We have only one example of biology forming in the universe: life on Earth. But what if life could form in another way? How do you find alien life when you don’t know what it looks like?
These questions preoccupy astrobiologists—scientists searching for life beyond Earth. Astrobiologists have attempted to suggest universal rules that govern the emergence of complex physical and biological systems on Earth and beyond.
I am an astronomer who has written extensively on astrobiology. Through my research, I learned that the most abundant form of extraterrestrial life is likely to be microbial. This is because single cells can form more easily than large organisms. But just in case there is evolved extraterrestrial life, I am on the international advisory board of a group that designs a message to be sent to that civilization.
Detection of life beyond Earth
Since the first exoplanets were discovered in 1995, more than 5,000 extrasolar planets, or planets, have been discovered orbiting other stars.
Many of these exoplanets are small, rocky, like Earth, and located in the habitable zones of their stars. The habitable zone is the range of distances between a planet’s surface and the star it orbits, which allows the planet to have liquid water, allowing life as we know it to exist on Earth.
The exoplanet samples discovered so far point to 300 million potential biological experiments in our galaxy. That means there are 300 million places with suitable conditions for biology to occur, including exoplanets and other celestial bodies like the moon.
Researchers’ uncertainty begins with the definition of life. It seems easy to define life because we can know life when we see it, whether it is a flying bird or a microorganism moving in a drop of water. But scientists disagree on the definition, and some believe a comprehensive definition may not be possible.
NASA defines life as “self-sustaining chemical reactions capable of Darwinian evolution.” This refers to organisms with complex chemical systems that evolve to adapt to their environment. According to Darwinian evolution, the survival of an organism depends on its adaptation to its environment.
The evolution of life on Earth progressed over billions of years from single-celled organisms to large animals and other species, including humans.
Exoplanets are difficult to study because they are distant and hundreds of millions of times dimmer than their host stars. Using a method called spectroscopy, astronomers can examine the atmospheres and surfaces of Earth-like exoplanets to look for chemical signatures of life.
Spectroscopy can detect oxygen signals in a planet’s atmosphere, such as chlorophyll signals that indicate plant life or microorganisms called blue-green algae that were produced through photosynthesis on Earth billions of years ago.
NASA’s definition of life leads to several important but unanswered questions. Is Darwin’s theory of evolution universal? What chemical reactions could lead to biology outside of Earth?
Evolution and Complexity
All life on Earth, from fungal spores to blue whales, evolved from the last common ancestor of microorganisms about 4 billion years ago.
The same chemical processes can be seen in all life forms on Earth and these processes may be universal. It may also be radically different elsewhere.
In October 2024, a diverse group of scientists gathered to think outside the box about evolution. They wanted to figure out how to study the emergence of life completely different from life on Earth by taking a step back and exploring what kinds of processes, biological or otherwise, created order in the universe.
Two researchers have argued that complex systems of chemicals or minerals evolve to store greater amounts of information in environments where some configurations are better maintained than others. Over time, systems will become more diverse and complex, acquiring the features they need for survival through a form of natural selection.
They speculated that there might be laws that explain the evolution of various physical systems. Biological evolution through natural selection is just one example of this broader law.
In biology, information refers to the instructions stored in the nucleotide sequences of DNA molecules, which collectively make up an organism’s genome and direct the organism’s appearance and function.
Defining complexity from an information theory perspective, natural selection causes genomes to become more complex as they store more information about the environment.
Complexity can be useful in measuring the boundary between life and non-life.
However, it would be wrong to conclude that animals are more complex than microorganisms. Biological information increases with genome size, but evolutionary information density decreases. Evolutionary information density is the proportion of functional genes in a genome, that is, the proportion of total genetic material that expresses fitness for the environment.
Organisms that people think of as primitive, such as bacteria, have information-dense genomes that appear better designed than the genomes of plants or animals.
A universal theory of life remains elusive. Such theories include concepts of complexity and information storage, but do not relate to DNA or to the specific types of cells found in Earth biology.
Implications for the search for extraterrestrial life
Researchers have been exploring alternatives to terrestrial biochemistry. All known living organisms, from bacteria to humans, contain water, and water is an essential solvent for life on Earth. A solvent is a liquid medium that promotes the chemical reactions that can give birth to life. But life could potentially appear in other solvents as well.
Astrobiologists Willam Bains and Sara Seager explored thousands of molecules that could be related to life. Plausible solvents include sulfuric acid, ammonia, liquid carbon dioxide, and even liquid sulfur.
Extraterrestrial life may not be based on carbon, which forms the backbone of all life’s essential molecules, at least not here on Earth. You may not need a planet to survive.
Advanced forms of life on exoplanets may be so strange that they are unrecognizable. Astrobiologists have to get creative when trying to detect life beyond Earth.
One strategy is to measure mineral signatures on the rocky surfaces of exoplanets because mineral diversity tracks biological evolution on Earth. As life evolved on Earth, it used and created minerals for exoskeletons and habitats. The 100 minerals that existed when life first formed have grown to about 5,000 today.
For example, zircon is a simple silicate crystal that dates back to times before life began. Zircons discovered in Australia are known to be the oldest pieces of the earth’s crust. However, other minerals, such as apatite, a complex calcium phosphate mineral, are produced by biology. Apatite is a major component of bones, teeth, and fish scales.
Another strategy to find life forms different from Earth is to look for evidence of civilization, such as artificial lighting or industrial pollutants such as nitrogen dioxide in the atmosphere. This is an example of an intelligent life tracker called a technological signature.
It is unclear when and how life beyond Earth was first discovered. It could be inside our solar system, we could smell the atmosphere of an alien planet, or we could detect artificial radio signals from distant civilizations.
Search is a winding road, not a straight one. And that’s about life as we know it. As for the life we ​​don’t know, all bets are off.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Image credit: NASA’s Goddard Space Flight Center/Francis Reddy
