Exoplanet Transit Depth Calculator
The dip in a star’s light when a planet crosses it — from the two radii — in percent and parts per million, and the planet radius that a measured transit depth implies.
A transiting planet blocks a share of the star’s disc equal to the ratio of their areas: depth = (R_planet ÷ R_star)².
How the exoplanet transit depth calculator works
A transiting planet blocks a share of the star’s disc equal to the ratio of their areas: depth = (R_planet ÷ R_star)². Jupiter in front of the Sun would dim it by about 1%; Earth by 84 parts per million — the precision the Kepler and TESS missions were built for. Turn it round and a measured depth with a known stellar radius gives the planet’s size, which is how most exoplanet radii are known.
Formula: depth = (R_p ÷ R_★)²; R_p = R_★ √depth
Worked examples
| Inputs | Transit depth | Note |
|---|---|---|
| Earth across the Sun | 83.9 ppm | 84 ppm |
| Jupiter across the Sun | 1.01% | 1.01% |
| A 10,000 ppm dip round a 0.3 R☉ dwarf | 931.8 ppm | implies 3.3 Earth radii |
FAQFrequently asked questions
How small a dip can we detect?
Ground telescopes reach about 1,000 ppm (0.1%) on bright stars; Kepler and TESS about 20–50 ppm with repeated transits, enough for Earth-sized planets around Sun-sized stars. Small red dwarfs make planets easier: the same planet gives a deeper dip.
Does the depth give the mass?
No — only the radius. Mass comes from radial-velocity wobble or transit timing; together they give density and hence whether the planet is rocky, watery or gaseous.
Why are real transits not flat-bottomed?
Limb darkening: the star is dimmer at its edges, so the dip is deeper when the planet crosses the centre. The formula gives the geometric depth; fitted depths correct for the star’s brightness profile.
What about grazing transits?
A planet that only partly crosses the disc gives a shallower, V-shaped dip; the formula assumes a full transit. The impact parameter from the light-curve shape tells them apart.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G and the speed of light
- IAU 2015 Resolution B3 — nominal solar and planetary conversion constants — the astronomical unit, solar mass and planetary radii
- Royal Observatory Greenwich — the historic home of the prime meridian
Last checked: September 2026. Constants are CODATA 2018 (G, c) and IAU 2015 nominal values (solar and planetary parameters).