n our solar system—the only group of worlds we've had a chance to study up close—the planets fall into a curious pattern: Moving outward from Mercury (the innermost planet), each planet is about twice as far away as the one before it. In other words, as long as you're willing to count the asteroid belt as a planet, the size of the orbit increases by a power of two each time. Even if the belt isn't a planet, there's increasing evidence that the largest asteroid—Ceres, a spherical rock about as wide as Oregon—has a differentiated interior. That is, its center is made of iron and other heavy metals, with a layer of lighter materials like silicon resting on top. If true, this would qualify Ceres as a planet, albeit a small and inhospitable one.
This orderly spacing was popularized by German astronomer Johann Bode in 1772, who swiped it from his colleague Johann Titius, who may have based it on the even earlier observations of Christian Wolff, circa 1724. At first the "Titius-Bode law" seemed like a pleasing coincidence, or possibly a sign of divine influence, but the truth is more complex: The moons of the outer planets are also regularly spaced. They don't follow the Bode law, but they do adhere to its underlying principle of orbital resonance. Basically, this means that heavenly bodies exert a gravitational tug, either pushing and pulling each other into non-interfering orbits or else destabilizing the weaklings so they crash or fly away. We don't know very much about other solar systems yet, but we can expect them to follow these rules to some degree; the spacing of their planets will definitely not be random.
This presents an interesting problem in the search for extraterrestrial life, because liquid water (believed to be a necessary ingredient, at least for life as we know it) can exist only in a narrow band around a star. Too far away and the water all freezes; too close and it boils away into vapor, but in the "Goldilocks zone" between these extremes, conditions are just right. This balance is so delicate that even here on Earth we have significant amounts of water, ice and water vapor all at the same time. Our nearest neighbors, Mars and Venus, are not so lucky. At first glance, given Titius-Bode spacing it seems unlikely that a single star could harbor two Earthlike planets.
Take me out, to the black ...
So what's the deal with the Alliance star system in Joss Whedon's upcoming film Serenity? How exactly do they cram 100 habitable planets into a small enough space that technology barely out of the 1800s can let people travel between them? Since these worlds all have mild climates, we have to assume their orbits are stable and roughly circular. I don't think the science of this mattered much to the creators of Serenity (and of Firefly before it), but I can think of eight reasons that might possibly explain it. Care to have a look?
1: Big Star
If the Alliance sun is a particularly hot star, its water bands will be wider and farther away than in our solar system. In theory this leaves more room for planets, although as you move farther out, the spacing of their orbits also gets larger, which reduces the advantage.
2: Hot First Planet
As our innermost planet, Mercury serves as the ground floor for the Titius-Bode law. If it were closer to the sun, the planets of our solar system would be spaced closer together. This would increase the chance of having more than one planet in the Goldilocks zone, and in the extreme case—with a planet orbiting just above the outer surface of the star itself—there might even be room for several.
3: Double Planets
The Earth's moon is too small to retain an atmosphere, but it isn't hard to imagine a moon being just as big as the planet it orbits. This would allow two Earthlike planets to occupy a space that would otherwise hold just one.
4: Habitable Moons
If a large planet is orbiting in the Goldilocks zone, it could have as many moons as Jupiter or Saturn. If the planet were large enough, there's no reason these couldn't be Earthlike, and if we replaced the large planet with a brown dwarf—a failed star too cold for nuclear fusion, but still hotter than molten iron—we could warm a fleet of moons even outside the Goldilocks zone. When moons are stretched and squeezed by the tidal forces of a large planet, they also tend to be volcanically active, which could help warm their surfaces. Iceland would be as cold as Greenland if not for its many hot springs, but instead it's a comfortable home for over a quarter of a million people.
Closer to the star, a large planet could also act as a sun shade, blocking the noonday heat and allowing its moons, and possibly even neighboring planets, to run cooler (see "Blue Moons for a Distant Jupiter"). This is supported by dialogue in Firefly, where some of the habitable worlds are referred to as planets and some as moons.
5: Convenient Atmospheres
Planets swathed in greenhouse gas could harbor liquid water even if they were technically outside the star's Goldilocks zone. Similarly, planets with thin, dry, dusty atmospheres could stay relatively cool even if they were technically too close. If we swapped the positions of Venus and Mars in our own solar system, for example, we might have three habitable planets instead of one!
6. Protective Coloration
A pale planet reflects more light (i.e., absorbs less heat) than a dark-colored one. If the planets closer to the star were white and the ones farther out were black, Earthly life could survive in a much wider range of orbits.
7: Small Planets
Dense, small planets can hold an Earthlike atmosphere under Earthlike gravity, even if their total mass is smaller. In the extreme case (see "Why Crush the Moon?"), these planets could weigh less than our own moon and still be habitable. This could be very helpful, since lower-mass planets are less likely to interfere with each other gravitationally. For example, the asteroid belt includes over a million small bodies orbiting in a band only ten times wider than our sun's Goldilocks zone. Like Saturn's rings, the asteroids fall into resonance bands of their own where they move in relative safety, neither crashing together nor being pulled out of their orbits. In principle, we could scale that up to include a smaller number of larger worlds.
Given the cozy, almost claustrophobic feel of some of the planets in the TV series, this explanation seems particularly likely. These worlds don't seem nearly as large or diverse as Earth.
8: All of the Above!
A hundred worlds is a lot of worlds, and there isn't a single one of these factors that'll get us there by itself. But a hot star with a lot of small, dense, carefully designed planets (the inner one orbiting very closely indeed), plus a gas giant orbiting inside the Goldilocks zone with a flock of dusty, arid moons, plus a brown dwarf orbiting in the cold and dark with some greenhouse moons of its own ...
That just might work.
Of course, the odds of such a thing occurring naturally are slim indeed, and the chance that all 100 planets, when terraformed, would end up looking exactly like California is ... well, zero. Whether Firefly lore admits it or not, the Alliance system has got to be an alien artifact, left behind by Nivenesque Ringworld Engineers for their own obscure purposes. I guess you can't take the sky from those guys, either.
Sources:
Wikipedia: ("Titius-Bode Law"): en.wikipedia.org
Roger Bate, Donald D. Mueller and Jerry E. White: Fundamentals of Astrodynamics, Dover Publications, Inc., 1971
Beatty, J. Kelly et. al.: The New Solar System, Sky Publishing Corporation, 1981
The Encyclopedia Britannica Ultimate Reference Suite, 2004 Edition ("Iceland")
Gillett, Stephen L.: World-Building, Writer's Digest Books, 1996
Wil McCarthy is a rocket guidance engineer, robot designer, nanotechnologist, science-fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots and some other "really cool stuff" he can't tell us about. His short writings have graced the pages of Analog, Asimov's, Wired, Nature and other major publications, and his book-length works include the New York Times notable Bloom, Amazon "Best of Y2K" The Collapsium and most recently, To Crush the Moonn. His acclaimed nonfiction book, Hacking Matter, is now available in paperback.
