Why ppm for Earth-sized planets?

For a Sun-like star, (R_⊕/R_☉)² is of order 10⁻⁴, so depths are roughly 100 ppm before noise and stellar variability.

Does this give the planet mass?

Not by itself — transits yield radius and period (with stellar mass). Mass typically needs radial velocities or timing variations.

Exoplanet Transit (light curve)

For a star modeled as a uniformly bright disk, a planet of radius R_p transiting at separation ρ between centers blocks flux in proportion to the geometric overlap area of two circles divided by πR_*². With ρ² = b² + (vt)² for impact parameter b in units of stellar radius, ingress and egress durations grow for grazing geometries. Limb darkening, stellar spots, blended binaries, and instrumental systematics are omitted.

Who it's for: Introductory exoplanet detection; pairs with radial-velocity pages and stellar parallax.

Key terms

transit photometry
impact parameter
transit depth
orbital period

Live graphs

How it works

When a planet passes **in front of** its star (**transit**), the observed flux drops by the **fraction of the stellar disk** covered, for a **uniform** brightness disk: **ΔF/F ≈ (R_p/R_*)²** for a **central** transit. **Grazing** transits (**impact parameter b** approaching **1**) have **longer** ingress/egress and **smaller** depth. **Time between transits** gives the **orbital period**; **depth** and **duration** constrain **size** and **inclination** together with stellar radius. Real light curves add **limb darkening**, **star spots**, and **noise**.

Key equations

F/F₀ = 1 − A_overlap(ρ) / (πR_*²) · ρ = √(b² + (vt)²)

Central: ΔF/F ≈ (R_p/R_*)²

Frequently asked questions

Why ppm for Earth-sized planets?: For a Sun-like star, (R_⊕/R_☉)² is of order 10⁻⁴, so depths are roughly 100 ppm before noise and stellar variability.
Does this give the planet mass?: Not by itself — transits yield radius and period (with stellar mass). Mass typically needs radial velocities or timing variations.