The first discovery of planetary companions to Sun-like stars was, along with the discovery of dark energy, one of the greatest breakthroughs in modern astronomy. These discoveries have excited the astronomical community and the broader public as well. Since then, the pace of exoplanet discovery has increased each year. There are now nearly 1000 confirmed exoplanets and Kepler has identified thousands of candidates that await confirmation. Nature has surprised astronomers with the enormous and unexpected diversity of exoplanetary systems, containing planets with physical properties and orbital architectures that are radically different from our own Solar System. Since the very first discoveries, we have struggled to understand this diversity of exoplanets, and in particular how our solar system fits into this menagerie.
WFIRST will advance our understanding of exoplanets along two complementary fronts. First, WFIRST will use microlensing techniques to expand our catalog of known exoplanets. Second, WFIRST will directly characterize exoplanets using coronagraphy. These two complementary surveys will provide the most comprehensive view of the formation, evolution, and physical properties of planetary systems. In addition, information and experience gained from both surveys will lay the foundation for, and take the first steps toward, the discovery and characterization of a "pale blue dot " -- a habitable Earth-like planet orbiting a nearby star.
The first exoplanets to be discovered were gas giants, but today it is becoming clear that there are probably many more "small" planets, in the Earth to Super-Earth range, than there are giants. Discovering the statistics of these planets is crucial for understanding their formation and commonality. Gravitational microlensing is an observational effect that was predicted in 1936 by Einstein using his General Theory of Relativity. When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the gravitational "attraction" of the foreground star. This star is then a virtual magnifying glass, amplifying the brightness of the background source star, so we refer to the foreground star as the lens star. If the lens star harbors a planetary system, then those planets can also act as lenses, each one producing a short deviation in the brightness of the source. Thus we discover the presence of each exoplanet, and measure its mass and separation from its star. This technique will tell us how common Earth- like planets are, and will guide the design of future exoplanet imaging missions.
More than 20 planets have been discovered from the ground using this technique. The WFIRST microlensing survey will detect many more such planets, including smaller mass planets since the planet "spike" will be far more likely to be observed from a space-based platform. This will lead to a statistical census of exoplanets with masses greater than a tenth of the Earth's mass from the outer habitable zone out to free floating planets. The results from the WFIRST microlensing survey will complement the exoplanet statistics from Kepler, and will provide answers to questions about planet formation, evolution, and the prevalence of planets in the galaxy.
Our understanding of the internal structure, atmospheres, and evolution of planets was originally developed through models that were tuned to explain the detailed properties of the planets in our own solar system.
Surveys of exoplanetary systems have led to the realization that there exists a diversity of worlds with very different properties and environments than those in our solar system. Models of planet formation and evolution have had to be expanded and generalized to explain the properties of these new worlds, often including new and uncertain physics. Our understanding of these new worlds therefore remains primitive. The best hope of understanding the physical properties of this diversity of worlds is through comparative planetology: detailed measurements of, and comparisons among, the properties of individual planets and their atmospheres. Understanding the structure, atmospheres, and evolution of a diverse set of exoplanets is an important step in the larger goal of assessing the habitability of Earth-like planets discovered in the habitable zones of nearby stars. It is unlikely that any such planets will have exactly the same size, mass, or atmosphere as our own Earth. Measuring a large sample of systems with a range of properties will be necessary to understand which properties permit habitability and to properly interpret these discoveries.
Direct imaging provides the critical approach to studying the detailed properties of exoplanets. Images and spectra of directly imaged planets provide some of the most powerful information about the structure, composition, and physics of planetary atmospheres. This information can in turn help scientists better understand the origin and evolution of these systems. The direct imaging technique is also naturally applicable to the nearest and brightest, and thus best-characterized, solar systems.
Advancing the technology for direct imaging of exoplanets was the top priority medium-scale space investment recommended by NWNH. Coronagraphy on WFIRST will be a major step towards the long-term goal of a mission that can image habitable Earth-mass planets around nearby stars and measure their spectra for signs of life.