Since Roman’s microlensing survey will monitor the light from hundreds of millions of stars, the mission will also reveal more than 100,000 transiting exoplanets. Astronomers find these worlds when a planet crosses the face of its host star as it completes an orbit, which blocks some of the star’s light.

The transit method for finding exoplanets has been wildly successful for NASA's Kepler and K2 missions, which have discovered more than 3,000 confirmed planets to date, and is currently used by NASA’s Transiting Exoplanet Survey Satellite (TESS).

The twin detection methods of microlensing and transits complement each other, allowing Roman to find a diverse array of planets. The transit method works best for planets orbiting very close to their host star. Microlensing, on the other hand, can detect planets orbiting far from their host stars. This technique can also find so-called rogue planets, which are not gravitationally bound to a star at all. These worlds can range from rocky planets smaller than Mars to gas giants.

Roughly three quarters of the transiting planets Roman will find are expected to be gas giants like Jupiter and Saturn, or ice giants like Uranus and Neptune. Most of the remainder will likely be planets that are between four and eight times as massive as Earth, known as mini-Neptunes. These worlds are particularly interesting since there are no planets like them in our solar system.

Some of the transiting worlds Roman captures are expected to lie within their star’s habitable zone, or the range of orbital distances where a planet may host liquid water on its surface. The location of this region varies depending on how large and hot the host star is – the smaller and cooler the star, the closer in its habitable zone will be. Roman’s sensitivity to infrared light makes it a powerful tool for finding planets around these dimmer orange stars.

Combining the results from Roman’s microlensing and transiting planet searches will help provide a more complete planet census by revealing worlds with a wide range of sizes and orbits. The mission will offer the first opportunity to find large numbers of transiting planets located thousands of light-years away, helping astronomers learn more about the demographics of planets in different regions of the galaxy.