They're big-eyed and slight of build. They're the grim, greenish beings that every moviegoer recognizes as aliens -- the inscrutable inhabitants of a distant world. Playing supporting roles in countless films and TV shows, these hairless homunculi have become iconic.

But we've never seen a real alien. Indeed, we don't even know if real aliens exist.

However, astronomers have recently discovered several thousand planets in our galactic neighborhood. Consequently, the idea of intelligent life in space has eased from the realm of science fiction to scientific plausibility. Telescopes could receive signals at any time that would tell us they're out there.

What would they be like?

Is it possible that real aliens would resemble the smooth-skinned anthropomorphs of the movies? Simple engineering arguments suggest that having two eyes, an upright stance and a small number of flexible appendages make sense for any active and intelligent animal. Consequently, some biologists have sided with Hollywood, and claimed that the process of convergent evolution, a tendency for design to follow function, will ensure that aliens might vaguely resemble us.

This line of argument, as flattering as it is, misses the boat. More likely is that most advanced alien civilizations will be a special kind of artificial intelligence ("A.I.") called "superintelligence" - synthetic cognition that vastly exceeds the best that humans can manage in every category: social skills, general wisdom, scientific creativity, and so on.

We suspect this could be true based on analogy with our own near future, as we may be near to developing superintelligence ourselves. Making such machines may be the most transformative event of the 21^st century, or perhaps of any century in human history. It could be as natural an evolutionary step as the emergence of land-dwelling animals 370 million years ago.

In a similar vein, we suspect that once a society creates the means to communicate across interstellar space, they are likely only a few hundred years away from changing their own paradigm of sentience from biology to A.I.

Given the age of the universe, some of these alien thinking machines could be millions or even billions of years more advanced than we are. Some may have achieved superintelligence by means of biological enhancements to their bodies, but we suspect that the majority would simply transition to silicon, rather than remaining carbon-based life forms.

Indeed, silicon appears to be a better medium for information processing than the brain itself. Neurons reach a peak data rate of about 200 Hz, which is about seven orders of magnitude slower than current microprocessors. While biological brains compensate for the slowness of neurons with efficient algorithms and massive parallelism, speedier silicon systems can borrow these design features from biological brains, and even improve upon them.

Further, focused, deliberative thought is incredibly slow in human brains, having a maximum capacity of only about seven manageable chunks of information. And the quantity of neurons in the brain is limited by cranial volume and metabolism. Computers, in contrast, could occupy entire cities, or even planets. They could survive environmental catastrophes that would wipe out biological life.

Once begun, intelligent devices can engineer their own descendants. These improved sentients can then go on to design yet another generation of machines, a scenario that may have played out many times in the history of the cosmos. This process unfolds so quickly that it seems inevitable that the majority of the intelligence now extant in the universe is built, not born. The real aliens won't be protoplasmic.

Even if we are wrong -- even if the majority of alien civilizations are biological -- silicon-based creatures are more likely to engage in space travel, having durable systems that are practically immortal. They may be the kind of the creatures we first encounter, if we encounter anyone.

What can we say about such thinking devices? Would their thoughts and interests be anything like our own?

We suspect that many alien civilizations will become post-biological either by scanning their brains and transferring the contents to a computer or by slowly replacing parts of their brains with silicon components (both amount to "uploading" the biological brain). If this is right, their thinking could be like that of the biological aliens they are derived from, though much faster, and enhanced in ways that enable them to solve problems and develop technologies that were beyond the reach of their biological ancestors.

Biological intelligences have evolved to survive and reproduce. Silicon descendants could inherit their primary motivations, such as find food, avoid injury, reproduce, cooperate, and so on. If these aliens are indeed still interested in reproduction, we might expect that given the vast computational resources at their disposal, they would create simulated universes stocked with artificial life and even other superintelligences. If the simulated beings were intended to be their "children," they may retain these same basic goals as well.

You may object that it is useless to theorize about superintelligent aliens, for they can alter their basic architecture in numerous, unforeseen ways. There are limits to this, however. If a superintelligence has survival as a primary goal, it may resist fundamental changes to its architecture, sticking to smaller improvements. It may think: when I alter my architecture in radical ways, I am no longer me.

Cognitive science could prove instructive in considering these non-biological beings. For instance, in trying to understand superintelligent minds, cognitive scientists can theorize about characteristics of biological intelligence that superintelligent creatures would want to retain and enhance, such as working memory and attention, and those they may prefer to jettison, such as decision-making biases. Further, connectomics, which seeks to provide a wiring map of the brain, could be informative. While it is likely that a superintelligent A.I. would not have the same kind of connectomes that members of the original species have, some of the functional and structural connections may be retained. And philosophers of mind have already debated whether A.I. is even capable of having inner experiences, such as curiosity or the feeling of awe when seeing a sunset.

Movie aliens, from the pointy-eared Vulcans of Star Trek to the childlike gnomes of E.T. are merely projections of ourselves. Real aliens could be as different from us as we are from the trilobites.