WFIRST Exoplanet Microlensing

Exoplanet Microlensing

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A microlensing light curve (left) produced by the relative movement of a star-planet system with respect to a background source (right).
Image Credit: Dave Bennett (Notre Dame)

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 provide 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.

Other Science Topics:

  • Dark Energy
  • Large Area Near Infrared Surveys