How a planet's shadow, cast across its star, reveals a world 10 billion times dimmer than the light source itself. The most prolific exoplanet discovery method in history.
Imagine standing a billion kilometers away from the Sun and watching Mercury pass in front of it. From that distance, Mercury would block only 0.011% of the solar disk. Yet from Earth, we can measure this dip with modern photometers. Now imagine that Mercury orbits a star other than our Sun — a star we cannot resolve into a disk, but can only measure as a point source of light.
This is transit photometry: the art of detecting planets by measuring the minuscule dimming of starlight as a planet orbits in front of its host star. A typical Jupiter-sized exoplanet blocks about 1% of starlight. A super-Earth blocks 0.01%. The technique is so sensitive that missions like Kepler and TESS have detected planets whose transit depth is only 0.001% — utterly invisible to the human eye, yet unmistakable to a space-borne photometer.
Since the first exoplanet was detected via transit in 1999 (HD 209458 b, observed by Henry Charbonneau), over 4,600 worlds have been discovered using this method alone. It dominates the exoplanet catalog not because it is the most sensitive to small planets — that honour belongs to astrometry — but because it works from space, it works for distant stars, and it works repeatably across thousands of targets simultaneously.
Adjust the planet size and orbital period to see how the transit depth and frequency change.
Gas Giant
Hot
Transit Duration
2.7 hrs
Transits per Year
37
Simulated light curve
The mathematics of a transit is deceptively simple. When a planet of radius R_p passes in front of a star of radius R_*, the cross-sectional area blocked is:
where ΔF is the change in flux and F is the original stellar flux.
This elegant relationship means we can infer the planet's radius directly from the transit depth — if we know the star's radius. For a Sun-like star (R_* ≈ 1 R_☉) and an Earth-sized planet (R_p ≈ 1 R_⊕), the transit depth is about 0.01%, or 1 part in 10,000.
But measuring a 0.01% dip requires extraordinary precision. A 2-meter telescope observing from the ground experiences atmospheric turbulence that induces noise of several percent. This is why space-borne observatories like Kepler, with its 0.95-meter aperture but exquisite atmospheric stability, could outperform ground-based instruments by orders of magnitude.
The light curve — the graph of stellar brightness over time — contains far more information than just depth. The shape of the transit reveals the planet's orbital inclination. The duration (from first contact to last contact) constrains the orbital period and planet size jointly. The ingress and egress slopes depend on limb darkening — how the star's edge is dimmer than its center. Distortions in the curve can signal atmospheric refraction, rings, or moons.
Launched in 2009, NASA's Kepler mission fundamentally changed exoplanet astronomy. Its photometer was stable to better than 1 part in 100,000 — capable of detecting a candle on the Moon from Earth. It monitored over 150,000 stars continuously, collecting a 30-minute cadence measurement every half hour for four years.
By staring at the same patch of sky (Cygnus-Lyra, a region about 115 square degrees), Kepler discovered over 2,600 confirmed exoplanets — nearly 70% of all known worlds as of 2025. It found Earth-sized planets in habitable zones, hot Jupiters, super-Earths, and mini-Neptunes. It detected planets as small as 0.5 R_⊕ and as large as 15 R_J. It revealed that small planets are far more common than large ones, overturning a decade of bias toward massive, easy-to-detect worlds.
Kepler's portfolio included 4,711 planet candidates when it finally exhausted its fuel. Of these, 2,662 have been confirmed as of late 2024. The remainder await validation through follow-up observations (radial velocity, imaging, or statistical vetting), but the Kepler dataset remains the gold standard for exoplanet statistics.
Planets Discovered
2,662
confirmed
Smallest Planet
0.55 R⊕
Kepler-37b
Shortest Orbit
0.34 days
KOI-2700b
Not every dip in starlight is a planet. Transits can be mimicked by eclipsing binaries (two stars orbiting each other), starspots rotating in and out of view, or instrumental artifacts. This is why follow-up confirmation is essential.
Stellar Activity
Sunspots and stellar rotation can mimic transit signals
Blending
Light from a nearby star can dilute the transit depth
Planetary Companions
Multiple planets can cause complex timing variations
Atmospheric Effects
Ground-based observations suffer from scintillation and seeing
Radial velocity follow-up can confirm the planet's minimum mass (M_p sin(i)), ruling out stellar binaries. Direct imaging can verify the planet's temperature. Timing variations can reveal the presence of additional planets through their gravitational perturbations (Transit Timing Variations, or TTVs).
The Transiting Exoplanet Survey Satellite (TESS), launched in 2018, observes nearly the entire sky — 85% of it — with a focus on nearby, bright stars. This enables follow-up observations from the ground and with space-borne spectrographs like JWST. TESS has discovered over 350 confirmed exoplanets and thousands more candidates, including rocky worlds in habitable zones around M dwarfs.
Future missions will push precision even further. The PLATO mission (Planetary Transits and Oscillations, launching in 2026) will characterize thousands of Earth-sized planets in habitable zones. The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will discover thousands of transiting planets through wide-field, time-domain imaging.
Transit photometry remains the workhorse of exoplanet detection. Its strength lies in precision, repeatability, and simplicity. Every night, space telescopes and ground-based observatories collect new transits, refining our census of worlds and bringing us closer to answering the question: are we alone?