Exoplanet Transit Depth Calculator
The transit method detects exoplanets by measuring the tiny dip in a star's brightness when a planet crosses in front of it. The depth of this dip equals the ratio of the planet's disk area to the star's disk area, which simplifies to the square of the radius ratio: depth = (Rp/Rs) squared. This clean geometric relationship is one of the most powerful tools in exoplanet science, having led to the discovery of thousands of confirmed exoplanets by the Kepler and TESS missions. Enter planet and star radii to find the transit depth, or enter a measured depth and stellar radius to recover the planet's radius.
Transit depth formula
depth = (Rp / Rs)^2
Rp (from depth) = Rs * sqrt(depth)
depth (ppm) = depth * 1,000,000
Where Rp is planet radius, Rs is stellar radius, both in the same units. The depth is dimensionless (a fraction of total flux). ppm = parts per million. The formula assumes a uniform stellar disk (no limb darkening), which is an idealization; real transit light curves have slightly rounded bottoms due to limb darkening.
Transit depths for the solar system
Jupiter transiting Sun: (71,492/695,700) squared = 0.01055 = 10,550 ppm = 1.055%. Saturn: (58,232/695,700) squared = 0.007005 = 7,005 ppm. Neptune: (24,622/695,700) squared = 0.001252 = 1,252 ppm. Earth: (6,371/695,700) squared = 0.0000838 = 84 ppm. Mars: (3,390/695,700) squared = 0.0000238 = 24 ppm.
Exoplanet transit depth: frequently asked questions
What is a transit depth?
When an exoplanet crosses in front of its host star, it blocks a fraction of the star's light proportional to the ratio of their disk areas. Transit depth = (planet radius / star radius) squared. A depth of 0.01 means the star's brightness dims by 1% during the transit.
How is the transit depth formula derived?
The planet blocks a circular area of pi * Rp squared. The total stellar disk area is pi * Rs squared. The fractional dimming is therefore (pi * Rp squared) / (pi * Rs squared) = (Rp/Rs) squared. This assumes the planet is fully on the stellar disk (no limb crossing).
What transit depth does Earth produce on the Sun?
Earth's radius is 6,371 km and the Sun's radius is 695,700 km. The ratio is 6,371/695,700 = 0.00916. Squaring: depth = 0.0000838, or 84 parts per million (ppm). This is a tiny signal; detecting it requires space-based photometry like the Kepler or TESS missions.
What transit depth did Kepler look for?
NASA's Kepler mission could detect transit depths as small as 20 ppm for bright stars. Jupiter transiting the Sun would produce a depth of (71,492/695,700) squared = 0.01055, or 1.055% (10,550 ppm), easily detectable from the ground for nearby stars.
Can we determine the planet's radius from transit depth?
Yes, if the stellar radius is known. Rp = Rs * sqrt(depth). This is how thousands of exoplanet radii have been measured. The stellar radius must be determined independently from stellar models or interferometry.
Official sources
- NASA Exoplanet Archive: exoplanetarchive.ipac.caltech.edu.
- NASA Exoplanet Exploration (transit method): exoplanets.nasa.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.