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.
Model experiment
How does planet size change the light curve?
Change the orbital phase and planet radius. Predict how much starlight is blocked and compare the light curve.
Observer's sky view · relative planet/star radii to scale; orbital spacing schematic.
Hypothetical light curve · x: orbital phase 0–360° · y: brightness 100–90% (fixed scale).
The static model is readable while controls initialize.
Try phase 90° and compare 10% to 20% planet radii: blocked light increases from 1% to 4%. At 270°, the planet is behind the star and causes no dip.
Hypothetical calculation, not telescope observations. Uniformly bright stellar disk, opaque planet, exactly edge-on circular orbit; no limb darkening, noise, blended stars or grazing paths. Central depth = (planet radius ÷ stellar radius)². Orbit spacing not to scale; phase is not time. Sources: NASA transit explanation and NASA light-curve interpretation.
Read the light curve
Three clues turn a brightness graph into evidence.
Astronomers graph measured starlight over time. The experiment above instead plots an idealized calculation against orbital phase, not actual observations or measured time.
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
Small dips are hard to see
An Earth-sized planet crossing a Sun-like star dims it by roughly 0.01%. Our graph's brightness axis stays fixed from 100% to 90%; smaller planets can make the line look almost flat. Read the numerical value to detect subtle changes.
02
Only some orbits align
Transits require a nearly edge-on view from Earth; many orbit inclinations never cross the visible disk. This model fixes an exactly edge-on orbit; grazing-transit paths and orbital inclination are not simulated.
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 dip is a planet candidate, not proof by itself. Eclipsing binary stars or instrumental artifacts can mimic transits, so astronomers validate candidates with follow-up evidence.
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.