Exoplanet hunters have found thousands of planets, most orbiting close to their host stars, but relatively few alien worlds have been detected that float freely through the galaxy as so-called rogue planets, not bound to any star. Many astronomers believe that these planets are more common than we know, but that our planet-finding techniques haven’t been up to the task of locating them.
Most exoplanets discovered to date were found because they produce slight dips in the observed light of their host stars as they pass across the star’s disk from our viewpoint. These events are called transits.
NASA’s Nancy Grace Roman Space Telescope will conduct a survey to discover many more exoplanets using powerful techniques available to a wide-field telescope. The stars in our Milky Way galaxy move, and chance alignments can help us find rogue planets. When a free-floating planet aligns precisely with a distant star, this can cause the star to brighten. During such events, the planet’s gravity acts as a lens that briefly magnifies the background star’s light. While Roman may find rogue planets through this technique, called gravitational microlensing, there’s one drawback – the distance to the lensing planet is poorly known.