A deep dive into one of science’s great unknowns: Extraterrestrial life

05/05/2024

Of all the unknowns in science, the greatest may be extraterrestrial life. Despite stunning advances in astronomy and planetary science, alien life forms — whether microbial and slimy, animated and bounding around, or highly technological and spacefaring — remain conjectural. That doesn’t mean they aren’t a worthy subject to write a book about.

Lisa Kaltenegger proves that with “Alien Earths,” which explains for lay audiences the young field of astrobiology and describes intriguing and potentially habitable worlds that have only recently been discovered around distant stars. This is a science book through and through, and will disappoint anyone who wants to hear that the aliens are already here, secretly zipping around.

“So far, despite wild claims to the contrary, we have not found any definitive proof of life on other planets,” Kaltenegger, an astrophysicist at Cornell University, writes in her introduction.

What she can report is that astronomers now know that the universe is jammed to the rafters with planets. Although most are not Earth-like (something we know just from scanning our own solar system), many newly discovered planets are close to Earth in size and orbit their stars in what scientists consider the habitable zone. In the near future, employing current and planned telescopes, astronomers could detect signs (“biosignatures”) of extraterrestrial life by studying the atmospheres of those planets.

That would be a tremendous discovery. It might generate robust debate about whether such biosignatures were truly signed by living things. And it wouldn’t tell us about extraterrestrial intelligence. When people ask, “Are we alone?” I don’t think they’re wondering if there is algae on some distant world.

Kaltenegger wisely avoids litigating the topic of UFOs (or UAPs — Unidentified Anomalous Phenomena, as the Pentagon calls them), which in recent years has enjoyed a rebound in public interest, fueled in part by unexplained sightings by military pilots. (The claim that strange things seen in the sky are spacecraft from another world is a leap of reasoning lacking the safety net of evidence and logic. Don’t get me started.)

Kaltenegger’s skepticism about alien visitors is appropriate given her status as the founder and director of the Carl Sagan Institute at Cornell. She cites Sagan repeatedly as an inspiration.

Sagan was the most influential and popular promoter of the theory that the universe is rife with intelligent life. He estimated that our galaxy alone could have a million civilizations. But, like Kaltenegger, his pole star was science, and he spent his career throwing his body in front of UFO conjectures and pseudoscience more generally. To his dying day in 1996, Sagan acknowledged that we still did not know what was out there.

Lisa Kaltenegger.

Like Sagan, Kaltenegger is a science popularizer. She approaches her subject with the air of someone who does not want to scare off readers averse to books that get too technical, too down in the weeds of astronomy, planetary science or biology.

Some of the text reads like a very basic primer. It includes a tour of the solar system that explains how the sun works and introduces us to Mercury, then Venus and onward to Neptune, the Kuiper belt and the Oort cloud. As a space nerd, I got a bit fidgety.

But hold on: Kaltenegger kept surprising me with nuggets about science and nature. She seems to know a little bit about everything, which is probably required for an astrobiologist.

Among those surprising takeaways:

Earth was not always a blue marble. Billions of years ago, Earth’s continents had rusted, and “may have looked like a large, wet Mars, with red continents separated by blue oceans.”
When the moon formed more than 4 billion years ago after Earth slammed into a Mars-sized object, the moon was more than 10 times as close to Earth as it is today, a huge object in the sky, its face blackened by cooling lava, with red veins of hot lava breaking through to the surface. That’s a much scarier satellite than the one in “Goodnight Moon.”
The mountains on Saturn’s moon Titan, which has such a dense atmosphere that “if you strapped wings to your arms, you could fly,” are named after the fictional world of Middle-earth created by J.R.R. Tolkien.

She meticulously outlines what we actually know about planets, their habitability and the evolution of life on Earth — which, as she points out, took its jolly time to produce anything as elaborate as a clam, much less a space-faring technological species.

(She can also attest that the evolution of human intelligence is incomplete: She describes the maelstrom of sexism that she encountered as a young female scientist in a field dominated by men, some not fully emerged from the Stone Age. “How many brilliant ideas and discoveries have already been lost because gifted young women had to use most of their energy to fight to be even allowed to do research?” she asks.)

There is an obvious reason to assume we are not the only intelligent civilization in the universe: It’s a big universe. What happened here could, in theory, happen elsewhere. Biology is built upon chemistry, and the chemistry of the universe today is fundamentally congenial to life.

To my ear, Kaltenegger sounds far less optimistic than Sagan about the likelihood that our galaxy abounds with intelligent life. Complex life doesn’t emerge instantly when you add water to dirt. “Alien Earths” walks us through the fundamentals of what life is. This takes us back to the beginning of time: the Big Bang that created a hot young cosmos made of hydrogen and helium and not much else. Stars ignited, cooked up heavier elements in their cores, and then exploded, spewing material into deep space. The vast clouds of gas and dust, now laden with a variety of elements, condensed into new stars and planetary systems. We know that on at least one planet, the third rock from its parent star, life — chemistry in its most interesting form — took hold and evolved.

And then, after about 3 billion years, there were forms most beautiful: clams, jellyfish, worms. Eventually trees, flowers, dinosaurs. Furry critters. Astrophysicists. Readers.

That, at least, is the scientific origin story, one that Kaltenegger explains patiently and clearly. There is nothing supernatural about this story — but if it doesn’t blow your mind just a little bit, I don’t know what will.