ASTROBIOLOGY

The Origins and Evolution of Life in the Cosmos

From the moment people first realized that there were planets around other stars, we have wondered if those places contain life. The civilizations of antiquity had stories about alien worlds populated with gods, but these were more mythological than science-based. In fact, the Greek philosopher Aristotle flatly ruled out the idea of life on other worlds, and his Earth-centered cosmology held sway for more than 1,200 years.

With the advances in scientific thought made possible by the Copernican Revolution, people began to understand that there could be other worlds “out there” and that the idea of life elsewhere in the cosmos is not so strange. The advent of the Space Age allowed humans to put space probes in orbit around other worlds and eventually to land on them. The question of whether or not life exists elsewhere fostered a new discipline involving chemistry, physics, astronomy, molecular biology, planetary science, geology, and geography. This branch of science is called astrobiology.

The Roots of Life

Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This scientific discipline uses research tools to search out life-friendly neighborhoods on the other worlds of our solar system. Since the discovery of the first exoplanets (worlds orbiting other stars) in 1995, astrobiologists have been devising ways to search for signs of life on those worlds, too.

Life on Earth has a long history, rooted in a complex biochemical evolution that started with simple compounds and molecules that found their way to our planet’s oceans and surfaces. Under the right conditions, those compounds combined and eventually gave birth to the first primitive forms of life. The prebiotic chemistry known to exist on early Earth illustrates what the environment was like during the planet’s formative period. Furthermore, astrobiologists can apply what they have learned about our own planet to analyze conditions on Mars or Titan, for example, to see if those worlds provide environments conducive to life.

Twenty-First-Century Astrobiology Missions

Today astrobiology is a very active branch of scientific inquiry. In the United States, NASA funds the Astrobiology Institute, which was developed in the late 1990s to provide guidelines in the search for life elsewhere. A number of universities are also involved in the multidisciplinary science, and the European Space Agency is actively researching topics in astrobiology as well.

A large part of astrobiology is the study of what are called extremophiles—life forms that can exist and thrive in really hostile conditions. For example, there are microbes that can live quite happily in hot springs or in the deep sea in proximity to underwater volcanic eruptions. Other tiny life forms adapt well to freezing conditions or in places where water is extremely scarce. One of the more interesting life forms is a species of deep-sea worm that appears to live quite nicely while burrowed into methane ice deposits. The existence of these and other extremophiles gives some hope that life could be flourishing elsewhere in the solar system under similar conditions.

As astronomers search for life on worlds beyond our solar system, astrobiologists are focusing on Sun-like stars because they could well have evolved similarly to our own. The main assumption is that any life out there is carbon-based (as we are). It’s not so far-fetched—carbon is a very versatile element, and it combines with many other elements. It’s also very common in regions where stars form and thus could be one of the main building blocks of life on other worlds. It’s only a matter of time before we find a Sun-like star and its orbiting world with traces of water in its atmosphere.