World Builder: a modest program for designing strange new worlds. Stephen Kimmel.

World Builder: A Modest Program for Designing Strange New Worlds

Poluxxy Araxxanas raised his head and looked at the dismembered corpse of his foe with nictitating eyes. He clacked the chitin of his mouth opening and raised his four arms to the huge red sun. The sound tied in his throat uncarried by the thin air. On this cold day, victory was his. And the taste of it was bitter.

That is what computers and science fiction are about.

Both computers and science fiction offer the possibility of exploring worlds and futures that could never be explored in reality. Or which would be instantly fatal if you could. Anyone who uses the computer strictly as a means of freeing valuable time to work more efficiently is missing part of what computers can do. World Builder for the TRS-80 is designed to help bridge the gap and perhaps enrich both science fiction and computers.

Why a World Builder?

As a science fiction writer, I have an obligation to my readers to design consistent, believable worlds. This can be a tedious process. There is always the temptation simply to change the color of chlorophyll from green to blue and let it go at that. Giving into temptation results in weaker science fiction so I designed World Builder to help out. After seeing two recent articles in Creative dealing with astronomy, I decided to modify it for general use.

World Builder is of interest even to those who don't read science fiction. It has possibilities for use as an educational science program. Consider it a game, if nothing else. Your first goal is to design a planet on which life can occur. That is hard enough. The second goal is even harder: create a world that could be inhabited by humans. You will find that the parameters for life in general and humanity in particular are tight.

Most of what follows is rather technical. I won't be upset if you wish to skip it. I will, however, suggest that you read Planets for Man by Stephen Dole and Isaac Asimov. It is easy reading and fun. Much of this program is based on that book and on How to Build a Planet by Poul Anderson.

In the Beginning There Was Light

We begin with a star. The sun of any planetary system is the great thermonuclear engine that drives the system. Everything else depends on it. The star may be so large that its formation consumed all the nearby matter. If that is the case, then there can be no planets. Another possibility is that the star is so small that its gravity won't hold a planet of any appreciable size. Again, our quest for a habitable planet has come to a quick end. When the star dies, it will almost certainly destroy any planet close enough to support life. Therefore we can't have too old a star. The larger stars come to this end at an early age and in a most spectacular way--the super nova.

World Builder has two built-in assumptions concerning the star. First, the program assumes that the star is on the "main sequence' of the Hertzsprung-Russell diagram (a plot of spectral classes vs. the absolute magnitude or luminosity of stars). The 1% of all stars that don't fall on the main sequence are stars being born, which certainly won't have evolved life; stars that are dying, which have destroyed any life that might have evolved; and a few odd stars that we can probably ignore safely. The vast majority of stars settle into a position on the sequence according to their mass and spend most of their lives there. These are the stars that have the potential to host life.

The second assumption in World Builder is that the star either is not a binary star or can be treated as a single star. If the two stars of a binary are close enough, the planet can orbit both of them at once in what will probably be a highly eccentric orbit. Life may be possible, although the planet probably won't be habitable by people due to the extreme seasonal temperature variation. If the binary stars are far enough apart, then the planet could orbit one while feeling little impact from the other.

Consider what life on earth would be like if, instead of having Pluto, we had a small star. Pluto receives so little energy from the sun that its surface temperature is just a few degrees above absolute zero. Likewise, our second star would contribute little energy to us. The result would be a more eccentric orbit and a light in the sky brighter than anything but the sun and the moon. The main effect would be on our mythologies.

World Builder contains data on 36 stars including 17 nearby stars felt to have a reasonable chance (greater than 1% but never more than 6%) of having inhabited planets. The rest of the stars on the list are well known. The complete list includes most of the brightest stars and a few that can't be seen with the unaided eye.

Most of the stars are on the main sequence illustrated in Figure 1.

The most significant exceptions are the giants, Rigel, Deneb, Betelgeuse, Antares, Capella, Aldebaran, Arcturus, and Pollux. Obviously this doesn't begin to exhaust the eighty gajillion stars, so the program has the option of using stars not on the list. To use a star not on the list requires the use of either the spectral class of the star or its absolute magnitude.

The Business of Designing a Planet

Once we have settled on a star, we can get down to some of the more immediate matters. Planets come in all sizes and locations. Our own solar system has an interesting assortment of them. It should be said that the science isn't precise. Much of what this program does is speculation and approximation. We have only visited four bodies that could be considered planets. The others we have either studied from Earth or from "close-ups' of several thousand miles. At the range of some of these close-ups, we might be able to resolve objects as small as New Jersey. There would be uncertainty about whether Earth was inhabited by intelligent beings. Come to think of it, I know some people who have the same doubts about Oklahoma.

After settling on a star, the most important discretionary item available to us as planet builders is the average temperature of the planet. Temperature is probably the single most important factor in determining habitability. It also sets the approximate radius of the orbit of the planet about its sun. That, in turn, determines the length of the planet year, and how big the star will appear from the planet.

The size and location of any moons and the sun will determine how rapidly the planet rotates. We have a 24-hour day because the tidal forces of our moon have slowed the Earth down to that point. Jupiter, whose moons are much smaller in relation to it than our moon is to us, has a day that is less than 10 hours long. Jupiter is also much farther from the sun, so its tidal forces have little effect.

The rotation of Venus, despite its having no moon, has been essentially stopped by tidal forces caused by the sun. The importance of this to the temperature of the planet is great. The dark side of Mercury gets down to -380~F, while the temperature on the sun side gets over 600~F. A planet with a shorter day will have smaller variations in its high and low temperature.

The tilt of the planet on its axis also has a profound impact on the temperature of the planet. As every school child knows, winter occurs when the portion of the Earth we happen to be on is tilted away from the sun. Earth has an axis tilt of nearly 24 degrees. If the tilt were less, then summer and winter would be more alike. A greater tilt would make a hotter summer and a colder winter.

The final factor contributing to the temperature profile of the planet is its orbital eccentricity. Planets do not travel in perfect circles but in ellipses. Eccentricity is a measure of the deviation from a perfect circle. It determines the perihelion (the point of closest approach to the star) and aphelion (the point of greatest distance from the star). The closer the planet is to the star the warmer it is; the further away, the colder.

On Earth, which has a small eccentricity, the variation is a few degrees. Summers are slightly warmer and winters slightly colder south of the equator. If we had an eccentricity as high as that of Pluto, the difference would be substantial.

The gravity of the planet is the last important characteristic that we are free to set. This contributes significantly to determining the nature of the atmosphere and the nature of life on the planet. If there is life on Jupiter, and some scientists are serious about the possibility, then it certainly won't look human. And it won't be able to survive on Earth.

Gravity directly affects the makeup of the atmosphere. If the gravity is too high, the planet won't lose its primordial hydrogen; it will be a gas giant. If the gravity is too low, the planet won't be able to hold onto its oxygen in gaseous form. The result is a planet with an extremely thin atmosphere similar to that found on the moon and on Mars.

World Builder does not consider the chemical composition of the atmosphere, and that is probably a major weakness of the program. World Builder would predict for Venus an atmosphere slightly thinner than that of Earth. That is not the case. Venus has a massive atmosphere with a composition radically different from Earth's, due in part at least to high concentrations of carbon dioxide. In defense of World Builder it must be said that Venus could be terraformed into retaining an Earth-like atmosphere.

The Question of Life

For purposes of this program, I assume that life is a carbon-based chemistry that requires liquid water in its environment. There are several other possibilities although we currently have no proof that any of them exist. Perhaps the most likely are those which don't require liquid water, either converting ice or water vapor to water in their bodies. However, I don't see how such life could evolve without a period of liquid water on the planet.

There are four requirements for life in the restricted context of this program. There must be liquid water at least some of the time. The atmosphere must contain some oxygen. The star must not have died. And there must be enough time for life to evolve.

What will that life be like? That is the question. To some extent, we expect it to look familiar. Function pushes evolution along the same lines. Thus dolphins, goldfish and sharks have similar form despite having radically different heritages. A classic example is bats and birds. We should be able to recognize fish and birds as such no matter what planet they come from.

Land creatures offer a much wider range of variation. No one can say that every possible combination has been tried on Earth. Some other arrangements might be highly successful in other environments. Still we should be able to recognize eyes, legs and mouths. Hands may be a different story, although our design should work in almost any environment.

Gravity is the primary factor affecting life that is dealt with in World Builder. High gravity makes it harder to stand up. Anything that isn't hugging the ground will be more heavily reinforced than it would be on Earth. Bodies and trees will be thicker and shorter. Rising up on two legs may be impossible.

Low gravity would have the opposite. effect. With less pull to overcome, rising on two legs is easier. Creatures need less reinforcement to overcome gravity. The air should be thinner, reducing the effect of the wind and the ability to carry sounds. Beings in a thick, high gravity atmosphere would place a premium on a streamlined form. Creatures in a low gravity, thin atmosphere won't consider this such an advantage.

And humans? Will they be able to live on the planet? I have taken the restrictions to be those shown in Figure 2. One requirement is for other life. Without an existing biosphere with all its delicate balances and support systems, we would be living in an artificial environment. Without photosynthesis to regenerate oxygen, we would have to generate it ourselves. Again, there is little point in leaving the relative comfort and safety of our space craft if we must rebuild it on the surface of the planet. Of course, you might terraform an otherwise uninhabitable planet, but that is a matter beyond the scope of this program.

Thoughts on Earth

At first glance, Earth appears to have a unique position in the cosmos. You couldn't adjust the parameters of World Builder too much without making it uninhabitable. This really doesn't come as a surprise. Of the four bodies we have visited, Earth, Mars, Venus, and the moon, only Earth has what is unquestionably life.

There probably is no life anywhere else in our solar system. The only other candidates are long shots. Isolated pockets on Mars and the clouds of Jupiter don't seem particularly promising and Saturn's moon, Titan, is only slightly better. If the assumptions of this program are correct, we have no better than a fifty-fifty chance of finding a habitable planet within 25 light years of Earth.

Earth is the nearly perfect home for humanity but not because it was created that way. Humanity was created, by whatever means, to be perfectly at home on Earth. We grew up here. We wouldn't expect to do any better on most other planets than we would expect a sparrow to do under water. If we had grown up on Titan, a -200~F methane pool would seem perfect for a hot summer's swim.

Is Earth the sole harbor of life in the black sea of space? I can't believe that it is. There are billions and billions of stars (sorry, Carl) out there and at least an equal number of planets. If there is no life anywhere else in the universe then that is a waste unparalleled by anything in creation. The probability that Earth is the exclusive home for life is well over a billion to one. It might be, but I wouldn't bet on it.

Table: Figure 1. The Hertzsprung-Russell Diagram.

Table: Figure 2. Requirements for human habitability.

Table: World Builder Listing

Table: World Builder sample run.