Quiz: Do you know your aliens?
TENTACLED monsters, pale skinny humanoids, shimmery beings of pure energy鈥� When it comes to the question of what alien life forms might look like, we are free to let our imagination roam. The science-in-waiting of extraterrestrial anatomy has yet to acquire its first piece of data, so nobody knows what features we will behold if and when humans and aliens come face-to-face. Or face to squirmy something.
Despite this lack of hard evidence, a blend of astronomy and earthly biology offers some clues to what is out there. A few bold scientists are even willing to make an educated guess at the nature of aliens that might exist on faraway worlds.
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What these extraterrestrials will be like depends on where and how we expect to meet them. Barring the appearance of flying saucers, there are two broad possibilities: either we have a close encounter with our neighbours by visiting the planets and moons next door; or we make an interstellar phone call to creatures inhabiting much more distant planets that circle alien suns. These two options have different implications for the shape of what we find living there.
鈥淲hat extraterrestrials will look like depends on where and how we meet them鈥�
If first contact turns out to be within our solar system, then at least we have some prior knowledge about the available habitats. Several spots might be suitable for Earth-like life based on carbon biochemistry and using water as a solvent. The subsoil of Mars may be warm enough to host microbes akin to Earth鈥檚 bacteria, for example, and there could be larger beasts swimming in the watery oceans of some outer moons of the solar system 鈥� especially Jupiter鈥檚 moon Europa. There鈥檚 every chance that a deep aquatic ocean lies beneath Europa鈥檚 ice, stretching right down to the moon鈥檚 rock core, where volcanic vents pump out hot, nutrient-rich water.
Astrobiologist of Washington State University in Pullman calculates that the energy supplied by these vents could feed a large population of microbes, which in turn could support a pyramid of predators. Europa鈥檚 top predator, the equivalent of our great white shark, would be a fearsome creature with a mass of 鈥� wait for it 鈥� about 1 gram. 鈥淓uropa could support a shrimp-sized organism,鈥� he says. There would not be enough prey to feed a viable population of predators bigger than that.
Shrimp-sized doesn鈥檛 have to mean shrimp-shaped, of course. 鈥淚t is kind of difficult to say anything about how it would look,鈥� says Schulze-Makuch. Even on Earth animals have evolved an astonishing diversity of shapes and body plans, but Schulze-Makuch is nevertheless prepared to speculate. 鈥淚 would make a guess at something worm-like,鈥� he says. 鈥淭hat is a pretty successful kind of organism on Earth.鈥�
While the hypothetical ice worm of Europa would be swimming about in boring old water, a few astrobiologists are pushing the boat out and pondering the possibility of life that is not water-based. Most places in the solar system are too hot or too cold for liquid water to exist, but there are several other liquids that might host some kind of biochemistry, says of the Foundation for Applied Molecular Evolution in Gainesville, Florida.
The clouds of Venus hold droplets of sulphuric acid, and billions of years ago there may have been pools of the stuff on the planet鈥檚 surface. Though pretty destructive to bags of water like ourselves, it could be a refreshing draught for beings with the right biochemistry. These acid-dwellers would have to be formed of chemically resistant materials. 鈥淢ulticellular Venusians living in liquid sulphuric acid could have veins made of glass,鈥� Benner suggests, conjuring up visions of delicate, transparent glassware creatures, rolling carefully over the Venusian rocks. But glass is not the only option: more mechanically robust materials would also fit the bill. 鈥淭here are flexible polymers that are acid-stable, such as Teflon, polyethylene and silicone,鈥� Benner points out.
Elsewhere in our solar system, surface lakes and seas exist to this day 鈥� though not watery ones. On Saturn鈥檚 moon Titan, they are formed from a chilled hydrocarbon cocktail of ethane and methane, and Schulze-Makuch speculates about what forms of life they might harbour. 鈥淭hings might be bigger,鈥� is his unexpected conclusion. 鈥淲ater has a high surface tension, which constrains the volume of single cells. That鈥檚 why bacteria on Earth are so small.鈥� The surface tension in a methane-ethane blend is much lower, so single cells could be enormous, a possibility that Schulze-Makuch has explored in his novel Voids of Eternity. 鈥淚 have boulder-sized microbes moving over the surface and guzzling up hydrocarbons,鈥� he says. 鈥淭hat is science fiction of course, but there may be something in it.鈥�
In our eyes, the Titanians might seem pretty laid back. At around 93 kelvin, Titan鈥檚 seas are very cold and that makes chemical reactions super slow. 鈥淭hings could be very slow-moving and slow-growing,鈥� says Schulze-Makuch. 鈥淭he lifetime of such an organism may be 10,000 years, or perhaps as much as a million.鈥�
Who鈥檚 there?
As strange and marvellous as it would be to encounter these beasts, they are probably not going to be very interesting to talk to. For aliens on our intellectual level 鈥� or indeed far above it 鈥� we will almost certainly need to look beyond the limits of our own solar system. In the search for extraterrestrial intelligence, a few astronomers have been straining Earth鈥檚 radio ears for the sound of alien transmissions. SETI researchers are now debating whether to become more proactive and start sending out messages (see 鈥淲e鈥檙e over here!鈥�). If we do make contact, what kind of creature is going to be on the other end of the line?
Even without knowing the details of their chemistry or habitat, it is possible to hazard a few guesses. For a start, they may have a taste for flesh. 鈥淧redators tend to be more intelligent,鈥� says evolutionary biologist of NASA鈥檚 Ames Research Center in Moffett Field, California. 鈥淭hey have to do more moving around to outsmart the other guy. You don鈥檛 have to be terribly intelligent to grab a leaf of lettuce.鈥� If that applies to alien ecosystems, we can expect to be talking to carnivores 鈥� or at least omnivores like ourselves.
And then there is the fact that in order to contact us, ET must be able to send and receive radio waves or laser beams, or use some other medium to reach across the light years. So either they are vast creatures that have evolved natural radio-wave organs to talk and listen to each other, or they have developed technology. For that, intelligence alone is not enough. 鈥淭he thing that advances us as a species is that we are social,鈥� says Schulze-Makuch. 鈥淥ne of us alone is not very smart 鈥� I鈥檓 so dumb I can鈥檛 even build a radio. It is by working together that we got on the moon.鈥�
So message-sending aliens will probably have some form of society. It need not be anything like human societies, however. 鈥淭here are meta-intelligences in the societies of bees and termites. I can imagine something like a termite or ant colony that gets really intelligent,鈥� says Schulze-Makuch. This does not tell us, however, whether they will be furry, scaly or slimy. Even on Earth, clever brains come in a wide variety of packages: dolphins and primates, parrots and crows, sea otters, honey badgers, octopuses and squid.
The principle of convergent evolution could give us some ideas, though. 鈥淪ome things have evolved many times on Earth such as hearts, eyes and jointed limbs, and the four 鈥淔鈥漵 鈥� flight, fur, photosynthesis and sex,鈥� says , a reproductive biologist who has helped science fiction authors design plausible aliens for their books. 鈥淭hey happened independently in different branches of the tree of life. If you ran Earth again from the start, you鈥檇 likely get these again.鈥�
So if our aliens come from a planet with a range of habitats not too different from those on Earth, they might well have some of the same characteristics. A well-lit world like ours would probably produce beings with eyes 鈥� so maybe a recognisable face after all. And our cosmic correspondents would presumably need some manipulating organs to fiddle with the nuts and bolts of their technology. They might even have hands, but then again why not a prehensile tail or a trunk instead? 鈥淢aybe it鈥檚 an antenna, maybe a tentacle,鈥� says Rothschild. 鈥淢aybe an octopus would look at us and think 鈥楬ow can you expect this organism to develop technology with its two clumsy front limbs?'鈥�
Whether such creatures would be aquatic, like Earth鈥檚 octopoids, is questionable, however. Much of our technological development, from curing meat to smelting metal, is based on fire, and learning to use fire is likely to prove an insurmountable obstacle even for very clever water dwellers. Far better for the talented, tentacled monsters to slither onto land before building their interplanetary empire.
Putting it all together, the daring astrobiologist might be prepared to make a very small bet that SETI-type aliens will be social multicellular predators with eyes, sexes, and sticky-out bits of some sort. Unless, of course, the aliens were usurped by smart machines or decided to modify themselves using biotechnology. In that case, we might find tentacled monsters, pale skinny humanoids, shimmery beings of pure energy鈥�
Quiz: Do you know your aliens?
