The celestial hierarchy works as follows:
You have sectors. Sectors take a 64-bit seed.
Currently, we have 3 sectors defined, for release we're planning on 10.
Each sector contains systems (currently only star systems). There is a 1 in 100 chance that any given XY coordinate contains a system, and the coordinates range from -100 million to 100 million. Meaning there are on average 200 million ^ 2 / 100 systems per sector, or 400 trillion. (Z coordinate also ranges from -100 million to 100 million and is calculated as a checksum of the first two numbers iirc).
Each system can contain up to 12 planets, the exact probability depends on the type of the system, but average number of planets in a system is currently (2.0 * 0.2 * 12 + 3.0 * 0.6 * 11 + 1.0 * 0.4 * 10 + 1.0 * 0.4 * 10) / 7.0 ~= 4.657
Each planet could either be a terrestrial planet or a gas giant. And may have up to 12 moons (I believe?). Each moon is a terrestrial body and landable, gas giants are not.
The average number of moons for a planet is ( 2.0 * 0.1 * 12 + 1.0 * 0.2 * 12) / 3 = 1.6 moons / planet. And the probability that the planet itself can be landed on is 2/3.
This gives 3 * 400 trillion * (4.657 * 1.6 + 4.657 * 2 / 3) = 12.667 quadrillion planets in the game currently. 422.22 quadrillion planets expected for release. And if you're into modding the game and unlocking arbitrary sectors you can access up to 77.88 decillion different worlds.
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