How do we find exoplanets using their star’s speed?

It’s a way of finding distant worlds, so how does it work?

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An artist’s impression of Upsilon Andromedae b, a gas giant that was discovered using the radial velocity method

An artist’s impression of Upsilon Andromedae b, a gas giant that was discovered using the radial velocity method. Image Credit: NASA

Asked by David North

When a planet moves in its orbit, its parent star will also move in its own small orbit in response to the alien world’s gravity. This then causes the speed – known as its radial velocity – at which the star moves towards or away from the Earth to vary.

It’s this variation that tells astronomers that a planet could exist around a star and they quite fittingly call this technique the radial velocity method. Instruments affixed to ground-based telescopes such as ESO’s La Silla Observatory in Chile and the Keck telescopes on Mauna Kea in Hawaii employ this method to find exoplanets. So far, over 300 planets, over 30 of which have been confirmed as exoplanets, have been uncovered using the radial velocity method.

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