Humans have gazed up at the sky in wonder since before the dawn of civilization, and the age-old question of “are we alone?” has occupied philosophers and priests for centuries on end. Today the interdisciplinary field of astrobiology strives toward an answer to this mystery by examining this history of life on Earth in an effort to search for life elsewhere and better understand our future.
Astrobiology is a collaborative effort among scientists of different fields to examine the origin, distribution, and future of life in the universe. Earth is the only known example of an inhabited planet, but this at least provides astrobiologists with a rich geological and biological history to examine the conditions that led to the formation of life. Investigation into the interplay between life and climate can lead toward a more fundamental understanding of exactly what is needed for an environment to be “habitable”. This in turn allows exoplanet-hunting astronomers to interpret their remote observations and hone in on planets that represent the best potential candidates for extraterrestrial life.
The “habitable zone” describes a range of orbits around a star where a planet could (in principle) sustain liquid water on its surface. Too close of an orbit would cause any water to boil off into steam, while too far of an orbit would cause the atmosphere to eventually plummet into global glaciation. The region between these two limits is of particular interest for astrobiologists who seek to find a rocky planet with oceans and continents, about the size of Earth, that would be capable of supporting life.
Further remote observations using spectroscopy can infer the composition of the atmosphere and even the presence of clouds, which can provide further evidence to assess the habitability of a planet. If (for example) we observed a rocky planet in the habitable zone that contained oxygen, ozone, methane, and water vapor in its atmosphere, then this could suggest evidence for the presence of life on the surface. This argument is based on our knowledge of atmospheric chemistry that finds methane is readily depleted in the presence of oxygen, so if we observe both, then there must be sources of these gases on the surface. Most methane on Earth is produced by life, so the question becomes: are there abiogenic (non-living) processes that explain the observations, or must the observations be due to biogenic (living) processes? Studying the long-term climate histories of Earth, Mars, and Venus is one way to understand the diversity of processes that can determine the ability of a terrestrial planet to support life.
Ground-based surveys continue to improve in their ability to detect smaller planets than ever before, while new space telescopes, such as the James Web Space Telescope (JWST) and Transiting Exoplanet Survey Satellite (TESS), will provide the ability to characterize the planetary atmospheres our closest neighbors. What we learn from these studies will not only help provide better targets in the search for life, but they also help us better understand the context of the cosmic environment that is home to our planet.
Examining the range of habitable environments in which life can thrive provides a window into the possible trajectories of our own civilization’s future. We currently struggle to conceive of long-term solutions to environmental and economic issues that require much deeper foresight than our civilization has yet demonstrated. Our civilization has demonstrated its ability to alter global-scale processes for centuries or millennia to come, which has even been named as the epoch of the “Anthropocene”. The discovery of an actually inhabited planet—if we ever find one—will be one of the most philosophically profound discoveries in all of human history, yet in the meantime the exploration of the worlds around us allows us to better understand the complex dynamics of the interplay of Earth’s systems with the processes of life.
How will humanity evolve with the biosphere, and how will we overcome the challenges that face us in the future? Astrobiology provides a lense into the diverse planetary processes that help us better understand our own, and it stretches our minds to conceive of the billion-year timescales that shape the history of each and every planet. The challenge of astrobiology is not only to discover life afar but also embrace our lives here in an effort to shape a better future that we have ever known.