How Scientists Find Planets by Watching Stars Wobble

In 1995, two Swiss astronomers — Michel Mayor and Didier Queloz — noticed that the star 51 Pegasi wobbled back and forth every 4.2 days. They figured out something remarkable: a planet the size of Jupiter was pulling the star toward and away from Earth as it orbited. This discovery earned them the 2019 Nobel Prize in Physics, and the technique they used — the radial velocity method — has since found over 1,000 exoplanets.

By the end of this lesson you will be able to: • Explain why stars wobble when a planet orbits them • Describe how the Doppler effect lets us measure that wobble from Earth • Read a simple radial velocity curve and find the planet's orbital period • Explain what makes a planet easier or harder to detect with this method

We say a planet orbits a star, but both the star and the planet actually orbit their shared center of mass — called the barycenter. In our solar system, the Sun is so massive (99.86% of the total mass) that the barycenter is usually just inside the Sun's surface. But the Sun still wobbles! Jupiter tugs it about 742,000 km back and forth every 12 years — a distance bigger than the Sun's own diameter. If alien astronomers watched our Sun from another star system, they would see that wobble and could figure out Jupiter exists without ever seeing Jupiter directly. That is exactly how we find exoplanets.

You have heard the Doppler effect before. When an ambulance approaches, the siren sounds higher-pitched; when it moves away, the pitch drops. Sound waves bunch up (higher frequency) when the source moves toward you and stretch out (lower frequency) when it moves away. Light does the same thing. When a star moves toward Earth, its light is squeezed toward shorter, bluer wavelengths — called blueshift. When it moves away, the light stretches toward longer, redder wavelengths — called redshift. Astronomers use a spectrograph — an instrument that splits starlight into a rainbow — to measure these tiny color shifts. For 51 Pegasi, the wobble speed was only about 56 meters per second (roughly the speed of an Olympic sprinter), yet the ELODIE telescope in France could detect it from 50 light-years away.

When astronomers plot a star's measured speed toward or away from Earth over time, they get a wave-shaped graph called a radial velocity curve. What the graph tells us: • Period (time for one complete wave cycle) = the planet's orbital period — how long its year is • Amplitude (height of the wave) = how fast the star wobbles, which reveals the planet's minimum mass • Shape of the curve = the orbit's shape (circular orbits make smooth sine waves; elliptical orbits make lopsided waves) For 51 Pegasi b: • Period = 4.23 days — it circles its star in less than a week • Amplitude = 56 m/s • Calculated minimum mass = about 0.47 times Jupiter's mass The word minimum matters — if the orbit is tilted relative to us, we only see part of the wobble, so the planet could be heavier. Astronomers call this measurement m sin i (mass times the sine of the inclination angle).