The Search for Life Beyond Earth in Space and Time
James L. Green, Ph.D.
Director, Planetary Science Division
NASA Headquarters, Washington DC
About 4.5 billion years ago a supernova exploded, causing a nearby interstellar cloud to collapse—creating our solar system. What emerged first was our sun, blowing the lighter gases outward. This allowed the heavier elements to remain in the inner solar system, forming our terrestrial planets. We are lucky to have Venus and Mars, two terrestrial planets that are very similar to the Earth and with significant atmospheres.
Planetary scientists have developed the capability to model how these planets have evolved since their birth and what may happen to them in the distant future. Comparative planetology tells us that terrestrial planetary atmospheres have been in a process of continual change. We are finding some startling parallels that suggest both Venus and Mars had environments that would have been habitable for life in their distant past. In the outer part of our solar system, Europa and Enceladus are thought to have an ocean of liquid water beneath their icy crust in contact with mineral-rich rock. These icy moons may have the three ingredients needed for life as we know it: liquid water, essential chemical elements for biological processes, and sources of energy that could be used by living things. With these discoveries in mind, we are looking for potentially habitable exoplanets and have made some significant discoveries. Some of these exoplanets must be ocean worlds!