An exoplanet calculator for transits, temperatures and habitable zones
When a planet passes in front of its star the starlight dips by a tiny fraction — the transit depth. This exoplanet calculator turns that depth into the planet’s size, the orbital period into a distance, and the star’s temperature and size into the planet’s equilibrium temperature, the starlight it receives and whether it lies in the habitable zone. Add the planet’s mass and you also get surface gravity, density and escape velocity.
Use it with numbers from the NASA Exoplanet Archive, a class exercise or your own TESS light curve. Four presets — an Earth twin, a hot Jupiter, a super-Earth around an orange dwarf and a small planet around a red dwarf — show typical cases, and everything is computed in your browser.
How to use the Exoplanet Calculator
- Enter the Star: radius in solar radii, effective temperature in kelvin and mass in solar masses. Leave luminosity empty to derive it.
- Enter the Transit depth in ppm or per cent.
- Give the orbit by Period in days or by Distance in AU.
- Set the Bond albedo and how the planet spreads heat, and optionally its mass in Earth or Jupiter masses.
- Read the radius, temperature, starlight, habitable-zone verdict and gravity, then the table of habitable-zone limits.
Exoplanet Calculator features
Transit depth to radius
Rp = R★√δ, in Earth radii, Jupiter radii and kilometres.
Period to distance
Kepler’s third law with the IAU nominal solar GM.
Equilibrium temperature
With albedo and a choice of whole-planet or day-side heat spreading.
Kopparapu habitable zone
Recent Venus, runaway greenhouse (0.1, 1 and 5 Earth masses), maximum greenhouse and early Mars limits.
Gravity, density, escape speed
From an optional mass, using IAU nominal Earth and Jupiter constants.
Transit geometry
The probability that a random orbit transits and the longest possible transit duration.
Formulas and a worked example
Radius. The fraction of light blocked is δ = (Rp/R★)², so Rp = R★·√δ. An Earth-sized planet crossing the Sun blocks (6,378/695,700)² ≈ 84 ppm, which is why finding Earth twins is so hard; a Jupiter blocks about 1%. Limb darkening and grazing transits change the observed depth slightly, so this is the simple geometric estimate.
Distance. Kepler’s third law gives a = (GM★P²/4π²)1/3; 365.25 days around one solar mass is 1.000 AU. Temperature. Teq = T★·√(R★/a)·[f(1 − A)]1/4, with f = ¼ when heat is spread over the whole planet. For Earth, 5,772 K × √(695,700 km ÷ 2 AU) × 0.7¼ = 254.6 K, about −19 °C; the real average is warmer, about 15 °C, because of the greenhouse effect, which this formula leaves out.
Habitable zone. Kopparapu and colleagues (2013, updated 2014) fitted the starlight Seff at each edge as S☉ + aT + bT² + cT³ + dT⁴ with T = Teff − 5,780 K; the distance is √(L/Seff). The coefficients here are the authors’ published 2014 values. For the Sun they give a conservative zone from 0.95 to 1.68 AU and an optimistic one from 0.75 to 1.77 AU. The fit is valid for stars from 2,600 to 7,200 K.
Limits and good practice
For other orbital questions see the orbital period calculator, and for gravity on solar-system worlds the escape velocity calculator.
- Equilibrium temperature is not surface temperature: atmospheres, clouds and tidal locking can change the real climate by hundreds of degrees.
- Being in the habitable zone means liquid water is possible for an Earth-like planet, not that the planet is habitable or has life.
- Stellar radii often carry 5–10% uncertainty, and the planet radius inherits it directly.
- Orbits are assumed circular; for eccentric orbits use the semi-major axis and expect the temperature to swing.
Exoplanet Calculator FAQ
How do you calculate an exoplanet’s radius from the transit depth?
Take the square root of the depth (as a fraction) and multiply by the star’s radius. A 1% (10,000 ppm) dip around a Sun-sized star means a planet one tenth of the Sun’s radius, about 0.97 Jupiter radii.
What is the equilibrium temperature of a planet?
The temperature a planet would have if it simply balanced the starlight it absorbs with the heat it radiates, with no greenhouse effect. Earth’s is about 255 K.
How is the habitable zone calculated?
From the star’s luminosity and temperature using the Kopparapu et al. climate-model fits: each edge has a starlight level Seff, and the distance is √(L/Seff) AU.
How do I work out an exoplanet’s gravity?
Enter its mass. Surface gravity is GM/R², shown in m/s² and in Earth gravities, together with density and escape velocity.
Why is my planet in the optimistic but not the conservative zone?
The optimistic edges assume Venus and Mars once had liquid water; the conservative edges come from runaway and maximum greenhouse limits of climate models.
Is this exoplanet calculator accurate enough for research?
It uses the standard formulas and constants but ignores limb darkening, eccentricity and uncertainties, so use it for learning, checks and quick estimates.