Life and the Dark Side of Biology on Exoplanets

Gene Roddenberry populated his Star Trek universe with a wide variety of aliens. Budget constraints dictated that most were variations of humans, with skin tinted odd colors or antennae sticking out from their heads. Even the silicon-based Horta appeared to be a stagehand lurking under a decorated carpet. George Lucas treated us to a similar menagerie of off-world inhabitants in Star Wars, especially in his Mos Eisley Cantina. Aliens—and our concept of them—became more sophisticated as budgets soared and science grappled with the great question posed by Enrico Fermi: “Where is everybody?” Some were terrifying, like the creatures in the Alien movie series or H. G. Wells’s conspiring Martians in War of the Worlds. Perhaps our propensity for seeing extrasolar life as terrifying is our natural fear of the unknown. But others were far more benign and advanced, as witnessed by Arthur C. Clarke’s 2001: A Space Odyssey, Steven Spielberg’s cuddly E.T., Edmund H. North’s guardians of the worlds in The Day the Earth Stood Still, or the time-jumping beings of Eric Heisserer’s Arrival. But at the heart of a good story is a good conflict, and aliens provide natural fodder for such a plot device.

NASA Ames astrobiologist Chris McKay is definitely a fan of science fiction. When asked about the search for life in some corners of our own solar system, he paraphrases Star Trek’s famed physician, Dr. McCoy: “It’s dead, Jim.” But McKay doesn’t think this is the case for all planets out there, or he wouldn’t be in the business he’s in. In fact, he is optimistic.

McKay cites several reasons for his positive outlook. First, evidence for life on Earth has been found in the oldest sedimentary rocks on Earth, dating back to about 3.8 billion years ago. Life got started early and under harsh conditions.

Second, we seem to be surrounded by environments similar to the early Earth, with the key ingredients of liquid water, nitrogen, and carbon. Specifically, in our own solar system, we have the warmer, wetter early conditions on Mars and we have the current global seas of Enceladus. Lurking beneath the ice crust of Jupiter’s moon Europa may also be an environment similar to Earth’s deep oceans. “There are probably many hundreds of millions of exoplanets and exomoons that fall into the same class,” McKay says.

The third reason for optimism concerns the elements of life, McKay says. “The four elements that make up 99.5 percent of the elements in life are hydrogen, oxygen, carbon, and nitrogen, and these are the first, third, fourth, and sixth most abundant elements in the universe, respectively. Note that numbers two and five are helium and neon and it’s not surprising that life does not use them.” McKay’s fourth reason for optimism is that organic material is easily formed from carbon in a variety of environments. “We see this in meteorites, in the interstellar medium, and in the Miller-Urey experiments.” Those experiments took a primordial brew of methane, carbon, and other elements, introduced radiation or electricity similar to lightning, and decanted a product of complex chains of organic material like amino acids.

Lastly, McKay points out that when life starts in one place, there appear to be ways it can be carried to other worlds through meteor impacts that blast life-bearing material into interplanetary space. These meteors eventually make landfall on other planets, as we saw with the Allan Hills meteorite. It may be that such journeys are possible from one star to another, with life in suspended stasis for the eons it takes to travel between solar systems. In many regions, stars are more densely packed than they are in our neighborhood, making such a delivery even more likely.

McKay concludes, “To quote Calvin to Hobbes. ‘Let’s go exploring’.

Some of the most ancient microfossils are complex one-celled microbes. Examples from the Columbia River valley have been documented with external sheaths and segmented membranes. A team in Greenland uncovered fossil evidence of structures left behind by ancient microbes in 3.7-billion-year-old rocks. Those rocks were from a shallow sea, while other primordial microfossils resided in superheated water of deep-sea hydrothermal vents. Life must have begun before these, and the two very different environments in which the fossils lived shows that life diversified early in the Earth’s history.

How did life start on our planet? The question plagues biologists, but they understand, at least, the elements that it takes to make life. The early Earth’s environs had no free oxygen in the air, presenting a truly alien environment. The Moon was closer, looming large in the sky with its seas of molten lava from great impacts of asteroids. Earth’s atmosphere contained a mix of water, nitrogen, and carbon, all readily available to living systems. Lightning was in the air and the active Sun added its own influences, pouring radiation into all those carbon atoms. The energy from both sources triggered chain reactions in the chemistry of atmosphere and ocean, fusing carbon atoms with other materials to produce complex chains of organics, the building blocks of life.

We do not know whether intelligent life exists across the universe (even if we count ourselves), but if it does, will we find friend or foe? Will we find, as H. G. Wells had it, “intelligences greater than man’s and yet as mortal as his own”? Or when we stumble upon life, will it be so fundamentally different from ours that we will not even recognize it? In his 1961 novel Solaris, author Stanisław Lem pens a global alien entity whose thoughts are so different from human intellect that any meaningful dialogue proves unattainable.

Another disturbing possibility is that advanced life always evolves to face a crisis point, and that many do not survive it. For humanity, that crisis may come in the form of nuclear or biological warfare, fatal powers that we unleash upon ourselves. Whatever the crisis, it may be that advanced civilizations across the galaxy are rare because they do not survive into an old age.

Most astrobiologists suggest that debating intelligent life may be getting ahead of ourselves. Simple and basic life-forms, some argue, are far more likely to be discovered than ones building shopping malls. Microbial creatures may not necessarily lead to complex ones as they may not survive the long-term changes in habitable zones, shifts in steady orbits, disturbances of stable environments, and extinction events like large impacts, all of which may cut short the progression of life from simple to complex. Microbial creatures or other simple life may be the common denominator throughout the exoplanets. Life may cling to the shorelines on planets within habitable zones, but it also may thrive in submarine environments on ice planets or moons.

Envisioning Exoplanets is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from Envisioning Exoplanets © 2020 by Elephant Book Company Limited