Designing Alien Worlds
Month 5, Week 4, Episode #6
Well, besides the various calamities that occurred this last week with the Boston bombings and manhunt, the failure of a gun reform law, Syria reportedly using chemical weapons, and North Korea saying that they won't talk with the US unless we give them pudding, there was one ray of sunshine for the science fiction guru. Once again, there's big news on the planet hunt launched by the Kepler telescope with the discovery of two planets well within the 'habitable' zone of their parent stars: Kepler-62 and Kepler-69.
Huzzah!
As a science fiction author currently working on a fairly 'hard' sci-fi project, news like this is often a great boon to the faltering muse. It also makes the universe a lot smaller... or at least less mysterious.
The Kepler-62 star system with its five planets is over 1,200 light years away, and Kepler-69 are 2,700 light years distant... Which is really, really far. I often wonder when I get news like this why we're not detecting more planets closer to us. Actually this is a really rather amusing discussion to me as the vast majority of stars are red-dwarfs and we're lucky to detect them at a few dozen light years thanks to how dim they are... which I guess leads me to what I wanted to put forth today, some notes and thoughts on designing alien worlds, and inhabitants.
I.) The Environment
Now, I'm going to be doing this in very broad strokes, as going into any exacting detail will likely drive almost anyone completely mad. A large amount of the planet's enviroment will be determined by factors that can well be easily understood and envisioned by anyone with at least a basic understanding of physics. A larger world will have higher gravity, a closer world will be warmer with a shorter year, and a more eccentric orbit will replace seasons from axial tilt with seasons based on orbital position. The very first step however should be to determine a few things.
1.) Determine the Parent Star's qualities.
Now, if you're using a computer program like Astrosynth you can go into much more detail then I'm going to be giving right now. Essentially what you really need to determine are the following things.
Stellar Class - Stars are classed by their primary emission lines as a star resulting in one of seven main types:
Class O Stars are the hottest and largest form of main sequence star, the vast majority of their light is emitted at the Ultraviolet wave lengths. As a result you can probably guess that UV exposure is much higher. Most have a luminosity of several hundred thousand or more times that of our sun, as a result they will have the largest habitable zone. Thanks to the inverse square law, a rather simple equasion you can use to estimate the distance of an optimally placed earth like planet is this...
square root (star's Luminosity in solar luminosties) = Optimal terrestrial orbit in astronmical units.
So the ideal orbit for an Earth-like world around a 1,000,000 luminosity O type star would be 1,000 AUs (1,000 Earth - Sun distances). 1,000 AUs is undetectably distant with current methods unless the planet wanders in front of its parent star.
Sadly, it's likely that most O type stars will not last long enough to produce complex life on their planets as these, the largest of stars, are also the shortest lived. In most circumstances they might not even live long enough for a world to coalesce in orbit around them before dying.
Class B stars are bright and big, primarily releasing most of their energy as blue-white light. Many named and well known stars are of this class, Regulus, Rigel, Bellatrix and so forth. Like Class O stars, they are mostly large objects (with the exception of dwarf stars) with short life spans. Vegetation on Class B stars is likely to have evolved to use blue-white light as it's primary source of energy, meaning leaves are more likely to be blue instead of green.
Class A stars are bigger intermediate stars that release most of their energy as white light. I actually use a number of these stars in my current project, the most obvious being Fomalhaut. Vegetation on an planet orbiting a Class A star will likely be bright white or blue for maximum use of the solar radiation. The habitable zone is likely to be much further out then around our star, but not excessively so.
Class F stars are just slightly bigger then the G type star that is our sun with a proportional shift toward the blue end of visible spectrum. They are slightly shorter lived then our sun.
Class G stars are like our sun and emit most of their radiant energy as green light. This is why chlorophyll is green. You would expect the habitable zone of such stars to be almost identical to our own.
Class K stars are slightly smaller then our sun and emit light in a dull yellow or orange color. Vegetation on such a planet is likely to be an equally yellow color. The habitable zone is smaller as well. There are exceptions to this from stars that are transitioning from main sequence stars to Red Giants will be quite different in nature.
Class M stars are the red dwarfs and red giants of the universe. They're dim unless they're big, long lived unless their giants, and the vast majority of stars in the universe fall in this category. Below them are brown dwarfs, which technically aren't stars as they lack the fusion furnace that powers their larger brethren and are really an intermediary between super jupiters and red dwarfs.
Then you need to decide if the system is a single star system, a binary, or a more complex system like say the Alpha Centauri system with three stars and at least one planet.
In my experience, it's best to imagine complex systems like that as essentially being like the solar system we know, but with planets like Jupiter and Saturn being replaced by smaller stars and their moons as planets unto themselves. Now that we got that star system in our mind, lets think about the planet itself.
2.) Determining the Planet's qualities.
A very important bit for designing a planet is to determine it's type. Now, I'm going to ignore gas giants for the moments, as while there are ways such worlds could be habitable (giant floating jellyfish anyone?) they're not likely to develop life which we'd interact much with.
So, this leaves us with rocky worlds and ice balls... as essentially there are three types of planets, rocky, icy, and gaseous. An ice ball in the habitable zone is probably the ultimate in terms of viability for life to form, but like gas giants would not likely result in the evolution of life which our heroes would regularly interact with (though the idea of a ship going down on such a world would be interesting).
An ice ball in the habitable zone is not a terrestrial world, but an almost purely liquid world. The depths may have a stony, rocky, or metallic core, but that area would be unreachably deep. It is likely that such worlds would also have a substantial atmosphere from evaporation of the surface liquids.
Highly intelligent life could develop, but without land there is no place to create fire, and therefore no means of forging metals which would likely be very rare on such a world anyway. As a result, while culture may develop, and even language, there would likely never be a water based lifeform that industrialized or even moved beyond the equivalence of the 'stone age', though how they even manage that without stone is beyond me.
This leaves rocky terrestrial worlds like our own. When designing a world of this sort, the first matters of any significant concern are it's size and age. An extremely young world is not likely to have any life, nor significant atmosphere from outgasing, or even significant surface water. A larger world will have higher gravity, a smaller world will have lower gravity.
The smallest a planet can be and still be habitable is probably best demonstrated by one of our neighboring worlds, Mars. Mars is 10.7% Earth's Mass, 53.2% Earth's Diameter, 28.4% Earth's Surface Area, and 0.376 Standard Gravities. There is significant evidence that there was once significant surface water on Mars, and perhaps at one time life. These oceans have evaporated, and the planet itself has a crack running along it's equator from it's interior contracting as it cools. The atmosphere is 0.6% as thick as ours.
Overall Mars is a world that once was once hospitable but is no longer. It would require massive terraforming efforts to restore it as a viable world for humanity's use. In short, it is the border line between what would support life and what wouldn't.
Most of Mars's shortcomings are directly related to its size, as a larger world would hold its atmosphere better, be more volcanically active, and likely hold onto its surface water. Global cooling is accelerated thanks to its smaller size and resulted in the solidifcation of the world's core (causing Mars's magnetosphere to collapse), the contraction of the surface which created the Great Rift Valley, and so forth.
Assuming a planet is roughly the same density of Earth, calculating its mass (in Earths) will give you most of the most important values. Gravity is affected by the inverse square law, as a result increases in mass results in a non-linear increase in surface gravity.
Kepler-62e and Kepler-62f have been estimated at being around 1.5 the mass of Earth. If they have an Earth-like density, it means their surface gravity will only be around 22% greater then Earths. I'd think the maximum adaptable gravity for Humans would probably be around 2Gs (with significant side-effects to adapting), meaning that worlds as large as 4 Earth Masses could be colonization targets.
When you consider Mars a lower limit of viability at 0.1 Earth Masses and 4 Earth Masses as the upper limit for size, it quickly becomes apparent that there could well be hundreds of worlds within relatively sensible reach of Earth. While Kepler-37b is the smallest world yet detected at 1% of Earth's Mass, such small worlds can only be detected by the Kepler space telescope watching for planets transiting before the star. This means that only a tiny sliver of the sky has yet been searched in this manner (almost all Kepler targets are in the constellation Lyra) and that only a tiny portion of planets orbiting observed stars could be identified.
At my last count less than five planets have been identified with a mass of less than 1.5 Earth Masses, meaning that there could potentially be thousands of worlds with masses between 0.1-1.5 Earth masses within the limited reach of a sub-light generational ship.
Back to what all this means...
Determining the mass of the world determines it's surface gravity. Worlds with higher surface gravity are more likely to have denser atmospheres, and proportionally smaller lifeforms. Worlds with lower surface gravity are more likely to have less dense atmospheres and proportionally larger lifeforms.
Now, you need to determine the age. I'm not going to go into a great amount of detail on exact ages, but more on the stage of Earth-like planet evolution. I divide the stages thusly.
Stage I Primordial - Planet is inhospitably due to asteroid or comet bombardment, the planet's crust is likely molten.
Stage II Pregarden - Planet has cooled and oceans have formed atop its surface. Oceans are likely a green color due to oxidation and iron deposits, no significant life has evolved. The atmosphere is likely a result of out-gassing alone and formed primarily of carbon-dioxide. A planet which fails to have sufficient water deposits to absorb the out-gassing of carbon-dioxide will develop a run-away greenhouse effect much like Venus.
Stage III Garden - The evolution of plant-life has greatly transformed the atmosphere, resulting in the conversion of large amounts of carbon-dioxide into oxygen and thus paving the way for animals and other organisms.
Stage IV Fully Developed - The evolution of animal life has caused an explosion of life on the world, given time the ascendancy of intelligent life is possible, though the evolution of a technologically adept species will take a great deal of time.
3.) Details
Details are what most people think of when they talk about planets in science fiction. There are many things I'd call mere details, but here's a partial list...
Oxygen Content - Higher Oxygen Levels or Lower Oxygen Levels can profoundly effect the life that evolves. A study of life on Earth shows that higher Oxygen Levels in the carboniferous period is likely the cause of the evolution of truly massive insects and invertebrate life like dragonflys with three foot wingspans.
Day - Longer or shorter days greatly effects the way life lives on a planet. Studies show that during Earth's youth, the planet had a day that was a mere eight hours long, which was slowly reduced to due to tidal effects with the moon to a 24 hour period. Many simulations of planet development around red dwarf stars indicate a high probability of what is known as 'tidal lock'. A 'tidally locked' world has a day the same length as its year, causing one side to constantly face its star or other companion (if a moon). This would likely result in a permanent storm on one side of the world and an icy night on the opposite, with the ideal environment only existing in the twilight zones.
Water Proportion - How wet is the planet? Earth is around 70% covered in water, having more or less water then this would greatly influence the weather and enviroment of a world. Some lessons from planetary evolution show that pre-garden worlds are often times only begining to form continents and therefore will be much more water covered then Earth.
Moons, Companions, and Rings - Years ago I designed a world for a Star Wars campaign that featured what I have to say was the single most evil depiction of what having a ring could do to a terrestrial world. If composed of large objects, well... I would expect a near constant bombardment along the equator. If composed of small objects, dust, or debris then meteor shower like effects could reliably be witnessed if looking in the direction of the equator.
Moons have an even more profound effect on a world thanks to tides. A large moon could significantly slow a planet's rotation, create large tides, or increase tectonic and seismic activity thanks to tidal effects.
Axial Tilt or Orbital Seasons - Unlike with Earth, seasons can in part be determined by orbital position rather then axial tilt. It comes as a surprise to most people in the Northern Hemisphere that the closest point Earth is to the sun is in February. Our planets axial tilt greatly influences the seasons we have, but if tilt was negligible and our orbit more eccentric, then orbital position could determine the season for the entire planet instead of just a hemisphere.
Magnetosphere Strength - Earth's Magnetosphere isn't constant, and neither will most worlds you create. There will be periods of lower strength, or perhaps simply an overall weaker magnetosphere. Some scientists have posited that the reason for the 'Punctuated Equilibrium' that is shown in the evolution of life on earth coincides with periods of weakness to the magnetosphere and increased radiation exposure.
A weak magnetosphere increases radiation exposure, but it also has some beautiful effects as a weak magnetosphere will result in much brighter and more common aurora which could extend globally.
Next time... II.) Designing the Ecology
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