Habitability Zone Calculator

Inputs

Planet

Mass 1.00 M⊕

Radius 1.00 R⊕

Albedo 0.30

Orbit

Distance 1.000 AU

Star

Type

Atmosphere

Composition

System

Age 4.5 Gyr

Load preset

Computed outputs

Click any row to see the calculation.

Equilibrium temperature — K

Surface temperature — K

Water phase at surface —

Atmospheric retention —

Magnetic field —

Stellar habitable zone — AU

Planet in HZ? —

Habitability framework

Six criteria, each MEETS / PARTIAL / FAILS / UNCERTAIN. Click any row to see the 5-field reasoning. Mirrors the Habitability Framework Template you fill in Block 7.

Liquid water —

Atmosphere —

Temperature —

Magnetic field —

Energy source —

Stable orbit —

The calculator uses the formulas, lookup tables, and constants below. Every output above corresponds to one of these — click any row to see the calculation with your current numbers substituted.

§3.1 Equilibrium temperature

T_eq = T_star × √(R_star / (2 × d)) × (1 − A)^0.25

Where T_star is host star effective temperature (K), R_star is star radius (m), d is planet orbital distance (m), A is bond albedo (0–1). Earth's T_eq ≈ 255 K (well below freezing — the greenhouse fills the gap).

§3.2 Surface temperature (greenhouse adjustment)

T_surface ≈ T_eq + ΔT_greenhouse

Atmosphere	ΔT (K)	Pressure (bar)
None / vacuum	0	0
Thin CO₂ (Mars-like)	+5	0.006
Earth-like (N₂/O₂ + GHGs)	+33	1.0
Thick CO₂ (Venus-like)	+500	92
H₂/He thick (gas giant)	+200	1000

§3.3 Atmospheric retention (Jeans escape, simplified)

Lookup by surface gravity (relative to Earth). Adjust downward one band if T_surface > 500 K (hot planet loses atmosphere faster).

g/g⊕	H₂	H₂O	N₂/O₂	CO₂
<0.2	No	No	No	Marginal
0.2–0.4	No	No	Marginal	Yes
0.4–0.7	No	Marginal	Yes	Yes
0.7–1.5	Marginal	Yes	Yes	Yes
>1.5	Yes	Yes	Yes	Yes

Surface gravity ratio: g_ratio = (M / M⊕) / (R / R⊕)²

§3.4 Water phase at surface

T_surface (K)	Pressure (bar)	Phase
<273	any	Frozen (subsurface liquid possible if internal heat)
273–373	≥0.1	Liquid
273–373	<0.1	Sublimation (no stable surface liquid)
>373	<50	Gaseous (water vapor)
>373	≥50	Supercritical water (Venus-like)

§3.5 Magnetic field plausibility

Rough heuristic based on planet mass + system age. Real magnetic field generation depends on rotation, composition, and interior dynamics that v0.1 does not model.

Mass (M⊕)	Age (Gyr)	Magnetic field?
<0.3	any	Unlikely (cools too fast, like Mars)
0.3–0.5	<2	Marginal
0.3–0.5	>2	Unlikely (Mars-like loss)
0.5–2	<8	Likely
0.5–2	>8	Marginal
>2	<10	Likely (more internal heat)

§3.6 Stellar habitable zone

HZ_inner = 0.95 × √(L_star / L☉) · HZ_outer = 1.67 × √(L_star / L☉)

For G2V (Sun-like) star, HZ ≈ 0.95–1.67 AU. M-dwarfs have very tight HZs (TRAPPIST-1: 0.022–0.032 AU).

Star type defaults (Explore mode)

Type	T_eff (K)	R_star (R☉)	L_star (L☉)
F8V	6200	1.10	1.50
G2V	5778	1.00	1.00
K2V	5000	0.85	0.30
M3V	3050	0.15	0.0017
M5V	2800	0.12	0.00056

Physical constants

G = 6.674 × 10⁻¹¹ m³/(kg·s²) · M⊕ = 5.972 × 10²⁴ kg · R⊕ = 6.371 × 10⁶ m · M☉ = 1.989 × 10³⁰ kg · R☉ = 6.957 × 10⁸ m · T☉ = 5778 K · L☉ = 3.828 × 10²⁶ W · 1 AU = 1.496 × 10¹¹ m

Saved state

Your inputs are saved in your browser only — never sent anywhere. Clear them below to start fresh from Earth defaults.

Teacher URL parameters

You can preload the calculator to a specific scenario by adding these query strings to the URL. Useful for emailing class links or embedding in a launcher.

URL	What it does
`?preset=trappist1e`	Opens Explore mode with TRAPPIST-1 e loaded
`?preset=kepler22b&view=explore`	Opens Explore with the Block 8 worked-example planet
`?compare=earth,mars,trappist1e`	Opens Compare mode with default Goldilocks loadout
`?compare=earth,mars,k218b`	Compare with K2-18 b as the chosen exoplanet

Valid preset keys: earth, mars, venus, trappist1e, proximab, k218b, kepler186f, kepler452b, lhs1140b, toi700d, gliese581d, kepler22b. URL params override saved state.