Doppler spectroscopy
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Doppler spectroscopy is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star.
As of November 2022, about 19.5% of known extrasolar planets have been discovered using Doppler spectroscopy.
Doppler spectroscopy detects periodic shifts in radial velocity by recording variations in the color of light from the host star. When a star moves towards the Earth, its spectrum is blueshifted, while it is redshifted when it moves away from us. By analyzing these spectral shifts, astronomers can deduce the gravitational influence of extrasolar planets.
Methods of detecting exoplanets
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Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the exoplanets reported as of January 2024 have been observed directly, with even fewer being resolved from their host star.
Theoretical transiting exoplanet light curve. This image shows the transit depth (δ), transit duration (T), and ingress/egress duration (τ) of a transiting exoplanet relative to the position that the exoplanet is to the star.
This image shows the relative sizes of brown dwarfs and large planets.
Artist's impression of the pulsar PSR 1257+12's planetary system.
The Kepler Mission, A NASA mission which is able to detect extrasolar planets