Star System Generator

What this generator does

Starts from the star. Its brightness follows from its mass, its habitable zone from its brightness, and the distance beyond which water was already ice when the system formed from its brightness again. Planets are then placed outwards with enough space between neighbouring orbits to be stable, each one rock or gas according to which side of that line it formed on, and each one given the year its distance implies.

How to use this tool

1. Choose how many planets to place and how heavy the star is.
2. Build the system, and read the star’s figures before the planets — everything else follows from them.
3. Check the habitable zone and the snow line against each planet’s distance.
4. Copy the link to keep this system, or the text to paste into your notes.

Understanding the controls

Planets

Three to nine. More planets means a wider system, because neighbouring orbits have to stay far enough apart to leave each other alone.

Star mass

In hundredths of the Sun’s, from half to a little over twice. The range stops where the brightness rule used here stops being defensible.

Repeat word

The word this system was built from. Anyone opening the same link gets the same star and the same planets.

Common use cases

• A home system for a science-fiction campaign that holds up when a player asks how long a year is
• Background for a novel where the astronomy is meant to survive a reader checking it
• A setting brief a games master can hand to a table from one link
• A worked example for teaching how a habitable zone is worked out
• Somewhere to start when a blank page is the problem

How this generator works

The check is a separate reading of the finished system rather than a record of what was intended. Each year is worked out again from the planet’s distance and the star’s mass and compared with the one shown; the same law is then applied the other way round, so the check is not one function trusting itself. Distances and years are held at full precision and rounded only where they are printed, so the check is testing the arithmetic and not the rounding.

Randomness and fairness

The system is built from a repeat word taken from your browser’s secure generator unless you type one of your own, so the same link rebuilds the same star. A repeat word exists to make a system reproducible; it is not a security measure, and nothing here is suitable for anything that needs to stay secret.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

• The star and its planets are invented. No real system is being described.
• Two of the relations are physics used as physics: the law linking a planet’s year to its distance, and the rule linking a star’s brightness to its mass. The second is an approximation, a rough one, and only holds over roughly the mass range offered here.
• The habitable zone is taken as the band receiving between 0.53 and 1.1 times the sunlight Earth gets. Real estimates depend on a planet’s atmosphere and are argued over; this is one conservative pair of limits, not a settled figure.
• The snow line is placed by the usual order-of-magnitude rule of thumb, which is a stylisation and not a measurement.
• Orbits are circular, evenly spread and never resonant, tilted or eccentric, and no planet has rings, a captured moon or a companion star.
• Atmospheres, temperatures and whether anything actually lives on a world are not modelled. A planet in the habitable zone is one where liquid water could stand, which is a long way short of habitable.

Privacy and your data

The system is built on your device and nothing about it is transmitted or stored. The repeat word is a short label this page made or you typed, and it is the only thing that appears in the address; it is never measured.