"The Biosphere Variations" is an ongoing set of world-models so unlike Earth most biologists would write them off as havens for life--prematurely.

The series began as 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. Nearly any gas greenhouses a world--to some extent.
• albedo (reflectivity). Enough clouds, haze, or ice can create an anti-greenhouse effect. Titan may be 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 even a bit farther 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? Way simpler than Fermi's formulation! And it's intriguing, just how big the livable parts of these curves are.

TEMPERATURE

Big rocky planets well outside the classic liquid-water zone can have dense air warming them quite nicely. My example, Lyr, not only has extensive life on its small continents; it has a Europa-like moon, Oisin, whose thinly ice-covered sea lets light penetrate, so it too harbors complex life--another model Ward ignores.

I've started an even colder world--Xanadu is a somewhat larger, warmer, wetter, older Titan. Its ethane seas have had time--in their cool, leisurely way--to evolve complex life.

I've finished 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 have crawled out of its many shallow seas.

I'm now working in another model Douglas dismisses as impossible, with seas from 50-100 degrees Celsius (hot to boiling). Even on Earth, multicellular life adapts just fine to hot undersea vents; there's every reason to assume hot seas would evolve a whole ecology. Maybe I'll call it Sauna or Jalapeña or Blisteria or Ovenia or...Capsica

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!

But my real objection to world-seas is esthetic. They're no fun to sculpt. Slather a globe blue and you're done. No, the real potential--both exobiologic and esthetic--is 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. Why? Well, liquids spread. Even small volumes can create significant surface areas, and it's surface water where most life is--and that generates rain.

Aside from world-seas so deep they're 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.

And of course what we want isn't really water but surface liquid in which life can evolve; not quite the same thing! Other liquids might do the job. Xanadu is a somewhat larger, warmer, wetter, older Titan. Its ethane seas have had time--in their cool, leisurely way--to evolve multicellular life. Unexplored as yet is the idea of a cool world with ammonia seas. Ammonia and methane/ethane seas greatly widen the range for potential life at the cold end. And lots of worlds will be cold. There's a lot of mass and elbow room out there.

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, or liquids other than water. And even if we restrict ourselves to close-in worlds, spin depends on more than tidal drag: 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. Example: Pegasia.

Nor must every tidelocked world be half-charred, half-frozen, with only a narrow ring of tolerable temperatures. In the real world, orbits are elliptical; libration and nutation mean a tidelocked world may have quite broad regions with diurnal cycles. And denser atmospheres flatten the temperature gradient between day and night; Venus, though slow-spinning, still has very even temperatures. I'm building an example now of a livable tidelocked eccentric: Libratia.

In short: we can't write off small stars, and they're everywhere. So all of my Biosphere Variations have smaller, cooler suns than Sol; the only variation here is how much smaller.

COMET SHIELDS?

A fifth assumption Douglas makes 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 many of 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 I just saw a study claiming 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.

ROTTEN LUCK

An different sort of assumption underlies a lot of thinking about exoplanets--that if the numbers look Earthlike the planet will be Earthlike. My next experiment, Kakalea, explores how a very Earthlike world can go wrong. Similar sun, age, mass, gravity, chemistry, insolation, tectonics, atmosphere, water content, land area, multicellular life... and yet Kakalea's mostly barren. Its current geography just happens to have placed its contintents so they're mostly Australian or Antarctic. Only coastal strips are fertile.

And yet... civilization thrives. Or at least Barbie-rism.

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. Six world-models, and more under construction. 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?	KAKALEA Earthlike by the numbers but Australian bad luck!

PEGASIA a huge Earthlike moon of a hot gas giant	THARN a living Marslike moon with just 0.2% of Earth's water	OISIN a giant Europa walking on thin ice

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!

KAKALEA tours a world so Earthlike by the numbers that it should be ideal. But through sheer geographical bad luck, its continents are mostly dry and barren--wrong latitude, wrong shape, mountains blocking storm tracks. Still, life flourishes on coasts and islands. For the first time, not just the globe but the native life-forms are sculpted not drawn. Too bad I sculpted them from Barbie. But this is the planet where everything goes a bit wrong...

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, the deal with NASA is on hold again. Cutbacks.

THARN is a smaller, Mars-sized moon orbiting the same hot gas giant as Pegasia. It's a study in extremes--in drought (0.2% of Earth's water) and thin air (1/4 atmosphere on most of the surface). 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! And abundant life. Tharn's proof how little water is needed--if the temperature's right.

UNDER CONSTRUCTION

CAPSICA explores life in the hot half of the Goldilocks Zone. Well, the middle at least: average temperature 323 K, 50 C, 122 F. See? Juuuuust right! Halfway between freezing and boiling, what could be more comfortable? So far, my only firm prediction about a world of scalding-hot tropical seas (and body temperatures) is this--Capsican scientists know intelligent life will never evolve on frost-damaged worlds like Earth! Our deepsea vent communities prove that complex life can adapt to extreme heat; but exobiologists still focus on cool worlds like Earth. Would life differ on hot world? Just as some of Earth's iced over, some of Capsica is Earthlike; would these biological "coolspots" be oases... or backwaters?

Is Capsica a steambath or oven, wet or dry? Wet seemed too easy: our Carboniferous squared. So I went with hot but drier, with shallow seas and patchy rain patterns like Earth. Or are they like Earth? Evaporation's fast; small seas generate rain, but rivers shrink as they run...
(50% complete--planet built and most tours written, but life-forms unsketched and some regions still untourable.)