Exoplanet Transit Explorer
Two modes. The Transit Method tab calculates transit depth, orbital period, duration, and impact parameter from star and planet parameters. The Habitable Zone tab shows inner/outer HZ boundaries, equilibrium temperature, and whether a planet falls within the zone. Six presets from Earth-Sun to TRAPPIST-1e.
Mode
Presets
Transit Diagram
Transit Results
Reference Guide
Transit Method
When a planet passes in front of its host star as seen from Earth, it blocks a tiny fraction of the starlight. The fractional dip in brightness is called the transit depth.
An Earth-sized planet crossing a Sun-like star produces a depth of roughly 84 ppm (0.0084%), while a hot Jupiter can block over 1% of the light. The transit duration depends on the orbital speed, stellar radius, and the planet's impact parameter.
The impact parameter b describes how far from the stellar center the planet's chord passes. b = 0 is a central transit; b > 1 means no transit.
Kepler's Third Law
Kepler's third law relates the orbital period to the semi-major axis and the mass of the host star.
For the Earth-Sun system with a = 1 AU and M = 1 M☉, P = 1 year (365.25 days). A hot Jupiter at 0.05 AU orbits in just a few days.
Habitable Zone
The habitable zone (HZ) is the range of orbital distances where liquid water could exist on a planet's surface. The boundaries depend on the star's luminosity.
For the Sun (L = 1 L☉), the HZ spans roughly 0.95 to 1.37 AU. Earth at 1 AU sits comfortably inside. The equilibrium temperature is the blackbody temperature a planet would have without an atmosphere.
A = Bond albedo (fraction of energy reflected). Earth's actual surface temperature (~288 K) exceeds T_eq (~255 K) due to the greenhouse effect.
Notable Exoplanets
Over 5 500 exoplanets have been confirmed as of 2024, most discovered by the transit method (Kepler, TESS) or radial velocity.
TRAPPIST-1 System
Seven Earth-sized planets orbiting an ultra-cool red dwarf 40 light-years away. Three (e, f, g) are in the HZ.
Kepler-22b
Super-Earth (2.4 R⊕) in the HZ of a Sun-like star. Orbital period 290 days.
51 Pegasi b
The first confirmed hot Jupiter (1995). 0.47 Mⱼ, orbits at 0.05 AU with a 4.2-day period.
Proxima Centauri b
Nearest known exoplanet (4.24 ly). Roughly Earth-mass, in the HZ of a red dwarf.
