In the late 1800s Italian astronomer Giovanni Schiaparelli pointed a telescope at Mars and saw something curious: linear features that he called canali, meaning “channels” or “grooves.” A mistranslation of that word helped lead to a widespread belief that the planet closest to Earth hosted a civilization.
American astronomer Percival Lowell took Schiaparelli’s observations and ran with them. He became obsessed with the Martian markings, which he interpreted as evidence of a sophisticated network of water-transportation channels. “That Mars is inhabited by beings of some sort or other we may consider as certain as it is uncertain what those beings may be,” Lowell wrote in his 1906 book Mars and Its Canals.
It sounds ludicrous now, but it wasn’t back then. At the time, ideas about life were evolving rapidly, says David Baron, author of the new book The Martians: The True Story of an Alien Craze That Captured Turn-of-the-Century America. In 1858 Charles Darwin published his theory of natural selection. One year later German scientists Robert Wilhelm Bunsen and Gustav Robert Kirchhoff invented the spectroscope, which they and others used to analyze the chemical signatures in light from the sun and the planets. These studies revealed that other worlds are made of the same elemental constituents as Earth. If life evolves by a natural process, and all planets form in similar ways, why wouldn’t life take hold on the Red Planet, too?
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More than 100 years later scientists searching for extraterrestrial life are guided by the same reasoning: The universe is vast, and it’s all made of the same basic stuff we are, so why wouldn’t there be life elsewhere? Yet the evidence for intelligent life beyond Earth has taken several turns. In fact, the only constant has been hope: the desire that many people have to prove we are not alone. The question of extraterrestrial life’s existence isn’t just a neutral scientific debate—it matters to humans, including the humans searching for that life. And our optimism that we’ll find it has tended to flip on and off.
The idea that Mars is home to canal-digging civilizations began to lose its sparkle in 1909, when French astronomer Eugène Antoniadi observed the Red Planet during one of its biannual close approaches. The lines, he found with a better telescope and a more intimate view, were an optical illusion. Those data didn’t convince Lowell, and it didn’t put the theory to rest—in 1916 Scientific American managing editor Waldemar Kaempffert was still convinced the canals were real. Nevertheless, belief in advanced life on Mars faded in the following decades. When the Mariner 4 spacecraft flew by Mars in 1964, relaying images of a dry and desolate world, the Martian hypothesis died for good.
And the signs weren’t promising for extraterrestrials elsewhere, either. In 1950 physicist Enrico Fermi had pointed out what he called the “Great Silence”: If life is likely to be plentiful, then where is everybody? The fact that humanity hadn’t heard from other intelligent beings became known as the Fermi paradox. Maybe life is common, but advanced life is rare, scientists suggested. Or perhaps other civilizations arise often and then destroy themselves, as humanity seemed newly capable of doing after the invention of the atomic bomb in 1945.
Astronomers began a more systematic study of the question. In 1960 Cornell University researcher Frank Drake started Project Ozma, which used a radio telescope to scan for broadcasts from two distant star systems. In 1977 astronomers caught a batch of radio waves that blasted out for 72 seconds, looking more like a hugely powerful cosmic radio station than something natural. They called it the WOW! Signal and got excited. But the same transmission was never heard again. So far the search for extraterrestrial intelligence (SETI) has not found convincing evidence of broadcasting aliens.
Yet lately there are new reasons to hope. In 1992 astronomers Aleksander Wolszczan and Dale Frail discovered two rocky worlds circling a dense, rotating star called a pulsar. Although those planets are bombarded with too much radiation to be habitable, more exoplanet discoveries trickled in through the 2000s. Then the Kepler space mission launched in 2009. It revealed thousands of worlds beyond this one, with more than 5,900 total confirmed as of publication time. “Planets became the rule, not the exception,” says Nathalie Cabrol, director of the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute.
This wealth of worlds once again changed the calculus on the likelihood of life beyond Earth. Back in 1965 Drake developed a formula to calculate the odds of communicating with extraterrestrial civilizations. It factored in the rate of star formation, the fraction of stars with planets, the fraction of those that are habitable, the proportion of habitable planets that actually develop life, the proportion of that life that becomes intelligent, the fraction of civilizations that develop communications technology, and the length of time they are likely to be transmitting. Most of those variables were unknown at the time—and still are—but the exoplanet boom helped to narrow down the second variable, and it’s making headway on the third. We now have a much better idea of how many stars host planets, and it’s at least most of them.
We still don’t know how life started here on Earth, so we don’t know how it might happen elsewhere. And we don’t know how likely advanced civilizations are to destroy themselves—a pressing question for reasons beyond SETI. But we do now know that primitive life can thrive in profoundly inhospitable conditions, and that means that microbial aliens may be a lot easier to find than intelligent ones.
In 1966 ecologist Thomas Brock discovered the first extremophile, Thermus aquaticus, living in the hot pools of Yellowstone. Since then, scientists have found microscopic organisms in hydrothermal vents at the bottom of the ocean and in toxic mine waste, in the interiors of rocks and in radioactive water. Just because a planet looks barren doesn’t necessarily mean that it is. There is good reason to think primitive life could survive in the buried oceans of Jupiter’s moon Europa and the geysers of Enceladus, a moon around Saturn. There might even be microbes in the pools of meltwater under the ice caps of Mars. More than a century after Percival Lowell and his illusory Martian civilization, science has given us plenty of reason to think we’re not alone, even if aliens turn out to be single-celled organisms rather than canal-building architects.
