Where do we begin? To answer this question, we must look to the reaches of the universe and the mysteries it contains. The dawn of life on Earth is believed to have begun with the first cells burning energy in a hollow on the Earth’s newly formed surface, or a superheated vent at the bottom of an ancient sea. But understanding life does not stop there. We must look to the stars and their formation from protoplanetary disks, as well as the energy and chemistry of planetary surfaces and atmospheres to gain a deeper understanding of life. All these cosmic forces conspire to create a complex web of life, stretching from a single cell on Earth to distant stars and planets beyond.
The discovery of over 5,000 exoplanets in our Milky Way galaxy has sparked a great deal of interest in the possibility of other life forms existing beyond our own planet. With more sophisticated telescopes being developed and used to scan the sky, we are presented with better tools than ever before to gain insight into these distant worlds. To find answers to that timeless question “Are we alone?”, we must first understand what such an answer would mean. This means considering the conditions of a planet that would make it hospitable to life, what evidence there might be for life existing on those planets, and how we would go about detecting it. While these tasks may seem daunting, the availability of better tools for studying the universe is making them more manageable than ever before.
Mary Voytek, director of the NASA Astrobiology Program at the agency’s headquarters in Washington, once said, “We don’t know where to look or what to look for if we don’t understand what happened on Earth.” Voytek’s statement highlights the importance of understanding our own planet before we can search for life in other places. To answer this challenge, Voytek suggests breaking it down into pieces and starting with what we already know about Earth. With this knowledge, we can use the clues available to us to piece together the puzzle of how life began and evolved.
NASA’s astrobiology research is a fascinating exploration into understanding the origins and requirements of life in the cosmos. It begins with understanding our very own star, the Sun, which was born out of a swirling cloud of gas and dust containing the essential ingredients for life such as carbon, water, ammonia, methane, and other molecular building blocks made from elements created inside previous generations of stars. The study of these components in distant stars and planets throughout the universe can help us to identify whether they have the necessary conditions for life to exist.
Voytek’s comment about the start of life begins with the star. He explains that planets formed out of a disk of dense gas surrounding the newly formed star, and this relationship to the system is what made Earth habitable and supported the emergence and evolution of life. In addition to this, the habitable zone, the orbital distance from a star that allows liquid water to pool on a planet’s surface under a suitable atmosphere, is another condition favorable to habitability. Liquid water is essential for life on Earth, and scientists believe it is also essential for life on other worlds. The diversity that we see on Earth in terms of temperature conditions and extreme environments are all dependent on liquid water, which is why it is so essential to the start and sustainment of life.
Venus, Earth’s twin in size and rocky composition, orbits too close to the Sun. On its surface, the temperature is hot enough to melt lead, making its current conditions inhospitable to liquid water. It is believed that liquid water may have existed on Venus in the past. Mars is located at the outer edge of the habitable zone, and its thin atmosphere and frozen surface make it highly unlikely that liquid water persists there today.
The icy moons of the outer solar system, with their hidden oceans of liquid water, have the potential to provide habitable conditions despite being well outside the traditional habitable zone. While exomoons are beyond the reach of our present remote sensing technology, astronomers can use the habitable zone and the possibility of surface water as a guide to help identify potential life-bearing targets. Such environments, similar to those found in our solar system, could exist elsewhere in the galaxy, providing new opportunities to explore the possibilities of extraterrestrial life.
Chemistry in the origin of life.
The search for the origin of life often involves a focus on molecules and chemistry. Scientists have proposed many theories as to how primitive, energy-consuming materials emerged on Earth some four billion years ago. An understanding of this process requires an exploration of how microscopic interactions and geochemistry could create a package of material that is defined as “alive”. Professor Betül Kaçar and her team at the University of Wisconsin-Madison’s Molecular Paleobiology Laboratory, as well as the NASA Interdisciplinary Consortium for Astrobiology Research (ICAR) project, Metal Utilization and Selection across Eons (MUSE), are researching the relationship between evolution, geochemistry, and early life in an effort to unlock the mystery of life’s origin. By getting the right chemistry, scientists can further understanding of the life cycle and how it began.
Dr. Kaçar has an extensive research background in the field of biology and the origins of life. She believes that life began through comet impact, shock synthesis, or hydrothermal vents, and believes these to be the most popular theories. To further explore ancient biological systems and how life used its environment to thrive, her research group investigates enzymes, proteins, and metabolism. They are using available DNA to explore the past, going back billions of years, to see how life began in the first place. Through their research they hope to uncover how cells were able to construct and break down cellular material.
Kaçar Voytek’s research into astrobiology has seen a shift in recent years towards exploring the behavior of ancient aggregations of molecules that may be seen as life-like, rather than simply synthesizing the chemical compounds associated with early life. These molecules, referred to as proto-molecules, are far more primitive than the efficient RNA and DNA that we are familiar with today and are able to store information or catalyze reactions, though less efficiently. According to Voytek, these proto-molecules should be regarded as life-like but not exactly life, indicating an new area of research in the field of astrobiology.
Voytek and Kaçar see a shift in our view of the history of life on Earth, expanding from the bottom of the deep ocean at hydrothermal vents to potential life-generating chemistry on the earliest land surface. This gives us a wider variety of energy sources, mineral diversity, and wet-dry cycles that can be incorporated into our understanding of the origin of life. Location and chemistry are of utmost importance in considering these possibilities. It is also possible that the components and functions of life could have arisen piecemeal over hundreds of millions of years before coming together to form recognizable living organisms.
What can we learn from the planets?
As advances in technology and observation of the universe grow, so does ability to find and learn more about exoplanets. Telescopes have revealed exoplanets of all varieties – rocky, gaseous, and in between. Examples include super-Earths, which may or may not be scaled-up, rocky worlds, and mini-Neptunes, which are less massive versions of own Neptune. Hot Jupiters and hot Saturns, which orbit their stars closely and are scorched by the heat of their nearby stars, are also part of the mix. Perhaps most intriguingly, there are also rogue planets that are free-floating through the depths of space without a parent star to call their own. As continue to uncover new exoplanets and new insights about them, understanding of the universe continues to expand as well.
NASA’s retired Kepler and the active Transiting Exoplanet Survey Satellite (TESS) have enabled to locate planets, while the James Webb Space Telescope has provided us with an awe-inspiring amount of images and atmospheric data. Coming soon is the Roman Space Telescope which, when launched in 2027, is predicted to discover an additional 100,000 exoplanets and test new technology for direct imaging of such planets. This telescope can be seen as a beacon that promises new revelations about universe and the secrets that it holds.
Future space telescopes that are even more powerful could search exoplanet atmospheres directly for signs of life, a process known as biosignature detection. This is a very exciting prospect for astrobiologists. To be able to look at the atmospheres of distant planets and perhaps find evidence of life would be groundbreaking. But even if these future telescopes could detect biosignatures in exoplanet atmospheres, still need to be able to recognize signs of life in planets that may not resemble Earth as it is now. In order to find biosignatures on these planets, must be able to recognize them even if the conditions on the planet are very different than today’s Earth. This requires a deeper understanding of the wide range of conditions in which life can exist and flourish.
Timothy Lyons is a biogeochemistry professor at the University of California, Riverside, and he leads the Alternative Earths Team. This team is focused on studying how Earth might have appeared to a distant observer in its 4.5 billion years of existence. This research has been funded by both the NASA Astrobiology Institute and the ICAR project, and Lyons is passionate about learning more about the past of our planet. He believes that Earth has been many different planets over its history, and these are what the Alternative Earths Team is striving to uncover.
Before oxygen was abundant in the atmosphere, life forms that did not rely on oxygen existed for billions of years, making it difficult for any external observer to detect them. Even after life began producing oxygen, the accumulation of it in the atmosphere was likely too low for detection. It is possible that oxygen remained undetectable until 800 million years ago, right when animal life appeared and complex life cells that had a nucleus began to appear. This suggests that before this time, Earth was unrecognizable to external observers due to its lack of oxygen, yet life still thrived on our planet.
Lyons’ research team is attempting to develop a kind of catalog of gaseous profiles for the many phases of Earth’s past. This catalog is created by using chemical measurements of ancient rocks, as well as computer models. This would allow them to better understand and imagine possibilities on distant planets, even if they are very different from Earth. If Webb and future space telescopes detect matching profiles in the atmosphere of an exoplanet, it could be a strong indication of a “biosphere” – a world that has environmental conditions and changes that are both driven by, and drive some form of life.
The ultimate goal of research into planets is to understand how they can create and maintain a detectable biosphere, not just theorize that life could possibly exist there, but be able to confirm its presence. According to Lyons, this work will influence the designs of new telescopes and lead to a deeper understanding of the atmospheric composition of planets situated in habitable zones. Additionally, researchers need to be aware of non-biological processes that could be responsible for generating gases that are usually associated with life. Examples include photochemistry which could result in an abundance of oxygen on a planet without any life.
Taking a holistic view of the potential for life requires an interdisciplinary approach combining multiple fields of expertise, such as biology, geochemistry, geology, and exoplanet research. This is exactly what Dr. Leslie Lyons and Dr. Ayşe Kaçar have done, when they formed a team to tackle this problem. Dr. Kaçar has even likened the team’s approach to “ordering a smoothie,” as it combines many disciplines to uncover the code of life detection on other planets. The growth of interest in this subject is very encouraging, with more and more students getting involved every day. This has filled Dr. Kaçar with optimism, believing that we are close to unlocking the mystery of extraterrestrial life.