The Transit Method: Finding Planets by Watching a Star Blink

Watching a star blink

Most confirmed exoplanets were never directly imaged -- they were found by watching a star's brightness and catching it dim slightly on a regular schedule. If a planet's orbit happens to be aligned edge-on as seen from Earth, the planet passes directly in front of its star once per orbit, blocking a small fraction of the starlight for a few hours. That dip, repeating with exact periodicity, is a transit, and the method built around detecting it is how missions like Kepler and TESS have found the large majority of known exoplanets.

How deep the dip tells you the planet's size

To first approximation the star is a uniformly bright disc and the planet is an opaque circle crossing in front of it, so the fraction of light blocked is simply the ratio of the two discs' areas:

delta F / F  ~=  (Rp / Rs)^2

delta F / F   fractional dip in brightness during transit
Rp            planet radius
Rs            star radius

This is why the transit method is so sensitive to planet size rather than mass: a Jupiter-sized planet (Rp/Rs around 0.1 for a Sun-like star) produces a dip around 1%, easily seen from the ground, while an Earth-sized planet transiting a Sun-like star produces a dip of roughly 0.008% -- eighty parts per million -- which is only measurable with a space telescope's photometric precision, free of atmospheric scintillation. This size-squared relationship is also why the method is naturally biased toward finding large planets on close orbits first, and why it took a dedicated space mission (Kepler) staring at over a hundred thousand stars continuously to build up the statistics needed to find smaller, longer-period planets.

How long the dip lasts tells you the orbit

The transit's duration depends on the planet's orbital speed and the geometry of the crossing, which in turn depends on the orbital period P, the star's radius, and the semi-major axis a. Longer-period planets orbit slower and transit a star of fixed size more slowly, but they are also farther out, so duration scales with the ratio of stellar radius to orbital velocity. Combined with Kepler's third law linking period and orbital distance, a light curve's period and duration together let astronomers reconstruct the planet's actual orbital distance, not just its size.

Why the dip is not flat-bottomed: limb darkening

A real transit light curve does not drop instantly to a flat minimum and pop back up -- it eases in, bottoms out with a slightly curved floor, and eases back out. Part of that shape is simply the finite time the planet takes to slide fully onto and off the stellar disc (ingress and egress), but the curved bottom itself comes from limb darkening: a star's edge (limb) appears dimmer than its centre, because our line of sight into the limb passes through cooler, higher layers of the stellar atmosphere at a shallower angle. A planet crossing the bright centre blocks more light than the same planet crossing the dimmer limb, so the transit is slightly deeper mid-transit than at the edges -- and fitting that exact curvature is one of the ways astronomers independently constrain the impact parameter (how central the crossing is) and inclination of the transit.

ingress   planet begins sliding onto the stellar disc, brightness falling
mid-transit  planet fully in front of star, dip near its deepest point
             (curved floor from limb darkening, not perfectly flat)
egress    planet slides off the disc, brightness recovering

Confirming it: radial velocity as the second witness

A transit alone can be a false positive -- an eclipsing binary star in the background, diluted by the target star's light, can mimic a shallow periodic dip. The standard confirmation is the radial velocity method: a planet's gravity tugs its star into a small orbit of its own, Doppler-shifting the star's spectral lines back and forth with the same period as the transit. Combining both methods is powerful specifically because they measure different quantities -- transit depth gives planet radius, while radial-velocity amplitude gives planet mass -- and together they yield the planet's density, the first real clue to whether it is a rocky world, a gas giant, or something in between.

Frequently asked questions

Try it live

Everything above runs in your browser — open Exoplanet Transit and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.