Illustration depicting a brown dwarf, which range from about 4,000 to 25,000 times Earth’s mass. They’re too heavy to be characterized as planets, but not quite massive enough to undergo nuclear fusion in their cores like stars.
When it launches in the mid-2020s, NASA’s Nancy Grace Roman Space Telescope will explore an expansive range of infrared astrophysics topics. One eagerly anticipated survey will use a gravitational effect called microlensing to reveal thousands of worlds that are similar to the planets in our solar system. Now, a new study shows that the same survey will also unveil more extreme planets and planet-like bodies in the heart of the Milky Way galaxy, thanks to their gravitational tug on the stars they orbit.
Roman will primarily use the gravitational microlensing detection method to discover exoplanets – planets beyond our solar system. When a massive object, such as a star, crosses in front of a more distant star from our vantage point, light from the farther star will bend as it travels through the curved space-time around the nearer one.
The result is that the closer star acts as a natural lens, magnifying light from the background star. Planets orbiting the lens star can produce a similar effect on a smaller scale, so astronomers aim to detect them by analyzing light from the farther star.
Since this method is sensitive to planets as small as Mars with a wide range of orbits, scientists expect Roman’s microlensing survey to unveil analogs of nearly every planet in our solar system. Miyazaki and his colleagues have shown that the survey also has the power to reveal more exotic worlds – giant planets in tiny orbits, known as hot Jupiters, and so-called “failed stars,” known as brown dwarfs, which are not massive enough to power themselves by fusion the way stars do.
This new study shows that Roman will be able to detect these objects orbiting the more distant stars in microlensing events, in addition to finding planets orbiting the nearer (lensing) stars.
Animation depicting the xallarap effect. As a planet moves around its host star, it exerts a tiny gravitational tug that shifts the star’s position a bit. This can pull the distant star closer and farther from a perfect alignment. Since the nearer star acts as a natural lens, it’s like the distant star’s light will be pulled slightly in and out of focus by the orbiting planet. By picking out little shudders in the starlight, astronomers will be able to infer the presence of planets.
Xallarap is parallax spelled backward. Parallax relies on motion of the observer – Earth moving around the Sun – to produce a change in the alignment between the distant source star, the closer lens star and the observer. Xallarap works the opposite way, modifying the alignment due to the motion of the source.
While microlensing is generally best suited to finding worlds farther from their star than Venus is from the Sun, the xallarap effect works best with very massive planets in small orbits, since they make their host star move the most. Revealing more distant planets will also allow us to probe a different population of worlds.