Radial velocity method

The radial velocity method detects orbiting planets by measuring periodic Doppler shifts in a star’s spectral lines. The shifts reveal the star’s motion around the system’s shared center of mass.

Doppler spectroscopy: A technique that measures motion along the line of sight from shifts in spectral features. It turns the host star’s changing line-of-sight motion into a measurable wavelength shift.
51 Pegasi b: An exoplanet orbiting the Sun-like star 51 Pegasi, discovered in 1995 as the first confirmed planet around a Sun-like star. Its discovery by stellar wobble established the method’s power to find close-in giant planets.
Transit method: A planet-detection technique that measures the periodic dimming of a star when a planet crosses its disk. Transits measure planet-to-star size ratios, while velocity measurements constrain mass.
Stellar jitter: Apparent radial-velocity variation caused by changing stellar surface features rather than motion of the star as a whole. It is the central noise source when searching for low-amplitude planetary signals.
Barycenter: The center of mass around which two or more bodies orbit under their mutual gravity. A planet and star both orbit this point, making the star wobble.
Minimum mass: The lower bound on an exoplanet’s mass inferred when its orbital inclination is unknown. A velocity signal alone usually gives mass multiplied by the sine of orbital inclination.
Astrometry: The measurement of celestial positions and their changes across the sky. It detects a star’s sky-plane wobble, complementary to the radial motion measured here.
Magnetic activity cycle: A recurring change in a star’s magnetic activity, often accompanied by variations in spots and emission. Long activity cycles can mimic the slow velocity trend of a distant planet.
Keplerian orbit: An idealized orbit governed by Newtonian gravity, usually represented by an ellipse. Its parameters shape the periodic velocity curve produced by an orbiting planet.
Hot Jupiter: A gas giant orbiting very close to its host star, with an orbital period typically under ten days. Large, rapid stellar wobbles made these planets especially accessible to early surveys.

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