Derive a surface for every body that was never photographed

Fifteen bodies here have a real photograph. Every exoplanet does not, and
never will on current instruments — none has ever been imaged — and nor do
several of the solar system's own moons. Those all shared one crude
stand-in: a few noisy bands tinted by category, cached per colour, so
every exoplanet in the app was literally the same picture.

They now get a surface reasoned from what has actually been measured.

The chain is standard at every link. A host star's luminosity comes from
its catalogued apparent magnitude and its parallax distance — that pair is
exactly an absolute magnitude — plus a bolometric correction for its
spectral class. The correction is not optional: an M dwarf radiates most
of its light in the infrared, so its visual magnitude understates it more
than tenfold, and M dwarfs are what most nearby planet hosts are.
Luminosity and the semi-major axis then give an equilibrium temperature,
mass and radius give a bulk density, and size, temperature and density
together give a class of world.

Checked against the solar system the temperatures land on Earth 255 K,
Jupiter 112 K, Neptune 46 K, all within a kelvin or two of published
values, and 51 Pegasi b comes out at 1227 K against a published 1200.

Each class carries a palette reasoned from its chemistry — methane absorbs
red light, which is why the ice giants are blue — and a structure: zonal
bands for a body with a fluid envelope, because a rapidly rotating
atmosphere organises into them, and fractal terrain for one with a solid
surface. Polar caps grow and shrink with the derived temperature, which is
the clearest visible consequence of the whole chain.

The generator samples three-dimensional noise along the sphere rather than
a flat field, so there is no seam to stitch at the antimeridian and no
pinching at the poles, and it writes into a byte array rather than a
canvas — a pure function, testable, with no 2D context to be unavailable.

Two things the derivation cannot do, both stated on screen next to the
measurements it rests on. Equilibrium temperature ignores greenhouse
warming and internal heat, so Venus comes out at 300 K against a real
surface of 737 K and Io, kept molten by tides, classifies as ice. And
these are illustrations: reasoned, but not observations.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WaySiNst4HhDXBHnMy8p5G

README.md

@@ -49,7 +49,8 @@ drop line from each body, so eccentricity and inclination read against a circula
 instead of having to be inferred from a shape in space.
 
 **Body detail** — a dedicated close-up scene and info panel for one planet, moon or exoplanet,
-with real photography where NASA/ESA/USGS imagery exists.
+with real photography where NASA/ESA/USGS imagery exists, and a surface derived from the body's
+own measurements where it does not. See "On surfaces that were never photographed" below.
 
 **Search** — name search across stars, solar-system bodies and exoplanets, navigating to the
 same place an in-scene click would.
@@ -87,6 +88,48 @@ same place an in-scene click would.
 - **No backend.** Every dataset is baked at build time into `src/assets/data/` and served as a
   static asset. Nothing queries an astronomy API at runtime.
 
+### On surfaces that were never photographed
+
+Fifteen bodies here have a real photograph. Everything else does not, and never will on current
+instruments: no exoplanet's surface has ever been imaged, and a few of the solar system's own
+moons have no usable map in this asset set either.
+
+Those bodies get a surface reasoned from what *has* been measured, in a chain that is worth
+following because every link is standard:
+
+1. The host star's **luminosity** comes from its catalogued apparent magnitude and its
+   parallax distance — that pair is exactly an absolute magnitude — plus a bolometric correction
+   for its spectral class. The correction is not optional: an M dwarf radiates most of its light
+   in the infrared, so its visual magnitude understates it by more than tenfold, and M dwarfs are
+   what most nearby planet hosts are. Good to about a factor of two, which matters less than it
+   sounds: temperature goes as the fourth root.
+2. Luminosity and the planet's semi-major axis give its **equilibrium temperature**, the standard
+   blackbody balance. Checked against the solar system it lands on Earth 255 K, Jupiter 112 K,
+   Neptune 46 K — all within a kelvin or two of published values — and puts 51 Pegasi b at
+   1227 K against a published 1200.
+3. Published mass and radius give **bulk density**, which is the difference between a ball of
+   iron, of rock, of water and of hydrogen.
+4. Size fixes the family, temperature the state within it, and density overrides both at the
+   extremes. That yields a class — molten, scorched, iron-rich, rocky, temperate, ice, sub-
+   Neptune, ice giant, gas giant, hot gas giant — each with a palette reasoned from its chemistry.
+   Methane absorbs red light, which is why the ice giants are blue; ammonia cloud tops are cream
+   and ochre; silicate cloud decks over a glowing interior are why hot Jupiters are drawn deep red.
+5. The surface is then painted from that class: **zonal bands** for a body with a fluid envelope,
+   because a rapidly rotating atmosphere organises into them, and fractal **terrain** for one
+   with a solid surface. Polar caps grow and shrink with the derived temperature.
+
+The generator samples three-dimensional noise along the sphere rather than a flat field, so
+there is no seam to stitch at the antimeridian and no pinching at the poles, and it writes into
+a plain byte array rather than a canvas — which makes it a pure function with no DOM to depend on.
+Each body's surface is seeded from its own id, so it looks the same on every visit.
+
+The limits are worth stating. Equilibrium temperature ignores greenhouse warming and internal
+heat, which is why Venus comes out at 300 K against a real surface of 737 K, and why Io — kept
+molten by tidal heating — classifies as ice. Luminosity classes are often missing from the star
+catalogue, so a red giant read as a dwarf will come out too bright. And the surfaces are
+illustrations throughout: the info panel says so on every body that has one, next to the
+measurements it was reasoned from.
+
 ### On the Galaxy model
 
 Every other dataset here is measured. The Galaxy is the exception, and not for want of trying: