The discovery of many hundreds of exoplanets over the last two decades has confirmed that our solar system is not unique in our galaxy. While the majority of the discovered extra-solar planets are gas giants, similar to Jupiter or Saturn, we have also found many large Earth-like planets called "Super-Earths", which are thought to come in a variety of flavors, depending on whether they are primarily made of iron, silicates, water, or carbon compounds. Understanding the statistics of these planets and the formation processes that produced them are crucial for understanding how our own Solar System formed and how common Earth-like planets may be.
Gravitational microlensing is an observational effect that was predicted in 1936 by Einstein using his General Theory of Relativity. When the angular separation of two stars on the sky becomes sufficiently small, the light rays of the background source star become bent due to the influence of the gravitational field of the foreground star. This acts in the same way as a magnifying glass in amplifying the brightness of the source star, and we hence 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, producing a short spike in the brightness of the source. Thus their presence can be inferred and we can measure both the planetary mass and the star-planet separation. Even planets as small as Mars can be detected using this technique and so this may be used to determine how common Earth-like planets are in order to guide the design of future exoplanet imaging missions.
Numerous planets have been discovered from the ground using this technique. The WFIRST microlensing survey will cover ~2.81 square degrees in the Z and W bands (see baseline survey characteristics ), providing an opportunity to detect many more such planets and of much smaller mass since the brightening of the source star due to a planet 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 at distances at 0.5 AU and beyond. The results from such a survey will complement the exoplanet statistics from the Kepler mission and will provide invaluable information on answering the questions of planet formation, evolution, and their prevalence within the galaxy.
High-contrast, high-angular-resolution 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 diagnostic information about the structure, composition, and physics of planetary atmospheres, which in turn can provide constraints on 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. High contrast imaging is also ideally suited to studying the diversity and properties of debris disks around the nearest stars; these disks serve as both fossil records of planet formation, and signposts of extant planets through their dynamical influences.
Advancing the technology for direct imaging of exoplanets was the top priority medium-scale space investment recommended by NWNH. Developing a coronagraph with active wavefront control for WFIRST accomplishes this objective and, thanks to the 2.4-m telescope, achieves far more real science than would be possible on a technology demonstration mission with a much smaller aperture. 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.