Over an illustrious 50-year history, Star Trek TV shows and movies have introduced audiences to some of the most famous fictional aliens. Be they Klingons, Cardassians, Vulcans, or tribbles, the Trek universe abounds with distinct and diverse extraterrestrials.

For now, such exotic civilizations remain in the realm of science fiction. But scientists have plenty of ideas about what real alien life might be like, and what our chances are of ever discovering whether we are alone in the universe.

Andrew Fazekas is the author of 'Star Trek': The Official Guide to Our Universe.

In the past few decades, scientists have developed exquisitely sensitive equipment to search for other habitable worlds and to hunt for greetings from across the cosmos. To date, astronomers have found more than 3,500 planets beyond our solar system, and they calculate that many more remain undiscovered. (See how aliens might contact us using giant laser doodles.)

Even more exciting, we’ve found handfuls of planets that seem to be in the so-called habitable zone—the region around a star where the temperature is just right for liquid water to exist on a planetary surface. Water is a key ingredient for life as we know it, and its presence seems to have driven biological development right from the start.

“The fact that on Earth life emerged quickly just after conditions became habitable suggests that this could happen often in other systems, too,” says Daniel Apai, an astronomy and astrobiology researcher at the University of Arizona and the NASA Astrobiology Institute.

“Life persisted for about four billion years on Earth and survived a variety of planetary challenges, suggesting that it is very difficult to ‘sterilize’ a planet once it’s taken a foothold,” he adds. “So chances are good that life is relatively common in the galaxy.”

Spreading Seeds

If we ever do find aliens, would they be anything like the beings that populate Star Trek? While the show writers have imagined some highly unique creatures, from non-corporeal energy beings to intelligent clouds, most aliens encountered by the Enterprise crew have been carbon-based life-forms that are all too often humanoid.

The striking similarity between humans and the show’s most famous aliens, including Romulans, Vulcans, and Andorians, has been a point of contention with many fans over the years. In Star Trek: The Next Generation, the franchise writers attempted to explain this convenient lack of biological diversity using a real scientific theory called panspermia.

View Images Captain Kirk deals with troublesome tribbles in Star Trek. Courtesy CBS

Proponents of this theory argue that life on Earth may have been seeded by hardy microbes—or at least, by raw ingredients such as amino acids—that traveled here via comets or asteroids. While there’s no direct evidence to back up this claim, missions such as the Rosetta orbiter have found the building blocks of life on comets, and we know tough organisms such as tardigrades can survive unprotected in space.

Scientists also believe early Earth was bombarded by meteors, which could have delivered a “starter kit” for life from elsewhere in the galaxy.

Building on the panspermia concept, Star Trek suggested that an ancient humanoid life-form intentionally seeded worlds around the Milky Way, creating new species that assumed their basic shape. While each of these species may have split off on its own evolutionary path, they are all essentially long-lost relatives. (Find out how Star Trek is right about almost everything.)

While there is a case for panspermia, chances are slim that we will encounter humanoid life-forms in the real universe, Apai argues. Some theories suggest that local environmental conditions and evolutionary events left to chance will most likely shape the future of any life-form, making it totally unique.

“While there are examples for parallel evolution in the terrestrial biosphere—for example, similar eyes evolving in very different, unrelated species—it seems unlikely to me that we would encounter alien life that would be humanoid,” says Apai.

It’s a Small Galaxy?

Instead, given the only example we have, some researchers suggest that the most common form of life across the galaxy may be what’s most abundant here on Earth: microbes.

“On Earth, it took only a relatively brief period for life to emerge and reach the capability to photosynthesize, but it took billions of years for complex animals to emerge, and intelligence and technical civilization have only been present for about 0.000001 percent of Earth’s past,” says Apai. In other words, our planet’s history has been dominated by a wide array of microscopic life.

There’s also a good chance that the basic molecular building blocks we see on our world will be common throughout the universe.

View Images The horta was a silicon-based life form in Star Trek. Courtesy CBS

“Given the availability and its chemical properties, it is a good guess that carbon may often be the key species that provides the backbone, the molecular scaffolding, for biomolecules,” says Apai.

Still, astrobiologists are trying not to be too Earth-centric in their theories about alien life—there’s always the chance that if we do manage to find extraterrestrials, we’ll stumble upon silicon-based creatures like the horta or sentient minerals like the crystalline entity.

“We are looking for everything we can find and trying to exclude no possibilities that we can conceive,” says Apai. “Of course, I am still certain that we will be surprised when we will first be able to study extraterrestrial life.”