Before 1976, when Viking 1 and 2 became the first spacecraft to successfully land and operate on the surface of Mars, the global imagination longed desperately for a red planet that harbored life. The Viking landers were designed to test for microbes, but the real hope, held by even the most jaded planetary scientists, was that NASA’s spacecraft would discover complex life on Mars—something that scurried, or maybe a scraggly shrub. Mars, after all, was our last, best hope after astronomers (and the Mariner 2 spacecraft) forever vanquished the notion of dinosaurs stamping across humid, Venusian bogs. It was Mars or bust; Mercury was just too close to the sun, and beyond the asteroid belt, it was believed, lay a no microbe’s land of gas giants and frozen moons.
The exploration of the solar system since Viking has represented a world-by-world grasping for something—anything—that might suggest life as we know it (or life as we don’t). Today the oceans of Jupiter’s moon Europa are what the swamps of Venus and canals of Mars were for the twentieth century: perhaps the best option for annihilating human loneliness. NASA’s next outer planets flagship mission, Europa Clipper, will attempt to determine the icy moon’s habitability. Some future lander or swimmer will have to find the life if it’s there. The habitable zone of the solar system now includes, potentially, every planet in the solar system. Enceladus and Titan, circling Saturn, are good candidates, as is Triton around Neptune. Like water, life might be everywhere.
And yet we have found it only here, where it teems—where it is seemingly indestructible, despite multiple extinction-level events. An asteroid collides with the Earth and wipes nearly everything out? Microbes make a home in the cracks caused by the killer impactor, and it all begins again. Based on our sample of a single world, once life begins, it is very, very hard to make go away. And so we keep searching.
The sparking of life from lifelessness—known as abiogenesis—is a process that scientists are only beginning to understand. Astronomers, biologists, chemists and planetary scientists work together to painstakingly piece together a puzzle that crosses disciplines and celestial objects. For example, carbonaceous chondrites—some of the oldest rocks in the solar system—were recently found to harbor pyruvic acid, which is essential for metabolism. When chondrites rained down on this planet as meteorites, they may well have fertilized a lifeless Earth. This theory doesn’t answer the all-consuming question, “Where did we come from?” But it does represent yet another clue in the search for how it all began.
Abiogenesis doesn’t even require DNA—or at least, not DNA as it exists in all known lifeforms. DNA consists of four nucleotide bases, but earlier this year, geneticists created a synthetic DNA using eight bases. (They dubbed it hachimoji DNA.) This strange genetic code can form stable double-helixes. It can reproduce. It can even mutate. The scientists did not create life; they did, however, prove that our conception of life is provincial at best.
While work in laboratories will help define how life could spring from inanimate matter, space telescopes like Kepler, which ended operations last year, and TESS, which launched last year, are finding new planets to study. These spacecraft search for exoplanets using the transit method, detecting minute decreases in a star’s light as a planet passes between it and us. Twenty-five years ago, the existence of planets orbiting other stars was hypothetical. Now exoplanets are as real as those circling our sun. Kepler alone discovered at least 2,662 exoplanets. Most are inhospitable to life as we know it, though a handful are sometimes characterized as “Earth-like.”
“When we say, ‘We found the most Earth-like planet,’ people sometimes mean that the radius is right, the mass is right, and it has to be in the habitable zone,” says John Wenz, author of The Lost Planets, the story of early exoplanet hunting efforts, to be published later this year by MIT Press. “But we know that most of those discovered exoplanets are around red dwarf stars. Their environment isn’t bound to be very Earth-like, and there’s a good chance a lot of them aren’t going to have atmospheres.”
It’s not that Earth is the most special planet in all the universe. In our solar system, Venus would easily register to alien exoplanet hunters as Earth’s twin. But planets truly like Earth are more difficult to find, both because they are smaller than gas giants, and because they don’t orbit their host stars as closely as planets around red dwarfs.
“It could be that true Earth-like planets are incredibly common, but that we don’t have the resources to dedicate to their search,” Wenz says. The most promising Earth 2.0 exoplanet found so far is Kepler-452b, which is somewhat larger than Earth, with a bit more mass, and has a pleasing 385-day orbit around a sun-like star. The problem is that it might not exist, as a study suggested last year. It might simply be statistical noise, as its detection was on the margins of Kepler’s capabilities, and the spacecraft died before further observations could be conducted.
Once it launches in the early 2020s, the James Webb Space Telescope will target many of the exoplanets discovered by Kepler and TESS. It will only be able to resolve the distant worlds to a pixel or two, but it will answer pressing questions in exoplanet science, such as whether a planet orbiting a red dwarf star can hold on to its atmosphere despite the frequent flares and eruptions from such stars. JWST might even present indirect evidence of alien oceans.
“You won’t see continents,” Wenz says. “[But] you might look at something and see a blue dot, or the kind of off-gassing you would imagine from a continuous evaporation cycle.”
The Abiogenesis Zone
The Habitable Exoplanet Catalog currently lists 52 worlds outside our solar system that might support life, though the news might not be quite so thrilling as that. Being the correct distance from a star for surface temperatures to hover above freezing and below boiling is not the only requirement for life—and certainly not the only requirement for life to start. According to Marcos Jusino-Maldonado, a researcher at the University of Puerto Rico at Mayaguez, the correct amount of ultraviolet (UV) light hitting a planet from its host star is one way that life could rise from organic molecules in prebiotic environments (though not the only way).
“For reactions allowing abiogenesis to appear, a planet must be inside the habitable zone because it needs liquid surface water,” Jusino-Maldonado says. “According to the primordial soup theory, molecules and salty water react and eventually originate life.” But those reactions are believed to spark only in a place called the abiogenesis zone. “This is the critical area around the star in which precursor molecules important to life can be produced by photochemical reactions.”
UV radiation may have been the key to sparking reactions that lead to the formation of life’s building blocks on Earth, such as nucleotides, amino acids, lipids and ultimately RNA. Research in 2015 suggested that hydrogen cyanide—possibly brought to Earth when carbon in meteorites reacted with nitrogen in the atmosphere—could have been a crucial ingredient in these reactions driven by UV light.
To test the theory further, last year, as reported in the journals Science Advances and Chemistry Communications, scientists used UV lamps to irradiate a mixture of hydrogen sulfide and hydrogen cyanide ions. The resulting photochemical reactions were then compared to the same mixture of chemicals in the absence of UV light, and the researchers found that UV radiation was required for the reactions to produce the precursors to RNA necessary for life.
For UV photochemistry to produce these cellular building blocks, the wavelength of UV light must be around 200 to 280 nanometers. Jusino-Maldonado says that in his work, this concept was applied to the habitable exoplanet model. “Of all the habitable exoplanets, only eight of them are found within the habitable zone and the abiogenesis zone.”
Although all eight are in both habitable zones and abiogenesis zones, none are particularly favorable for life, Jusino-Maldonado says. Each of the eight worlds is either a “super-Earth” or a “mini-Neptune.” The most likely candidates are Kepler-452b (if it exists) and maybe τ Cet e (if its radius is appropriate). No Earth-sized worlds have yet been discovered in both the habitable and abiogenesis zones.
As the search for a truly habitable alien world marches on, astrobiologists are attempting to create a framework to categorize, discuss and study these planets. Big scientific endeavors to work require standards of definition and measurement. Astrobiology is a young field of study, relatively speaking, and one of the pressing, nontrivial questions it faces is, how do you define habitability? How do you define life?
“I have been working on this problem for ten years,” says Abel Mendéz, a planetary astrobiologist and Director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. “I knew the habitability problem needed work. Everybody was dealing with how to define it.” Earlier this year, at the 50th annual Lunar and Planetary Science Conference in Houston, Texas, Mendéz presented his recent work on a global surface habitability model applicable to planets both in our solar system and outside it.
After combing through the literature, he realized that astrobiologists weren’t the first to run into problems of definition, categorization and uniformity with regard to habitability. Forty years ago, ecologists were dealing with the same challenge. “Everybody was defining habitability as they wished in different papers,” Mendéz says. In the 1980s, ecologists came together to create a formal definition. They hammered out averages to measure habitability, developing a system with a range from 0 to 1, with 0 being uninhabitable, and 1 being highly habitable.
Having a singular framework was critical to the advancement of ecology, and it has been sorely lacking in astrobiology, Mendéz says. Building a habitability model for whole planets began with identifying variables that can be measured today. “Once you develop a formal system, you can build systems from that, and create a library of habitability for different contexts.”
First, Mendéz had to deal with the only habitat suitability measurement of “1” in the known universe. “If you are proposing a habitability model, you have to make Earth work,” he says. His lab used his model to compare the habitats of various biomes, such as deserts, oceans, forests and tundra.
“If we calculate the habitability of a region—not considering the life, but how much mass and energy is available for independent life—it is more of an environmental measurement. We correlate that with an actual measurement of biological productivity in a region: our ground truth. That’s our test.” When his group charted environmental habitability and biological productivity, they found what Mendéz described as “nice correlations.”
Today, Mendéz’s model for habitability takes into consideration rocky planets’ ability to support surface water, the age and behavior of their stars, and the orbital dynamics and tidal forces acting on these worlds. The model considers the mass and energy within a system and the percentage of said mass and energy available to a species or biosphere. (That percentage is the hardest part of the equation. You couldn’t claim 100 percent of Earth’s mass, for example, is available to life.)
Limited to the “near-surface thin layer of a planetary body,” the model pegs the surface habitability of Earth at 1, early Mars to be less than or equal to 0.034, and Titan to be less than or equal to 0.000139. The model is independent of the type of life under consideration—animals versus plants, for example—and worlds like Europa with “subsurface biospheres” are not yet accounted for.
Such groundwork is invaluable, but it is still limited in its ability to predict habitability, partially because it only applies to life as we know it. In 2017, Cornell researchers published a paper revealing evidence of the molecule acrylonitrile (vinyl cyanide) on Titan, which, hypothetically, could be the key to methane-based life on an oxygen-free world—truly alien life, unlike anything we have ever known. Should life flourish on such a conventionally inhospitable world as Titan, and should we find it, Mendez writes in an abstract describing his model, “An anticorrelation between measures of habitability and biosignatures can be interpreted as an abiotic process or as life as we don’t know it.”
In any event, the lack thus far of worlds outwardly favorable for life means that humankind must continue improving its observatories and casting its eyes toward far-flung realms. It’s a big galaxy, filled with disappointments. We no longer hope for Martians digging waterways or dinosaurs reaching for moss on Venusian trees, but we still dream of squid swimming through Europan seas and who-knows-what lurking in the hydrocarbon lakes of Titan. If these worlds, too, fail to deliver, it’s up to the exoplanets—and they are just outside of our observational capabilities, and a very long way from home.