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)

Uniform disk overlap; R_p/R_*; impact b; F(t) vs period.

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About this model

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

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.

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