In the Avatar universe the geography of the Alpha Centauri system has been worked out in some detail.
All the three stars, Alpha Centauri A, B and C, have planets. Even C, the red dwarf, has a close-in gas giant and two rocky worlds. B has one gas giant and five rocky worlds, and an asteroid belt; B’s subsystem is perhaps most similar to our own solar system.
A, the largest star, has three gas giants and three rocky worlds. Polyphemus is one of the gas giants, with similar size and mass to Saturn in our system, though without the rings. It orbits at about the same radius from Alpha A as Earth does from the sun—unlike Saturn, which is about nine times further out from the sun than Earth. Interestingly, rather like the Trojan asteroids in our solar system (see Chapter 6), two rocky bodies share Polyphemus’ orbit, at points of gravitational stability sixty degrees ahead of and behind the planet: one significant rocky world and one planetoid. Polyphemus has fourteen moons (compared to Saturn’s astounding sixty-two, at the latest count, of which seven are spherical). All these (fictional) bodies have names, by the way. All of them await explorations of the imagination, in movies, books and comics…
The world we care most about is, of course, Pandora, fifth moon of Polyphemus.
The larger moons, like Pandora, probably formed from the same swirl of debris that formed Polyphemus itself; the smaller ones may be captured asteroids. There are limits on where big moons might be found in relation to the primary world. Sensible spherical moons need to be outside the primary’s “Roche limit,” within which tidal effects are so strong they pull the moon apart; inside the Roche limit you may get shapeless asteroid-like lumps of rock, but not round worlds. The precise distance depends on the mass and rotation of the primary, and on the composition of the moon, but as a rule of thumb the Roche limit is around two and a half times the primary’s radius, measured from the planet’s centre. Thus Saturn’s innermost spherical moon Mimas is three Saturn radiuses out. You can see from the onscreen size of Polyphemus in Pandora’s sky that Pandora is safely out beyond the Roche limit. Some close-in moons of gas giants are “tidally locked,” so that they keep one face permanently set towards the primary, as the moon does to the Earth. This isn’t the case with Pandora; during its twenty-six-hour day Polyphemus rises and sets.
In real life we’ve yet to detect any worlds of Alpha Centauri. But we have found an awful lot of worlds orbiting other stars.
One of the true scientific miracles of my lifetime has been the discovery of “exoplanets,” indeed in some cases whole other solar systems. When I was a boy not a single planet beyond the sun’s family was known. Some scientists maintained there were no other worlds—that the solar system was a freak, a matter of chance. Now, at the time of writing, we know of more than four hundred other worlds. We’re starting to learn a good deal about the distribution of planets and planetary systems, and are coming up with new theories of planetary formation. And we have new ideas of how planets may be habitable, suitable for life, even if in some cases they are dramatically different from our own Earth. It’s certainly timely for Avatar, a movie of travel to alien worlds, to appear just now. Suddenly we see a sky full of Polyphemuses—and, maybe, Pandoras.
The challenge of detecting worlds beyond our own is formidable, because planets are small and faint compared to their parent suns.
Suppose we were studying the solar system from a planet of the star Altair, in the constellation of the eagle (Aquila), about seventeen light years away. Even mighty Jupiter, the largest of the sun’s planets, would be lost in the sun’s glare. Jupiter’s apparent distance from the sun, from the point of view of an Altairean, would be only one-thousandth the width of a full moon seen from Earth, and its light, which is just reflected sunlight, only a billionth of the sun’s. It was once believed that you would need truly ginormous telescopes flying in space to resolve worlds like Jupiter out of the glare, let alone Earths, smaller, closer to the sun, even fainter. Not so.
While there had been tentative observations of planets orbiting pulsars (small supernova remnants) since the 1980s, in 1995 the scientific world was startled by the first observation of a planet orbiting a star called 51 Pegasi, a “main sequence” star (that is, a star in the middle of its normal lifetime, like our sun). The discovery was made not with giant telescopes but with improved instruments, careful observation and a dash of ingenuity.
An exoplanet is generally detected indirectly: not by observations of the planet itself, but by studying its effects on its parent star. The most productive technique to date has been the “radial velocity” method. As the planet orbits its star, the star itself is pulled out of position, just a little, and if some of this motion is towards or away from Earth you can detect it with a subtle shifting of the lines of the star’s light spectrum. This is the Doppler effect, the same phenomenon that causes the blue shift and red shift so familiar to hardened interstellar travellers like us. Alternatively there is the “transit” method. If the planet happens to pass across the face of its sun as seen from Earth—just like transits of Venus and Mercury, planets inside Earth’s orbit crossing the face of our sun—the dip in the star’s apparent brightness can be detected. Other techniques include using stars in the line of sight as gravitational “lenses.”
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 journeys.
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.