Bug-like, viscous masses or as green dwarves with big eyes, science fiction has presented us with many interpretations of aliens, but the stereotypical image of an extraterrestrial depicts them with bilateral symmetry (two legs, two hands, two eyes, etc.). Like us, they use their inferior extremities to move and the superior ones to use tools. Now we can predict, with recent scientific knowledge, how they would really look.
Certainly, they would have a "mouth" (for them to obtain nutrients) and a "nose" (for respiration). Also, they must have an organ that allows them to recognize volatile substances and odors and some sort of "anus" to dump their surplus.
Furthermore, an intelligent organism would need a good storage system (a brain) and sensors in order to relate to their environment (eyes, ears, nose). These would be near the encephalon, for the information to arrive and be processed quickly.
They must have at least two free appendices for them to be able to manipulate their environment, and especially, a metabolism capable of producing large amounts of energy for their brains.
Their height, determined by the gravity of their planets, is easier to establish. Here on Earth, giants from legends and folklore would not be able to exist because nature says it's so. There can't be anything in nature above a certain level with the same proportions and materials.
As Archimedes discovered, if we increase the size of any solid, its surface will grow to the square (length, wide and height) and its volume to the cube. For example, if you grow twice your size, the total surface of your skin will increase four times, and your volume, eight times.
Another thing would be their movement, greater gravity means bigger feel of weight. If we had appeared in a world with a gravitational force ten times from the Earth's, our bones would be thicker in order to sustain us.
When sizing increases, so does mass, meaning weight would be higher too. Thus, in places of intense gravity, their muscular system would need to be strong enough to move around. In contrast, on planets with low gravity, the size of its inhabitants is less complicated to determine, but there's the risk that if it is so low, the planet itself would not be able to maintain its atmosphere.
Upon a star
Some hypothetical ideas for the existence of life have been presented throughout the years by many respected and famous experts. Such as in 1973, radio astronomer Frank Drake proposed a subatomic life-form based in non-chemical systems that would use nuclear forces to maintain its nucleus and process certain radioactive disintegrations. Their perfect habitat would be the surface of a neutron star, the stellar corpse of stars four to eight times the size of our sun after a supernova. These same organisms wouldn't use radio frequencies to communicate with each other, but gamma rays.
Three years later (1976), astronomer Carl Sagan (1934-1996) and the astrophysicist, also expert in stellar evolution, Edwin Salpeter described a whole ecosystem similar to our oceans, but in the atmosphere of Jupiter. They imagined beings parallel to balloons inside clouds, which provides them necessary nutrients, and missile-like predators that feed on them.
Like two drops of water
Darwin's Theory of Evolution can be applied to understand possible aspects of aliens too. Paleontologist Simon Conway Morris proposed that, with everything we know about evolution today, there are restrictions for such aspects. First, there's their biochemistry, based on carbon, which can't differ much from ours because organic chemistry is constant in any place of the universe.
Their molecules and cells could be different, but their processes would be the same. At the scale of organisms, things get more difficult because the definition of bodies depends on environmental factors and random events. For example, mammals dominate the Earth because chance put an asteroid on a course of collision towards our planet.
According to Morris, every form of life on any habitable planet would evolve just like us. Such a comment is justified by convergent evolution, meaning that different species arrive at the same solutions independently. Here on Earth are many examples, such as the eye, which was developed by countless species. For him everything we consider important (cognitive sophistication, big brains, ability to create tools, etc.) is convergent.
But many scientists differ from the ideas of Morris, claiming that our superior intelligence and current appearance are a pure evolutionary chance. One of them was biologist Ernst Mayr (1904-2005), whose words were: "Nothing further demonstrates the improbability of the origin of high-level intelligence as the millions of phylogenetic lineages that have failed to obtain it".
From 50,000 million species on our planet, only one reached the type of expertise needed to establish a civilization. The question now is if there is something universal for all living beings, something that transcends evolutionary differences.
The only certainty is that if there are aliens, they will not only be stranger than we imagine, but they will be stranger than we can imagine.