Planets don't make their own light; they only reflect a little of their star's. Seen from light-years away, a planet like Earth would be billions of times fainter than the Sun and sit right beside it, lost in the glare — like trying to spot a firefly next to a searchlight from miles away.
So for a long time, nobody knew whether other stars had planets at all. The first confirmed exoplanets were found in 1992, orbiting a pulsar. In 1995, Michel Mayor and Didier Queloz found 51 Pegasi b, the first planet around a Sun-like star — work that earned the 2019 Nobel Prize in Physics. Today the count of confirmed exoplanets is past 5,000, and almost all of them were found without ever being seen. The trick is to stop looking for the planet and watch its star.
If a planet's orbit happens to be lined up edge-on from our point of view, the planet passes in front of its star once every orbit, blocking a tiny fraction of its light. A graph of the star's brightness over time — a light curve— shows a small, flat-bottomed dip each time. The time between dips is the planet's orbital period. The depth of the dip reveals the planet's size:
The depth is the fraction of the star's disk the planet covers. Jupiter is about a tenth of the Sun's width, so it would block about (0.1)² = 1% of the Sun's light. Earth, about 1/109 of the Sun's width, would block only about 0.0084% — 84 parts per million. That's why transit hunting takes space telescopes that can measure brightness with incredible steadiness: NASA's Kepler mission (2009–2018) found more than 2,600 planets this way, and TESS has been surveying nearly the whole sky since 2018.
Set a planet and a star, then slide the planet across. The graph on the right is the light curve a telescope would record — notice how the vertical axis zooms in on tiny dips.
A planet doesn't simply orbit its star. The planet and the star both orbit their shared center of mass. The star is so much heavier that its orbit is tiny, but it's real: the star moves in a small circle, toward us and then away from us, over and over. You already know how to catch that motion. It's the Doppler shiftfrom Activity 5.2.1: the star's spectral lines slide slightly toward blue, then toward red, once per orbit. Measuring that back-and-forth speed is the radial-velocity method.
A heavier planet, or one closer in, tugs harder and makes a bigger wobble. Jupiter makes the Sun wobble by about 12.5 m/s, over a 12-year cycle. Earth makes it wobble by only about 9 centimeters per second. 51 Pegasi b, a giant planet orbiting its star every 4.2 days, produced a wobble of tens of meters per second — which is why it was found first.
A planet and its star both orbit their shared center of mass. Change the planet's mass and distance and watch the star's wobble — its back-and-forth speed toward and away from us — grow or shrink.
Assumes a Sun-mass star, a circular orbit, and an edge-on view.
Each method alone tells only half the story. Size alone can't tell a dense rocky world from a fluffy one wrapped in gas; mass alone can't either. But mass ÷ volume is density— and density reveals what a planet is made of. In Earth units there's a shortcut:
Enter a planet's mass (from its wobble) and radius (from its transit), in Earth units.
For comparison: water is 1.0 g/cm³, rock about 3, Earth 5.5, Jupiter 1.3, Saturn 0.7.
Using both methods also guards against false alarms. A dip in starlight could come from something other than a planet — for example, two stars eclipsing each other in the background. A transit that repeats on schedule,plus a matching wobble with the same period, is strong confirmation that a planet is really there.
| Method | What it measures | What it tells you | Needs |
|---|---|---|---|
| Transit | Dips in the star's brightness | Planet radius; orbital period | An orbit lined up edge-on from Earth |
| Radial velocity | Doppler shifts of the star's lines | Planet (minimum) mass; orbital period | A big enough wobble to measure |
| Both | — | Density, and so composition | A planet that shows up in both |
Both methods favor the same kinds of planets: big ones (deeper dips, bigger wobbles) on short orbits (more transits and wobbles to catch in a short time). That's why the first exoplanets discovered were mostly hot Jupiters — giant planets orbiting closer to their stars than Mercury does to the Sun.
Other methods fill in the gaps. Direct imagingblocks a star's light to photograph large, young planets far from it. Gravitational microlensingcatches a planet's gravity briefly magnifying a background star. But transits and wobbles have found the large majority of known exoplanets.