Astrobiology

Astrobiology is the scientific study of extraterrestrial life and the search for planets and environments where that life may exist. Astrobiology combines elements of several scientific fields, including astronomy, microbiology, chemistry, atmospheric science, oceanography, and geology. While definitive evidence of life beyond Earth has yet to be uncovered as of the early twenty-first century, scientists have made numerous discoveries that suggest such life may be possible. Signs of liquid water have been found in Earth's solar system, thousands of planets have been observed orbiting distant stars, and the chemical building blocks that make life possible have been discovered to be common in the universe. Astrobiology is a relatively recent and evolving scientific field with more questions at the moment than answers. While those questions are many, astrobiologists are generally concerned with several broad inquiries. What conditions are needed for life to arise? Are there other planets capable of supporting life? Is there evidence life exists outside of Earth?

Brief History

To the people of the ancient world, the sky was the realm of gods and the stars that dotted the night were fires on the dome of a giant sphere. Earth held a special place in the cosmos, fixed at the center of existence while the universe revolved around it. As human knowledge increased, science discovered that Earth was one of several planets orbiting the Sun and that the points of light in the sky were other suns. In the late sixteenth century, Italian philosopher Giordano Bruno was one of the first to propose that other Earth-like planets circled those distant suns and that life must exist upon these worlds. For such "heretical" words, Bruno was burned at the stake; however, within a century, other philosophers and astronomers were openly speculating about the existence of life elsewhere. Eighteenth-century astronomers saw the craters of the Moon as evidence of a "Lunarian" civilization, while in the late nineteenth century the idea that Mars was inhabited by a race of canal-building beings was widely believed.

The dawn of the space age in the mid-twentieth century dispelled any notions of advanced life in the solar system. The environments of the planets were just too inhospitable to support anything close to a human-like being let alone a civilization. The Moon's craters were just the scars left behind by asteroid impacts, and the Martian canals were nothing more than an optical illusion. Yet, astronomers began to see the tools at their disposal as a way to search for extraterrestrial life in a reasoned, scientific manner. Scientists began an attempt in the 1950s to understand what kind of biological life could exist outside Earth. This science was dubbed exobiology. In 1959, the newly formed National Aeronautics and Space Administration (NASA) established its own exobiology program.

In the early 1960s, scientists began to listen to the heavens in hopes of detecting radio signals from alien civilizations. The project grew to become the Search for Extraterrestrial Intelligence (SETI). The first attempt to find signs of life on the surface of another world was undertaken in 1976 when the Viking 1 and 2 landers touched down on Mars. The probes returned tantalizing hints of the presence of microbes, but ultimately the evidence showed no life was present in the Martian soil. The disappointing results were a setback for NASA's exobiology program, but the search for life in the universe continued in other fields. In the early 1980s, astronomers detected planetary material around another star and a decade later had discovered the first planets outside Earth's solar system. Advancements in biological and planetary science plus a changing attitude toward space exploration prompted NASA to rethink its exobiology program in the mid-1990s. To focus on the many scientific disciplines involved in the search for extraterrestrial life, the agency rebranded the program as the field of astrobiology in 1995.

How Did Life Arise?

To understand how life could arise outside of Earth, scientists often look at the process of how it formed on Earth. Every organism on the planet, from the tiniest microbe to the largest dinosaur that ever lived, is made of six basic elements: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S). All life on Earth is carbon-based because carbon can easily form chemical bonds with other carbon atoms. These bonds create chain-like structures that link with other atoms, such as hydrogen, nitrogen, and oxygen, to form more complex organic molecules. Phosphorus and sulfur atoms are necessary to provide energy for the chemical processes that make life possible.

These elements are formed in the hearts of stars and are among the most abundant in the universe. Stars are powered by nuclear reactions that fuse together hydrogen atoms to form helium. A star about the size of the Sun has so much hydrogen fuel that it takes many billions of years to exhaust its supply. When it does, it begins to cannibalize its helium, using it as fuel to make more complex atoms such as carbon, nitrogen, oxygen, all the way up to iron. A star in this phase is in the process of dying, but before it does, it will expand into a red giant and eventually blow off its outer layers into space. A star much larger than the Sun undergoes a more violent end, blasting apart in a violent explosion called a supernova. Elements more complex than iron are created in a supernova explosion.

The atmosphere of the early Earth contained an abundance of the six elements of life in the form of hydrogen, water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), ammonia (NH₃), and sulfur dioxide (SO₂). Scientists have found that by adding an energy source—such as electricity to simulate a lighting strike—to a recreation of the primordial atmosphere, the chemicals form a number of amino acids. Amino acids are organic chemical compounds that combine to form proteins and are essential elements of life. Further experiments have demonstrated that amino acids can form under other conditions as well, such as those found near volcanic vents on the ocean floor.

How life took the next step from organic compounds to simple organisms is unknown. Some theories hold the change was sparked by ultraviolet light from the sun, while others speculate the catalyst was a chemical compound brought to Earth by meteorites. No matter what the answer, the basic building blocks of life are common not only on the early Earth but also throughout the universe. Astronomers have discovered clouds of organic molecules in regions where new stars are being born. These compounds are not alive or a sign of life. They are simply the basic elements needed for life to exist under the proper conditions.

Habitable Worlds

In January 1992, astronomers discovered the first planets outside our solar system. These rocky exoplanets orbited the shell of a dead star about 2,300 light-years from Earth. Life was not possible on the planets as they were under constant bombardment from radiation given off by the corpse of the star. Three years later, a planet was discovered orbiting a star about the size of the Sun, but it was so close to the star that life was impossible. In the ensuing decades, thousands of exoplanets have been discovered orbiting distant stars, but only a few are in a narrow region capable of supporting possible life. Astronomers call this habitable region the Goldilocks Zone, after the fairy tale character who liked things "just right."

Scientists have identified a number of factors that can make an exoplanet just right for life to form. That does not mean that life will develop on those worlds, only that the conditions are similar to those that allowed life to form on Earth. One of the main conditions necessary for life is the proper temperature. If a planet is too far from its stellar heat source, the chemical reactions needed for life will occur too slowly or water may be frozen in solid form. If the temperature is too hot, organic molecules and proteins will begin to break apart and water will evaporate. Astrobiologists estimate life can form only within a range of about 5 degrees Fahrenheit (-15 degrees Celsius) to 239 degrees Fahrenheit (115 degrees Celsius). The range is important because it allows water to remain in liquid form; even at the extremes, water can remain liquid under certain circumstances. Water is considered necessary for life because it helps dissolve and transport chemicals within living cells.

Life is also only considered possible on a planet with a suitable atmosphere. Not only does an atmosphere contain the chemical elements needed to make organic molecules, it can trap heat to warm the surface and protect it from harmful radiation. Smaller planets do not have enough gravity to keep an atmosphere, while large gas giants are made up almost entirely of atmosphere and have little or no solid surface. Even on a rocky world, if an atmosphere is too thick, it can trap too much heat and create a hot environment. Venus, for example, is about the same size as Earth, but its atmosphere is made almost entirely of thick, greenhouse gasses and has an average temperature of about 865 degrees Fahrenheit (463 degrees Celsius).

By 2025, NASA had cataloged more than 5,800 planets outside Earth's solar system. Of those planets, the overwhelming majority were gas giants or ice giants. Hundreds of Earth-sized planets have been found, but only a few of those orbited inside the habitable zone. The first, named Kepler-186f, was discovered in April 2014 and is about 500 light-years from Earth in the constellation Cygnus. In February 2017, a system of seven Earth-like planets was discovered orbiting within the habitable zone of a star 40 light-years away in the constellation Aquarius. The system was named TRAPPIST-1 after the Transiting Planets and Planetesimals Small Telescope that aided in its discovery. Since the discovery, scientists have been studying the TRAPPIST-1 planets and evaluating whether they could support life.

Evidence of Life

Kepler-186f and the TRAPPIST-1 system are only in the right place for life to form; no evidence of life has been found there or anywhere else in the universe. To find life on these worlds, scientists would need technology that could detect certain atmospheric chemicals, such as a combination of methane and oxygen or water vapor, that may signal life's presence. If life has evolved to a certain point, the planet's atmosphere may contain traces of hydrocarbon fuels, but finding such evidence is considered highly unlikely. Humans have been scanning the skies for radio signals from space for decades, and have so far found no evidence of other civilizations. While astronomers believe that life must exist somewhere outside of Earth, the universe is so large that finding it could take decades or centuries of effort.

Astrobiologists are also focusing their search for life on Earth's solar system, especially Mars and the moons of Jupiter and Saturn. While these places are believed too inhospitable to have given rise to complex life-forms, scientists speculate that microbes and primitive life may exist in some form on these worlds. Mars does contain water ice, and scientists have found evidence that water flows in liquid form in some spots, but the planet's atmosphere is very thin, making it difficult for liquid water to keep from evaporating into space. Evidence suggests that ancient Mars may have had oceans and lakes of water on its surface, raising the possibility that microbial life may have taken hold at one point; however, no proof has been found of its existence.

Despite being outside the Sun's habitable zone, Jupiter's larger moons, Europa and Ganymede, may have oceans of liquid water trapped miles beneath their frozen surfaces. Formed by gravitational tidal forces from Jupiter, these oceans would normally be too cold to support life, but some scientists believe microbes or other simple life could form near volcanic vents on the ocean floors. Saturn's moon Titan has an atmosphere with some of the same chemical elements as Earth and is covered with lakes of organic molecules. Another of Saturn's moons, Enceladus, is a geologically active world with geysers of water, ammonia, and organic molecules. The chemistries of these worlds have led to speculation life could form using other elements, such as liquid methane in place of water or silicon in place of carbon. While scientists have shown silicon-based life to be theoretically possible, the chemical bonds formed with silicon are less stable than with carbon and less likely to bond with oxygen.

Bibliography

Carney, Stephen, editor. "The Habitable Zone," NASA, 3 Feb. 2025, science.nasa.gov/exoplanets/habitable-zone/. Accessed 6 Feb. 2025.

Choi, Charles Q. "Possibility of Silicon-Based Life Grows." Livescience, Future US, 18 Apr. 2017, www.livescience.com/58727-silicon-based-alien-life-possible.html. Accessed 6 Feb. 2025.

Cockell, Charles S. Astrobiology: Understanding Life in the Universe. 2nd ed., Wiley-Blackwell, 2020.

"Exoplanets." NASA, 30 Jan. 2025, exoplanets.nasa.gov/. Accessed 6 Feb. 2025.

Gough, Evan. "How Will NASA Find Life on Other Worlds?" Universe Today, 10 Mar. 2017, www.universetoday.com/134228/will-nasa-find-life-worlds/. Accessed 6 Feb. 2025.

Kaufman, Marc. "A History of Astrobiology." Astrobiology at NASA, 7 Aug. 2022, astrobiology.nasa.gov/about/history-of-astrobiology/. Accessed 6 Feb. 2025.

Lemonick, Michael D., and Mark Thiessen, photographer. "The Hunt for Life beyond Earth." National Geographic, www.nationalgeographic.com/astrobiology/. Accessed 6 Feb. 2025.

"The Possibilities and Search for Life in Our Solar System." Las Cumbres Observatory, lco.global/spacebook/astrobiology/possibilities-and-search-life-our-solar-system/. Accessed 6 Feb. 2025.

Tillman, Nola Taylor. "Red Giant Stars: Facts, Definition & the Future of the Sun." Space.com, Future US, 29 July 2023, www.space.com/22471-red-giant-stars.html. Accessed 6 Feb. 2025.

"What Makes a World Habitable?" Lunar and Planetary Institute, www.lpi.usra.edu/education/explore/our‗place/hab‗ref‗table.pdf. Accessed 6 Feb. 2025.

Wolchover, Natalie. "What Are the Ingredients of Life?" Livescience, Future US, 2 Feb. 2011, www.livescience.com/32983-what-are-ingredients-life.html. Accessed 19 May 2017.