Transit Depth Formula & Exoplanet Transits
The transit method finds exoplanets from tiny dips in starlight. Learn the transit depth formula δ ≈ (Rp/R★)², with Jupiter- and Earth-sized examples.
The idea behind it
A transiting planet hides a fraction of its star’s visible disk. Larger planets block more light. The light curve plots the remaining brightness as the planet moves across the star.
The relationship to remember
Planet/star radius = 0.1026. Central full-overlap depth would be 1.052%; the chosen path gives a maximum 1.052% dip.
Make your own discovery
Compare Earth-sized and Jupiter-sized presets. Notice how much harder it is to detect the smaller planet.
Open the Exoplanet Transit Simulator & Depth Calculator ↗Key formulas
| Quantity | Formula | Note or example |
|---|---|---|
| Transit depth | δ ≈ (Rp / R★)² | Planet fully in front of a uniform star |
| Planet radius from depth | Rp ≈ R★ × √δ | Invert a measured dip |
| Earth–Sun radius ratio | R⊕ / R☉ ≈ 0.00916 | So δ ≈ 0.0084% |
Worked examples
Each row uses one of this lab’s presets and the same model as the interactive tool.
| Example | Maximum depth | Light received | Radius ratio |
|---|---|---|---|
| Jupiter-sized | 1.052% | 98.948% | 0.1026 |
| Earth-sized | 0.0084% | 99.9916% | 0.0092 |
| Grazing transit | 0.6448% | 99.3552% | 0.1026 |
What this model leaves out
An opaque circular planet, a uniformly bright star, and a straight path. Overlapping disk areas handle grazing transits. No limb darkening, starspots, noise, or orbital-speed effects.
Check your understanding
Does every planet transit?
No. The orbit must be aligned so that the planet passes in front of the star from our viewpoint.
Can a transit reveal a planet’s mass?
A transit primarily constrains size relative to the star. Mass usually requires additional observations.
What is the transit depth formula?
For a planet fully in front of a uniformly bright star, the fractional drop in brightness is about the square of the planet-to-star radius ratio: δ ≈ (Rp/R★)². A 1% dip around a Sun-like star implies a planet about one tenth of the star’s radius — roughly Jupiter’s size.
Continue with the formula reference or test yourself in space practice.