Playable Solar Systems and Earth-Sized Planets: From Orbit to the Ground, Explained

A playable solar system is a star system whose bodies are places, not pictures. You can reach them, stand on them and feel their gravity. Earth-sized planets are the hard version of that promise, because the numbers are large. This page puts numbers on the idea, explains how gravity follows from size, lists the four engineering problems behind it and says what COLONIZED, a space survival MMORPG for PC with a closed alpha first waves planned for winter 2026-2027, has built so far. It is designed around an entire star system with Earth-sized planets; we are careful about what that means today.

What “Earth-sized” means in numbers

	Radius	Surface area
Earth	about 6,371 km	about 510 million km²
The Moon	about 1,737 km	about 38 million km²

An Earth-sized planet has roughly 13 times the surface of the Moon. Walking pace is about 5 km/h, so the equator of an Earth-sized planet is about 40,075 km, or roughly 334 days of nonstop walking. Even a Moon-sized world, with a circumference near 10,900 km, is about 91 days. That is the scale a designer must fill or skip.

Gravity comes from size

For a rocky world, surface gravity follows from its density and radius: g = 4/3 × π × G × ρ × R. At the same density, halve the radius and you halve gravity. That is why a smaller world makes you jump higher and fall slower, and why a larger one is harder to leave.

For a game this matters in three places: how you move on the ground, how much fuel it takes to leave and how an atmosphere is held. A studio that generates worlds from their physical properties gets gravity, atmosphere and feel from the same data, which is the approach in how COLONIZED builds fifteen worlds from lore.

A worked example: Earth and the Moon

NASA’s fact sheets give surface gravity of about 9.80 m/s² for Earth and 1.62 m/s² for the Moon, about one sixth. Escape velocity, the speed needed to leave without further thrust, is about 11.19 km/s for Earth and 2.38 km/s for the Moon. Put plainly:

• On a Moon-sized world you jump about six times higher and fall six times slower than on an Earth-sized one.
• Leaving an Earth-sized planet takes about 4.7 times the speed that leaving the Moon does, which is why big planets are harder to leave and their atmospheres easier to hold.

For a game, this means planet size is not decoration: it sets the feel of walking, the cost of leaving and the kind of air a world can keep. When a studio says its planets are Earth-sized, ask whether its gravity, atmosphere and escape cost follow from that size.

From orbit to the ground

What turns a planet into a place is the path between sky and surface. The air thins as you go up and the gravity drops to nothing outside the atmosphere. A good implementation treats that as one continuous world, not as two levels with a loading gate in between. The details of how a game fits all of that in one space are in how to simulate a solar system at scale.

The four hard problems

1. Precision. Numbers that describe positions run out of digits far from the origin, so a planet-sized world needs special handling.
2. Distance. Real orbital distances are too long to travel, so a system has to compress them without breaking the feel.
3. Content. Hand-placing every rock on an Earth-sized surface is impossible; the ground must be generated from rules that make sense.
4. People. Players need to share the system, so zones and servers have to follow the geography.

What to ask about any “whole solar system” claim

• Are the planets places or backdrops? Can you stand on them?
• Is the ground part of the same world as the sky?
• How is distance compressed? Jumps, time acceleration or scaled-down bodies?
• How is the ground made? Hand-built, procedural or both?
• What exists today and what is planned?

What COLONIZED has built, and what it has not

COLONIZED is set in one star system, Aurin: a G1V star, four planets and eleven satellites in the canon. Fifteen worlds are generated from their lore: four planets, ten moons and a captured asteroid.

In the development build:

• Worlds are spheres with gravity that points to their centre, and outside the atmosphere there is zero gravity.
• Terrain comes from real elevation data and physics (impacts, craters, erosion), not noise.
• Each world has weather of its own: rain, snow, dust, fog and wind.
• Day and night, eclipses and volumetric clouds work from the ground to orbit.

In development or under review: the visual review of the fifteen worlds; ship flight, landing and multi-crew, which are meant to connect the worlds; and the design goal that planets reach the size of Earth. We do not say you can explore a finished planet today. Release date, price and requirements are not announced.

The numbers we can publish are in the Aurin system in numbers, and the canon is in the Aurin system guide. For the question of how big is big in terms of walking time, read huge open-world maps.

In short: scale is not a number on a store page but a set of consequences, in gravity, travel and ground to generate. When you read about a game’s planets, look for those consequences and for the state of each one in the build.

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