Star Systems and Generation

The universe is computed

Every system has an address. The star follows from it unambiguously, the zones from the star, the planets from the zones and what lies in them from the planets. One address always gives the same system.

It is computed by physics, if simplified, and it grows in the same order as a real one: star, disc, planets.

Until someone looks at a system, it exists as mathematics. The first visit writes it down, and from then on it lives its own life: what was mined stays mined, what was built stands. So an "undiscovered" system already is what it is. You just don't know it yet.

First, the star

It all starts with mass. From it follow the star's luminosity, its size and its lifetime. A heavy hot star burns out fast and so is always young. A red dwarf will outlive the present age of the Universe, and is in no hurry whatsoever.

The second number is metallicity, the share of heavy elements. It decides how much rock and metal the future planets will get.

The third is temperament. A young star spins fast and flares often; over the years it slows down and settles. The game works out how flare-prone a star is from its colour, age and rotation, by the same relations astronomers use.

Then, the disc

Around a young star lies a disc of gas and dust, and it lives only a few million years. In that time the planetary embryos have to grow. Close to the star they grow fast, out at the edge slowly, and the farthest never make it at all.

Past the snow line ice joins the dust, and there is several times more building material. A core that gathers enough mass out there starts sweeping up gas and becomes a giant, provided the disc has not dispersed by then. That is why metal-rich stars have giants more often. Real stars do too.

Close neighbours do not get along: two planets on orbits too near each other merge into one. The asteroid belt settles into the widest gap between orbits and gathers from whatever did not go into planets. A heavy disc gives many planets, giants and a rich belt all at once: a system has exactly as many planets as its disc grew.

Then, the details

From mass and composition follow a planet's size and density, its surface gravity and the length of its year, by Kepler's law. Its temperature is starlight plus the greenhouse effect. Whether a planet keeps its air is a contest between gravity and heat: a small hot world loses it.

The make-up of the air starts from the nearest real relative — Venus, Mars, Titan, Earth — and drifts away from it for reasons of its own. The magnetic field is derived too. Giants always have one, moons never do, and a rocky planet needs mass, a metal core and a not too venerable age: an old core cools and falls quiet.

Numbers you can see

A planet on the system map is painted by a program, from its own data. An overheated super-Jupiter smoulders red, an ice giant turns pale or blue depending on its temperature, and the coldest worlds wear methane frost. Clouds form only where there is air, and there are fewer over a desert. Over the poles of a planet with a magnetic field an aurora glows, and flares up when the star flares. Even the star's own disc darkens towards the edge, as the Sun's does.

In a probe flight the terrain is built from the same numbers. On a light planet the mountains stand taller. In vacuum craters keep sharp rims; under thick air the small ones fill in. On an airless, metal-rich world scarps run across the ground, as on Mercury. A moon kneaded by tides is warmed from within, and geysers burst from its cracks. The star in the sky is exactly the size it looks from that orbit, and under thick air it cannot be seen at all.

Where the model is simplified

The map squeezes distances and sizes: at true scale the planets would be lost as dots at the edges of the screen. Much of what astrophysicists write dissertations about comes down here to a couple of rules. What holds the model together is something else: every number has a cause, and you can find it.

Where you live

The map is divided into districts, one per player, each holding a few dozen stars. The density is taken from the real neighbourhood of the Sun, so the next star is usually a few light years away — days of real travel.

Star class: the hint from afar

A star's class can be seen from any distance, and the class shares are real: most stars are dim red dwarfs, and Sun-like ones are few. A red dwarf's habitable zone is narrow and pressed close to the star, and the dwarf itself, a young one above all, is prone to flares that hit the stations nearby. Orange and yellow stars are rarer and calmer. Hot stars are rare, young and no good for life.

The decision to send an expedition away for several days is made on that one line. Class is a bet: what the system really holds is seen only on the spot.

Structure beats one good planet

Resources cannot leave a system, so a system's economy is its layout. Zones are measured from the star's luminosity: around a dim star everything huddles close, around a bright one it stretches out.

• rocky planets — metals;
• the habitable zone — water, atmosphere and sometimes life;
• the asteroid belt — metals with no gravity well, the best place in the system to mine;
• gas giants and their moons — gas, ice and metals;
• the icy outskirts — volatiles, which keep a colony's life support going.

The interface shows missing zones as plainly as present ones. No ice — nothing to feed a colony. No asteroid belt — metals have to be hauled up out of a gravity well. That forecast is worth reading before you leave.

What you see from afar

Information opens in layers. The star map shows a star's class and luminosity, and for the nearest stars their largest planets. An orbital observatory widens the map's radius. The system's layout, its sites and finds open when a copy arrives; a planet's interior, when a probe flies over it.

So an expedition leaves for several days knowing roughly one thing about its target: "orange star, a gas giant". The rest turns up on arrival, when it is too late to turn back. That is why the observatory gets built before the expedition: it won't make the ship any faster, but it gives you more targets.

On the spot, every body also has "Curious facts" — cards that turn dry figures into a place: what a tonne of cargo weighs here, how big the star looks in the sky, how long the local year lasts.

Every system has a history

A system's chronicle is built from causes: depletion leads to revolt, revolt to evacuation, evacuation to abandoned equipment. And if the history mentions something, that something is in the world. The first such history is waiting very close to where you wake.

The right to a name

Stars and bodies carry the names the generator gave them until somebody stays in the system. The one who has put a station there may name the system — once and for good. The discoverer stays in the records; the name is given by the one who stayed.