The series is a response to Peter Douglas Ward's book "Rare Earth," which claims worlds with microbial life may be common but multicellular life is rare, and intelligent life vanishingly rare. Ward lists all Earth's quirks that influenced our unique path of evolution, then demands other worlds match every one. This approach is common in exobiology, but specious. Even if Earth were absolutely ideal for life (patently untrue) and no other path was possible (fantastically unlikely), a world may be unearthly in several ways that effectively cancel out! For example, here are just some of the factors affecting surface temperature

• Insolation, the most obvious, hinging on several factors: solar luminosity, average distance from the sun, orbital eccentricity, cloud cover, and the absence or presence of rings
• greenhouse gasses--and this in turn hinges on primal composition, gravity, life's activities, tectonic activity and vulcanism (in turn affected by sheer size, spin, tidal stress from sun and any moons, lubrication from any seas)
• sheer atmospheric density. Every gas is a greenhouse gas--to some extent.
• albedo (reflectivity). Enough clouds or haze can create an anti-greenhouse effect. Titan is colder than it would be with no atmosphere at all.

SWAP VENUS AND MARS

Move Mars where Venus is now, and put Venus is Mars's place, or perhaps a bit further out, toward the inner edge of the asteroid belt. Assume they were there from the beginning, and estimate their current surface temperatures! You'll find that life on Hot Mars probably would develop early, when the sun was cooler. Life would suck up CO2 and keep the planet from runaway greenhousing. Hot Mars might still end up thin-aired and dry, with small seas at most, and maybe just a planetary desert... but with sparse life. Thin air can be a virtue!

Meanwhile, our Cool Venus, swathed in a thick atmosphere, would be warm enough for life but lacks enough water for large seas. With the air so rich in CO2 and water as the limiting factor, sparse plant life might be all we'd get, but who knows? CO2-tolerant oxygen-breathers might evolve--Terran life's adapted to stranger chemistries.

And of course, our mutant worlds would have somewhat different chemistries--Venus, as an outer world, would have been richer in water; seas would dissolve much of the CO2, leaving a cool world, but with true seas. How cool? Even if every molecule of CO2 were scrubbed out of Venus's atmosphere, it has four Earth atmospheres of other gasses--and the seas add water vapor, a strong greenhouse gas. So would our Cool Venus really be cool--a big, wet Mars? Or would it greenhouse up to Earthlike temperatures? You see how quickly such questions tangle into complexity.

One way to reduce the problem to manageable simplicity is to declare all these contributing factors are beyond us right now, and just assume that size, temperature, and water-content distributions of bodies in space are all pretty smooth curves--more small ones, more cold ones, with more water ice in the cool part of the range--then guesstimate what proportion of bodies look friendly for life (not specifying how they got that way, or even if they're moons or planets). Anyone out there care to try? Simpler than Fermi's formulation! And it's intriguing, just how big the livable parts of these curves are.

TEMPERATURE

Just for example, I'm building a world well outside the classic liquid-water zone, Lyr, whose dense air warms it quite nicely. It has a Europa-like moon, Oisin, whose thinly ice-covered sea lets light penetrate, so it too harbors complex life--another model Ward overlooks. I'm also designing an eccentric little Marslike world sweeping in and out of the liquid zone, Serrana; high tidal stress led to active tectonics, high CO2 levels, and livable temperatures. Very livable--in fact, many different kinds of life crawl out of its many shallow seas. In the future, I hope to design another biosphere Douglas dismisses as impossible, with seas from 50-100 degrees Celsius (hot to boiling). Even on Earth, life can thrive in this temperature range; there's no reason to assume a whole ecology couldn't develop in it. Maybe I'll call it Blisteria. Or Jalapeña.

HOT JUPITERS

Ward also assumes that Pegasian planets (gas giants close to their suns) would sweep away Earthlike planets as the big ones spiral in--or keep Earths from evolving at all! He's overlooked at least two scenarios I explore. First is a true hot giant nearly double Jupiter's mass, with two moons supporting life: Earthlike Pegasia and stark little Tharn. Ward never mentions moons of Pegasian worlds--he writes off their whole solar systems prematurely. Sure, gas giants may destroy smaller planets--instead, you get families of huge moons! Gravity giveth, and gravity taketh away. My second Pegasian scenario isn't a classic "hot Jupiter", but something likely to be even commoner (just indetectable at the moment)--a "tepid Neptune" (Lyr again). It's bigger and further out than our rocky planets, but smaller, closer and warmer than our gas giants. A world between.

WATER

A third assumption is that life needs an Earthlike amount of water. I present one wet scenario I think is common, Lyr again, with 15 times our water. I haven't bothered to build a landless world with seas up to 100 miles deep--it's the one time I'll concede Ward may be right. A world-sea with no land at all will grow mineral-poor, with sparse life like our deep seas--and though multicellular life may develop, even intelligent life, how far can it progress in that limited environment? No solids but living bodies--anything heavier than water (such as bone) drops into the abyss, its minerals lost forever to the sunlight surface. It makes even my icy Europan moon Oisin sound good--at least its world-sea has shallows, reefs... rocks!

The real potential lies in dry models! Pegasia (1/3 our water) and Serrana, (1/15th), and my terraformed version of Mars (in the earlier series Futures) show how little water it takes to sustain a biosphere--as long as it's warm enough to be liquid! Last and most extreme is the desert moon Tharn, with just 0.1%-0.2% of our water. Yet even Tharn STILL has small seas--and abundant life.

Aside from the occasional world-sea that's truly landless, I'm slowly coming to the conclusion water levels hardly matter. Most worlds with surface temperatures allowing water at all will have both land and sea. And life! Temperature, not chemistry or water content, is the key factor.

SMALL SUNS

A fourth assumption: Peter Ward Douglas writes off the vast majority of stars because they're too small and dim. To stay warm, planets would have to orbit them so close that tidal forces would stop or slow their spin. Roasting dayside, freezing nightside! But as I've argued above, livable planets can orbit a star further out than he thinks if they have denser atmospheres. Spin, too, depends on chance, impacts, a world's mass (big worlds tend to spin fast and resist tidal drag much longer), and whether it IS a planet--many large moons will always rotate relative to their sun, since they always face their planet instead. In short: we can't write off small stars, and they're everywhere. So all five of my Biosphere Variations have smaller, cooler suns than Sol.

COMET SHIELDS?

A fifth assumption is that any well-made solar-system needs a Jupiter as a shield against comet-strikes. I'm growing skeptical of this, too. After all, Jupiter's gravity disturbs the outer system, thus causing the very comets it supposedly shields us from. Besides, it's now looking like we need some comets--our water may come from early bombardment, and the latest studies show that the rise of multicellular life coincides with a large increase in meteoric impacts on Earth! A certain number of small catastrophes may be good for evolution.

A system with no planets bigger than Neptune (or with its only giant hugging the sun) may have more catastrophic impacts but not fatally more, or have familiar cratering rates, or be more peaceful than our system, and we don't know what rates are healthy, either. Nor are we likely to have answers until we've thoroughly explored a large sample of solar systems. But I'll stick my neck out. I predict multicellular life, interesting life, will be quite common; 5% to 50% of all solar systems.

CONCLUSIONS?

I've convinced myself that many different kinds of worlds are possible biospheres; Earth is not the only path. The question is, to what extent can I convince you?

Well, anyway. Five world-models. Enjoy your trips! Don't forget to write--particularly if you think of other intriguing biosphere variations.

SERRANA Cross Earth with Mars-- How low can you go?	LYR Not a hot Jupiter, but... a tepid Neptune?

OISIN a giant Europa walking on thin ice	THARN a living Marslike moon with just 0.1% of Earth's water	PEGASIA a huge Earthlike moon of a hot gas giant STILL UNDER CONSTRUCTION

LYR has a mass seven times Earth's and is further from its sun. Not a hot Jupiter, but a tepid mini-Neptune. These worlds, too, may be quite common--after all, we have two worlds only twice Lyr's mass in our own solar system. Dense atmosphere, deep seas... but does that preclude continents and their fertilizing minerals, land animals, and technological civilizations? Nope. We forget big worlds are so damn big. Lyr is 95% ocean--but that last 5% is nearly as much land as Earth!

OISIN's a huge, ice-covered moon of Lyr, looking like a warmer, larger Europa with small Martian uplands rising from a frozen sea. But the ice skin is mere yards thick, like our Arctic Sea--algae flourishes on the underside, feeding a complex life-web. Rich reef communities teem in the rocky shallows, algae grazers swim the open sea, and weird vent communities dot the deeps.

PEGASIA's a nearly Venus-sized moon of a hot super-Jupiter. It's tide-locked with its primary, so one side has a huge "moon" permanently overhead, sometimes sunlit, sometimes glowing dim red, radiating so much infrared it substantially warms that hemisphere of Pegasia. Rather than climate belts, Pegasia has a fascinating pattern of nodes of hot and cold. Not just a north and south pole, but an east and west pole, and an in and out pole. With only 25% of Earth's water, Pegasia still has a shallow world-sea studded with continents. My most Earthlike model yet--and it's not even a planet!
By the way, I'm looking for plausible inhabitants of Pegasia. Invent an intelligent species and win your own continent! You'll have to get there on your own though.

THARN is a smaller, Mars-sized moon orbiting the same hot gas giant as Pegasia. It's a study in extremes--in drought (0.1% of Earth's water) and thin air (1/4 atmosphere). Tharn's warmer than Mars, but that's about all. Life? Perhaps you'd accept oases--but how about savannas, rainforests, and seas? Yes, seas! One thousandth of Earth's water is enough for seas--and abundant life.

UNDER CONSTRUCTION

XANADU will be an experiment in non-water-based life. A slightly larger, warmer, "wetter" version of Titan, Xanadu has identical geography--except that its dark basins aren't dry lakebeds but hyrdrocarbon seas. Ethane rivers carve canyons through continents of ice... what would life be like?

Three related sets of world-models: Tilt! -- Futures -- Caprices -- Home page for all four sets: Planetocopia