When people talk about the search for extraterrestrial life, they often think of flying saucers and what science now calls unidentified anomalous phenomena (UAP). However, if scientists ever discover life beyond Earth’s borders, it will likely be in the places where it originated, rather than flying across Earth’s skies.
When NASA released its report on UAP, it argued that people needed to start taking the analysis of these phenomena seriously. But UAPs are one thing—unidentified phenomena likely unrelated to extraterrestrial life—and the search for life on other planets is another.
This search has long been taken seriously by space agencies and researchers around the world, so much so that it has its own associated scientific field: astrobiology.
The discipline’s name is self-explanatory, but it’s not easy to understand the questions it must answer, the methods it must use, and the potential impact of its findings.
When humanity sets out to search for life elsewhere in the universe, perhaps the first question to ask is where. The most frequently repeated answer is Mars. This planet is, in many ways, the most Earth-like in the solar system. In terms of size, climate, geology, and atmosphere, Mars is tied with Venus as the most Earth-like planet.
In the popular imagination, Mars has long been the home of extraterrestrials. Still, it’s also a prime candidate for harboring life because it contains water deposits, remnants of a geological era when the planet was partially covered by water.
Mars is the only place where researchers have searched for extraterrestrial life in situ. They’ve done so with Viking, two uncrewed missions to the planet, and continue to do so with the Perseverance rover, which just reached the rim of the Jezero Crater after 3.5 months and a 1,640-foot vertical climb.
Carl Sagan poses with a model of the Viking lander in Death Valley, California. Image | NASA
Venus is a hostile planet, so exploring its surface is one of the outstanding tasks of space agencies. A few years ago, the discovery of a gas with biomarker potential—phosphine—briefly revived the possibility of bacterial life on the planet. The study was mired in controversy, and subsequent studies have failed to find the gas. Although phosphine can be produced by biological processes, its existence could also be due to abiotic processes.
Over the years, other solar system bodies have attracted astrobiologists’ attention: icy moons. Oceans of liquid water exist beneath the icy layers of some of the solar system’s natural satellites, and experts suspect that many more such seas and reservoirs exist.
These moons could have all the “ingredients” for life. In addition to liquid water, these satellites could have hydrothermal activity capable of transferring thermal energy from the moon’s interior to the ocean and generating geochemical processes that could, in principle, trigger abiotic processes culminating in life’s emergence. Astrobiologists are interested in whether life exists and the conditions under which it can develop.
Several moons of Jupiter and Saturn, as well as some of Uranus’s satellites, are candidates. Enceladus is a good example. Experts have known for almost a decade that Saturn’s moon has liquid oceans and have detected not only hydrothermal activity but also key elements for life on Earth, such as carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur.
These icy moons may even be joined by a dwarf planet: Pluto. Its geology is more complex than you might think.
The search for life is more exhaustive than the far reaches of the solar system. In recent decades, astronomers have discovered more than 5,000 exoplanets, and the Milky Way may contain millions.
Today, experts cannot study exoplanets for biomarkers. Still, they screen for habitability based on whether the planets are in their star’s so-called “habitable zone,” where liquid water can exist. They’re also conducting studies to estimate the size and composition of these planets.
How Do You Search for Life on Other Planets?
Earth is the only place where life exists, so it’s the first place of interest. Experts rely on a mid-20th-century measurement: the Miller-Urey experiment.
The experiment’s idea was to use simple compounds like water, ammonia, or methane to simulate prebiotic conditions on Earth. By adding electrical energy to these simple compounds, they found amino acids could be produced.
Astrobiology isn’t only experimental but also involves fieldwork. Earth has many extreme ecosystems that allow astrobiologists to test the limits of life. The truth is that tenacious organisms on Earth can colonize everything from the bottom of the oceans to places of extreme temperature.
Work done from Earth can also be directed outward. One example is the SETI project, which shows that not all astrobiology focuses on microorganisms: Researchers also want to know if intelligent life exists elsewhere in the galaxy and beyond.
Radio telescopes, antennas capable of picking up waves in radio frequency bands and the surrounding electromagnetic spectrum, are particularly useful in this search. These radio telescopes have paved the way for countless discoveries in astrophysics and may one day lead experts to the discovery of other civilizations.
Not all telescopes are on Earth, some are in orbit. Although telescopes like Hubble or the James Webb Space Telescope (JWST) aren’t designed to discover life on other planets, they help scientists study exoplanets’ properties and learn important facts about them.
Image | NASA
The change may come thanks to a project still in its infancy: the Habitable Winged Observatory (HWO). This project has similarities to the JWST, but its function would be to search for habitable planets in our galactic neighborhood.
Going further, astrobiologists also have tools in the places they search. From probes like Cassini to rovers like Perseverance, they’ve sent dozens of vehicles to explore planets and satellites. While any information can be helpful, some of these rovers are more specialized in the search for life.
An example of a mission focused on a planet’s habitability is the Perseverance rover, which is studying the geology of Mars in a crater formed by water millions of years ago.
So far, however, the only missions specifically designed to search for life are the Viking missions. Some hypothesize that these probes found life on Mars and destroyed it—at least in the sample they analyzed unsuccessfully.
I said at the beginning that the name astrobiology is self-explanatory, but its scope goes far beyond that. Understanding the conditions under which life might have arisen elsewhere can also help answer a question that still hasn’t been fully resolved: How life arose on Earth.
A recent example of this is NASA’s OSIRIS-REx mission, which returned to Earth with the most significant sample of asteroid material ever collected by a space mission. Preliminary studies have already shown that the asteroid Bennu, from which the samples were taken, contains both water molecules and carbon fixed in organic molecules.
Regolith captured by the OSIRIS-REx probe. Image | NASA | Erika Blumenfeld | Joseph Aebersold
Researchers speculate that some of the key organic molecules for life and all the water in the oceans may have reached Earth through successive impacts of asteroids like Bennu.
But knowing how life originated on Earth isn’t enough. It’s also essential to understand how it evolved, what factors contributed to the emergence of complex life forms, and ultimately how intelligent life originated on Earth.
Until astrobiologists study enough planets, it will be impossible to know whether Earth’s circumstances are the exception or the norm in the universe. Until then, any answer to whether we’re alone in the universe will be a guess.
Images | NASA
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