Moon(s)

What better way to signify that a someone is not on earth than to have them look up and see a different moon, or even more than one?

There are a couple ways the moon(s) will effect you planet besides making the night sky prettier.

So what range of Gravity is Habitable for Humans?

Low gravity (relative to earth) seems to give people no problems once they get used to it. We've had people in weightless environments for months. However the human body can easily adapt to those conditions, and gets rid of what it perceives as excess muscle, bone mass, and fluid volume. Which is fine, unless the person wishes to return to a higher gravity world. Astronauts use rigorous exercise programs to stay in shape enough to return to earth, though the longer they stay in space at zero G, the longer it takes to return to full health on Earth.

How Heavy & how Big?

The next step is to figure out the size of your planet, and the strength of its gravity.  This information is derived from your planet's mass, radius & density. With only 2 of these variables you can figure out gravity, and anything else relating to your planet's size. Given the same mass, the denser a planet is, the higher the gravity.  Jupiter with over 300 times the mass of earth has a gravity of less than 3 times that of earth's (on the theoretical, unobserved surface of Jupiter) because it is very diffuse, made out of gas rather than iron and stone.

Mass can be pretty much whatever you want, within certain bounds.  Dole estimates (see Habitable Planets for Man, free PDF.) that a planet needs at least 0.4 Earth Masses to retain a human-breathable atmosphere.  Depending on who you ask, 5, 10, or even 14 earth masses is the upper limit for a terrestrial planet. Beyond that it will retain so much atmosphere that it crosses over into gas giant territory.

Radius can also be whatever you want, as long it doesn't force the other variables into something silly. Just be aware that radius has a strong effect on the surface area of your planet.  If you double the radius, the surface area increases four times.

Density is a little trickier. But here are the basics.  Density is primarily determined by what a planet (or moon) is made out of.  Gas, such as helium and hydrogen, tends to be least dense and then in increasing density are ice (water), rock, and iron.  These are the main ingredients of the known planets.  It is easy to find the densities for these materials at normal, surface of the earth conditions.  But as the mass of a planet increases, so does the pressure inside, and thus the density of various materials.

Freebie: The colors of Gas Giants

Of the lifeless planets, gas giants, or jovians are the most beautiful.  So i've wondered --the extrasolar giants we've been discovering but haven't yet got a good look at-- do they look like the our tan and striped jovians, or are they different?

Apparently they've been able to infer some things about how they appear.  See this Extrasolar Visions page.

I may have to swap out the first planet in this Orb's solar system for a Jovian.  I think it would be a Class II, white water-cloud jovian.

At closest it would be ~0.2 AU away, as compared to Jupiter's closest 5.2 AU, so at closest to Orb it would appear to have about half the diameter that the Sun does from Earth— assuming it is as big as Jupiter.  That would be quite a cool sight.

a water cloud jovian from Extrasolar Visions, apparently with a life-bearing moon.

Celestial Architect Spreadsheet

To simplify the effort for myself, & for others who may be interested i've created a spreadsheet for moderate control without much effort.

Download Celestial Architect Spreadsheet 1.1 below:

for Open Office
Microsoft Office Excel 95

EDIT:   Updated to 1.1. Renamed the Spreadsheet and added another page dealing with the moon.

Using The Sheet

The "Sheet" is a spreadsheet-- a file you can open with Microsoft Office, or free alternatives such as OpenOffice or it's Mac port, NeoOffice.  All you need to do is enter numbers in the yellow boxes, & all the other figures will be instantly recalculated.

This Orb's Solar System.

Well, i had decided that i wanted this Orb to be less hospitable than earth.  The simplest way to do that was to make it hotter or colder.  I decided to go with colder, since i could then mostly ignore those tricky parts of the texture map where it wraps around the pole and gets extremely distorted.

A yellow sun seemed somehow passé, so i tried some smaller stars on my Sun & Planet Creator Sheet.  It quickly became clear that this Orb's year was going to be significantly shorter.  I didn't want to bother with reckoning a year that was some weird fraction of an earth year, so i pushed the numbers around till this Orb's year was just over half an Earth year.

a screenshot of this Orb's system from StarGen with appropriate alterations.

Elusive Accuracy

Please note, that not all the sources i'll link to precisely agree.

• Sometimes the calculations are only rough approximations of complex systems.
• Sometimes scientists are making their best guesses, but aren't all making the same guess.
• Sometimes it may be out of date information.
• Sometimes i won't know which of the above is the case.

I'd like to know about any out-of-date info i've included, but i expect to find sources that don't exactly agree.

While i do strive for accuracy and understanding, it's also important not to get carried away with details that aren't important for one's purpose.

How to get Started

If you are interested in a normal fantasy world, you may not findf anything useful in this post.  By "normal fantasy world," i mean one where the temperature, length of the year & the day are about the same as on Earth.  Often fantasy worlds have archaic or unique cosmologies, so that the nature of the sun, stars, moon, etc. are quite different from our own.  Somehow these cosmological differences seldom change the *effect* of the sun on the fictional world, but i suppose if you are re-writing the laws of physics, you can get any result you want.  I often find these fictional cosmologies quite interesting, but i won't digress further into these infinite possibilities.

Since i'm building a realistic sci-fi planet, & not assuming the sun & other features are just like earth’s, i need to find a good place to start.  A lot of the factors interrelated.  The only truly independent factor is the sun.  It will greatly effect your planet, but is not effected by it.

There’s a huge variety in the size of stars.  For “normal”, main sequence stars, the larger & brighter stars are bluer, while the smaller, dimmer stars tend to red. Wikipedia has a good overview of the star types.

Main sequence stars.  From Wikimedia.

Life, but Not as We Know it

When i mention "life", or more specifically "earth-like life", i will generally mean the same thing: carbon-based life-forms.  It’s something built from organic chemicals using mostly carbon, hydrogen,oxygen, & nitrogen, and requires access liquid water.  It’s pretty much “life as we know it”.

While i don't deny that other biochemistries might exist— that breath chlorine, ammonia, or even something that no one has thought of—  i will mostly ignore these possibilities.

1. Firstly, because i’m not a chemist and can’t evaluate how plausible different proposed biochemistries are.
2. I believe it’s all just a slightly educated guess.  Scientists know a lot about earth life, but not what makes something alive—certainly not to the degree needed to know what other forms life might take.
3. Finally, it would simply be too complicated to take into account all theoretically plausible types of life, and all the different types of planets they could live on.

digital sketch of a random gas-giant dwelling alien

Feel free to devise life based on any biochemistry you want, just realize that any statements about life and habitability in this blog probably won’t apply.

My Objectives for this Orb

For this project i will often follow the process & accept the outcome, weather expected or unexpected, but sometimes i'll fudge a little to get the results i want.  Since much of the process of even the most rigorous world-building is outrageous simplification, & bold guesstimation, there is a lot of wiggle room.  No doubt other world builders will choose to be loose where i will be strict, & consider factors that i don't understand or don't care about.  Though i am interested to hear about any mistakes i might make— especially before a lot of additional work is based on them.

This Orb is an out-of-the way planet in a sci-fi setting where some form(s) of FTL are possible.  Several waves of sapient explorers, refugees, conquers, & colonists have settled there.

The following are my objectives for this Orb.

• While humans can live on it unaided, it is not as hospitable as Earth.
• This Orb is not conducive to global communication and rule.
• The oceans are completely separated by land, so it's impossible to sail round this Orb.
  
• Mountains, rivers, currents & coast make it generally less convenient to cross a continent.
• There is one location that is geographically destined to be the crossroads of the whole planet.
  
• Some factors prevent the inhabitants from rebuilding a spacefaring civilization.

I'd recommend that aspiring world-builders jot down the traits that they want their new planet to have.  This will make it easier to navigate through the myriad possibilities in the following steps.

Hello World(s)

I've always been fascinated by maps, globes & planets.  As a child, i drew maps of all kinds.  Wanting to make maps realistically, i became curious about the reasons that natural features are arranged as they are.  I accumulated little bits of information from books & inferred patterns by looking at maps.  But i wasn't satisfied with what i understood.

A year or so ago, i searched the web for world-building, & though i had to dig a bit, i found nearly all the answers i was looking for.  Using a more rigorous process, i designed a planet up from the orbit, until getting stalled while figuring out the climate.  I'd leaned a bit more through the process, & so was no longer happy with the result.

Now i'm starting over. Via this blog, interested world-builders can watch the process unfold.  I've found some good examples of constructed worlds, but not much that really shows the process of building that world.

I'll be linking to the best sources i've found, showing how i apply that information, & giving some ideas of my own.  Hopefully this blog will make it easier to design plausible worlds for people like myself, less mathematically inclined.

Once i clear up the introductory matters, & finish tweaking the appearance & settings, we'll start with orbital mechanics.

an old version of this Orb taken with Celestia