The year 1877 was a big year for Mars. Earth and Mars passed closer than usual in their orbits, making the red planet stand out as unusually large and bright for observers looking at it through telescopes.

One of those observers was the Italian astronomer Giovanni Schiaparelli, who used the opportunity to map the surface of Mars. Straining at the eyepiece to make out features at the very edge of visibility, he made what might have been the greatest of all extraterrestrial discoveries — if it hadn’t turned out to be a mistake.

Schiaparelli sketched what appeared to be a network of lines connecting darker features on the planet’s reddish surface. He described them as channels, or, in Italian, canali.

Canal Builders of Mars

In Italian, canali is used to describe both natural and artificial waterways. In English, we use two related but distinct words. So when Schiaparelli’s report was translated, canali was translated not as channels, carved perhaps by natural forces, but as canals — hinting at the presence of canal builders. (The straight lines they formed on Schiaparelli’s map were also suggestive of deliberate construction.)

The impact was sensational. Imaginative writers had placed beings on other planets before, but only as a whimsical fantasy. Schiaparelli’s canali were a scientific observation — at least they seemed to be — and implied not just life but intelligence and civilization.

As Slate recounts, a whole theory of Martian civilization and history was worked out to explain the canals, as well as other features of Mars, notably seasonal color changes including waxing and waning ice caps.

Mars, the theorists suggested, was an old and dying world of dwindling water and expanding deserts. Martians built the canals as irrigation works to sustain their crops and make the most of their precious and limited water. Only a mighty civilization could have undertaken such a vast project.

There had been no extraterrestrials in the earliest science fiction, such as the works of Jules Verne. But now science fiction had aliens — and writers knew just what to do with them.

Invaders from Mars

In 1898, British author H. G. Wells wrote the most famous of all Mars stories, “War of the Worlds.” Per Slate, his ancient Martians, with “intellects vast and … unsympathetic,” came up with a new solution to their water problem: conquer their water-rich neighbor, Earth.

Martians weren’t always scientific supervillains. As noted by Space.com, pulp writer Edgar Rice Burroughs, creator of “Tarzan,” wrote a series of books set on Mars — Barsoom, as the natives called it — in which the declining civilization of the canal builders is being overrun by local barbarian hordes, until earthman John Carter comes along to save the day.

Later, science fiction writer Robert Heinlein featured wise, peaceful and mysterious Martians in several of his stories, including “Stranger in a Strange Land.” These three themes — scientific supervillains, colorful barbarians and wise demigods — still account for the great majority of alien theories favored by Hollywood and pop culture.

From Mars to the Stars

Schiaparelli’s canals turned out to be an optical illusion, says Slate. Mars has real channels, dry riverbeds, but they do not correspond to anything he showed on his map. Space missions showed that Mars is indeed a geologically dying world that has lost most of its water. But while it may still have life, it offers no hint whatsoever of an advanced civilization, living or dead.

In fact, we haven’t found any positive evidence of extraterrestrial discoveries, like ruins or signals, that people in the late 1800s thought they had discovered on Mars.

What we have found is indirect evidence for the possibility of alien life, and therefore alien intelligence. NASA’s TESS mission has confirmed that some planets orbiting other stars are close to Earth-sized and in orbits that allow them to retain liquid water.

In fact, alien theories are now part of the toolbox of possible solutions that astronomers examine when seeking to explain unusual phenomena in the sky. Over the last few years, says Science Daily, peculiar flickering light changes drew attention to an otherwise ordinary-seeming star, officially designated KIC 8462852, but much better known as Tabby’s Star, for lead investigator Tabetha Boyajian.

The flickering pattern of light changes by Tabby’s Star is so unusual that researchers examined the possibility that an alien megastructure surrounding the star might best explain what they observe. Alas, the alien theory did not fit the observations; the current best guess is that Tabby’s Star is surrounded by clouds of dust.

But the search continues for evidence that we are not alone in the universe — for hints that someone really is out there. And perhaps looking back at us.