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It seems like the astronomical side of DU is still in its infancy, or at least from what I can see. So I would like to present a two-sided series of both suggestions and educational posts. Though I will not be going into too much detail. So if there are any other Physicists or Astronomers out there, I'm simplifying for a general audience and possibly have these parameters included into the game. Planet Orbits: Depending on the mass of a star the typical orbits of planets around it have been found to have certain properties. The is an inner and outer limit. Too close and the planet (or rather that section of the proto-planetary disk during formation) will be pulled into the star. Too far and the star will not have enough gravitational pull. Before we proceed, an Astronomical Unit, or AU, is the average distance between the Earth and the Sun, approximately 150 million kilometers or 93 million miles. The inner limit follows this equation: I = 0.1 * M Where I is measured in AU and M is the mass of the star. Similarly, the outer limit can be generalized as: O = 40 * M Though with some caveats on the mass and velocity of individual planets, as particularly fast or less massive planets would have a significantly smaller outer limit. Frost Line: The simplified frost line can be found based on the star's luminosity. F = 4.85 * L^-2 Where L is luminosity and F is measured in AU. Keeping in mind that a planet barren of atmosphere will retain nearly no heat from their star and certain atmosphere compositions will have a greenhouse effect. Stable Orbits: Planets have an almost infinite combination of orbits, but there are some rules of stable orbits when multiple planets are involved. Each further planetary orbit will always be between 1.4 and 2 times the distance of the previous or be unstable and something will somewhere destabilize. This, however, is in relation to the star. IF we are looking at orbits close to the parent star, planets themselves can be no closer than 0.15 AU of each other (generalization, gets a bit more complex). Otherwise, the planets will affect each other and one or more planets will destabilize and go into the sun or settle into a further orbit which may cause a chain reaction of other orbital interactions. Orbital Resonance: Dwarf Planets: Criteria: Orbits a star Roundish in shape Has not cleared its orbital path of debris Not a satellite Terrestrial Planets: Ice Giants: Hot Giants: Moons: Major/Minor: Moons are currently classified by two types, Major and Minor Moons. Major moons are those who have enough mass to collapse into a spherical shape. As mass is not always the same density at certain sizes a minor moon may, in fact, have more volume than a major moon. However, this is a limited range and is extremely rare. This range of overlap resides almost solely in the 200-300 km range. Though theoretically, some really odd and really rare highly dense or the oppositely composed moons could exist. Rocky/Icy: Moons also come in two varieties, predominantly rocky and icy. Why this is has a few theories based on we believe system formation occurs. Moons located within the systems frost line will be predominantly rocky and those outside the frost line will be predominantly rocky. Abundance: Terrestrial planets tend to have very few moons and often none at all. It is not uncommon to capture a few asteroids which reclassify as minor moons, but it is particularly rare to have major moons. Additionally, terrestrial planets closer to a star tend to have fewer moons than those further away. Hill Sphere: The Hill Sphere is the range in which a smaller mass within it will gravitationally bound to the larger mass. This can be calculated by the equation: H (outer): a * (m/M)^(1/3) * 235 Au , m is the smaller mass, and M is the larger mass. The inner limit is the Roche limit, more details on that later, but for now, the equation is: d = R * ( 2 * pM / pm )^ (1/3) Where R is the radius of the major body, pM is the density of the major body, and pm is the radius of the minor body. It must be kept in mind that these are simplified equations for static, or solid moons with an ideal orbit. Moons with fluids or elliptical orbits will have modified equations. Simplified Orbital Period: P = 0.0588 * ( R^3 / (M + m ) )^(1/ 2 ), Where P is in days. Moon Systems -- If you found this interesting comment below and if at least a couple people are interested I will continue with a lot more info. Specifically types of planets and realistic ranges of their properties.