Solar System and Planets

Unless your really interested in this part I'd just skip this section, assume your world is in Earth's position, has Earth's size and Earth's distance from the sun. That would make the year 365 days, Size approximately 25,000 mi in circumference, with seasons and temperatures the same as ours.

The parent body(ies)

The first thing our world needs is a stable orbit around another body. That body could be a single star, a single star of multi-star system, a multi-star system, or a larger planet which is orbiting something else. The world needs to revolve around that parent body (its year) in either a clockwise or anti-clockwise direction in roughly the same plane and direction as the other bodies. Putting your world in a high degree of tilt to other bodies, or having it revolve in the opposite direction from other bodies wont work in the long term. Its orbit will become unstable and it will be tossed out or end up crashing into another body.

Single Star Systems

The easiest is the single star just like Earth. A single sun rises and sets each day, it takes a year to revolve once around the sun.

Circumbinary planets

A multi-star system where the two (or more) stars revolve around each other and the world revolves around their common center of mass functions about the same as the single star system. Any calculations are done with the 2 stars counted as one body and the center of mass of the two as the point the world orbits. The only concern will be that the two stars aren't too big or to far apart. The stars should orbit less than 3 AU of each other. Farther than 50 AU apart and a planet should orbit only one of the two. Systems where the stars are between 3 and 50 AU seem to lack the dust that planets form from. In the sky the two suns would always be close together like Tatooine's suns. Research presented at the 223rd American Astronomical Society meeting show that binary stars dampen each others solar radiation and winds creating a more hospitible environment. Two stars also more the habitable zone outward reducing other negative effects.

Multi-star Systems, planet orbits 1 star

For a world revolving around a single star in a multi-star system the two stars must be separated by enough not to disturb the world. This means one star will most likely be much further away and fainter than the primary star the world revolves around. Seen from the planet it would appear that there is a normal sun that rises and sets each day, and a very bright star that might take years to travel the night or twilight sky with an occasional retrograde motion. Stars seperated by more than 50 AU could have planets orbiting only one of the stars.

An example could be a planet in the Centauri System. Alpha and Beta Centauri orbit each other at an average of 23.7 AU with a close approach of 11.4AU. A planet could orbit either star within 3AU of the star stabally, or it could orbit both beyond 70AU. However as noted above stars that orbit between 3 and 50 AU tend to lack the dust that forms planets unless they are far out so there probboly isnt a planet in the system orbiting only one of the stars. There is some speculation that Alpha Centauri B has a planet only 0.04AU from it. That is outside the habitable zone (0.5AU-0.9AU). Alpha Centauri B would appear 300 times brighter than our sun, and Alpha Centauri A would be -21.9 mag or 1% of our sun.

Habitable Moon

The habitable moon scenario just makes the world in question a moon of a larger planet. The larger planet will need to fall into the habitable zone described below. In the sky of the moon the larger planet will loom large, and it probably hovers over one spot on the world since the moon is most likely tidally locked. The moon does need to be far enough from the planet not to experience tidal heating but remain in the Hill radius. The sun will rise and set once a month as the moon goes around the larger planet so days are long. But years are dictated by the larger planets revolution around the sun. Red dwarfs less than 20% the mass of the sun cant have habitable moons.

Star Type

Stars are designated by a system of letters O,B,A,F,G,K,M which categorizes them from hottest (O) to coolest (M). The hottest stars (O, B and A) are generally too short lived for life to develop and should probobly be excluded from consideration. Stars at the cool end (M and some K) would have planets that are tidally locked. These stars also tend to be varible stars. Varible stars present a problem because they change in luminosity and generate lots of lethal gamma and X-rays, they would also buffet the atmospheres of close in planets away deprving them of protection from those rays.
ClassApparent ColorLuminosityMass in solar massesCHZ% of stars
FWhite1.5-5L1.04 - 1.41.6AU - 3AU3%
GYellowish white0.6 - 1.5L0.8-1.041AU - 1.6AU7.6%
KPale yellow orange0.08 - 0.6L0.45-0.80.4AU - 1AU12.1%
MLight orange red<0.08L0.08 - 0.45<.4AU76.5%

The Circumsteller habitable zone

Once the primary is determined the next step is to determine where a habitable planet can exist around that star. The habitable zone is the area where liquid water can exist on the surface. A basic calculation is that the CHZ is centered on the square root of the luminosity of the star * 1.34 AU (see chart). This is the center of the zone, planets on the inner edge are warmer, those on the outer edge cooler. Earth is at 1AU around a G2 star whose zone is centered on about 1.34 AU (.95AU - 1.37AU in some models), so we are on the warmer side. The distance from the star then determines the length of the year.

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