World Building 101: Today's Topic: That's Heavy, Man!

Gravity is what holds the universe together on the grand scale of things. All mass has gravity, which means, everything has gravity, because nothing, so far, has been found to be without mass. What is mass? Mass is energy, in one form or another. So all the universe is but an ephemeral net of energy, bent and twisted by the four basic forces: Gravity, Electromagnetism, Weak and Strong. I'm going to concentrate on the heavy subject of Gravity in today's lecture.

Gravity holds the Earth together. It keeps the atmosphere on the surface of the planet, and the oceans, and all the stuff living on it. The steady pull of gravity is what makes us weighable. If you step on a scale, you are measuring the pull of the Earth. If you were on Mars, the pull would be less, since Mars is less massive than the Earth.

What is the gravity on your fantasy world? Can it be less? Can it be more? Yes, and yes, within a certain range.

If your world is as massive as Mars, which is 38% the size of the Earth, then the pull would be 0.38 that of Earth's. Meaning a 100 pound rock would weigh only 38 pounds on your planet. It would still have the same mass, it would just be more buoyant... as if you had help hefting it.

This is where things can get confusing. Mass and weight are two different things. Only on Earth does a 100 lbs rock have 100 lbs of mass. Out in space it may be weightless, but it's not massless. It is always a 100 lbs mass.

Life on a world with less gravity would provide for larger growth. Large creatures are curtailed in their growth by the square-cube law. Larger objects get bigger by volume, as in cubic feet, while they must support themselves as a cross-section of their skeletons. Cross-sections are measure in square inches. If an animal doubles its size, then it has become eight times more massive (2x2x2), while its bone support has only increased by a factor of four (2x2). So the bones have to become even more massive to support the extra mass. This becomes progressively more difficult as the animal gets bigger and bigger.

(Learn more about The Square-cube law)

The whole subject of large creatures is still debated, with gravity providing but one of many limitations on being big, including landmass and biodiversity.

On a planet more massive than Earth, the animals would have a lesser upper limit, since the square-cube law applies stronger. On a planet twice as massive as Earth, you probably wouldn't have elephants, or if you did, they would have much thicker legs.

Gravity also greatly determines the atmospheric pressure of the planet. The lighter the planet, the lighter the atmosphere; the heavier the planet, the thicker the air. (Smaller planets can have thick atmospheres, from volcanic eruptions and other chemical events, but it will be short-lived, geologically speaking). I'll get into atmospheric composition in another lecture.

So what is a fantasy author to do? Make your planet smaller, larger, or about the same as Earth? Make it too small and there's not enough gravity to hold an atmosphere. Make it too large and the surface becomes slushy from the strong internal convections.

A small world makes feats of strength more believable -- but remember that mass stays the same. Lifting a car would be easier, but stopping a speeding train would be just as hard. And a less massive planet would make it easier to fly, except that smaller planets usually have less air (If you want to have large flying creatures, you'd have to have a world that is still producing its atmosphere, making it thick enough for flying, and coupled with the lesser gravity).

A large world would likely have a thicker atmosphere, smaller animal and plant life, and more chaotic plate tectonics. Cloud formation is easier under more pressure, and wind dynamics are more persistent. A storm may persist for several years on a heavy atmospheric world, circling the globe many times as it rages.

World Building 101: Today's Topic: Holy Magnitude!

Space is huge. Really huge. Mind-bogglingly, oh-my-gawd huge. Trying to wrap your brain around just how huge space is is a daunting task. But we'll give it a try.

Why does one need to know just how huge space is? Well, it allows you to do something amazing. Winning the lottery sort of amazing. You can imagine your world just about anyway you want, and if it can theoretically exist, then chances are it does, or did, or will. The numbers are on your side.

Our solar system is comprised of our lonely sun, a single star, and eight decently large planets, four rocky and four slushies, and a multitude of smaller icy planetoids. (Nine? You say? Well, not really, since Pluto hardly qualifies... but I digress).

How many more stars are out there in our galaxy? About 200,000,000,000. That's two hundred billion suns in our local Milky Way galaxy alone. And how many galaxies are there? Well, current estimates put the value around 100 to 300 billion galaxies. Our Milky Way is considered pretty large on the ol' galaxy scale, so we can't use it as an average, but assuming an average of only 1 billion stars per galaxy we end up with 100,000,000,000,000,000,000 stars in the known universe! That is a lot of stars! That is ten raised to the twentieth power, also called a hundred quintillion.

So, assuming for the moment that only one percent of those stars have planets in stable orbits, and assuming only one percent of those have been around long enough for life to form on the planets, and further assume that only one percent of those actually have planets in the right orbital temperate zone, and assume that one percent of those have the right collection of elements to start life, and in the right proportions. How many planets does that give us? That reduces our huge number solar systems by eight powers of ten, leaving 1,000,000,000,000 planets that have all the right stuff for life. One trillion planets. Odds are, one of them fits your profile.

Now, my 'one percent' assumption could be off, it could be off a lot, but even if it's off several magnitudes, we still have plenty of life producing planets to choose from. (This thought experiment is brought to you by something called the Drake Equation.)

What I'm getting at is that a fantasy writer doesn't have to worry about the odds of a habitable planet existing. Chances are, it does. Somewhere out in the vastness that is space, a planet probably exists with that blue sun, three distinct rings, and two moons that you want.

The real problem is distance. Space is huge, as I've said. And most of it is filled with nothing (or dark matter, which is still mostly nothing). Traveling from one solar system to another is a monumental undertaking.

Most stars are light years apart. A light year is about six trillion miles.

How long would it take to travel a light year? Well, Voyager 1, our own unmanned space probe sent out 28 years ago (in the summer of '77 - and still running, I might add!), is now 8,700,000,000 miles away. That's about 0.15% of a light year. At that speed (about 38,000 mph), it will be another 18 thousand years before Voyager 1 is one light year away.

Even assuming you made a craft that travels a thousand times faster than Voyager 1, it would still take you a good 18 years to get one light year. And our nearest neighbor, Alpha Centauri, is four light years away! That's 72 years at 38 million mph. And, of course, once you got there and tried to phone home, you'd have to wait four years for the signal to reach Earth, and another four years for the answer to come back. It's hard to carry on a conversation with an eight-year lag time! Can you hear me now?

And, traveling very fast creates new problems, problems that are weird and spooky, which I'll save for a some future lecture.

All this humongous distance of space is the reason why sci-fi writers invented warp drive. Flying from star system to star system via faster-than-light space travel is de rigueur. It is so much nicer to be sitting around playing holographic chess for a few hours while hurtling through space at super-light speed rather than waiting a whole lifetime just to reach your stellar neighbor.

In summary, space is huge and empty, yet contains billions and billions of stars, some of which probably have a habitable planet orbiting it that can be your fantasy world. But good luck getting there.

World Building 101: Today's Topic: Color Me Blue!

Stars come in various colors, depending upon how hot they are burning. Blue is the hottest, followed by white, then yellow and red.

Can your fantasy world have a blue, white, or red sun, rather than a 'normal' yellow sun? Possibly. I'll talk about each one.

A blue star is burning hydrogen just like our yellow sun, but at a much faster rate. It must be many times larger than our sun in order to have enough gravitational pressure to up the temperature to the blue range. This means it won't last as long (the whole a candle that burns twice as bright lasts half as long thing).

So the question becomes, could a planet orbiting a blue star be around long enough for life to form? It's estimated that a blue giant star has a life of about 10 million to 100 million years. Hardly enough time for a planet to cool and form life. But, spectral maps of star clusters reveals a fair number of 'blue stragglers' -- blue stars that for some reason haven't shifted to the red giant phase. Some could be as old as a billion years. This puts it in range of life forming planets, so, it's theoretically possible. Particularly if you have a blue star that's only just blue, that is, about ten times as massive as our sun, as opposed to hundred or two hundred times more massive.

What would a blue sun look like? Well, it would be a smaller disc, since the temperature is so much higher the planet needs to be father away to have a similar climate. But it would very bright, like the difference between a yellow incandescent and a blue-beam headlight.

White stars come in two flavors: main and dwarf. A white main sequence star is just a star like our own, but a few times larger. It won't live as long as our sun, but should still be good for a few of billion years worth of stable solar energy. So no problem having a world orbiting a main sequence white star.

White dwarves, on the other hand, are old stars that have already gone through the red giant phase. This means that any planet orbiting the star has been engulfed when the star expanded to a super-size red sun. So, there wouldn't be any stable planet orbiting a white dwarf star close enough for the solar energy of the dwarf to keep the world temperate.

Red stars also come in two flavors: Giant and main. A red giant is a blue, white, or yellow star that has used up all of its hydrogen and is now burning helium. Red giants are huge, puffed up versions of the original star. Our own sun will become a moderately large red giant in about 5 billion years. It will expand and engulf the four inner planets (Mercury, Venus, Earth, and Mars). At that point, moons orbiting Jupiter might become life-supporting. But that phase will only last for a few million years at most. So, one could have such a world, but the life would probably need to come from somewhere else.

Main sequence red stars are stars with a mass slightly less than our sun, 80% to 30% of our sun's mass. It burns cooler, and therefore has a red hue. These types of stars are quite common, probably the most common, and are very stable. So a fantasy world orbiting a main sequence red is no problem at all.

What would a red sun look like in the sky? It would be a larger disc than our own sun, and have an amber or orange hue. The infrared spectrum would be stronger than our own yellow sun, and therefore the sunlight would 'feel' hot.

What about smaller than a small red main sequence? Suns that small don't have enough gravitational pressure to cause nuclear fusion, and therefore don't give off light. So-called 'brown' and 'black' stars (not to be confused with a black hole), these stars give off infrared light only. A planet could orbit around such a star, and life could possibly exist, for the infrared heat could keep a planet from being a frozen hunk of rock.

More info on stellar classification can be found here.

World Building 101: Today's Topic: You've Got Twins!

Having a twin solar sunset has to be one of the cooler images in the science fiction/fantasy genre. That and dragons...

A solar binary, a system where two stars orbit each other, is actually quite common in our universe. Our closest neighbor, Alpha Centauri, is a binary (actually, it's a triple, with a third red dwarf star out far from the main pair). Star pairs, and double pairs, occur throughout the galaxy in high numbers.

The question becomes, can a planetary system exist in a binary (or higher) system? Or can only the lone stars, like our sun, manage to keep planets in a stable orbit?

The answer is, sometimes. The trouble is that many of the binary systems have stars that are very close to each other, so close that the two gravity wells are overlapping to a huge degree. This makes it highly unlikely that a planet could develop any sort of stable orbit about either sun. If a planet was circling one star, then each time it came near the other star the tidal effects would be large enough to shear the planet apart, or at least pull it out of orbit.

So, to have a planet orbiting a sun that's part of a binary system necessitates that the two suns are a good distance apart. How far apart? Well, there's a lot of math involved, but once it is all said and done it works out that the second sun must be at least five times the distance from the planet to the primary sun.

For example, Alpha Centauri's two main suns are 11 AU (Astronomical Unit: The distance from the Earth to the Sun, about 93 million miles) apart from each other on their closest approach. Meaning that a planet could be orbiting one of the suns as long as it was less than 2 AU from it. So, the Earth could exist in an orbit about Alpha Centauri A or B.

But, as viewed from the surface, the second sun would look a hundred times smaller and fainter than the primary. This is because that while the Earth is one AU from the primary sun, it would be ten AU from the secondary, and light drops off in brightness at the square of the distance (10 x 10 = 100). This is still much brighter than our moon, even during a full moon, and would lend an eerie orange glow to the landscape. The third sun, Proxima Centauri, would just be a little red dot, as its distance is 13,000 AU from the two pairs, and its less bright to begin with.

So, as a fiction writer, binaries, trinaries, and more are certainly possible, but one needs to take into account that the planet will be circling a primary sun, and all the other suns will be progressively further out, by a factor of at least five.

But, Professor Thule, what if the second sun is a giant star, you ask? That's certainly another possibility. Suns come in many sizes, from the little red dwarf to the mighty blue super-giant. This implies a chance where a planet circles a medium-yellow star like our sun, but then have a second sun that is a giant, and while farther away, is many times bigger, thereby making it appear as if the two suns are equal in the sky. Now, giant stars have giant gravity wells, and our ballpark factor of five becomes a factor of ten or even twenty.

We can try this using Betelgeuse, a red giant some 400 light years from our solar system. It's a huge sun, 270 million miles across, big enough that if was dropped down in our system the Earth would be inside of it!

Assuming, for the moment, that one put our sun in orbit about Betelgeuse, could you get a double sunset? Yes. Betelgeuse is about 13,000 times brighter than our sun. So, if it was a bit more than a hundred AU from Earth, it would look as bright as our sun. So we have a factor of 100, more than enough to insure that the planet doesn't get pulled apart from the tidal effects of the second sun. Note that the Earth would now be receiving twice as much solar energy, and would therefore get much hotter. So, as a fantasy writer, you'd have to back off both suns a bit so that the total energy is about the same as one sun.

The problem, however, is that giants and super-giants have a tendency to be unstable. Meaning they often change in brightness, or go supernova into oblivion. This would be devastating for a planet's inhabitants.

In summary, a world can be part of a double sun system. In most cases, the second sun would be a dimmer, smaller sun to the primary. But, in a special case, both suns could appear to be the same size.

World Building 101: Today's Topic: Tilt a World!

Our Earth is tilted at a 23.5-degree angle in its orbit about the sun. We spin like a tilted top and this accounts for our seasons. When the northern hemisphere is tilted away from the sun it’s winter up there. Six months later it’s summer because the tilt is now towards the sun. In the southern hemisphere, it’s exactly six months different: When it’s summer in the United States, it’s winter in Australia.

Some assume that during the winter the sun is farther away, and in the summer it’s closer. But that has very little impact on the seasons. The Earth’s orbit is only slightly elliptical. It varies from about 92 to 94 million miles away from the sun.

This tilt of ours creates a wide temperate zone where we get four distinct seasons: Winter, Spring, Summer, and Fall. A larger tilt would create a harsher difference between a cold winter and a hot summer. A smaller tilt, or no tilt, would pretty well eliminate the seasons. If, however, the planet had a large elliptical orbit, then that would create seasons that would affect the whole planet at the same time in the planet’s year.

The planets in the solar system vary considerably in tilted-ness. Mercury isn’t tilted at all, while Uranus is at 82 degrees (almost lying on its side), and Mars is very similar to Earth at 25 degrees.

So a fantasy world could really have any tilt you’d like, and an eccentricity (a measure of elliptical-ness) greater than Earth’s. The more tilt you give it, the more extreme the temperatures are going to be. If you had a planetary tilt like Uranus’s, then for half the year you wouldn’t see the sun at all. It would be a very cold, very long, night. Then the sun would start to peek on the horizon, making little circles in the sky. As the year wore on, the sun’s little circle would get higher and higher in the sky, and then finally set on the opposite horizon. The summer would be one long, very hot, day. Such extremes would beg the question of how plants and animals would survive. Presumably, life would find a way.

The tilt of a planet slowly rotates about itself like a tilted top… very slowly. It is called the Precession of the Equinoxes, and for Earth, the loop takes 26 thousand years. This moves the seasons slowly backward in the calendar year.

All of this can play heavily in your fantasy world building. From the tilt of the planet and the eccentricity of its orbit you can have massive winters and unrelenting summers, or mild, practically non-exist seasons.

And as for the philosophical and religious implications of a tilted planet, just look at how the Mayan’s viewed it all.

Related to tilt, we have spin. A planet spins (rotates) on its axis as it orbits the sun. The rotation, over time, will slow down. The Earth’s spin is currently at about 365 spins per orbit, giving us our 365-day year. A planet could spin faster, or slower. If it slows down completely then it will rotate exactly once per year, meaning one side (the heavier, or denser side) will face the sun, while the opposite side is forever in the dark.

This allows your fantasy world to have very short days, or very long ones. A planet can even spin in the opposite direction to Earth’s, meaning the sun would rise in the West and set in the East. (Of course, if the denizens of your fantasy world know nothing of Earth, then ‘opposite’ of it means nothing. The sun merely rises and sets.)

World Building 101: Today’s Topic: Darn Those Constellations!

Look up on a clear night and you’ll see a million stars splashed across the sky – provided you’re not in the middle of a large city. But even in those light-polluted boroughs, you should be able to pick out the Big Dipper (Ursa Major) or Scorpio.

In a fantasy realm the constellations will be completely different (unless that realm is a mirror-universe of this one, of course). Your world will be sitting somewhere else, or somewhen else. That means the view at night of the neighboring universe will look different.

Our night sky has been mapped into 88 different regions. Each houses a collection of stars that are together in the sky – or at least apparently together. The only problem is, this map was drawn as if all the stars were the same distance, as if there was a celestial sphere about the Earth – a dome of stars. But we know that while some stars are close, others are very far away. It’s like looking at a tank of glowing fish, some may look like a group, but if you walked around to the side, they might be all spread out. Also, like fish, stars move. Some are moving away from us, others towards us, and many are moving across the sky. The movement is relatively slow, in that it takes hundreds, or even thousands of years before the movement is obvious. If your world is the Earth a million years ago, or a million years hence, the arrangement will have changed.

These regions – constellations – were mostly named in ancient times. Today we still retain many of the names used by Western Civilization. Orion, Draco, Cassiopeia, Leo, and so forth were so named thousands of years ago. Some are newer: Little groups of stars that didn’t really fit in the main constellations. Musca (the Fly) is a good example. (Here's a good source list of constellations, if you are curious.)

In a fantasy world, what will the characters see when they stare up at the night sky? Well, it all depends. If the world is in a galactic arm of a spiral galaxy, very likely they’ll see a sky much like our own, except all the stars will be scattered across it in its own unique way. Likely, your character’s social history included a similar connect-the-dots sort of naming convention. Then, too, your character will see constellations in the sky, but named after his or her own mythic or real heroes of the past. It has been mankind’s tendency to see the divine in star patterns, whether gods, heroes, or mythic animals. Your world’s beings may be similar, or completely different.

But perhaps your world isn’t located in the outer neighborhood of a nice spiral galaxy. Maybe yours is closer to the center, or in a small, dense cluster of stars, or in the middle of a gaseous nebula, or a dark dust cloud. These conditions would cause the sky to look very different, very different indeed.

In a dense-star neighborhood, the sky would be filled with brightness. There would be so many shining stars that it would be brighter than our own full moon. Some stars could be close enough to be shining beacons unto themselves. Which could be true even in less populated areas of the universe. But I'll talk more about close stars on another day...

Or, put your system in a gaseous nebula, and the sky would be aglow with wispy gossamer trails and whorls of faint light. Like some eerie abstract watercolor, the night sky could be a surrealistic artist’s dream.

One could combine the two, for many dense star clusters are full of gas and dust nebula.

And lastly, the world might be isolated in a dark cloud, an area of space filled with dust. Though there may only be one particle per cubic yard, if it goes on for ten light years, that’s enough to block out all neighboring light. The world’s night sky would be total darkness. There would be no constellations to name.

Choosing your night sky, and any constellations, will help to reveal the history of your fantasy culture. A sky full of animals will indicate a society where animals are highly revered. A night sky of war heroes, gods, and monsters tells us something else.

Constellations were important to the early navigators. The stars were used to tell the position of ships in the night, and to confirm the seasons of the year. And of course, constellations have been used to predict the future behavior of people (astrology). The twelve signs of the zodiac are the twelve constellations that the sun passes through during the year (It doesn’t literally pass through, it’s all relative. The Earth moves around the sun, which means the background of stars behind the sun changes throughout the year.)

Images from Portraits of Stars and their Constellations and Astronomy Picture of the Day.

World Building 101, Today's Topic: Rings are cool!

Saturn, the second largest planet in our solar system, has a spectacular ring system. Beautiful icy rings that hug the gas planet's equator. So, as a fantasy writer wanting a cool world, rings might be just the thing!

Actually, all four of our solar system's large slushy planets have rings: besides Saturn, Jupiter, Neptune, and yes, even the pun-inducing Uranus has rings (nine dark ones!). So, could our smaller, terrestrial planets have rings? Sure, but it's less likely. Mars might one day end up with rings, as its inner moon is right on the edge of something called the Roche Limit. More about that later...

Rings are formed when dirt, dust, and ice gets captured by a planet's gravity. If the stuff is outside of the Roche Limit it will slowly gather together like a snowball due to its own gravitational pull and eventually form a moon. If the stuff is inside the limit it will either fall to the planet's surface, or continue to orbit planet. The stuff can't collect into a ball to form a moon because the tidal forces of the planet pulls it apart... The explanation is a bit technical, but basically, it's because closer objects have to spin around a planet faster than farther items... So, the outer part of a moon moves slower than the inner part. If the moon is far enough away the tidal force isn't enough to break it apart. But if its close in then the inner part has to move so much faster than the outer that it literally flies apart.

So, if enough debris collects around a planet and orbits it inside the Roche Limit, a ring will form. Over time, rings will form a nice flat disc high above the planet's equator (See this explanation as to why).

The four inner planets of our solar system (the terrestrial or 'rocky' planets) don't have rings mostly because the four outer giants act as vacuum cleaners, sweeping up a lot of the debris as it falls in on the sun's gravity well. And then there's the sun itself, which pulls rather mightily on all this dust and ice. Also, our own moon is quiet large for our size of planet, so it too sweeps up debris that might otherwise fall into an orbit about the Earth, given the chance.

A fantasy world could have a ring. All it takes is enough dirt and ice to collect about the planet. It would also help to have a solar system with fewer large planets in the outer orbits, thereby reducing the amount of clean-up that occurs.

A ring can be bright or dim, gaseous or rough, banded or smooth. It all depends on what it's made up of, and whether there are any 'shepherds'. A Shepard is a large hunk of dense rock -- a tiny moon really, that is orbiting inside a ring. Since it is dense the tidal forces don't pull it apart, and it acts like a little broom, sweeping up the debris in its orbital path. These Shepard moons give a ring two or more distinctive bands (You can read about Saturn's Shepard moons here). Bright rings are composed of ice particles, dark ones of rocky debris. High iron content will give it a reddish color, while other more rare metals can give it a blue (cobalt) or green (copper) sheen. And the planet's magnetic field can have an effect on the metallic rocks as well, moving then into positions in the ring that give rise to faint dark spokes and braided banding.

Millions of ice particles in the ring can refract and diffuse the sun's light, making the ring scintillate with color. Beautiful to behold indeed!

But since a ring will form about a planet's equatorial belt, it will look differently viewed from the planet's surface, depending on what latitude (north-south) position you are at. If you are right on the equator, you'd look up to see a thin line across the middle of the sky, neatly dividing it in two east to west. Rings are quite thin, usually only a few kilometers thick. Meaning that looking at it edge-on from a thousand or so miles away would make it look like a fine demarcation, or even disappear entirely, depending upon just how dense the ring is.

The further one gets away from the planet's equator, the more of the ring will become visible. In the northern hemisphere the ring will look like a giant band going across the southern part of the sky, east to west. In the southern hemisphere one will see the ring in the northern sky.

If the planet has a tilt like Earth's then the band will rise and fall in the sky throughout the year, and the sun will cross behind the ring twice a year. Depending upon just how large the band is, the sun might spend several months behind it. If it is a mostly an ice crystal ring, then the whole band would be diffused with the sun's rays. A brilliant band across the sky for several hours before dawn to after sunset.

As for moons, your fantasy planet could still have them, they would just have to be outside the Roche Limit. The Earth's Roche Limit is about 40,000 km, and our moon is about 400,000 km, easily beyond the limit.

The Earth does have a ring of sorts. Our man-made satellites and various space trash, orbits the Earth in a band of junk... if enough builds up then we'd have our own visible ring! Read more about space trash here.

That finishes my lecture on World Building: Planetary Rings.