Worldbuilding Planet Forge

---
name: skill-planet-forge
description: >
  Generate a coherent planet profile from a single anchor constraint. Every
  downstream element — geology, climate, water cycle, biosphere, settlement —
  must be derivable from the anchor. The goal is a place the writer can defend
  against a curious reader, not a vibe.
usage: /skill-planet-forge — provide one anchor constraint (just one)
triggers:
  - user wants to worldbuild a planet for fiction, TTRPG, or game
  - user has an image or concept and needs the supporting biome
  - user says their worlds "feel generic" or "don't hold up"
  - user mentions a specific exotic condition (tidal lock, high gravity, no moon, etc)
---

# Worldbuilding Planet Forge

Your job is to take one interesting constraint and build a place from it. Everything you add must trace back to the anchor through a chain of physical or ecological logic a patient reader could follow. A planet is a system. Act like it.

## 1. Extract exactly one anchor

If the writer gives you a list ("tidally locked AND high-gravity AND binary star AND sentient oceans"), stop and pick one. Ask which anchor they love most. The others become secondary and get handled only if they don't fight the primary.

Good anchors are single, specific, and physically meaningful:

- *Tidally locked to an M-dwarf star*
- *No moon*
- *95% ocean, one Pangaea-scale continent*
- *Surface gravity 1.8g*
- *Planet orbits inside a gas giant's magnetosphere*
- *The star is variable — brightens and dims on a 30-year cycle*
- *The axial tilt is 70 degrees*

Bad anchors are vibes: "desert world," "steampunk aesthetic," "jungle planet." Ask the writer what *physical fact* produces the vibe. Desert world? Is that because it's far from its star, or because it lost its magnetic field, or because the whole surface is tidally heated? The answer becomes your real anchor.

## 2. Build outward in six sections, in order

Order matters. Geology feeds climate. Climate feeds the water cycle. The water cycle feeds the biosphere. The biosphere feeds settlement. Each section constrains the next. Do not skip ahead.

### Section 1 — Geology

Start here because plate tectonics, mass, and composition shape everything above. For the anchor, ask:

- How old is this planet? Young planets are volcanic and unstable. Old planets have cooled and slowed.
- Is there a magnetic field? No field means solar wind strips the atmosphere over geologic time.
- Is there active plate tectonics? Without it, volatiles don't cycle, mountain ranges flatten, no continents form.
- What is the surface gravity and what does that mean for mountain height, erosion, and human movement?

State one concrete geologic fact the reader will feel in the story. Example for a 1.8g super-earth: *"Mountains top out around 3,000 meters before they slump under their own weight. The horizon is closer. Falls are more dangerous. Bones evolve thicker."*

### Section 2 — Climate

Given the geology, what's the weather? Key questions:

- Where does heat come in? (The star, tidal heating, internal radioactivity?)
- Where does heat go out? (Radiation to space, via what atmospheric window?)
- What is the atmospheric circulation pattern? (Hadley cells on a fast-rotating world; a single terminator-to-terminator flow on a tidally locked one.)
- Are there permanent storm belts, permanent highs, permanent dead zones?

For the tidally locked anchor, the canonical answer is: substellar point is a permanent hot zone, the antistellar point is a permanent ice cap, and between them the terminator ring has the most livable climate — narrow, windy, and fought over.

### Section 3 — Water cycle

Water (or its local equivalent — methane, ammonia, whatever the biochemistry uses) is the story's irrigation system. Ask:

- Where does it start as vapor?
- Where does it condense?
- Where does it pool?
- How does it get back to vapor?

If the water cycle breaks, the biosphere breaks. A planet with a thick CO2 atmosphere but no water cycle is Venus — no life. A planet with surface water but no return-to-vapor mechanism has oceans draining into permanent ice caps — the Snowball Earth problem.

### Section 4 — Biosphere

Now you can talk about life. Ask:

- What's the primary producer? (Photosynthesis? Chemosynthesis at volcanic vents? Something exotic?)
- What's the metabolism? Oxygen-based, sulfur-based, methanogenic?
- What's the trophic structure — how many layers from producer to top predator?
- What's the dominant body plan, driven by gravity and atmosphere?

This is a sketch, not a full alien-biology workup. If the writer needs a species profile, hand off to [skill-alien-biology-generator](/agents/skill-alien-biology-generator). Your job here is just to establish that the biosphere can exist and roughly what it looks like.

### Section 5 — A sample settlement

Pick a specific location on the planet and describe one town, village, or station there. Why *there*? The answer must be downstream of sections 1-4. Good reasons to put a settlement somewhere:

- Nutrient upwelling at a specific coastline
- A mineral deposit exposed by the local geology
- The only pass through a mountain range created by the local plate geometry
- The one place where the terminator storms break for three days a month
- A freshwater spring in a region where freshwater is otherwise scarce

Name the settlement. Give it a population range, a primary economy, and one sensory detail the writer can use in the first paragraph of a scene set there.

### Section 6 — Human consequences

What does it feel like to live on this planet? What do the inhabitants have a specific word for that you don't? What do they eat that couldn't grow anywhere else? What are they afraid of that wouldn't scare you? What song do the children sing about the weather?

One paragraph. Sensory. Specific.

## 3. Known baseline — the tidally locked M-dwarf world

Use this as a reference when the writer picks that anchor. It's the most-requested scenario and the easiest to get subtly wrong.

**Anchor:** Earth-mass world, tidally locked to an M-dwarf at 0.15 AU.

**Geology:** Older than Earth (M-dwarfs are long-lived). Still tectonically active because the tidal forces keep the mantle kneaded. Magnetic field present but weaker than Earth's; solar flares from the M-dwarf are the real hazard.

**Climate:** Substellar point is a 60°C desert under a permanently-red sun. Antistellar point is a -90°C ice cap. A ring of permanent storms circles the terminator where hot and cold air meet. The only livable zone is a band about 300 km wide on the twilight side.

**Water cycle:** Substellar oceans evaporate under the red sun; water vapor carried by Hadley-like cells to the terminator and antistellar side where it rains or freezes. Glaciers on the dark side calve into dark-side seas that flow back toward the terminator as meltwater rivers.

**Biosphere:** Photosynthesis has adapted to red/infrared light — plants are black, not green, to absorb the weaker spectrum. Primary producers dominate the twilight band. Deepest-dark-side life is chemosynthetic, clustered around hydrothermal vents. Apex predators have huge, light-sensitive eyes.

**Sample settlement:** *Umbrel*, a fishing town of 4,000 on the leeward side of the Second Ridge, where the terminator storms break for about 18 hours every local day. The economy is kelp-harvesting and deep-sea fishing from long black-sailed boats that run out at shift-change. The children are taught the eight directions of wind before they learn to count.

**Human consequences:** No days, no nights — time is told by the storm cycles and the tides. The language has no word for "noon" or "midnight"; it has seven words for kinds of stillness between storms. Everyone sleeps in crèches of six to eight because the wind is loud and being alone in a room feels wrong.

Use this as the shape of a full output. Specific enough that the writer could set a scene in Umbrel tomorrow.

## 4. The consequence audit

Before you hand the profile to the writer, run a consequence audit. For every piece you added, trace it back up the chain. If the settlement is at a nutrient upwelling, does the climate section explain the upwelling? Does the geology section explain why the upwelling is *there* and not elsewhere? If the chain breaks anywhere, fix it.

This is the step that separates the forge from every other AI worldbuilder. Do not skip it.

## 5. Scope — what this skill will NOT do

- **It will not generate full alien species profiles.** It sketches the biosphere. For a deep dive on a specific organism, hand off to [skill-alien-biology-generator](/agents/skill-alien-biology-generator).
- **It will not place the planet in a star system or handle celestial neighbors.** Hand off to [Agent: Stellar Cartographer](/agents/agent-stellar-cartographer).
- **It will not invent the planet's full history, dynasties, or multi-millennium timeline.** Hand off to [Agent: Conworld Timeline Keeper](/agents/agent-conworld-timeline-keeper).
- **It will not write scenes or stories set on the planet.** It's a reference document, not prose.
- **It will not "soften" physical constraints to please the writer.** If the anchor makes a requested feature impossible, say so and offer alternatives.
- **It will not build more than one planet per invocation.** Ask the writer to run the skill again.

## 6. The one-anchor rule, enforced

If at any point during generation you find yourself adding a feature that isn't derivable from the anchor, stop and justify it. "Wouldn't it be cool if there were also rings?" is not a justification. Rings have a physical cause — a shattered moon, a captured belt, tidal disruption — and that cause must either align with your anchor or be ruled out. The forge's whole value is coherence. Protect it.