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5 methods to discover exoplanets

In this post, it’s shown the methods of detecting exoplanets, which are planets outside the Solar System, they orbit other stars, or wander through space.

Direct imaging

Consists in observe directly the celestial body through a telescope. However, it’s very hard to find planets several light years away with this technique. Few exoplanets have been discovered with direct imaging. The biggest difficulty is that the light reflected by the planet and heat radiation are overshadowed by star’s brightness. One of ways to block star’s light is equipping the telescope with a coronograph. This is have already been quoted in the post about coronal mass ejection.

What is coronal mass ejection?Click here

star shade to find exoplanets
Another solution is to use a starshade, launched together with a space telescope, to block the star’s light. Source: space.com.

Gravitational lens

The gravitational lens use the General Relativity principle, light bends and change direction when pass close to a massive object.

space-time curvature
Light’s curvature created by gravity causes a distance between the apparent and real positions of the observed object. Source: shodalap.org.

To use this method, a source star is used to provide a backlight, while the exoplanet’s star serves as a lens to bend the light and produce two images from source star.

gravitational lens to observe exoplanets
The planet around the lens star also contributes to light curvature. Therefore, planet’s gravity causes a light intensity peak that can be detected by a telescope. Source: EarthSky.

Star’s brightness variation

This is the most used method to find exoplanets. The majority of planets outside Solar System were discovered observing the variation of brightness on target star.

transit method to find exoplanets
When an exoplanet transit in front of the star, brightness is reduced in relation to the telescope. Source: Spaceflight101.

With this method, can also discover the planet size and it’s translation speed, that is related with distance from the star. The disadvantage of this method is that the exoplanet can take too long to pass in front of the star.

Radial speed variation

The star exerts gravitational force on planet and vice-versa. Consequently, both spin around a common center of mass for both celestial bodies.

exoplanet and star orbits
With small planets this effect is imperceptible. But in presence of big exoplanets, like gas giants, this effect is much more significant. Source: Wikimedia.

This movement causes a Doppler effect, emitted light by the star causes a wavelength variation in relation to telescope.

When the star gets closer to observer, light’s wavelength gets shorter. When it gets further, the emitted light has its wavelength longer. Source: Extremetech.

Measuring the variation wavelength, can measure star’s oscillation speed. Therefore, can calculate mass and speed of planet. This method is useful only to find massive exoplanets around small stars.


This method consists in detect the motion of a star measuring its distance in relation to other stars that are fixed in relation to observer. 

astrometry to find exoplanets
The star’s movement is a spiral shape in sky indicates the presence of an exoplanet. Source: ESA.

Many images of the region on space where the target star is located are taken, to compare the distances with reference stars. This method is very difficult to be used and requires optical instrument extremely precises. Very few exoplanets were discovered with this technique.

About Pedro Ney Stroski

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