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

1.000 L☉
0.0011000 L☉
1.000 R☉
0.01100 R☉
1.000 R⊕
0.120 R⊕
1.000 AU
0.0110 AU
89.97 °
8090 °
1.000 M☉
0.0850 M☉
0.50
01

Transit Diagram

b = 0.11Star (1 R☉)Planet (1 R⊕)Time (phase)Flux1.0

Transit Results

Transit Depth
83.86 ppm
Transit Depth
83.9 ppm
Orbital Period
365.250 days
Impact Parameter
0.1126
Transiting?
Yes
Transit Duration
13.01 hours
R_p from depth
1.000 R⊕
Transit Depth
84 ppm
Orbital Period
365.25 d
Impact Param
0.113
Transiting
Yes

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.

δ=(RpR⋆)2\delta = \left(\frac{R_p}{R_\star}\right)^2

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.

b=acos⁡iR⋆b = \frac{a \cos i}{R_\star}

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.

P2=a3M⋆(years, AU, M⊙)P^2 = \frac{a^3}{M_\star} \quad (\text{years, AU, } M_\odot)

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.

Earth365.25 days Hot Jupiter (0.05 AU)~4 days TRAPPIST-1e~6.1 days Kepler-22b~290 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.

dinner=L⋆/1.1,douter=L⋆/0.53d_{\text{inner}} = \sqrt{L_\star / 1.1}, \quad d_{\text{outer}} = \sqrt{L_\star / 0.53}

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.

Teq=T⋆R⋆2a (1−A)1/4T_{eq} = T_\star \sqrt{\frac{R_\star}{2a}}\,(1-A)^{1/4}

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.