(https://s1.postimg.org/5amc1g46m3/HZs.png) (https://postimg.org/image/5amc1g46m3/)
Habitable Zones
The inhabited IS comprises less than one tenth of one percent of all the stars within the same volume -- thousands (of inhabited systems) out of millions (all systems). Thus, BT star systems are inherently exceptional, affording a rather wide latitude of artistic license. However, the following guidelines are probably applicable to the statistical majority of systems within several hundred light-years of Earth.
Planets & Moons
- "blue giant" stars (O,B) have no planets
- white stars (A) have at most one "Super Jupiter" gas giant planet (100s-1000s of earth masses) with many many moons (?)
- green stars (F) have at most several gas (mini-)giants (10s-100s of earth masses), widely spaced (at most 1 in the HZ), with many moons (?)
- yellow & orange stars (G,K) have the most extravagent planetary systems (much like ours), up to 1-2 in the HZ, with up to 1-2 moons each around the larger stars of this group
- "red dwarf" stars (M) also harbor extensive planetary systems, however...
- the stars are fully convective, with stellar "thermals" rising all the way up from the fusion core to the surface, whereat powerful super-flares frequently erupt, showering everything within ~0.1AU with high doses of x-ray & particle radiation
- the stars are so dim that their HZs reside within easy reach of those super-flares
- planets within those HZs would also experience powerful Tidal effects, quickly becoming "Tidally Locked (TL)" to their parent star, much as the Moon is to Earth, or Mercury is to the Sun
- so one face of the planet would be frequently fried by flares, and the other face would be perpetually dark and frozen nitrogen & CO2 cold
- recharging Jumpships or Docking Stations would also be continually blasted by those super-flares, at almost point-blank range
Stellar Lifetime & Planet "geologic age"
(https://s1.postimg.org/313bhy65qz/Geologic_Age.png) (https://postimg.org/image/313bhy65qz/)
The biggest potentially-habitable stars (A,F) only shine for 1-3Gyr before "self destructing" in a "slow-motion super-nova" (dispersing themselves into a mis-named "Planetary Nebula" surrounding a central white dwarf stellar core remnant).
But it takes an earth-mass planet ~2.5Gyr to cool sufficiently to be easily habitable. Most of that heat "budget" derives from Uranium and other radioactive elements in the planet's core. The cooling time of a rocky, terrestrial type planet increases with that mass of heavy heat-generating metals (MZ), which in turn is proportional to the total planet's mass; and decreases through radiation from the exposed surface area of the world (4pi R2):
T ~ MZ / 4pi R2 ~ M / R2 ~ g = surface gravity
For the planets in our Solar System, the expected "geologic lifetime" of scales as its surface gravity. Thus, Mars (g = 0.38) ages about ~2.5x as quickly as Earth, and only lives ~40% as long. Its sequence of geological ages (Pre-Noachian / Noachian / Hesperian / Amazonian) -- defined & bounded, respectively, by the progressive freezing of the Crust / Mantle / Core -- occurred about two-and-a-half times faster than the corresponding series of ages on Earth (Hadean / Archaean / Phan-proterozoic...). (Earth's core is only half frozen, still has another billion years of geologic life in it, or so.)
Colonizing and terraforming a "Hadean" world could be impossible (the Crust has not even completely solidified), and an "Archaean" planet possibly impractical (incessant seismic, volcanic and other geologic activity on Crustal "rafts" floating on a churning subterranean sea of molten Mantle magma). So, there is plausibly a "minimum age requirement" for settleable planets.
SUGGESTED GUIDELINES:
In sum, the usual scenario for settleability may well be the following:
A,F stars
- cosmologically "young" systems <1-3Gyr old
- habitable "Moon-to-Mars-mass" moon(s) of a "Uranus/Neptune or bigger" gas giant
- 1/6 - 1/2 g
- "Pandora / Endor / Warm Europa" scenario
- primary planet's orbital is 2-3 Earth years
G,K stars
- cosmologically "mature" systems up to 5-10Gyr old
- habitable "Mars-to-Earth-mass" planets
- 1/2 - 3/2 g
- maybe a moon or two circling the larger planets orbiting the larger stars of this class
- "regular Earth" scenario
- orbits take 6-18 months
M stars
- frequent super-flares
- marginal colonies ?
- orbits last weeks to months
- Mercury-like orbital resonances, with 20-day days (spin, rotation) & 30-day years (revolution) such that the apparent day is 45 Earth days long, 3 weeks of sub-Arctic night and 3 weeks of perpetual day?
Dry Deserts vs. Water Worlds
(https://s1.postimg.org/7hlv6s72bv/water_worlds.png) (https://postimg.org/image/7hlv6s72bv/)
Note that there are very few "super-Earths" more massive than 2-3 Earth masses with surface gravities greater than 1.5-1.7 g because such worlds have strong enough gravity to retain a massive atmospheric envelope and so become the cores of a Neptune-like mini-giant instead. But "never say never" obviously :)
Moreover, another consideration is vertical terrain contrast -- how high are the "highs" and how low are the "lows" on some world, and what is the difference in elevation between them? Scaling up the Moon or Mars would ceteris paribus make for taller mountains and deeper valleys. However, eventually, the stronger gravity of more massive planets tends to pull mountains down, and push material into valleys. In our Solar System, Mars actually has the optimal combination of "size without high gravity", such that Mars has both the tallest mountains (Olympus Mons) and deepest valleys (Vallis Marinaris).
Meanwhile, more massive worlds tend to have more & more water, which inundates the surface more & more deeply. Mars in its heyday had >30 km of vertical relief between mountain peak and valley trough, and perhaps enough water to theoretically submerge the whole surface to a depth of ~1km. So, Mars was mostly land, with a few large lakes and shallow regional seas.
Whereas Earth is mostly water, with a global ocean system ~5km deep on average, inundating ~half as much vertical terrain contrast.
The chart above estimates average "vertical contrast (km)" (green line) & "ocean depth (km)" (blue line) as a function of planet size (earth radii). The normalized radius of the Moon R~1/4 (M~1/100, g~1/6) & Mars ~1/2 (M~1/10, g~1/2).
Observationally, smaller worlds are more common than larger ones, much as smaller stars are more common than more massive ones.
SUGGESTED "LOWEST ORDER" GUIDELINES:
- most IS worlds (MKG system planets & FA system moons) are like Mars
- surface gravity ~0.5 g
- mass ~0.1 Mearth
- radus ~0.5 Rearth
- 1/4 the surface area
- mostly land with a few large lakes & shallow seas ("Great Lakes & Black Seas, Mediterranean at most")
- rain clouds never reach 20-30% of the surface, which remain dusty deserts
- "very vertical" with towering sheer-sided mountains & deep cliff-encased chasms
- many IS worlds are like Earth (~1.0 g)
- most "super Earths" (~1.5 g) are featureless & fully flooded "water worlds" with deep oceans, no land, and dense atmospheres
- almost all "hyper Earths" (>2 g) have retained thick atmospheres to become the cores of (mini-)giants resembling Neptune / Uranus / Saturn / Jupiter