beamed emission similar to the search light on a lighthouse, are incredibly precise in the timing of their rotation. They are so precise and predictable, in fact, that even the tiny wobble produced by the gravitational influence of an orbiting planet will cause a big enough change in the timing of their passing strobe light for astronomers to detect. Although the first exoplanet was found using this technique, the harsh conditions around a pulsar mean such planets would not be suitable hosts for life like us.

The search for exoplanets is driven by our desire to understand how common or unique our tiny corner of existence is and ultimately if we are alone in the universe. Astronomers have used these current techniques to identify exoplanets and can compare these planets to our Earth to first order by determining the planets’ sizes as well as whether or not the planet is in the star’s habitable zone. However, much more information is needed to determine habitability. For example, here on Earth, we would not survive without our atmosphere, and our efforts to understand the atmospheres around exoplanets are only just beginning.   

Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

Image courtesy of nasa.gov


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About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.