Transit Probability Calculator
Calculate the geometric probability that an exoplanet will transit its host star as seen from Earth.
Shows why nearby stars are best targets.
Not every planet transits its star from Earth’s perspective. Whether a transit occurs depends purely on geometry — the orbital plane must be nearly edge-on as seen from Earth.
Transit probability formula:
p ≈ R_star / a
More precisely, including the planet’s radius:
p = (R_star + R_planet) / a
Where a is the orbital semi-major axis, all in the same units.
Why this formula? For a randomly oriented orbit, the probability that the inclination brings the planet across the star’s disk is proportional to the ratio of the star’s (and planet’s) apparent size to the orbit size.
Real-world transit probabilities:
| Planet Scenario | Probability |
|---|---|
| Earth-Sun equivalent | 0.47% |
| Hot Jupiter (0.05 AU) around Sun | 9.3% |
| Venus analog (0.72 AU) | 0.65% |
| Mars analog (1.52 AU) | 0.31% |
| Jupiter analog (5.2 AU) | 0.09% |
Why close-in planets are detected more:
- Higher transit probability (shorter orbit = more likely to transit)
- More transits per year = more opportunities to detect and confirm
- Deeper transit signal (hot Jupiters block more light)
Survey strategy: TESS focuses on nearby stars because they are brighter, making small transits (from Earth-sized planets) easier to detect. Kepler stared at one patch of sky to detect long-period planets with low transit probability.
The detection bias: Most known exoplanets are hot Jupiters (large, close-in) not because they are common, but because they are the easiest to detect by transit. Surveys must apply careful statistical corrections.