About

Generating a believable fictional planet requires more than random numbers. Each property constrains others: a body with mass M below 0.5 M_⊕ cannot retain a thick nitrogen atmosphere at temperatures above 400 K. Surface gravity g follows directly from M and radius R. Orbital period depends on semi-major axis via Kepler's Third Law. Getting these relationships wrong produces planets that feel implausible to anyone with basic astrophysics literacy. This generator enforces those constraints so writers, game designers, and tabletop GMs get worlds that hold up to scrutiny.

The tool classifies output into four archetypes - terrestrial, gas giant, ice giant, and dwarf - each with distinct composition rules. Atmosphere mixtures respect molecular escape velocity thresholds. Temperature accounts for stellar luminosity, orbital distance, and albedo. Limitations: the model assumes circular orbits, single-star systems, and chemical equilibrium atmospheres. Tidal heating, magnetic field strength, and plate tectonics are not modeled. Use the seed field to reproduce any planet exactly.

Formulas

Surface gravity is derived from Newton's law of gravitation applied at the planet's surface:

g = G ⋅ MR²

where g = surface gravitational acceleration, G = gravitational constant (6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻²), M = planet mass, R = planet radius.

Escape velocity determines which atmospheric molecules the planet can retain:

v_esc = √2 G MR

A molecule with thermal velocity exceeding v_esc6 escapes over geological timescales (Jeans escape criterion).

Orbital period follows Kepler's Third Law:

T = 2π √a³G M_☆

where a = semi-major axis, M_☆ = host star mass.

Equilibrium temperature approximation:

T_eq = T_☆ ⋅ √R_☆2a ⋅ (1 − A)^1/4

where T_☆ = stellar effective temperature, R_☆ = stellar radius, A = Bond albedo.

Reference Data

Planet Type	Mass Range	Radius Range	Typical Composition	Atmosphere	Surface Gravity	Example Analog
Dwarf	0.001 - 0.1 M_⊕	0.03 - 0.5 R_⊕	Rock, Ice	Trace or None	0.02 - 0.3 g	Pluto, Ceres
Small Terrestrial	0.1 - 0.5 M_⊕	0.5 - 0.85 R_⊕	Silicate, Iron core	Thin CO₂, N₂	0.3 - 0.7 g	Mars
Terrestrial	0.5 - 2.0 M_⊕	0.85 - 1.5 R_⊕	Silicate, Iron, Water	N₂, O₂, CO₂	0.7 - 1.5 g	Earth, Venus
Super-Earth	2.0 - 10 M_⊕	1.2 - 2.5 R_⊕	Silicate, Volatiles, H₂O	Dense H₂, He, H₂O	1.0 - 3.0 g	Kepler-442b
Mini-Neptune	10 - 30 M_⊕	2.5 - 5.0 R_⊕	H/He envelope, Ice core	H₂, He, CH₄	0.8 - 2.0 g	Neptune (analog)
Ice Giant	14 - 50 M_⊕	3.5 - 6.0 R_⊕	H₂O, NH₃, CH₄ ices	H₂, He, CH₄	0.8 - 1.5 g	Uranus, Neptune
Gas Giant	50 - 1000 M_⊕	6.0 - 12.0 R_⊕	H₂, He dominant	H₂, He, NH₃, H₂O	1.5 - 3.5 g	Jupiter, Saturn
Hot Jupiter	100 - 3000 M_⊕	10 - 20 R_⊕	Inflated H/He	H₂, Na, K, TiO	0.5 - 4.0 g	51 Pegasi b
Rogue Planet	0.1 - 500 M_⊕	Varies	Any	Frozen, minimal	Varies	CFBDSIR 2149
Ocean World	0.5 - 5.0 M_⊕	0.9 - 2.0 R_⊕	Deep H₂O mantle	H₂O vapor, CO₂	0.6 - 2.0 g	Europa (larger)
Carbon Planet	0.5 - 8.0 M_⊕	0.8 - 2.0 R_⊕	SiC, Graphite, Diamond	CO, CH₄	0.8 - 2.5 g	55 Cancri e (hyp.)
Lava World	0.5 - 3.0 M_⊕	0.8 - 1.5 R_⊕	Molten silicate surface	SiO vapor, Na	0.8 - 2.0 g	CoRoT-7b

Frequently Asked Questions

The generator computes the planet's escape velocity and equilibrium temperature, then applies the Jeans escape criterion. For each candidate gas (H₂, He, N₂, O₂, CO₂, CH₄, NH₃, H₂O, Ar, SO₂), the mean thermal velocity at the planet's temperature is compared against v_esc ÷ 6. Gases whose thermal velocity exceeds this threshold are excluded. Low-mass hot worlds therefore lose hydrogen and helium, while massive cold worlds retain everything. This mirrors the real mechanism that stripped Mars of most of its atmosphere.

Ring probability scales with planet mass: bodies above 30 M⊕ have approximately a 45% chance, dropping to under 5% for terrestrial worlds. This reflects the correlation observed in our solar system where only the giant planets possess significant ring systems. Moon count follows a Poisson-like distribution anchored to the planet's Hill sphere radius relative to its host star distance. Dwarf planets receive 0-2 moons, terrestrials 0-3, and gas giants 5-80+.

Yes. Every generated planet has a unique seed string displayed in the result. Enter that seed into the seed input field and click Generate. The tool uses a deterministic Mulberry32 PRNG seeded from a hash of the input string, so identical seeds always produce identical planets across sessions and devices. The seed and full planet data also persist in localStorage between page reloads.

Surface gravity g = GM/R². Gas giants have very large radii that partially offset their large mass. Jupiter, at 318 M⊕, has a surface gravity of only ~2.5g because its radius is 11.2 R⊕. The generator models this radius inflation realistically: above ~100 M⊕, additional mass compresses the interior rather than expanding the radius (electron degeneracy pressure), so radius plateaus or even decreases, a phenomenon observed in transiting exoplanet data.

No. The formula T_eq = T★ × √(R★/2a) × (1−A)^(1/4) gives the blackbody equilibrium temperature assuming uniform redistribution and no atmospheric greenhouse. Earth's equilibrium temperature by this formula is ~255 K, while its actual surface average is ~288 K - a 33 K greenhouse offset. The generator notes this limitation in the output. For worldbuilding purposes, add 10-50 K for thin atmospheres, 50-500 K for dense CO₂ or H₂O vapor atmospheres.

Names are built from a phoneme table of 24 onset consonants and 12 vowel nuclei inspired by Greek and Latin astronomical naming conventions. The generator picks 2-4 syllables, applies phonotactic constraints (no triple consonants, no identical adjacent syllables), then appends a catalog-style suffix (e.g., "-b", '-IV') with ~40% probability. The PRNG seed controls selection, ensuring name reproducibility.