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How are Exoplanets Detected?

Author: Emily Jiang

Editors: Sophia Chen and Emily Yu

Artist: Jade Li

Are we alone in the universe? This question drives our search for planets with signs of life or the potential to support it. It inspires us to explore space, with a particular focus on the planets that orbit outside of our solar system–exoplanets. But how do we detect something so far away? Exoplanets are incredibly difficult to see directly with telescopes since they are hidden by the intense light of the stars they orbit. Despite these difficulties, there are currently several methods that have been developed to identify exoplanets. These methods include the transit, radial velocity, and direct imaging methods. 

The transit method involves observing a planet as it passes in front of its star (transit), akin to a solar eclipse where the moon obstructs the sun’s light. When the exoplanet transits its star, the star’s light diminishes temporarily. By measuring the decrease in brightness, scientists can determine the size of the planet, as well as confirm its existence. Larger planets obscure more light, resulting in a more significant dimming of the star. Moreover, the transit method can determine how far the exoplanet is from its star; the farther away the exoplanet is, the longer it takes to pass in front of the star, resulting in a longer transit event. By analyzing the starlight’s colors as  it passes through the planet’s atmosphere during transit, scientists can determine the composition of its temperature or atmosphere. To date, this method has successfully identified 4151 exoplanets.

The radial velocity method is used to find wobbles in stars caused by the gravitational influence of orbiting planets. The stars’ wobbles change the color of light which astronomers can observe. Larger planets exert greater gravitational force, leading to more pronounced wobbles, while smaller planets produce subtle shifts. These wobbles inform scientists about the number and size of planets orbiting the star. This method relies on the Doppler shift phenomenon, where the light waves either compress or stretch out as the planet moves towards or away from the sun, resulting in a change of color. When visible light waves bunch up together, they appear more blue, and when visible light waves stretch out they appear more red. The resulting color change, known as redshift, enables scientists to observe the wobbling of stars. Through the Doppler shift, scientists have discovered 1075 exoplanets.

The direct imaging method involves capturing images of the exoplanet by blocking the light of the star it orbits. Two primary techniques are employed for this purpose: coronagraphy and starshade devices. Chornography involves using a device inside a telescope to block any light before it reaches the telescope’s detector. The second method is by using a starshade device, positioned to intercept starlight before it enters the telescope.

 

Citations:

5 ways. “5 Ways to Find a Planet.” NASA, NASA, 20 June 2019, exoplanets.nasa.gov/alien-

Hederstrom, Alexandra. “What Is an Exoplanet?” NASA Space Place, NASA, 10 Jan. 2022,

spaceplace.nasa.gov/all-about-

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