Baseline
Normal stellar brightness
The flat part of a light curve is the star's usual brightness when no planet is crossing its visible disk.
Exoplanets · Question
Exoplanet transit simulator: how light curves reveal planets
Most confirmed exoplanets have never been photographed — they're too small, dim, and close to their blinding-bright star. Instead, astronomers watch a light curve for a tiny, regular dip in the star's brightness as a planet transits in front of it.
Visual model
Drag the planet around its orbit.
Read the light curve
Three clues turn a brightness graph into evidence.
The graph is a measurement of starlight over time. Its shape tells astronomers more than a single image can.
Transit depth
How much light is blocked
The deeper the repeating dip, the larger the planet is relative to its host star. An Earth-size planet crossing a Sun-like star produces only about a 0.01% dip.
Period
Time between repeated dips
The interval from one matching transit to the next gives the planet's orbital period — its year around that star.
What's simplified
Real transit signals are far smaller.
01
The dip is exaggerated
A real Earth-sized planet transiting a Sun-like star dims it by less than 0.01% — this diagram uses a far larger dip so it's visible at all.
02
Only some orbits align
Transits are only visible if the planet's orbit happens to be lined up edge-on to Earth — most exoplanets orbit at the wrong angle to ever be detected this way.
03
One dip repeats
Astronomers look for the same dip repeating on a precise, regular schedule — a single dip could be measurement noise; a repeating one is a planet.
Misconception check
Quick check.
Quick check
Go deeper
Keep exploring.
Next in this series →
EM spectrum ↗
What infrared and X-ray reveal.
Orbits
Why don't planets fall into the Sun?
The orbital mechanics behind the planet in this diagram.
Learn
How to read space images
A light curve is a measurement, not a photograph — the same distinction this page covers.