Asked, “What is the most important experiment in modern biology?” at a public talk in 2012, renowned Harvard biologist and Pulitzer Prize winner E.O. Wilson responded, “Searching for extraterrestrial life.” His answer highlights the transformation underway in our study of planets within and beyond our solar system.
What was once the realm of science fiction has been brought into reality by our examination of potentially habitable environments in our solar system and our ongoing discovery of planets around other stars. For the first time in human history, the ability to find evidence of biological activity on planets circling nearby stars is within reach. Today, the scientific and technological capabilities of the United States are uniquely positioned to lead this transformational program, to answer that ancient question, “Are we alone in our galaxy?”
Over the last decade, scientists have discovered how persistent and adaptable life is across a wide spectrum of environments. On Earth, we have gained deep insights into the complex interaction of life’s multifaceted expressions with the terrestrial ecosystem. Through examination of life in extreme environments on our own planet, we have expanded our understanding of the limits of life and are getting closer to understanding the conditions in which it may have arisen. In space, NASA missions have discovered thousands of planets around other stars in just the last five years. Furthermore, we have begun to measure the chemical composition of some of those planets’ atmospheres, finding possible evidence of water.
Soon we will be able to isolate a number of stars in Earth’s solar neighborhood that could potentially harbor Earth-like planets in habitable zones, using the upcoming NASA Transiting Exoplanet Survey Satellite (TESS) mission and the James Webb Space Telescope (JWST).
But TESS is not designed to measure the atmospheric composition of any of the planets it finds. JWST will be able to make such observations using a method called transit spectroscopy, in which observations are taken of a star both alone and while a planet is eclipsing it. By comparing the spectra taken at these two different phases, we can reconstruct the spectrum of the planet and use that information to determine the makeup of its atmosphere.
JWST will only be able to study a limited number of worlds in this way – exoplanets orbiting small, cool stars quite different from our own Sun. We will also need a remote sensing survey of the conditions on Earth-sized planets orbiting within the “Habitable Zones” of nearby stars that are similar to our own Sun. This will require a telescope that is larger than JWST and allows much more precise control of its optics. Such a telescope will be able to detect “biosignatures” – features in the planet’s spectrum that suggest the presence of life. These signatures include the presence of water, molecular oxygen, ozone and smaller amounts of carbon dioxide and methane. Discovery of these signatures would indicate chemistry out of geological equilibrium, suggesting the presence of a planet whose makeup may be similar to our own: an Earth 2.0.