Alien-scapes

Procedurally generated landscapes under other suns, where every colour on screen is computed from a spectrum rather than chosen by hand.

Pick a star, an exoplanet and an atmosphere, and the page works out what the sky, the ground, the water and — if anything can live there — the vegetation would actually look like. Then it shows you the spectra behind that answer and lets you export the whole thing as a data sheet.

→ Open Alien-scapes

No build step, no dependencies, no network calls. It is a handful of ES modules and a canvas.

---

What it does

• 14 stellar presets on a spectral-class grid (O–M, dwarfs and giants) plus 28 more named stars including Wolf-Rayet, white dwarf and brown dwarf classes — and a free temperature slider from 1,000 K to 50,000 K.
• 15 exoplanet presets — Earth, Mars, Venus and Titan as controls, then TRAPPIST-1 e, Proxima b, Kepler-186 f, LHS 1140 b, TOI-700 d, K2-18 b, Kepler-452 b, GJ 667 C c, GJ 1214 b, Gliese 581 g, and the circumbinary Kepler-16 (AB) b, which really does have two suns in the sky.
• 10 atmospheres, from airless through Earth Rayleigh, Mars dust, Venus acid cloud, Titan tholin haze, methane-rich and hycean.
• Leaf colours per spectral type, derived from a pigment model rather than a lookup table — including the purple result for a retinal biosphere.
• Three charts: the surface spectrum, the pigment absorption curves (retinal, chlorophyll and the nitroplast antenna, plus whatever the model selected), and the resulting leaf reflectance with its red edge marked.
• Export to PNG, JPG or PDF as a full data sheet — the landscape plus every number behind it — or the landscape on its own.

---

How the colours are computed

Nothing here is art-directed. Each colour is the result of an integral.

Starlight

The star is treated as a blackbody at its effective temperature. Planck's law gives the emitted spectrum, an atmospheric transmission profile attenuates it, and the result is integrated against the CIE 1931 2° standard observer to get tristimulus XYZ, then converted to sRGB through the D65 primaries and the sRGB transfer function. This path is ported from starlight-filter, which uses the same integral to drive a display's gamma ramp.

Because Beer's law is exponential in path length, a lower star simply means the zenith transmission raised to the air mass. That one relationship is why the same model produces a blue noon and a red sunset without being told about either.

Sky, ground and water

A Rayleigh atmosphere scatters short wavelengths out of the beam, so the sky glows with exactly what the beam lost. Dust and haze absorb rather than scatter, so those skies take the transmitted tint instead of its complement — which is why Mars is butterscotch rather than blue.

Ground colour is the total downwelling light (direct beam plus skylight) times a mineral reflectance curve, integrated the same way. Out-of-gamut results are tone-mapped on luminance with the chromaticity preserved, so a deep Rayleigh sky comes out pale blue instead of clipping to flat white.

Vegetation

Photosynthesis is a quantum process, so the model counts photons, not energy. A reaction-centre pigment is chosen by weighing three things against each other:

1. Capture — how many arriving photons the absorption curve takes.
2. Thermalisation — every captured photon is worth 1/λ_trap, because energy above the trap is dumped as heat. This is what stops far-red pigments winning under every star.
3. Cost — a pigment absorbing everywhere is metabolically expensive.

Antenna pigments are then layered on, constrained by what can physically attach to what: a chlorophyll photosystem accepts chlorophyll b, carotenoids and phycobiliproteins, while a rhodopsin accepts only a carotenoid and bacteriorhodopsin runs bare. Phycobiliproteins carry a heavy cost penalty because a phycobilisome can run to half a cell's protein — which is why land plants never took that option, and without it the model paints Earth's forests red.

The leaf's reflectance is what survives that absorption, scaled by a structural reflectance term that reproduces the red edge. Its colour is the illuminant times that reflectance. Calibration check: Earth under the Sun comes out green, reflectance peaking near 550 nm with a red edge at ~705 nm, which is what real vegetation does.

---

Honesty about the model

This is a worldbuilding and teaching tool, not a radiative-transfer code.

• Pigment absorption curves are sums of Gaussians placed at literature peak positions. The peak positions are real; the band shapes are modelled, not digitised from measured spectra.
• Stellar luminosities and radii are rounded literature values, good enough to place a planet in or out of a habitable zone and no more.
• Habitable-zone bounds use the Kopparapu et al. (2013) runaway-greenhouse and maximum-greenhouse coefficients, which are fitted for 2600–7200 K. Outside that range the UI says the bounds are extrapolated.
• Atmospheric transmission profiles are deliberately simple — enough to reproduce each world's characteristic tint, not to model its chemistry.
• The nitroplast needs its own caveat. The nitroplast is a nitrogen-fixing organelle, derived from the cyanobacterium UCYN-A inside the alga Braarudosphaera bigelowii. It has lost photosystem II and RuBisCO, so it does not fix carbon. The curve here models the photosystem I antenna it still carries — what the organelle still absorbs, not a carbon-fixing pigment.
• Equilibrium temperatures use a greenhouse offset per atmosphere rather than a climate model. As a sanity check the Earth preset lands on 288 K and Venus on 727 K, against real values of 288 K and 737 K.

The reasoning behind the leaf-colour model follows Kiang et al. (2007), Spectral signatures of photosynthesis.

---

Running it locally

There is nothing to install, but the page uses ES modules, so it needs to be served over HTTP rather than opened from the filesystem:

python -m http.server 8123

Then open http://localhost:8123.

---

Layout

File	What lives there
src/cie.js	CIE 1931 CMF table, Planck's law, XYZ → sRGB, tone mapping
src/stars.js	Stellar presets, habitable zone, angular diameter, insolation
src/planets.js	Atmospheric transmission, exoplanet presets, biome classifier
src/pigments.js	Pigment spectra, suite selection, leaf reflectance solver
src/scene.js	Ties it together — every derived quantity and colour
src/landscape.js	The procedural renderer; shape only, no physics
src/charts.js	Spectrum, absorption and reflectance charts
src/exporter.js	Data sheet composition, PNG/JPG, and a hand-rolled PDF writer
src/noise.js	Seeded PRNG and fBm
src/main.js	UI wiring
fonts/	Atkinson Hyperlegible Next, self-hosted (SIL OFL 1.1, see fonts/OFL.txt)

Seeds are deterministic: the same seed always redraws the same terrain at any resolution, so an exported sheet can be reproduced exactly.

Licence

MIT.