Trying to find ET and our place in the universe

Toddlers, I know from experience, believe themselves to be very, very special — the center, in fact, of the entire universe. With maturity, however, most of us learn that we are not quite as special as we thought. Other people are important, too.

It’s similar with Earthlings in general. As humans have learned about the size of the cosmos (and our not-so-central place in it), we have grappled with weighing our specialness against the possible existence of alien beings on other worlds. Given the amazing diversity of life on Earth, it makes sense that life here is just one example from a cosmic multitude. If so, the next question becomes whether we can find the others.

That quest is at the heart of this special issue, which explores our long fascination with the idea of extraterrestrial life and then examines more practical matters, such as how to go about searching for it. We already monitor radio signals with projects like SETI. So far, we’ve heard nothing. We have sent rovers to Mars to dig in the soil. So far, we have found nothing that resembles a living thing. But we persist in the search.

That persistence has deep roots in human history, as Tom Siegfried describes in his essay. Originally, the question of whether life exists on other worlds was linked to questions of divine power. But in the 20th century, the search for aliens became an issue of logic and probabilities. Siegfried reports on new ideas about why, even if they’re out there, we haven’t yet heard from them.

In contrast to the superintelligent, high-tech aliens that inhabit film and TV, most scientists expect aliens to be some form of microbial life — perhaps not even carbon-based. Tina Hesman Saey discusses the latest thinking about what alien life might look like, and the challenges we may face in recognizing it even if we do encounter it. Telling such life from nonlife is not trivial, even on Earth, Saey reports. Desert varnish, a dark stain on earthly rocks, may be made by living things that have so far escaped detection. Finding signs of past or present life on another planet will be even more difficult. But the possibility of life as we do not know it energizes the debate about how to find ET, and a broader discussion about what counts as living.

The broader search for ET assumes alien life (and a planet) similar to Earth’s. Christopher Crockett discusses new and existing tools that exoplanet hunters can use to peer at faraway worlds. Analyses of exoplanet atmospheres might reveal signatures of life, hints, if not proof, of ET. Geoscientists have also gotten in on the action, trying to understand what planetary processes make Earth habitable, and, in so doing, helping to identify livable worlds in space. Thomas Sumner reveals new findings about alternatives to Earth’s carbon cycle, plate tectonics and other processes that could keep exoplanets temperate and inform the hunt for other life-forms.

It is our obsession with aliens, and whether they exist or not, that motivates much of our search for Earth-like planets and even the exploration of our solar system. And that obsession stems from a deeper, innate curiosity about whether Earth is special or just one of a billion.

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