Today, we stand on the threshold of a new discovery—a planet that humans might one day inhabit, perhaps one that some other being already calls home. A new race to find Earth’s twin orb is under way, led by NASA, whose Kepler space observatory went into orbit in 2009. Kepler is designed to discover large planets in or near the habitable zone of stars where liquid water might exist; the mission’s goal is to determine how many of the stars in our galaxy have such planets.
From a purely technical standpoint, there is no overwhelming benefit in discovering a true Earth analog, because the properties of worlds similar to ours can be found in all kinds of bigger, easier-to-find planets. But scientists and casual observers agree that we should—we need—to build future telescopes to study such planets for signatures of life. However, from a philosophical standpoint, the Kepler quest sparks quite a bit of soul-searching. Our work is in some ways the 21st-century update of the existential question of the Other: Who am I and how do I relate to others? If life exists elsewhere in the universe, how does that jibe with the age-old human need for meaning and belonging? Discovering life elsewhere has the potential to give humans a collective identity crisis.
The perception of who we are has been a part of our history for a long time. Mostly the question came up when we had “first encounters” with other tribes or races. Homo sapiens encountered Homo neanderthalensis somewhere in today’s Europe. Mayans encountered Spanish conquistadors in Central America. And there are numerous other examples. But the time of first encounters on our planet is over. For good or bad, we all now know about one another. The latest generations of Homo sapiens have a global awareness and a sense of connectedness that humans have never had before. Indeed, the end of the 20th century was a real watershed in this regard. That’s when our planet suddenly became small.
The search for new worlds orbiting distant stars affords us a fresh opportunity to contemplate a first encounter. Before we get too excited, however, we have some major hurdles to overcome. One big impediment to the search for life on other planets is the lack of a fundamental definition or understanding of the origins of life on Earth—or rather, how life emerges from chemistry. We do know that the molecular unity of Earth life may give us a clue. For example, we know that the chemistry of life on Earth is rather restrictive in its molecular permutations, and unnecessarily so, it seems, given the enormous choice of good options provided by chemistry for building bodies and function. The question is whether nature or nurture is the reason for the permutations of life we know. Our biochemistry could be universal, similar to gravity, where the same basic rules apply anywhere. Or our biochemistry could be one of many options—one that just happened to fit Earth’s environmental conditions. If the latter, how do we plan to search and what types of life could we expect to find first?
The question of whether alternative biochemistries exist now seems approachable in the lab and, although this does not directly answer the big questions of life’s definition and origins, it represents a giant leap in the search for alien life. With physics and chemistry being the same throughout the universe, we can work to define signatures that are unique indicators of a living planet. Half the battle is knowing how to search. If Earth and its history of life are any indication, we should be looking for a vibrant microbial biosphere that is capable of affecting the chemistry of the atmosphere and oceans.
Astronomers like me, and those I work with at the Origins of Life Initiative at Harvard, are now using powerful telescopes to discover entirely new classes of planets orbiting other stars. These telescopes (and others currently under construction) allow us to detect the signatures of atmospheric gases from distant planets by analyzing their light in different colors. Among them, we are looking for one that could teach us about alternative geochemistries—and ultimately lead us to the discovery of biochemistry and life. We will only succeed if we manage to combine what we see remotely with our work in the lab.
Because of this search technique, the first confirmation of a living planet outside our solar system is likely to be the geochemical signature of the biochemistry that thrives on it. It won’t be an image like the artist’s renderings in this magazine, but it will be a historic event nonetheless. It will be a cosmic first encounter for the people of Earth, and we will be faced once again with the question of who we are—this time a molecular level.
When that day comes, we’ll learn a lot, but whatever we end up learning, it is bound to transform our world in at least two ways. First, the technology that comes out of our work in the lab is going to be powerful. Much as the 20th century brought us synthesis in chemistry, the 21st century is bringing a synthesis in biology, with implications for new materials, therapies, industries, and applications. Second, these discoveries will represent nothing less than a revolution in our understanding of life and its place in the cosmos. We may again ask deeper questions about who we are and how we belong. Issues of theology and philosophy are bound to crop up. Whether the extraterrestrial life we discover is rare or ubiquitous remains to be seen, but either way the implications will be earthshaking.