The Science of Avatar

by Stephen Baxter

  As you can imagine, these effects, though detectable, are small and subtle. The more massive the planet, and the closer it is to its parent star, the larger the effect and the more likely it is that the planet will be detected. Thus the first exoplanets found tended to be more massive than Jupiter, yet orbiting (to everybody’s surprise) very close to their parent stars. The very first discovered, at 51 Pegasi, was a “Jovian,” in the jargon, a gas-giant planet like Jupiter, orbiting its sun in just four days (our closest-in world Mercury takes eighty-eight days). Polyphemus is another example, a gas giant not much further from Alpha Centauri A than the Earth is from the sun.

  There is an inevitable “observational bias” in our exoplanet detection. For a long time yet we are going to find more large, close-in worlds than small, further-out worlds, and the statistics of the planets we’ve found so far must reflect that. Nevertheless we have enough data now to start to classify the exoplanets and make some tentative predictions.

  For example, eighty per cent of the exoplanets discovered have been in multiple-planet “solar systems” (which can be detected by observing the multiple tweaks the planets apply to their parent star’s motion). It’s thought that about a third of all sunlike stars will host planets the size of Neptune (around seventeen Earth masses), or “super-Earths,” worlds somewhere between Earth and Neptune in size. A super-Earth, by the way, would be a spectacular place, despite the higher gravity; the larger the world is the more geologically active it is likely to be, as the Earth is much more active than Mars or the moon. Expect fiery worlds, tremendous volcanoes.

  The observational techniques are improving, but we’re still some way from being able to detect an “Earth,” orbiting at an Earthlike distance from a sunlike star. This would produce only a thousandth the deflection of the parent star of a close-in Jupiter (Jupiter has over three hundred times the mass of Earth).

  So suddenly we’re seeing all these planets. But what about life?

  It used to be thought that if it is to be liveable for creatures like us or the Na’vi, a world would have to be more or less Earth-sized, and would have to occur in the “habitable zone” of its parent’s star—orbiting at a distance from the star that would allow liquid water to occur on its surface, not too hot and not too cold, so at something like Earth’s distance from a star like the sun.

  But in recent years we have discovered life surviving in quite extreme environments on Earth: in the deep sea where no sunlight ever penetrates, in conditions of cold and heat, even subject to radiation. Maybe life is more robust and flexible than we used to think.

  And we have discovered new kinds of worlds, like Jupiter’s moon Europa, which under a crust of ice has a water ocean, kept liquid by tidal effects. Europa’s ocean seems a prime arena for life, even though it is far outside the traditional habitable zone.

  In Avatar’s fictional universe Pandora too is an example. Alpha Centauri A is about fifty per cent brighter than Sol, and its habitable zone is about twenty-two per cent wider than the radius of Earth’s orbit around the sun. Polyphemus with its moons follows an orbit about forty per cent wider than Earth’s, so is just outside the traditional habitable zone of Alpha A—but oxygen, a signature of life, was detected in Pandora’s air anyway. It turns out that Pandora is kept warm by complex effects include tidal heating, and by a greenhouse effect from an atmosphere thick with carbon dioxide, and by other aspects of its complex environment as a moon of a gas giant in a double star system. No doubt we will turn up many other exceptions to the habitable-zone rule in the future.

  These days, in fact, we no longer even think the parent star has to be like the sun to support a habitable world. Even red-dwarf stars, like Proxima Centauri, could conceivably have life-bearing planets. Such stars are small and dim, and the planet would have to huddle close to the central fire, probably so close that it would be “tidally locked” like our moon orbiting the Earth, with a single face perpetually presented to the star. You would think that the dark side, a place of eternal night, would be so cold that all the water, and even the air, would freeze out. But it’s believed that even a thin layer of atmosphere would transport enough heat around the planet to keep this ultimate chill-out at bay. From such a planet’s surface the sun would be huge—pink-white rather than red to the vision—and forever fixed in the sky, no sunrises or sunsets. The lack of tides, and the comparatively low-energy sunlight, would surely shape the origin and evolution of life. Perhaps plants would be characteristically black, to soak up all the energy available from the sunlight. It could be a dangerous environment, for stars like Proxima are prone to violent flares.

  This may not sound like much fun. But remember that not so long ago people thought that to have life you had to have a sunlike star, with planets at an Earthlike distance. Since, as noted in Chapter 12, seventy per cent of the Galaxy’s stars are red dwarfs, with this model we have multiplied the potential number of habitable worlds in the Galaxy many times over. Not only that, the dwarfs have very long lives as stable stars, perhaps a hundred times as long as the sun’s. Suddenly the universe looks a lot more hospitable for life.

  As it happens, the best candidate found so far of another Earth, the fourth planet of a star called Gliese 581, orbits a red dwarf. And as our nearest neighbour, Proxima, is a red dwarf, maybe it’s there we will find a “Pandora,” in reality, not orbiting the more glamorous Alpha A or B.

  We may detect signs of life even before we manage to image habitable worlds directly. Spectroscopy, the analysis of the light reflected by a planet, or of starlight passing through a planet’s atmosphere during a transit across the face of its parent, can show evidence of the gases making up the planet’s atmosphere. Some gas giants have already been shown to have methane in their atmospheres. Direct spectroscopy may be possible in the next decade or so, through such missions as ESA’s infrared telescope Spica (to be launched possibly in 2017). Detecting such gases as oxygen in a world’s atmosphere would be a good indicator that life was present, even before we could see the green. This, in fact, in the Avatar universe, was how Pandora’s life was first detected.

  The holy grail is to image an Earthlike world—to see its seas and polar caps and continents—as well as to detect the makeup of its atmosphere. This is the goal of future space missions including NASA’s proposed Terrestrial Planet Finder. And if such a world were discovered there would surely be pressure to develop and send a space probe. In the Avatar universe the first discovery of the Alpha Centauri planets prompted a rapid development of technology, leading ultimately to the sending of the first interstellar probes.

  But could Polyphemus and Pandora exist? And if they do, given Alpha Centauri is the nearest star system, why haven’t we seen them yet?

  Much of what we used to think we knew about Alpha Centauri has turned out to be wrong.

  We used to think that in a multiple-star system like Alpha Centauri you might get close-in rocky worlds, but the formation of Jovian gas giants could be inhibited because of the closeness of the suns. After all, Alpha B is sitting at an orbit where Alpha A’s Jovians should have formed, and vice versa. But in October 2002 astronomers in Texas announced the discovery of a Jovian planet orbiting a star of the Gamma Cephei binary system, about forty-five light years from Earth, a system with twin stars with the same kind of spacing as the two suns of Alpha. The Jovian they found is about twice as massive as Jupiter, orbiting happily about twice as far as Earth is from the sun.

  Then we used to think that even if multiple star systems like Alpha Centauri grew planets the stars’ gravitational perturbations would destabilise their orbits and throw them out of the system altogether. But recent studies have shown that for planets as close to Alpha A as Earth is to the sun, B’s gravity would have no significant effect on their orbital stability. So Alpha Centauri may not just have twin stars. It may host twin solar systems: two planetary systems just a few light-hours apart, so close that if humans had evolved there we might already have made interstellar journey
s.

  And we used to think that we would never find a giant planet like Polyphemus so close to its star, as close as Earth is to the sun. When we only had the example of our solar system to study, we believed that gas giants would only be found far from the parent star, beyond the “snow line,” where, out in the stillness and cold and dark, the worlds grow immense, misty, stuffed with light elements like hydrogen and helium that were boiled out of worlds like Earth that formed close to their sun’s heat. Thus in our solar system the closest-in Jovian, Jupiter itself, is five times as far as Earth is from the sun. But as we’ve studied the new exoplanets we’ve found endless examples of gas giants orbiting much closer to their suns than was thought possible. Indeed, as I noted earlier, it’s the very closeness of these huge worlds to their suns that allow us to detect them in the first place.

  It seems a Jovian may well be born out beyond the snow line, but then it can suffer a kind of friction with the sun-surrounding disc of dust and gas from which it formed, causing it to lose orbital energy and spiral inwards. Several such planets may be eaten by their sun until at last the growing sun’s radiation and solar wind, or perhaps a blast from a nearby supernova, clears away the last of the debris, leaving the survivors to settle where they are. In our system, perhaps Jupiter and the other three giants are the last survivors of a flock of gassy worlds, most of which were consumed by the young sun.

  In other systems we’ve seen “hot Jupiters,” left stranded in stable orbits much closer to their suns than Jupiter is to the sun. The most extreme example found so far, reported in 2010, is a planet of a star called WASP-12, nearly nine hundred light years from Earth. While Jupiter takes around twelve years to orbit the sun, this wretched world orbits in a mere day. The star’s gravity will have pulled it into an egg-shape, its surface temperature must be thousands of degrees, and the star’s heat, boiling away its atmosphere, will some day ensure its break-up altogether.

  Even without being a hot Jupiter, being close in would make a difference to a gas giant’s formation, to its weather, and ultimate fate. And indeed Polyphemus has a different composition to Saturn—it is smaller and denser—and it is lot more stormy, with a “great red spot” storm larger than the red spot on Jupiter.

  So it’s entirely possible that a Jovian like Polyphemus could indeed be found at an Earthlike distance from Alpha Centauri A, with a nice spherical moon like Pandora. But even if we found Polyphemus using exoplanet-tracking techniques, would we be able to see Pandora? Maybe. One recent computer simulation, of an Earth-sized “exomoon” orbiting a Neptune-sized giant, showed that the moon’s orbit would affect the giant’s path sufficiently for it to be detected by a “transit” observation by a future space telescope.

  In reality we haven’t yet detected a Polyphemus orbiting Alpha Centauri, or indeed any worlds in that system, despite its closeness. In the Avatar universe the explanation is simple. The plane of the planets is tipped at sixty degrees to our own; our current detection methods, the transits and Doppler tracking, work best when the planets’ orbits are in our line of sight. There are other factors too, such as the comparative instability of planetary orbits within the system. This could well be the case. Planet-hunting is still a tentative game. But we are planning more subtle exoplanet searches, with powerful spaceborne instruments. I think we can be confident that if Poly-phemus and Pandora, or anything like them, do exist, some day we will see them.

  And, someday, maybe, visit them.

  14

  THE CASE OF THE CYLINDRICAL BIOLOGIST

  Polyphemus and Pandora: what evocative names!

  In giving these new worlds names from classical mythology, their discoverers followed a tradition that dates back to 1781, when the British astronomer Sir William Herschel discovered the solar system’s seventh planet, the first found beyond the wandering bodies visible to the naked eye that had been known since before humans were humans. Eventually the new planet was named Uranus, in Greek mythology the personification of heaven and the son and husband of Gaia, the Earth goddess—though Herschel had hoped to name it Georgium Sidus, in honour of King George III: a planet called George!

  In myth, Polyphemus, with a name meaning “very famous,” was a Cyclops, a cannibalistic one-eyed giant encountered by Odysseus in Homer’s Odyssey. It seems an apt name for a giant world dominated by a single glaring-eye storm. And to the Greeks, Pandora, whose name means “giver of all,” was the first woman. Out of curiosity she opened up the famous “Pandora’s Box” (actually a jar), thus releasing all the evils of mankind, leaving only Hope inside the box as consolation. Certainly it seems appropriate that a world as fecund as Pandora should be given the name of the Greek Eve.

  (There are in fact already two astronomical Pandoras in our solar system. One is a main belt asteroid discovered in 1858, a rock about sixty kilometres across. The other is the seventeenth moon of Saturn, an even more battered lump of rock around a hundred kilometres long by eighty wide, which shepherds the outermost of Saturn’s rings, following a very complex and chaotic orbit.)

  But if you were to follow Jake Sully down the ramp off the Valkyrie, it’s probably not the moon’s name you’d be thinking of in your first moments on Pandora, but its low gravity.

  Colonel Quaritch is suspicious of Pandora’s low gravity. He obsessively pumps iron to avoid being made “soft” as a result.

  Pandora’s gravity is about eighty per cent of Earth’s. Its diameter is three-quarters of Earth’s, and its mass about half; in size it’s a world somewhere between Earth and Mars, which has around one-third Earth’s gravity.

  But Pandora’s air is thicker, about twenty per cent denser than Earth’s. You might wonder how a low-gravity world can hold on to a thick atmosphere, as Pandora evidently does. On any world air molecules can be heated to “escape velocity” and just fly off into space, like tiny spacecraft. A battering by the solar wind, charged particles from the sun, adds to that leakage as well. The lower the gravity, the more air will escape to space. Thus our moon with around a sixth Earth’s gravity is all but airless.

  But gravity isn’t the only factor when it comes to a world keeping its air. Titan has around the same gravity as the moon, but, as we saw in Chapter 7, its atmosphere is more massive than Earth’s. This is because it is so cold out there at the orbit of Saturn; Titan’s air molecules move much more slowly, on average, and fewer escape. On the other hand Venus, only a tad smaller than Earth, has a much more massive atmosphere than the Earth because it’s too hot; all the heavy carbon dioxide that’s locked up in the rocks on Earth is baked out into the air on Venus.

  Leakage of an atmosphere can be surprisingly slow too. It’s thought that an Earthlike atmosphere somehow delivered to one-third-gravity Mars (perhaps as part of a “terraforming” project, making Mars into a second Earth) would take around ten million years to leak away. That’s a slow enough process for a civilisation to manage an artificial atmosphere if it had to (recall the air machines on Burroughs’ Barsoom). There are natural inputs to a planet’s air too, from outgassing via volcanoes, and impacts from comets. And there are other special factors. Pandora orbits between radiation belts surrounding its primary Polyphemus, which deflect the charged-particle wind from the sun. This is evidently a complex question; a world’s lower gravity does not imply it must have thinner air.

  But what effect would a different gravity have on living things?

  Galileo was able to figure out the basic physics of gravity and bodies back in 1638: “It would be impossible to fashion skeletons for men, horses or other animals which could exist and carry out their functions [proportionally] when such animals were increased to immense weight…”

  This work was the origin of the famous “square-cube law.” If you double the size of an animal, its cross-section goes up as the square of the size—four times—but its volume, and therefore its mass, goes up as the cube—eight times. This basic rule is central to “biomechanics,” the discipline of how living things are put together mechanicall
y. This means that you couldn’t just double the size of an elephant in some genetic-engineering brainstorm and expect it to function; its four-times-thicker muscles wouldn’t be able to raise its eight-times-greater weight.

  Ah, but what if you transported said elephant to a lower-gravity world, like Pandora?

  There’s a difference between mass and weight. Mass is a resistance to motion. You would have the same mass even in zero gravity, in space. You get weight in a gravity field. Weight is mass multiplied by the acceleration due to gravity, which is approximately ten metres per second per second on Earth. Weight is the load you have to carry around. In space you would still have mass, but no weight.

  We all have a maximum weight we can bear, given the strength of our bones and muscles. But in a lower gravity field, you could carry around more mass: higher mass times lower gravity comes out to the same weight. How much more mass depends on the weakness of the gravity.

  Humans have complicated geometries, so let’s simplify things. There’s an old joke about the farmer who’s having trouble with his milk production, and he calls in theoretical physicists from the local university to help. After weeks of intensive study, back comes the report which begins: “Consider a spherical cow…” (Well, it made me laugh.) The point is, to figure out basic principles, scientists will often make simplified models of the real world to make the calculations easier, even if the models are somewhat unlike the real thing.

  And in that spirit, consider a cylindrical biologist.

  Here’s Dr. Grace Augustine, standing tall on Earth, probably giving some RDA desk jockey a hard time. She could be represented by a pillar a bit less than two metres tall, say twenty centimetres diameter. The pressure she’s exerting on the bones holding her up is her weight divided by her cross-sectional area.