Extraterrestrial, titled Alien Worlds in the UK, is a British-American two-part Speculative Documentary. It was released in 2005, airing in the UK on Channel 4 and in the US on the National Geographic Channel.
The program alternates between CGI segments depicting hypothetical extraterrestrial lifeforms and environments and live-action ones describing real-life astronomy and biology. Two worlds are described, one for each of the series' episodes.
- Aurelia describes a tidally locked planet orbiting a red dwarf star, where native animals have to contend with periodic solar flares and a deadly microscopic predator.
- Blue Moon describes a nameless satellite orbiting a gas giant in a binary system, where high oxygen levels support towering forests and a complex airborne biota but fuel devastating wildfires.
This series contains examples of:
- Speculative Biology: The series describes imagined alien environments on two alien worlds, using them as ways to exposit on natural processes and on the possible nature of extraterrestrial life.
- Armless Biped: Gulpogs are terror bird-like animals without forelimbs, possessing only a pair of long, powerful legs.
- Blob Monster: In its collective form, hysteria resemble a creeping mass of yellow goo that crawls along the ground, seeking prey to infest and digest from the inside out.
- Extra Eyes: Gulpogs have a third eye on the top of their heads, which they use to detect the beginning of solar flares in order to take shelter before the worst of the flare comes.
- Fantastic Fauna Counterpart: Mudpods are essentially alien salamander beavers. They fell trees for food using a constantly-growing saw-like claw explicitly compared to a beaver's teeth, and build dams out of mud to create a network of lagoons that provides rich habitats for other species.
- Fungus Humongous: The forest floor of the Aurelian swamps is dotted with large mushrooms, ranging in size from ones the size of a person to a few that come close to matching the gulpogs' nine-meter height.
- Planimal: The stinger-fan trees of Aurelia are technically animal-like organisms that can photosynthesize. They still possess circulatory systems, with hearts in their trunks, and creep along the ground on mobile, tentacle-like roots.
- Stock Footage: A number of shots are reused throughout the episode, either unmodified or mirrored, including the mudpods felling and eating the trees, a gulpog eating a mudpod, the gulpog clan in the forest and the hysteria swarm gathering in the water.
- Tidally Locked Planet: Aurelia orbits a small, dim red dwarf star, and due to its physical proximity to it — a red dwarf's dim heat and light can only support life on planets very close to it — it's permanently locked onto its sun, always showing it the same side. Its dark side is cold, dark and covered in vast glaciers, while the day side is covered in a permanent storm endlessly lashing it with torrential rains. Between them is a temperate, humid twilight band home to flourishing swamp environments fed by rivers pouring from the storm zone.
- The Worm That Walks: Hysterias are normally microscopic aquatic creatures, but when food grows scarce gather in large swarms that act and move like singular beings.
- Alien Landmass: The surface of the moon is interspersed with occasional rocky hill topped with strange, spire-like structures, which the segment never describes.
- Alien Sky: The moon's is dominated by its parent planet, and the high quantities of aerial plankton color large patches of it green.
- Binary Suns: The moon's planet orbits a binary system. The narration notes that these systems were once thought unable to support planets in stable orbits, but that this is now believed to be possible if the planet is far enough from the suns or if it orbits only one.
- Eaten Alive: Once the stalkers incapacitate the skywhale, they tear it to pieces and eat it while it's still alive.
- Extra Eyes: The stalkers have three eyes in a ring around their heads, giving them 360 degrees of vision.
- Giant Flyer: The moon's dense atmosphere and high oxygen allows it to support immense flying creatures, many of which never land, including skywhales with ten-meter wingspans and kite-like creatures five meters across.
- Hive Caste System: The stalkers live in large hives compared to those of eusocial insects, and are divided into slender-billed scouts, which search for and mark potential prey; more robust workers, which bring it down and carry the meat back to the nest; and an unseen breeding queen.
- Living Gasbag: The balloon plants are a vegetal example of this. They consist of a number of large hydrogen-filled sacs connected to a central stem by tendrils, which raise the plant above the canopy of the immense pagoda trees. Mature sacs eventually break off and float into the sky, drifting for vast distances until fire or lightning burst them and scatter their seeds.
- Noisy Nature: The stalkers are extremely noisy beings, loudly screeching, cawing and croaking at all times, even when trying to find prey.
- Stock Footage: A number of shots are reused throughout the episode, either unmodified or mirrored, primarily the sky whale soaring in the sky, the stalkers in their nest, the stalkers flying through the forest and the stalkers devouring one of the whales.
- The Swarm: The stalkers are much smaller than the other creatures in the segment, and especially so compared to their immense skywhale prey. They make up for this by hunting in large, organized groups, overwhelming their much larger prey in a tide of screeching, biting creatures.
- Tree Top World: The dense atmosphere and low gravity of the moon allow its plant life to reach immense heights, and the dominant plants have found methods to extend their reach even further — balloon plants, for instance, support themselves with buoyant balloons, while the pagoda trees grow large bowls to catch rainwater (they're much too tall to draw water purely from the ground) and tangle their branches into a tight mesh to support each other's massive weights. As a result, the trees of the moon are over a kilometer in height, and almost all of the fauna in the episode consists of flying or arboreal animals living within and above the forests' roof-like canopy; further down, among the pillar-like trunks, almost no sunlight penetrates, and the ground is a lightless world of luminescent scavengers feeding on detritus falling from above.