Estimate whether an exoplanet transit campaign has enough photometric precision and observing time for a specified detection goal.
The model converts planet and star radii into an expected transit depth, then relates that depth to a target signal-to-noise ratio across repeated transits. It is useful for early mission trades, telescope time proposals, and quick feasibility checks before a full instrument simulation.
1. Enter the planet radius
Use Earth radii for the transiting planet.
2. Enter the stellar radius
Use solar radii for the host star.
3. Set the detection target
Choose a combined SNR and number of observed transits.
4. Add time requirements
Enter transit duration and out-of-transit baseline time per event.
5. Review the feasibility outputs
Compare predicted depth, required single-transit precision, and total scheduled hours with the instrument concept.
Transit depth = (Rplanet / Rstar)²
Combined SNR = (depth / σsingle) × √N
Required σsingle = depth × √N / target SNR
Total scheduled time = N × (transit duration + baseline)
Radii are converted using 1 R⊕ = 6,371 km and 1 R☉ = 695,700 km. The precision result is expressed in parts per million (ppm).
A deeper transit or more repeated events relaxes the required per-transit precision for the same combined SNR.
This is a screening model, not a substitute for a mission-specific noise model.
Given: 1 R⊕ planet, 1 R☉ star, target SNR 7, 3 transits, 3 h transit duration, and 1 h baseline per transit.
Calculation: Radius ratio = 6,371 / 695,700 = 0.009158. Depth = 0.009158² = 0.00008387 = 83.9 ppm. Required single-transit precision = 83.9 × √3 / 7 = 20.8 ppm. Scheduled time = 3 × (3 + 1) = 12 h.
Result: A campaign would need about 20.8 ppm single-transit precision and 12 hours of scheduled observing time for this simplified detection target.
The planet blocks a larger fraction of a smaller stellar disk, so the radius ratio is larger and its square increases.
No. It is a simplified per-transit photometric precision target after the data for one transit are combined.
No. The geometric depth model does not include limb darkening, starspots, correlated noise, or instrumental systematics.
No. The calculator treats transits as whole observing events and requires at least one.
Use it as an early allocation estimate. Real programs also need overhead, visibility constraints, calibration time, and contingency.