In Search of New Worlds
The NASA Exoplanet Exploration Program (ExEP) science and missions represent an undertaking of unprecedented scope and ambition, promising insight into humankind's most timeless questions: What kinds of planetary systems orbit other stars in our galaxy? How common are solar systems like our own? What are exoplanets like? Are we alone?
The primary goals of these explorations are to take a census of planetary systems in our galaxy, characterize the diversity of other worlds, and search for solar systems like ours. Ultimately, we aim to discover and characterize Earth-like planets around our nearest neighbors, search for habitable conditions on those planets, and uncover signatures of life. These investigations and the missions that carry them out are designed to build on each other's success, each providing an essential step forward toward the goal of discovering habitable planets and providing definitive evidence of life beyond our solar system.
The exoplanet zoo: exploring the diversity of planetary systems
The first phase of exploration entails building an understanding of how many and what kinds of planetary systems exist in the galaxy. This work began with ground telescopes around the world, using various detection techniques and a panoply of surveys to push the limits of our ability to detect ever smaller planets despite Earth's turbulent atmosphere.
Ultimately, space missions, which avoid the deleterious effects of the Earth’s atmosphere, have dramatically increased our knowledge of exoplanetary systems. In particular, the wildly successful Kepler mission has now discovered thousands of candidate exoplanets, uncovering a surprising diversity of planetary systems very unlike our own. Thanks to Kepler, as well as the aforementioned ground-based surveys, we now know that planets are incredibly ubiquitous: the majority of the stars in the galaxy are typically orbited by at least two planets. And, in a hopeful sign for astronomers seeking analogs to our own planet, Kepler has uncovered an abundance of small, rocky planets similar in size to the Earth.
Future NASA and international space missions, as well as larger and more sensitive ground observatories, will be able to continue the census-taking process by finding ever more exoplanets, surveying all the planetary systems in our stellar neighborhood, and observing solar systems in both young and advanced states of evolution. In particular, the WFIRST mission will use the technique of gravitational microlensing to thoroughly survey the outer regions of planetary systems, and thus complete the statistical census of planetary systems in the galaxy begun by Kepler.
What are exoplanets like? Characterizing exoplanets
Already with the Spitzer and Hubble telescopes, researchers using NASA data have been able to begin the process of characterizing “hot” exoplanets that transit in front of their parent stars. Spectroscopy, albedo, and infrared measurements have allowed scientists to make temperature maps and determine the composition and dynamics of the atmospheres of these strange worlds. Future missions such as the Transit Exoplanet Survey Satellite will identify the most promising targets for transit characterization, including (if we are lucky) rocky planets with temperatures similar to that of Earth, albeit orbiting low-mass stars. These “transiting temperate terrestrials” could then be characterized in detail by the James Webb Space Telescope, which may uncover evidence for habitable conditions.
Future instruments and methods, and in particular the WFIRST mission equipped with a coronagraph or accompanied by a starshade, will be able to directly image and thus characterize cold giant planets like those in our own solar system, a feat that would have been unthinkable just a few decades ago. Ultimately, using similar methods and technology, larger and more capable missions will enable direct detection of “pale blue dots”: planets like the Earth orbiting stars like the sun. Spectra of these planets may uncover evidence of habitable conditions, and, potentially, the first evidence of life outside our solar system.
Concurrently, NASA astrobiologists are busy studying the conditions and processes that shape planetary environments. This work is essential in guiding planet hunters toward targets that have the most potential to possess the conditions for life as we know it, and identifying the key signatures (“biomarkers”) that provide the most definitive evidence for biological activity on these planets.
The technological challenge of direct imaging of exoplanets is essentially one of high dynamic range detection coupled with high angular resolution imaging. This is because exoplanets appear in the sky as extremely faint objects (108 to 1010 times dimmer than the host star, depending on wavelength and type of planet) located in extremely close proximity to their host stars. The light from the planet must first be resolved separately from the starlight, and the glare of the starlight must then be suppressed to allow atmospheric spectroscopy of exoplanets.
Large, light weight mirrors that can achieve the required angular resolution in visible light is one key area of technological development. High dynamic-range imaging requires near perfect control of the “wave fronts” of light so that speckles of starlight, created by subtle aberrations in the telescope optics, won't create a glare that would obscure faint planets.
A current major area of emphasis for ExEP is the development of these starlight suppression methods, including coronagraphy and starshade technology. Various in-telescope occulting techniques are being researched for the future WFIRST mission, which is currently planned to include a coronagraph as part of the mission design. External starshades, which suppress starlight via an occulter component that flies separately from the main telescope, are also being investigated.
Optical/infrared interferometry is another key technology area of the ExEP program for the second generation of space-based exoplanet missions. Traditional telescope designs have an achievable resolution that is limited by the diameter of the telescope's primary mirror. When telescopes are combined in an array, the achievable resolution is limited instead by the separation of the telescopes. A space-based infrared interferometer would allow access to an additional suite of biomarkers, and would enable a measurement of the radius and atmospheric temperature of these pale blue dots.
Looking even further in the future, arrays could be built that would have the resolution of a single telescope whose primary mirror was several hundred meters in diameter. When implemented in space, free from the distortions of the turbulent atmosphere, such interferometers hold the promise of resolving the surfaces of planets around nearby stars, potentially enabling the identification and characterization of oceans, continents, and weather and climate patterns. ExEP has supported research in formation flying technology necessary to keep such an array precisely positioned in space.
Big Questions and Exciting Answers
The explosive growth of the exoplanet field has been stunning. Questions once thought unanswerable are now being resolved. Just a few years ago, astronomers were unsure if planets were common in the galaxy, and if small ones like Earth represented any significant fraction of them. The wide net cast by the Kepler mission has taught us the exciting truth that exoplanets, especially ones the size of Earth, are ubiquitous. Future missions will complete the census begun by Kepler, providing a full accounting of the ubiquity and diversity of planetary systems, including a determination of how common solar systems like our own are in the galaxy.
At the same time as astronomers seek to find an Earth-like world, numerous other discoveries have indicated an exotic and nearly limitless galaxy of possibilities, from enigmatic gas giants that orbit mind-bogglingly close to their stars, to planets around binary and trinary systems that experience multiple sunrises and sunsets.
As a new frontier of astronomy, the young science of exoplanets has witnessed an unprecedented explosion in knowledge and discovery. NASA’s Exoplanet Exploration Program is dedicated to furthering the pursuit of the answers to some of humanity’s biggest questions--questions that only this field can answer.