Faculty Editor: Dr. Heather Olins
Are we alone in the universe, or is there life elsewhere? Furthermore, if we were to find life elsewhere, what would it be like? Would it have the same carbon-based biochemistry as us, or would it be completely different? Society has always questioned the existence of extraterrestrial life. With recent advancements in astrobiology, scientists are closer than ever to answering these questions.
Before we can begin to look for life elsewhere, we must first characterize some of the fundamental features of life on Earth. Understanding how these features enable life on Earth allows us to better comprehend what we are looking for in the universe. All life on Earth is composed of (often chiral) carbon-containing organic molecules in a water medium. Chirality can be thought of as the existence of “right and left handed” organic molecules, and is important because it helps give rise to the specificity of reactions of organic compounds. Furthermore, carbon can stably bond with many other atoms, including up to four other carbons, which allows it to form complex polymers. These highly specific reactions and complex molecules provide a foundation for metabolism, a process central to life (Dessy, 1998). Additionally, this carbon-based biochemistry allows genetic information to be stored in the form of DNA, which can easily be replicated and passed to offspring. Thus, when searching for life, scientists typically search for environments similar to Earth, which provides ideal conditions for carbon-based life.
While carbon-based life prevails here on Earth, it is natural to wonder if there is another alternative. The existence of silicon-based life has been hypothesized by many, as silicon is directly below carbon on the periodic table and has similar properties. Just like carbon, silicon also contains four valence electrons, can form polymers, and can bind to oxygen (Dessy, 1998). However, there are several problems with the idea of silicon-based life. For example, carbon is easily oxidized into carbon dioxide, allowing life on Earth to easily get rid of the byproducts of cellular respiration. On the contrary, the oxidized form of silicon is silicon dioxide, which is a solid and not easily excreted. Additionally, silicon does not form as many chiral compounds or as strong of bonds as carbon does (Dessy, 1998). Therefore, while it is theoretically possible for extraterrestrial life to be silicon-based, carbon-based life is still thought to be the most plausible.
Given that extraterrestrial life will likely have a biochemistry similar to that of life on Earth, one intriguing area of research concerns the formation of nucleobases in space. Nucleobases are an important component of the nucleic acids that make up DNA, and they have already been detected throughout space, such as on meteorites. Understanding how these nucleobases form in space, and under what conditions they do so, can tell us more about where we should be looking for life. One recently published Nature Communications article reports a mechanism for the synthesis of nucleobases by exposing H2O, CO, NH3, and CH3OH to UV light at 10°K, conditions comparable to those found in interstellar ices during the evolution of molecular clouds (Oba et al., 2019). This mechanism stands in contrast to previous publications that have only demonstrated nucleobase synthesis at warmer temperatures. This discovery is very exciting, as it expands the range of environments where nucleobases could plausibly be synthesized in space. It also suggests new environments where life may be found, and that nucleobases may be more abundant in space than previously expected.
Another method used to search for life is the investigation of exoplanets located in habitable zones. Exoplanets are any planets outside our solar system, and habitable zones are zones around stars in which liquid water could theoretically exist. This strategy is important because the presence of liquid water is one of the greatest indicators of the potential to support life (Brennan, 2019). Recently, NASA’s Hubble telescope detected water vapor in the atmosphere of an exoplanet in a habitable zone, the first discovery of its kind (Garner, 2019). This exoplanet orbits around a red dwarf star 110 light-years away, and is approximately eight times more massive than Earth. While the exoplanet’s composition is still unknown, and would have greater levels of radiation and gravity than Earth does, it’s existence is nevertheless a very promising discovery (Garner, 2019). Thus, through exoplanet analysis, scientists can identify and investigate possible environments that may support life.
In contrast to those researching exoplanets, many scientists are searching for signs of life right here in our own solar system. Mars does not currently support life, but there has been much speculation about previous life on Mars. Although it has less gravity than Earth does and little atmosphere, Mars is very similar to Earth in that it is roughly the same size, has a rocky composition, and is a comparable distance to the sun. Most excitingly, it used to contain liquid water on its surface (Dunbar, 2009). In 2018, NASA’s Curiosity rover discovered organic molecules near Mars’ surface, as well as variations in Mars’ atmospheric methane levels (Kate, 2018). Although it is likely that these findings are solely the result of geological processes, it is also possible that they are remnants of previous life on Mars. NASA’s future missions to Mars will further investigate these findings (Potter, 2018). If Mars is found to have supported life in the past, this would lead to a greater understanding of what life might be like elsewhere in the universe.
Although we still haven’t found proof of extraterrestrial life, new discoveries are being made every day. The field of astrobiology and the search for life is enormous, and the next breakthrough could come from any one of the many exciting areas of research. Understanding the biochemical basis of life on Earth, how life can arise, and where in the universe it might have done so are important topics to understand. Some may wonder if life only exists on Earth, but the statistical probability of this suggests otherwise. If life arose on Earth, there are likely other life-supporting planets among the estimated 1 septillion (1*1024) stars ( Howell, 2017) in the observable universe. Thus, there is a strong likelihood that life exists elsewhere; it just hasn’t been found yet. Whether or not we will ever find proof of extraterrestrial life, and what this life will consist of, only time will tell. However, we may find answers to these mind-boggling questions sooner than we think.
- Brennan, P. (2019, October 14). How Do We Find Habitable Planets? Retrieved from https://exoplanets.nasa.gov/what-is-an-exoplanet/how-do-we-find-habitable-planets/.
- Dessy, R. (1998, February 23). Could silicon be the basis for alien life forms, just as carbon is on Earth? Retrieved from https://www.scientificamerican.com/article/could-silicon-be-the-basi/
- Dunbar, B. (2009, April 13). Exploratour: Life on Mars? Retrieved from https://www.nasa.gov/audience/forstudents/postsecondary/features/mars_life_feature_1015.html.
- Garner, R. (2019, September 11). Hubble Finds Water Vapor on Habitable-Zone Exoplanet for 1st Time. Retrieved from https://www.nasa.gov/feature/goddard/2019/nasa-s-hubble-finds-water-vapor-on-habitable-zone-exoplanet-for-1st-time.
- Howell, E. (2017, May 18). How Many Stars Are In The Universe? Retrieved from https://www.space.com/26078-how-many-stars-are-there.html.
- Kate, I. L. T. (2018). Organic molecules on Mars. Science, 360(6393), 1068–1069. doi: 10.1126/science.aat2662
- Oba, Y., Takano, Y., Naraoka, H., Watanabe, N., & Kouchi, A. (2019). Nucleobase synthesis in interstellar ices. Nature Communications, 10(1). doi: 10.1038/s41467-019-12404-1
- Potter, S. (2018, June 7). NASA Finds Ancient Organic Material, Mysterious Methane on Mars. Retrieved from https://www.nasa.gov/press-release/nasa-finds-ancient-organic-material-mysterious-methane-on-mars.