Astronomy
The Discovery of Exoplanets Using the Radial Velocity Method
Quick fact
The first exoplanet found around a Sun-like star, 51 Pegasi b, was discovered in 1995 and earned its discoverers the 2019 Nobel Prize in Physics.
Why this is interesting
You've probably seen artists' pictures of planets orbiting other stars—but how do we know they're there if we can't see them directly? The answer lies in a tiny, rhythmic wobble in the star's own light.
Read the full explanation
Understanding The Discovery of Exoplanets Using the Radial Velocity Method
Imagine swinging a weight on a string around your head. The weight pulls on your hand, making you move in a tiny circle too. Similarly, a planet's gravity tugs on its star, causing the star to wobble slightly as the planet orbits. We can't see this wobble directly, but we can detect it by looking at the star's spectrum—the rainbow of light it emits. When the star moves toward us, its spectral lines shift toward the blue; when it moves away, they shift toward the red. This is the Doppler effect, the same reason an ambulance siren sounds higher pitched as it approaches and lower as it recedes. By measuring these periodic shifts over time, astronomers can deduce the presence of a planet and even estimate its mass and orbit.
A deeper explanation
The radial velocity method relies on the fact that a star and its planet orbit their common center of mass. The star's motion is much smaller than the planet's, but it's detectable. As the star moves, its light's wavelength changes according to the Doppler formula: Δλ/λ = vr/c, where vr is the star's radial velocity (toward or away from us) and c is the speed of light. By recording spectra over weeks or months, astronomers plot the star's radial velocity curve. A periodic sinusoidal variation indicates a companion. The amplitude of the variation gives the planet's minimum mass (since the orbital inclination is unknown), and the period gives the orbital distance. The first success came in 1995 when Michel Mayor and Didier Queloz observed 51 Pegasi, a star like our Sun, and found a 4.2-day periodicity, implying a planet at least half Jupiter's mass and extremely close to the star—a 'hot Jupiter' that defied expectations. This discovery proved that planets could exist in configurations very different from our solar system, reshaping planetary formation theories and spurring a wave of exoplanet discoveries.