Just The Caffeine Talking

My WorldCon Schedule

In two and a half weeks I'll be in Chicago for ChiCon 8, the 80th World Science Fiction Convention. I offered to participate in programming events, and they're really making me sing for my supper this time.

Thursday, September 1

(all times are CDT)

2:30 P.M.: Writing and Story Development for Tabletop Games. This is a game design panel, obviously, focusing on fitting story to the game system and vice versa.

4:00 P.M.: Virtual Reading. I'll be reading from my forthcoming novel The Scarab Mission via video, so anyone in the world can watch.

5:30 P.M.: Science: The Core of Science Fiction's Sense of Wonder. This panel grapples with the central issue of why and how you can have a whole literary genre based on stories about science.

7:00 P.M.: Stroll With the Stars. Come for a walk with me and some other writers around downtown Chicago. I'll point out some of my favorite buildings and discuss how Chicago influenced my fiction.

Friday, September 2

10:00 A.M.: Really Big Things. A panel on megastructures. I've used some big ones in my fiction, but how big can structures really get?

1:00 P.M.: Table Talk. What used to be called a Kaffeklatsch or a Literary Beer, but now with masks on. I won't tattle if anyone cheats.

4:00 P.M.: March of Time. A virtual panel about the ways SF gets overtaken by real-world discoveries, and how to avoid it.

Saturday, September 3

11:30 A.M.: Re-Engineering the Solar System. A virtual panel discussing how humans — or our successors, whatever they may be — may rebuild the Solar System. 

2:30 P.M.: Tropes as Tools. Tropes aren't the same as cliches, nor are they something one must necessarily avoid. This panel will talk about how to use tropes intelligently to make better fiction.

4:00 P.M.: Autographing. I'll be signing books, bookplates, magazines, or anything my pen can make a mark on.

Sunday, September 4

11:30 A.M.: Science in Science Fiction: The Guesswork of 1946. The panel will look at how writers in 1946 envisioned the future, and try to understand why they made the assumptions and guesses they did. 

2:30 P.M.: The Science and Fiction of Droids. A combination science and literary panel about the long history of robots and AI in science fiction, and how everybody gets it wrong including me.

Whew! During the few moments I can snatch when I won't be doing convention programming, I plan to look up friends, relations and familiar sights in Chicago; satisfy my cravings for deep-dish pizza, proper hot dogs, fried chicken, and other Windy City delights; and hang out in the con suite or the hotel bar to see all the people I've missed for the past three years.

See you all in Chicago!

 

 

 

Notes on Notes on Worldbuilding

I've updated the entry Part 8: Planetary Characteristics to include a discussion of surface pressure, partial pressure of gases, and how that might affect habitability by humans. Thanks to Jim Torkelson for pointing out that I'd neglected that topic.

Notes on Worldbuilding Part 10: Weird Worlds

Most of the planets we've been talking about are analogous to the worlds of our own Solar System. But planetary scientists have come up with some planet types which might exist circling other stars. Small variations in element abundances when a system forms can lead to very exotic worlds.

Carbon Planets: Carbon is one of the most abundant elements in the universe — far more abundant than silicon or iron. Yet most of the solid planets in our Solar System are made of rock and metal, not carbon. What if a planet were to form with more carbon and less oxygen than Earth? It would have to be almost waterless, which suggests the most likely place to find a carbon planet would be close to its parent star.

The core would be steel (iron plus carbon), with a mantle of graphite or silicon-carbon compounds, and continents of diamond on the surface. The atmosphere would be pure carbon dioxide or carbon monoxide (and perhaps some cyanide compounds).

Here's the Wikipedia entry, which is about the most informative article I can find online: https://en.wikipedia.org/wiki/Carbon_planet

Chthonian Planets: A gas giant that migrates close to its parent star is going to lose atmosphere as diatomic hydrogen molecules get broken into monatomic hydrogen, and the extreme heat boils off those atoms. As the planet loses atmosphere, it loses mass, and thus its gravity decreases — which means the rate of hydrogen loss will increase.

The end result of this kind of runaway boil-off is the gas giant's dense core left exposed. ("Chthonic" or "Chthonian" has nothing to do with H.P. Lovecraft. It's the ancient Greek term for "Underworld" or "Earth-born" deities, as opposed to the gods of sky and air.) Essentially it's a "Super-Earth" or "Mega-Earth" with very high density, up to 1.5 times Earth's density. Such a world would be very hot, very dry, with massive gravity and an atmosphere of very dense gases (carbon dioxide, sulfur dioxide, and possibly things like mercury and sodium vapor).

As above, the Wikipedia entry is a good introduction: https://en.wikipedia.org/wiki/Chthonian_planet

Coreless Planets: Earth has a crunchy rocky crust and a chewy molten iron center. But if a planet were to form from oxygen-rich planetesimals so that all its iron is bound up in iron oxide compounds, the planet's interior would not differentiate into a metal core and a rock-metal mantle. Instead the interior would be a kind of uniform goo of hot iron-rich rock.

What would this mean in practice? No magnetic field, for starters. And that means a thinner atmosphere as more radiation boils off the lighter gases. It might also mean slower rotation, as the mass would be more uniformly distributed, rather than having a dense core. (I think. If anyone with a better understanding of physics can correct me, please do so.)

It would also mean less tectonic activity (again: I think) as there wouldn't be as much convection in the interior. Less tectonic activity means more elements like carbon and hydrogen get bound up in the crust. So you get a slow-turning, mostly flat world with a thin atmosphere and little or no water on the surface.

Gas Dwarf Planets: A "Gas Dwarf" or "Mini-Neptune" would be a planet with a mass of 5 or more Earths, with a dense rock-metal core, a very deep ocean of water or water mixed with ammonia, and a thick atmosphere of methane, ammonia, nitrogen, and diatomic hydrogen. Such a world would probably form in the outer system, though it might be right at the outer edge of the Goldilocks Zone, if it's big enough to hang on to a massive atmosphere. In the Goldilocks Zone a Gas Dwarf turns into an Ocean World (see below).

A Gas Dwarf would have low density (roughly the equivalent of water, about 1/5 that of Earth) and relatively low gravity at the cloud tops. The combination of a dense atmosphere and low gravity means it would be a perfect environment for flying.

Here's a paper about the idea: https://astrobites.org/2019/12/17/why-are-there-so-many-sub-neptune-exoplanets/

Iron Planets: A planet which forms from metal-rich planetesimals would have a higher iron content than Earth. The planet Mercury is an example of this: it's smaller than Mars but it's got a density more in line with bigger planets like Earth or Venus. Now imagine a Mercury with the mass of Earth, or even more.

An Iron planet would have high density, up to 1.5 times that of Earth, and thus high surface gravity. The abundant iron would soak up any free oxygen, so the surface would look rusty-red like that of Mars.

Ocean Worlds: Earth's oceans cover about three-quarters of the surface, but it's actually kind of remarkable that there's any dry land at all. Double the amount of water on the surface and Earth would have just a few islands. Double it again and there'd be nothing but water.

Now imagine an Earth which could boast thousands of times more water, like a small ice giant but warm enough for liquid oceans. The sea would be hundreds of kilometers deep, with absolutely no land at all. The planet's overall density would be lower (in the 0.5 to 0.75 Earth density range), which means an ocean planet could be substantially bigger than Earth with comparable gravity. Ocean worlds would likely have pretty dense atmospheres, and the enormous amounts of water vapor mean it would be best to stick them near the outer edge of the Goldilocks Zone to avoid a runaway greenhouse effect.

An ocean world would have distinct layers in its immense hydrosphere. The deep layer would start about ten kilometers down and run all the way to the solid seabed. Most of this water would be stagnant and oxygen-poor, with a uniform temperature and little convection. Above that would be the more active ocean layer, just a couple of kilometers deep, where life might thrive.

With no land at all, ocean worlds might boast long-duration storm cells, hurricanes lasting for decades, circling the globe in the warm latitudes. The seas would be fresh or slightly brackish water, as there just isn't enough sodium in the crust to make them salty.

Notes on Worldbuilding Part 9: Moons!

I'm back! Let's talk about moons now.

Most planets have moons, ranging from little chunks of ice or rock to planet-sized bodies like Triton or Titan. Moons can affect their primary planet — and sufficiently large moons of a giant planet in the Goldilocks Zone might be habitable worlds in their own right.

How Big? Studies of the moons of the Solar System suggest that the upper limit for a natural moon's mass might be comparable to that of Mars, or 0.1 Earth — but observations of potential "exomoons" circling planets of other stars (we couldn't write sentences like that back in the 20th Century) suggest that moons could be Earth-sized or even massive "Super-Earth" size. So a "forest moon" like Endor in the Star Wars series is perfectly plausible.

Looking at the moons of the Solar System, it seems as though there's a sharp distinction between the rocky worlds of the inner system and the big gas and ice giants of the outer system. Within the Asteroid Belt, moons are scarce, and only Earth has a really large one. Beyond the asteroids, every planet has at least one big moon, and most have several, accompanied by swarms of tiny moonlets.

(More and more I find it useful to consider the inner Solar System as the Sun's "moons," with the sprawling moon systems of the outer giants as separate entities.)

Anyway, if you want to select randomly, I suggest giving planets in the inner zone just one or two moons each, and those should be small. If you're generating them randomly, roll 1 six-sided die and subtract 3 for the number of moons. For size, roll a 10-sided die: on a 1-6 the moon is a tiny asteroidal moonlet like Phobos or Deimos, with a diameter less than 50 kilometers. On a 7-8 it's a sizeable asteroid with a diameter up to 100 kilometers. On a result of 9 it's a big asteroid, 100 to 200 kilometers across; and on a 10 it's a giant moon. Giant moons have a mass of several percent of the parent planet: roll a 6-sided die and multiply by 1/100 of the planet's mass.

True double planets may exist. So far we haven't really seen any, but there's no reason why they can't happen. Assign a 1 in 100 chance that an inner-system world could be a double planet, in which case just give the planet a companion with a mass equal to a 10-sided die roll times 10 percent of the planet you've already got.

In the Goldilocks zone and beyond, rocky worlds can have more moons. Roll a six-sided die and subtract 2 for the number of moons. Determine size as above.

Ice Giants and Gas Giants all seem to have large families of moons. For either type, roll a 10-sided die for the number of large moons, and a 20-sided die for the number of small ones. If the result is 20, roll two dice and take the sum.

Large moons of giant worlds are basically icy dwarf planets. In the Solar System the outer moons have masses ranging from less than 1/1000 Earths to about 1/50. But it's certainly possible to have much bigger moons.

Determine the mass of a major moon (big enough to treat as a planet for our purposes) using the "exploding die" method employed by some roleplaying games. Most moons have a mass (in Earths) of one 10-sided die times 1/1000 Earth mass; if the die roll is a 10, reroll and multiply by 1/100 Earth mass instead; and if that roll comes up 10, reroll again and multiply by 1/10 Earths. If that roll is also a 10, then roll a six-sided die and divide by 2 to get the mass in Earths.

This is likely to be a long and tedious chore, so only do it if you have a reason to. If you've got a giant planet in the Goldilocks Zone, then obviously it's important to see if any of its moons might be habitable. (And remember: you're creating this star system — you can make them big enough if you want!)

How Far? As to orbital distance around the primary planet, moons can't form at an orbital distance less than about 3 planetary radii from the planet's center. This is due to tidal forces and is called the Roche Limit (https://space.fandom.com/wiki/Roche_limit). Within that limit you don't get a moon, you get a ring system. In the Solar System, most of the big moons huddle fairly close to the primary, typically between about twice the Roche Limit out to a couple of million kilometers away.

Beyond that the giant planets have a mess of small moons the size of asteroids or comets, most of them probably captured bodies. Many have highly tilted orbits, often quite eccentric as well. Jupiter and Saturn each have a family of retrograde moons orbiting clockwise.

If you really want to generate these distances randomly, start at the Roche limit. The first moon will be a six-sided die times the Roche distance. Each additional moon's orbit will be a distance equal to 10 percent times a six-sided die greater than the previous moon.

For a rocky world like Earth, it's simpler to just roll a 100-sided die and multiply by 10,000 kilometers.

Effects: Large moons seem to slow the rotation of the parent planet. For a big world like the giants of the outer Solar System, the effect is negligible. But Earth apparently rotated faster in its early history. If the planet has a big moon, maybe double or triple its day length. And if the moon's mass is bigger than 10 percent of the parent planet's mass, then assume the two worlds are tidally locked, each presenting the same face to each other eternally.

Next time: Weird Worlds.

Not Lost in the Cosmos

For the next couple of weeks I'm going to be conducting a science fiction writing workshop for the Smith College summer Precollege Program. This means blogging will be very slow, and may not resume until the course is over. I haven't forgotten about the worldbuilding project. Have patience!

Notes on Worldbuilding, Part 8: Planetary Characteristics

This is where we figure out the details that actually determine what it's like on the planet. Some of these can be figured pretty precisely, others are up to the creator. (For gamers I've included random generators.)

Size: We know the mass (in Earths), and we've determined the density relative to Earth. Mass divided by density gives us volume (in Earths), and the cube root of volume gives us the planet's radius in Earths. Multiply that by 6,400 to get the planet's radius in kilometers. (Double the radius to get the diameter.)

Some comparisons: Mars's radius is 3400 kilometers, and Venus's is 6000 km. Note that even a "mega-Earth" with a mass of 10 times Earth's own mass would be only about twice Earth's diameter.   

Gravity: This is pretty straightforward. Relative mass (in Earths) divided by the square of radius (in Earth equivalents) gives you the surface gravity in Earth "gees" (often abbreviated g). Multiply that by 9.8 meters per second squared for the actual force, but keeping it in multiples of Earth gravity is probably more useful.

Mars's surface gravity is 0.38 g, Venus's is 0.9, and that hypothetical mega-Earth would have a crushing 2.8 gsurface gravity — about the same as at the cloud tops of Jupiter.

Escape Velocity: This is how fast you have to be going to get off the planet. It's useful to figure out how practical space travel is from that world, and for the atmospheric composition (see below). The simplest way to figure it is to divide the planet's mass (in Earths) by its radius (in Earth radii), take the square root of that, and multiply the result by Earth's escape velocity of 11 kilometers per second. 

Rotation: Planetary rotation is anyone's guess, really. Close to the Sun, tidal forces slow a planet's rotation. Mercury's rotation is locked into a 3:2 resonance with its orbital period (which fooled astronomers on Earth for years into thinking the planet kept one face permanently to the Sun). And there seems to be some vague connection between mass and spin rate: Jupiter's day is only 10 hours long, and Saturn's is about the same. But . . . Earth is ten times the size of Mars and yet they have almost identical day lengths. Meanwhile Venus takes 243 days to turn on its axis, and does it backwards, to boot.

So you can basically pick whatever day length you want. The absolute minimum would be somewhere around 3 or 4 hours for an Earthlike world, which would create a perceptible reduction of surface gravity at the equator due to centrifugal force effects. Slow the rotation in close orbits to some small multiple of the orbital period, and if the planet has a big moon you should aim for something in the 20-30 hour range. (In the case of an actual double planet, or the moon of a bigger planet, rotation and orbital period will be the same. We'll cover that later.)

If  you want to generate rotation randomly, I suggest simply rolling two 20-sided dice to get the day length in hours for an Earth-sized or larger rocky planet. If either die comes up 20, multiply by 10, and if both come up 10, re-roll and multiply the new result by 100.

Axial Tilt: This is another wild card. Most planets have their axis of rotation aligned with their axis of revolution around the Sun, within twenty or thirty degrees, at least. Except . . . Uranus's axis is deflected 90 degrees so it's lying on its side, and Venus seems to be entirely flipped over. Astronomers make vague noises about giant impacts during planet formation, but nobody really knows for sure why these weird exceptions exist.

So for "normal" planets pick an axial tilt in the range of about 20 to 30 degrees. To determine randomly roll a 20-sided die plus 10 degrees. If the die comes up 20, roll two 20-sided dice and multiply the sum by 10 for the tilt in degrees.

Atmosphere: The atmosphere composition depends on gravity, temperature and whether or not life exists on the planet. For now, assume that life is Earth-type life and can only exist in the Goldilocks zone.

I started to work out an enormous overcomplicated spreadsheet to provide data for different gases at different temperatures, but in the course of researching it I found this diagram showing what gases can exist at a given combination of temperature and escape velocity: https://en.wikipedia.org/wiki/File:Solar_system_escape_velocity_vs_surface_temperature.svg#file.

There are a few gases I would add to the list. Hydrogen cyanide (HCN) has approximately the same molar mass as nitrogen, and hydrogen sulfide (H2S) is close to oxygen.

Find the point on the chart where your planet's temperature and escape velocity meet. Any gases above that on the chart will be lost to space.

At low temperatures some gases condense and freeze out. Water freezes at 273 Kelvins, hydrogen cyanide at 260 Kelvins, ammonia at 195, carbon dioxide at 194, hydrogen sulfide at 191, xenon at 160, methane at 90, nitrogen at 63, oxygen at 54, and hydrogen at 14. Higher atmosphere pressures can make this happen at warmer temperatures, but you'll have to work that out on your own. I'm already at the limits of my chemistry knowledge.

The relative abundance of the gases in an atmosphere can vary quite a bit, based on the composition of the stellar nebula when the planet was forming. The simplest rule of thumb is to take the abundance of the elements in the universe as your guide: hydrogen is the most abundant, followed by helium, carbon, nitrogen, oxygen, neon, sulfur, and argon.

Oxygen won't occur as a free gas without life. It's rocket fuel and wants to burn the universe. On a lifeless world it will be locked up in water, carbon dioxide, and in the rocks of the crust.

Effects of Life: If a planet has plants (or plant-like organisms) doing photosynthesis, they may produce oxygen from carbon dioxide and water. As noted above, oxygen plays all too well with others: a planet with oxygen in its atmosphere won't have more than trace amounts of methane, ammonia, hydrogen sulfide, or hydrogen cyanide.

Atmospheric Pressure: There's an important concept in chemistry called "partial pressure." This is the percentage of a given gas in an atmosphere, multiplied by the atmospheric pressure. Earth's atmospheric pressure at sea level is 1 bar (by definition), or 1000 millibars. The atmosphere is 20 percent oxygen, so the partial pressure of oxygen at sea level is 200 millibars. Humans seem to need about 160 millibars of oxygen for normal metabolism. This means we could survive in a lower-pressure atmosphere if its oxygen content was higher, so that the partial pressure comes out the same — or conversely in a higher-pressure environment with a low oxygen concentration.

Pressure is a function of the atmosphere's actual mass, times the planet's surface gravity. The actual quantity of atmosphere around a planet can vary widely — Venus has a much more massive atmosphere than Earth does, even though it's hotter and has little magnetic protection, so its surface pressure is nearly 100 times what you and I are breathing. For rocky planets it does seem to be an all-or-nothing situation: small bodies have no pressure to speak of, Earth has 1 bar, and Venus has 100 bars.

To generate atmosphere pressure for rocky worlds randomly, roll a 6-sided die: 1 means vacuum, 2 means a trace atmosphere, 3 means a thin atmosphere, 4 means a medium atmosphere like Earth, 5 means dense, and 6 means massive. Pressure for a trace atmosphere is 1-10 millibars (use a 10-sided die to determine randomly) times the surface gravity (in Earth gravities). For a thin atmosphere it's 1-100 millibars (use percentile dice) times gravity. For a medium atmosphere, it's 100-1000 millibars times gravity (use a 10-sided die and multiply by 100). For a dense atmosphere it's 1000-10,000 millibars times gravity (roll a 10-sided die times 1000). And for a massive atmosphere it's 10 to 200 bars (roll a 20-sided die and multiply by 10 bars). 

If there's oxygen in the atmosphere, calculate the partial pressure to see if it can support humans and Earthly animal life. Presumably any native species are evolved to fit the local conditions. Note that since pressure drops off at high altitudes, a world with pressures too high at sea level might still have high plateaus with bearable conditions for humans. This involves a concept called "scale height" which is explained here: https://en.wikipedia.org/wiki/Scale_height. Pressure drops logarithmically as you increase altitude (that's the natural log, not log-10).

Next time: Moons!

Notes on Worldbuilding, Part 7: Temperature and Composition

From here on out I'm going to concentrate on solid planets which might be home to life we can interact with. These are bodies with a mass in the range of 0.1 Earth to 10 Earths, though I will include occasional nods to "Mega-Earth" type worlds even bigger than that. My plan is to work my way up the temperature scale to give some ideas about composition, density, and what atmospheric gases the world can retain.

Very Cold: These are planets with a temperature less than 100 Kelvins. They may have a little internal heat if they're big enough, but get almost no energy from the parent star. If they're small — a mass less than 0.1 Earths — they're likely low-density agglomerations of rock and ice. The very small ones may not even be differentiated inside (i.e. no distinct core, mantle, or crust). Density for those little ones will be low, from 0.2 times that of Earth (which is to say, about the density of ice) to perhaps 0.75 Earth (like Mars). Atmosphere will be very thin, a wisp of hydrogen and nitrogen with perhaps some helium.

Bigger worlds comparable to Earth (0.1 to 2 Earths) in that cold region can generate enough internal heat to have a distinct core and interior layers. I'm going to assume they are primarily rock and metal, so density will be close to that of Earth, 0.75 to 1. However, at the big end one might see "baby ice giants" which have swept up a lot of lighter elements and so have a low density of 0.3 or 0.4. Either way, the atmosphere will be mostly hydrogen with significant amounts of helium and nitrogen.

"Super-Earth" sized planets at those low temperatures are simply small ice giants like Uranus and Neptune. Density of 0.3 times that of Earth, with a massive atmosphere of hydrogen and helium, seas of liquid hydrogen, ice below that, then rock, then metal at the very center.

Cold: This is the temperature range from 100 to 200 Kelvins. Tiny planets less than 0.1 Earths are similar to their Very Cold counterparts — rock and ice, low density. The main difference is that methane and carbon dioxide become important atmosphere gases. Rocky worlds in the 0.1 to 2 Earth size can have hydrocarbon atmospheres like Saturn's moon Titan. Densities range from 0.6 to 1 for those worlds. The slightly warmer temperature makes "baby ice giant" planets less likely at this size.

Big "super-Earth" rocky planets in the 2 to 10 Earth size range may exist, with thick atmospheres and deep oceans of liquid methane. They might also have liquid water deep under the surface, melted by heat from the planet's core. Density would be 1 to 1.3 times that of Earth for these giant rocky planets. More likely, however, is that these massive worlds are ice giants with a density more like 0.3.

Chilly: Planets outside the Goldilocks Zone but with temperatures above 200 Kelvins. Ice is still a rock, but atmospheres of carbon dioxide, methane and ammonia are possible. Densities are about the same as above, but there's more options for atmospheric gases — and some of those are decent greenhouse gases so there's the possibility that a planet Earth-sized or larger in this region might be able to claw its way up to the melting point of water, at least some of the time. At the upper end of the size scale they would simply be "warm Neptune" planets with low density.

Goldilocks Zone: When water isn't a rock, planetary density changes abruptly. Having liquid water on a planet means two things. Yes, you might have life. But it also puts the planet at risk of losing its hydrogen. See, water molecules in the upper atmosphere can get broken into their constituent atoms by energetic particles from space. Some fraction of the free hydrogen can then escape the planet into space before it oxidizes again. Over geological time scales this can allow a planet to lose all its hydrogen. That means no water, and it also has knock-on effects on things like continental plate movement and vulcanism. It seems to have happened to Venus, and on Mars until that world lost so much atmosphere it got too cold for liquid water to exist on the surface. A powerful magnetic field like Earth's keeps the energetic particles at bay.

We don't have a very good understanding of how planetary magnetic fields work, but they seem to be generated in the dense metal core. Bigger planets are more likely to have a dense molten core, but it isn't universal. Venus seems like it ought to have a molten core but it doesn't generate much of a magnetic field at all. This may be related to Venus's weird slow rotation. As always, when science isn't sure, that means writers can indulge themselves. Choose or flip a coin to decide if your planet has a magnetic field as powerful as Earth's.

For small rocky bodies less than 0.1 Earths, the Goldilocks zone means no more water. All of it melted and got blown away. In the 0.1 to 1 Earth range, it all depends on how good a magnetic field the planet has. No planetary magnetic field means no water. Hydrogen might remain in heavy molecules like sulfuric acid. A working magnetic field means at least some surface water if the planet's big enough (more on that later). This does mean a higher density: Mercury's density is about the same as Earth's — 25 percent higher than Mars's density even though Mars has twice Mercury's mass. So simply assume all rocky inner-system bodies with a mass of 0.1 Earth or more have about Earth's density.

Bigger bodies, "super-Earth" and even "mega-Earth" sized can almost certainly hang on to their water even without magnetic protection, as their escape velocity is high enough to retain lighter atoms. For them the danger is runaway greenhouse effect due to all that atmosphere.

Hot: Within the inner edge of the Goldilocks zone, small worlds have all their water and volatiles cooked away, and big planets suffer a runaway greenhouse effect as water vapor is a major greenhouse gas. Expect a lot of airless Mercury-type worlds and a lot of Venusian and "super-Venus" planets, all with Earth-type density.

Above about 400 Kelvins, sulfur is a liquid. It's a reasonably common element, so some hot planets might have molten sulfur seas, especially if they have no hydrogen to bond with the sulfur and turn it into sulfuric acid. This raises some interesting possibilities for complex molecules of carbon, silicon and sulfur in a liquid sulfur medium.

A giant "super-Earth" type in the inner zone might be what's known as a Chthonian planet — the inner core of a gas giant which migrated inward but had all its lighter elements boiled away. Some exoplanets are thought to be this category, but we don't know for sure. A Chthonian world can be extremely dense, up to 1.5 times the Earth's density.

Molten: Above about 1000 Kelvins, rocks start to get gooey and planets basically don't have a solid surface any more. At those temperatures all the lighter elements can escape from any world smaller than a brown dwarf. Assume all bodies in that temperature range are made of dense heavy rock and metal with a density up to 1.5 times that of Earth.

Whew! It took a lot more space than I expected just to get the planetary density. Next time we'll really start putting it all together and figure out size, gravity, and maybe take a swing at atmosphere.

Notes on Worldbuilding, Part 6: Placing Planets

The two defining features for a planet are its mass and its temperature. Those data can pretty much predict its likely size, composition, and how suitable it is for life.

Temperature is a function of the world's distance from its primary star, and how bright that star is. By "bright" I mean the star's luminosity, which is a measure of how much energy it puts out. That is distinct from its visual magnitude, which describes how bright it looks as seen from Earth.

I'm not going to re-type data you can find elsewhere, so if you're using a real star, look up its luminosity. If you're making up a star system, look up the type of star you've got — F3, K9, whatever — and pick a value from the known range of luminosity for that kind of star.

To figure the temperature for each orbit around that star, you can do it the easy way or the hard way.

The hard way is to actually compute the black-body temperature based on the incoming energy flux. Read about it here: https://en.wikipedia.org/wiki/Planetary_equilibrium_temperature.

The easy way is to just compare the values for your fictional star system to the Solar System. Divide the star's luminosity (measured in multiples of the Sun's value) by the distance in AU squared. That gives you a value of how much energy the planet gets compared to Earth. Take the fourth root of that (hit the square root button twice) and multiply by 287, which is Earth's average temperature in degrees Kelvin. Then convert back into Celsius by subtracting 273 from the result. (You have to go through the Kelvin degrees step or it won't work properly.)

Note that this is the average temperature across the entire surface, and assumes that the planet reflects about as much energy as Earth does, retains as much heat as Earth does, and has a rotation rate that's roughly comparable. But it's good enough to make rough estimates of things like whether water can exist.

The Goldilocks Zone: Because water is not just important for life but a whole lot of other things, and because it's so common in the Universe, it's important to figure out the "climate zones" in a star system.

The inner zone is the space around the star where a planet's average temperature is too hot for liquid water to exist. Worlds orbiting in that zone won't have oceans, won't have ice, and unless they are very massive they won't have any atmospheric gases containing hydrogen (no hydrogen, no methane, no ammonia, etc.). Since gas giants and brown dwarfs are basically nothing but hydrogen, you're unlikely to have them in the inner zone — unless there's a "hot Jupiter" situation where a giant planet formed in the outer system and migrated inward over time.

I'm going to define the boundary of the inner zone as the distance at which a planet's temperature is 373 Kelvin — the boiling point of water at 1 atmosphere of pressure. A more dense atmosphere with higher pressure might keep water from boiling off — but such an atmosphere will also have a higher greenhouse effect and retain more heat, so I'm just going to let those factors cancel each other out.

Compute the boundary distance using the equation D (distance in AU) = the square root of (Star luminosity divided by 2.85). Orbits within that boundary can only have rocky worlds, except as noted above.   

The "Goldilocks Zone" (or in astro-speak the Circumstellar Habitable Zone) is the belt with temperatures between boiling and freezing. We've already figured the inner boundary, but the outer edge is a bit more fuzzy. A small world like Mars might be too cold at its current distance from the Sun, but if Mars was bigger and had a more dense atmosphere it might retain enough heat for liquid water. And we've seen that moons like Io and Europa orbiting a giant planet can get heat from tidal forces even if they're far from the Sun. So I'm going to figure the outer edge as the distance at which an Earthlike world would be below freezing, but keep in mind that there may be exceptions.

Compute the outer edge of the Goldilocks Zone using the equation D = square root of (Star luminosity divided by 0.82). Planets orbiting beyond that distance may have plenty of water, but most of it will be solid ice.

Mass: The other critical feature for planets is mass. Again I'm going to use Earth equivalents rather than kilograms, because the numbers are a lot handier.

We don't exactly know what determines how much mass a planet can gather up during early formation. Obviously, if you're using a real star system, just use the estimates for planetary mass that professional planetary scientists have worked so hard to come up with for you.

But for an imaginary star system, you're basically free to assign any value you want. The lower end is around the mass of Pluto (0.002 Earth mass), while the upper end is somewhere in the range of brown dwarf ojects (3000 to 16,000 Earth masses). Above that it's a star. Looking at actual exoplanet data, the biggest known exoplanet is a brown dwarf about 30 times more massive than Jupiter, or about 10,000 Earths.

Among planets, as among stars, it's a good rule of thumb to assume that there are a few big massive ones, a larger number of medium-sized ones, and a whole lot of small ones. If we look at the known multiplanetary systems (courtesy of Wikipedia: https://en.wikipedia.org/wiki/List_of_multiplanetary_systems), one can see that this is pretty accurate — although keep in mind that most of these systems probably have long-period planets we don't know about.

I'm going to lump planets into classes based on mass, and you can put them into your fictional star system as you like. Remember that massive worlds may interdict nearby orbits (see "Failed Planets" in the previous post).

Brown Dwarfs: Really big planets, with a mass above 1000 times that of the Earth. I doubt there will be more than one of these per star system, and they don't appear to be very common. If you're generating the system randomly, generate a random number from 1 to 10, subtract 9, and the result is the number of brown dwarf-sized bodies orbiting the star. The mass is a 10-sided die roll times 1000 Earths.

Gas Giants: Big planets like Jupiter or Saturn, with a mass from about 50 to 1000 times that of the Earth. Our own Solar System has two. If  you're randomly generating a star system, I suggest rolling a 6-sided die and subtracting 2 to get the number of gas giants in a star system. Put them beyond the outer edge of the Goldilocks Zone (unless you want a Hot Jupiter, in which case put it close to the star and eliminate all the planets between its orbit and the outer Goldilocks edge).

Generate the mass of a gas giant by rolling a 10-sided die and multiplying the result by 50 to get the mass of the planet in Earth masses. If the die result is 10, reroll the die and multiply the new result by 100 Earths instead. This method means most gas giants will be between 50 and 450 Earths, with a small number of really big ones.  

Remember to check for "failed planets" in orbits near your gas giant worlds.     

Ice Giants: This is what we now call planets like Uranus and Neptune, with several times the mass of the Earth but low density. I'm giving them a mass from 5 to 50 times Earth. This seems to be a pretty common planetary type, so just roll a 6-sided die to find how many to put in your planetary system. Put them anywhere in the star sytems — astronomers have identified several "hot Neptunes" orbiting other stars. Generate the mass of an Ice Giant by rolling a 6-sided die and multiplying the result by 5 Earths; if the result is a 6 then re-roll and multiply by 10 Earths instead.

Super-Earths: This class of planet has no representatives in the Solar System, unless perhaps Earth itself qualifies as a smalle example. It refers to dense worlds of rock or metal with a mass of 2 to 10 times that of Earth. There may exist "Mega-Earths" with masses of 10 or more Earths, but the observations are in dispute. To get the mass of a Super-Earth generate a result from 1 to 100 using percentile dice or a random number generator, and divide that by 10 (because we're getting into the regime where small differences in mass matter). If you want to allow for Mega-Earths, re-roll any resuilt of 100 and divide by 5 instead.

Rocky Worlds: This is the category that the four inner worlds of the Solar System fit into. Their masses range from 0.1 Earth (Mars) to 1 (Earth, obviously). Use them to fill up any remaining orbits in your star system.

I'm going to assume that rocky worlds can actually meet the lower bound of Super-Earths, so we're actually looking at a range of 0.1 to 2, but it seems likely that the majority of rocky worlds will be small. Generate mass for a rocky world by rolling a 12-sided die and dividing the result by 10. If you roll a 12, re-roll two dice and divide the result by 10.

Dwarf Planets: If you really, really want to spend a lot of time, you can generate dwarf planets for your star system. They have a mass of less than 1/100 Earth, and you can stick them into "failed planet" orbits along with a lot of asteroids, or beyond the outermost planetary orbit in the star's Kuiper Belt.  

Next Time: Planetary Composition!

Notes on Worldbuilding, Part 5: Planetary Systems

Until the boom in exoplanet studies, we really knew nothing about how planetary systems form. In the old days, with only the Solar System as our guide, it looked simple: small rocky worlds near the Sun, big giants in the outer regions. But then we began to observe things like "hot Jupiters" orbiting almost close enough to touch the parent star, giant rocky "super-Earths" and other weirdness. So now we really have no idea if there are any hard and fast rules for how planetary systems form.

Here are some rules of thumb, which aren't laws of nature but are pretty good guidelines.

Scale: The size of a planetary system seems to be roughly in proportion to the mass of the parent star. Our Sun's planets orbit in a range from about 1/3 Astronomical Units for Mercury (45 million kilometers) out to 30 AU (or 4.5 billion kilometers) for Neptune, with small bodies comets extending out to ten thousand times that distance.

An Astronomical Unit is the distance between Earth and the Sun, and is going to come up a lot in this post. It's 148,800,000 kilometers, and for most purposes you can approximate it as 150 million km.

By contrast with our Solar System, the red dwarf TRAPPIST-1 (http://www.trappist.one) has a tenth the mass of the Sun, and its planets are all crowded into a band between 1.5 million and 9 million kilometers (0.01 to 0.06 AU) — roughly the scale of the moons of a planet like Jupiter.

 So if you're creating a planetary system, multiply the mass of the parent star (in Solar masses) by about 35 AU and treat the result as the outer limit for planetary orbits in that star system. In the case of binary or multiple star systems, use that number or 1/3 of the distance between the primary star and its closest companion star, whichever is smaller.

How Many Planets?: The Sun has eight planets plus some dwarf planets and a lot of small stuff. Other stars with known exoplanets have fewer major planets, typically just two or three known. But that's probably an artifact of the methods we use to detect exoplanets: the periodic dimming of the primary star as orbiting planets pass across its disk as seen from Earth is going to be limited by the time astronomers have been watching. Distant, slow-orbiting worlds simply haven't been detected yet. We didn't know about Uranus and Neptune for most of human history.

In the absence of any other data, I'm going to use the Solar System as a baseline for how many planets a given star possesses. If you want to generate the number randomly using 6-sided dice, roll 3 dice and subtract 2 from the total, to get a number from 1 to 16 with the peak between 8 and 9.   

Spacing: The planets of the Solar System follow an interesting rule of spacing known as Bode's Law (https://www.britannica.com/science/Bodes-law), but other known planetary systems don't seem to have the same orbital spacing, and apparently it's just a bit of numerology. You can basically put planets wherever you want in another star system.

 It does seem to be true that the interval between planets increases with distance from the parent star, so my suggestion is to pick the distance for the innermost planet and then work outward as follows.

1. Innermost Orbit: multiply the mass of the parent star (in Solar masses) by a randomly-generated number between 0.1 and 1. My suggestion is to use a 10-sided die and divide the result by 10. The result is the orbital distance of the innermost planet in Astronomical Units.
2. Multiplier: determine a multiplying factor for the spacing of the next orbit. Again, I suggest using a 10-sided die roll divided by 10, and then add that to 1, so you have a number from 1.1 to 2.
3. Second Orbit: Multiply the innermost orbit by the Multiplier to get the distance of the second orbit around the star.
4. Recalculate the Multiplier: Roll a 6-sided die, subtract 1 from the result, and divide that by 10, to get a number from 0 to 0.5. Add that to the existing Multiplier from step 2 to get a new Multiplier. Use that to calculate the third orbit, and so on. Never use a lower Multiplier, and stop when you get a result greater than 2. If the distance exceeds the maximum size of the system, stop there. I tried it a few times and got star systems that look like this (with a parent star equal to the Sun in size). All distances are in AU.

Orbit:                   1         2         3          4          5           6          7          8          9      

Solar System       0.4     0.7       1        1.5        3*          5         9.5       19        30

System A             0.5     0.6     0.7       1.2       2.6        5.4       11.3     23.6  

System B             0.3     0.5     0.8        1.5       3.4        7.7       17.7  

System C             0.5     0.8     1.4        2.4       5.1       10.7      22.5  

*Note: the 3 AU orbit in the Solar System is the Asteroid Belt, which we will consider a "failed" planet as described below.

"Failed" Planets: There's a big gap in the Solar System between Mars and Jupiter — bigger than the distance from Mars to the Sun, in fact. How come? Jupiter itself is the prime suspect. In much of the space between Mars and Jupiter, an orbiting world will "synch up" with Jupiter every few years, and experience Jupiter's distant but powerful gravitational pull. The result (according to current theories) is that much of the matter in that part of the Solar System during the planetary formation era got knocked away by Jupiter, or captured by the giant itself. There simply wasn't enough left to form a planet.

This suggests that a large gas giant — anything bigger than about 100 times Earth's mass — will disrupt the orbits between about 0.4 and 1.6 times its own orbital distance. Leave those empty or make them asteroid belts.

Migrating Planets: One of the most startling exoplanet discoveries was the category of worlds called "Hot Jupiters." These are large, massive gas giants orbiting extremely close to the parent star. Current theories of planetary formation simply don't account for that — close to a star it should be just too hot for a gas giant planet to form.

So the current explanation is that in some systems large worlds form and then migrate inward as a result of interactions with other giants. So if you swap Jupiter and Saturn in the Solar System, the smaller inner world would get kicked into an eccentric orbit, gradually dropping down over millions of years until it orbited close to the Sun.

Needless to say, such a world would probably sling away or obliterate all the worlds whose space it passed through during that period. So if you want a giant world close to the star, pick an orbit in the outer system, beyond the "Goldilocks Zone" where water stops being a rock, and erase everything between that orbit and the one you want to put it in.

Nomenclature: Back in the good old days, science fiction writers had worked out a great convention for naming planets circling other stars. Obviously they'd be numbered, from closest to farthest out. So there would be planets like Tau Ceti III or Fomalhaut IX. Nice, simple, and elegant.

Naturally astronomers had to go and mess it up. Astronomers ruin everything. Their system (which has been around for a long time, actually) is that companion bodies of stars get labeled with letters in order of discovery, beginning with "b" because A is always the primary star. It's good for their purposes, but maddening for everyone else. So Tau Ceti has eight possible planets, and going outward from the star they are b, g, c, h, d, e, f, and i. Inelegant! Infuriating!

Fortunately, even astronomers think this is kind of lame, so the International Astronomical Union has started the Named Exoworlds project to bestow actual names on stars deemed sufficiently interesting, and on their planets. Wikipedia has a list of them here: https://en.wikipedia.org/wiki/List_of_proper_names_of_stars. The IAU retains the final approval authority so we're not going to get any objects named "Planet McPlanetface" any time soon.

So the star 55 Cancri A got renamed Copernicus, and its planets b, c, d, e, and f became Galileo, Brahe, Lipperhey, Janssen, and Harriot. A vast improvement.

For a fictional world, there are several options:

1. Use a real name if it's a real planet.
2. Use a plausible name (without duplicating one of the ones already taken). Something a bit stodgy and uncontroversial.
3. If the planet is going to have native intelligent life, use their name for it, or the closest approximation a human mouth can manage.

Next Time: Placing Planets!

Notes on Worldbuilding, Part 4: Stars!

Now we're going to get into the real nuts-and-bolts part, creating worlds and aliens. Note that you can start this process at either end: begin with the star and work your way through the planet to its life forms and their society, or start with what you need for the story and work backward. Either approach is fine. I'm using the cosmic to domestic progression simply because the steps build on each other more simply.

Real Planets: The simplest way to create a star system or a planet is to let the Universe do the work. Until about 20 years ago we only knew about the worlds of the Solar System, and there was still uncertainty about how common planets actually were. The first exoplanet was cornfirmed in 2002, and since then the floodgates have opened. Nowadays we have basic orbital data and at least an estimate of mass for thousands of planets circling other stars. I know of at least a couple of SF writers who have basically given up on creating their own alien star systems and just pick real ones.

You can find them in a number of places. NASA's Exoplanet Archive is here: https://exoplanetarchive.ipac.caltech.edu, and the European Extrasolar Planets Encyclopaedia is here: http://exoplanet.eu. There's also the Open Exoplanet Catalogue: http://openexoplanetcatalogue.com. And finally Wikipedia's various lists of exoplanets are pretty handy: https://en.wikipedia.org/wiki/Lists_of_exoplanets.

Stars: Unless you want a dark and cold "rogue planet" drifting in interstellar space, your alien world needs a star. We think that planets all form in stellar systems, though some get kicked out by interactions with other planets.* The choice of star is thus very important.

Stars are classified by temperature (color) which roughly correlates to mass, because the bigger a star is, the hotter and brighter it is. The classes range from O (huge and searingly bright blue-white giants) through B, A, F, G (yellow stars like the Sun), K, and M (red dwarfs); plus white dwarfs, brown dwarfs, and black holes.

Note: when I talk about stars here, I'm talking about stars on the "Main Sequence," the time when their energy output is relatively stable, which occupies most of a star's lifespan. Stars leave the Main Sequence as they deplete their fuel, swelling to red giants and then either collapsing or blasting apart in a supernova explosion, depending on how big they are. Those are not good candidates for lifebearing worlds, although one could certainly tell an interesting story of a civilization watching its star enter those last stages.

To have an Earthlike world a star needs to live long enough for planets to form and life to evolve. Big bright stars live fast and die young — which means that most of the stars you've heard of are unlikely to have lifebearing worlds. Star Trek loves to put planets around familiar stars like Rigel, Deneb, Spica, or Canopus — and all of them are bright short-lived stars unlikely to have any planets at all, let alone habitable ones.

Red dwarfs make up the majority of all stars, between 2/3 and 3/4, but they have problems, too: they are so dim that habitable worlds are likely tidally locked, and they tend to be flare stars. These aren't insoluble problems; a large moon orbiting a planet in the habitable zone of a red dwarf would have a day-night cycle, and there have been suggestions that most red dwarf flare activity is along their polar axes, where planets don't typically orbit.

The sweet spot for stars to have worlds capable of supporting life like our own appears to be in the F-G-K range. These stars have masses between 0.5 and 1.5 times that of the Sun, and brightness from about 0.1 to 7 times the Sun's output. They also have a long enough lifespan for solid worlds and complex life to form. Collectively they make up about 20 percent of the stars in the Galaxy.

Note that about a third of all star systems are binary or multi-star systems, with two or more stars orbiting around each other. These range from very close "contact binaries" to stars so distant it's hard to tell if they're actually orbiting or just passing nearby. They can and do have planets, so if you want a Star Wars style double sunset you can have it. For a close binary just treat them as a single star. Distant binaries function as separate star systems which just happen to be near each other.

If you want to use real stars (with or without known exoplanets), the best resource is SolStation.com: http://solstation.com/#sthash.IUQ23ZbY.dpbs. They've got encyclopedic descriptions of stars out to 100 light-years from Earth (http://solstation.com/stars.htm#sthash.htedKjWl.dpbs), plus star maps and very useful articles on planets and the potential for life.

Winchell Chung (his name's going to come up a lot) has not one but three very useful pages. The first is his "Atomic Rockets" discussion of stars and other stellar objects: http://www.projectrho.com/public_html/rocket/spacemaps.php, which includes some extremely useful "delta-V" maps of the Solar System, plus increasing scale maps out to intergalactic scale. His page on "Weird Astronomy" (http://www.projectrho.com/public_html/rocket/weirdastronomy.php) discusses real objects of interest. He also has a separate site for creating star maps, showing where things are in the Solar neighborhood. Very useful if you're creating a game setting or fictional "sandbox" with multiple inhabited star systems. It's at http://www.projectrho.com/public_html/starmaps/, and includes links to other star catalogues I haven't used.

And, again, Wikipedia is surprisingly useful, presumably because there aren't any aliens online engaging in edit wars. Their list of Lists of Stars is a good starting point: https://en.wikipedia.org/wiki/Lists_of_stars.

The most important features of a star are its mass and its luminosity. These are related: the bigger, the brighter. There are any number of sites listing the mass and brightness for different star types. If you want to make up a star rather than use a real one, just pick the type you want.

Brown Dwarfs: In recent years astronomers have gotten better at detecting "brown dwarf" objects — too small to be luminous, but big enough to generate some heat of their own. Basically they're the intermediate step between giant planets like Jupiter and the small end of red dwarf stars.

There's three very interesting things about brown dwarfs. First, there are probably an awful lot of them. In general, stars are more common the smaller they are. I've mentioned that nearly 3/4 of stars are red dwarfs. If this distribution holds true, then there are probably more brown dwarfs in the Galaxy than there are visible stars! Interstellar explorers and colonists might leapfrog from one brown dwarf to another across relatively short distances.

The second is that they do emit heat, and can have moons. A brown dwarf might have icy moons with subsurface liquid oceans, as is suspected of Europa and Enceladus in our own Solar System. That means a lot of places where life might arise.

And third, the atmospheres of smaller brown dwarf objects can be at "Goldilocks" temperatures where liquid water and — potentially — life might exist. As I mentioned, there are probably lots and lots of brown dwarfs, so even if it's unlikely for life to form on one, with enough rolls of the dice the right result might come up.

Wikipedia really has the best list of brown dwarfs I've been able to find, and includes links to full articles on some of them: https://en.wikipedia.org/wiki/List_of_brown_dwarfs.

*Or by the I.A.U.

Next Time: Systems!