Caption: Illustration showing the concept of gravitational microlensing. If the foreground (lens) star has a planet orbiting it, the planet can also act as a lens and cause further deviations to the light curve.
Credit: NASA GSFC/CI Lab.

The RGES is expected to discover over 1,400 bound microlensing planets with masses greater than 0.1M_earth (Penny et al. 2019). Of these, Roman should detect over 200 with mass approximately equal to 3M_earth, and should have sensitivity to planets with the mass of Ganymede (~0.02M_earth). A comprehensive microlensing simulation study was performed by Penny et al. 2019 to estimate the predicted yield of bound microlensing planets by Roman, assuming different mass functions and survey designs.

Caption: A comparison of the Roman survey sensitivity compared to Kepler and other radial velocity planets in the mass-semimajor axis plane.

As the figure shows, the microlensing technique with Roman will be most sensitive to colder planets that orbit their stars at a few astronomical units (AU), which previous exoplanet surveys and techniques are insenitive to. This fact leads to the exciting expectation that this survey will ultimately “complete of the census of exoplanets in our galaxy”.