A planet crossing the disk of its host star dims the starlight by a fraction equal to (R_planet / R_star)². State pills walk through pre-transit, ingress, mid-transit, and egress, with a live light curve cursor.
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Transit Method (Animated)
Before transit, we measure the full brightness of the host star. This is the baseline. Even a Jupiter-sized planet will only block a small fraction of the disk; an Earth-sized planet about 0.01%.
Host star
Transiting planet
Light curve
The transit method is the most productive technique in modern exoplanet science. NASA's Kepler and TESS missions both rely on it. The idea is simple: stare at a star for weeks or months, watch its brightness, and look for repeating dips. A planet crossing the stellar disk blocks a tiny fraction of the light, producing a characteristic flat-bottomed dip whose depth tells you the planet's size and whose timing repeats with the orbital period.
Light curve geometry is rich. The duration of ingress (when the planet edge is partially on the disk) and the flat-bottom duration together pin down the inclination of the orbit. Multi-band photometry catches the wavelength-dependent depth signature of an atmosphere. Repeated transits give the period precisely.
The transit depth is δ = (R_p / R_star)². For a Sun-like star (R_star ≈ 700,000 km), an Earth-sized planet (R_p ≈ 6,400 km) gives δ ≈ 0.0084%. A Jupiter-sized planet (R_p ≈ 70,000 km) gives δ ≈ 1.0%. The Kepler mission could reliably detect dips down to about 50 ppm (0.005%) across photometric noise, which is why most early transit detections were Jupiter-sized.
Probability of geometric alignment is also small: only about 0.5% of randomly-oriented Earth-Sun systems would happen to transit from our viewpoint. This is why surveys monitor hundreds of thousands of stars at once. False positives (eclipsing binaries, grazing transits, blended sources) are filtered by light-curve shape, multi-band photometry, and follow-up radial-velocity observations.
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Source & accuracy
This transit method (animated) is an editorial illustration built to represent the concept accurately. Where it shows figures, they are typical or representative values chosen to make the relationship clear, not a single underlying dataset. The diagram and its explainer are reviewed and maintained centrally, and updated over time as understanding improves.
How the transit method detects exoplanets
When a planet passes between its host star and an observer, it blocks a small fraction of the star's light, producing a periodic dip in brightness called a transit. The depth of the dip equals the ratio of the planet's area to the star's area, approximately the square of the planet radius divided by the star radius. A Jupiter-sized planet crossing a Sun-like star dims it by about one percent, while an Earth-sized planet causes a dip near 0.01 percent.
Because the dip recurs once per orbit, repeated transits reveal the orbital period, and the depth gives the planet's size relative to the star.
Reading a light curve: ingress to egress
A transit light curve moves through distinct phases. Pre-transit the brightness is steady at baseline. Ingress is the brief interval when the planet's disk begins crossing the stellar edge and brightness falls. Mid-transit is the flat-bottomed minimum when the planet is fully in front of the star. Egress is when the planet exits and brightness climbs back to baseline.
The shape carries information: a flat bottom and sharp ingress and egress suggest a clean central crossing, while a more rounded, shallower dip indicates a grazing transit near the stellar limb. NASA's Kepler and TESS missions used exactly this signature to find thousands of planets.
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