Exoplanet transit
Model a transit light curve and explore the projected star–planet geometry.
About this tool
Calculate the relative light of a star as an opaque planet crosses its disc. The default is an illustrative Jupiter-sized planet on a 3.5-day orbit, not a measured exoplanet. Press Calculate, then animate or move the time slider. The observer’s view and curve use the same geometry.
The circular orbit follows Kepler’s law using the star mass; planet mass is neglected. Stellar radius, planet radius and b set the transit chord. b is the projected centre-to-centre separation at mid-transit in stellar radii. T₁₄ runs from first to fourth contact; T₂₃ is the fully interior part, zero for a grazing transit. No transit is a valid result.
Under Star & limb darkening, set I(μ)/I(1) = 1 − u₁(1 − μ) − u₂(1 − μ)², μ = √(1 − r²). The blocked brightness is integrated over the actual disc overlap using 256-step quadrature; uniform discs and central transits use closed forms. Depth is the calculated flux loss at mid-transit, not merely the squared radius ratio. Boundary coefficients and a uniform disc are supported.
The curve is an ideal model: no eccentricity, spots, noise, finite exposure, planet emission or data fitting. Playback takes eight seconds across the displayed interval; its speed is illustrative. The optional 3D view is perspective, whereas the observer’s view is the parallel projection used for photometry.