Searching for Exoplanets

A team of scientists at the University of Exeter have been exploring the possibility that Proxima B—an exoplanet orbiting the nearest stars to our solar system—might be habitable. The rash of exoplanets discovered in recent years begs the question: how do astronomers find exoplanets?

Since Neptune was discovered in 1846, astronomers have realized that it is possible to find planets indirectly instead of just through observation (Neptune was discovered because of the gravitational effect it had on Uranus and was observed later).

Radial velocity method: Also known as Doppler spectroscopy. Just as stars exert a gravitational on planets, planets also exert a gravitational pull on stars. It may appear that a planet is orbiting a star, but really, both the planet and the star are orbiting a common center of mass. As a star orbits away from the Earth, the star’s light will red-shift, and as the star orbits closer to Earth its light will blue shift. Earth-sized planets are too small to be easily detected with this method, but as instruments have become more sensitive, even smaller planets have been found.

Transit: This method requires that a planet pass right in front of its star relative to the Earth. Obviously, this isn’t going to happen all the time, but astronomers have instruments sensitive enough that they can detect transiting exoplanets a few thousand light years away. This is how seven exoplanets were detected around TRAPPIST-1 early this year.

Gravitational lensing: This method was predicted by Einstein’s theories. If a luminous object passes directly behind a massive object relative to the Earth, the background object’s light will be bent by the foreground object, creating a lensing effect. The background object’s light will be magnified. If the foreground object is a star with a planet, the planet can be detected by how it contributes to the lensing. This is a complicated process, but there have been exoplanets discovered using this method.

Direct imaging: Considering how long it took the human race to find Uranus and Neptune, you can probably imagine what it’s like to directly observe planets in another solar system. Most exoplanets are imagined using infrared, which means only bigger, hotter planets at a good distance from their star can be imaged.

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