About
Ars Planet builds a fantasy world forwards from physics rather than backwards from a map. It starts with when in the history of the universe the world sits, and ends with county maps drawn in the manner of John Speed's atlas of 1611.
The rule the whole thing runs on
Science bends for fun. It does not bend for convenience. A world may be improbable — a sky with six suns in it, a planet that alternates between two of them — and the model will build it and tell you the odds. What it will not do is invent a number because working out the real relation was tedious. Where a figure is extrapolated past the data that supports it, the page says so and says how far.
Improbable is not impossible
Options are removed only when no mechanism produces them, and the removal always comes with its reason. Everything else stays on the table with a price against it. The threshold is roughly one in 1022: at that rate something has happened somewhere in the observable universe, and a world is allowed to be the somewhere.
Every choice is shown four ways
A picture, a plain description, the science behind it, and what it would mean to somebody living there. The last of those is the point of the exercise: an ultraviolet ratio decides whether a planet's oxygen means anything is alive, and a companion star decides whether shadows fall two ways at noon.
Where the numbers come from
Published measurements, cited on the step that uses them, with the disputes left visible rather than smoothed over. Where two papers disagree the page says which one the model follows and why. Where a relation has been extrapolated outside the range it was measured in, that is stated at the point of use rather than in a footnote.