NASA’s Nancy Grace Roman Space Telescope launched on Aug. 30, 2026, beginning a journey that will take it about a million miles from the Earth. Once there, Roman will study some of the biggest mysteries in astronomy, from dark energy to the formation of galaxies. It will also search for planets around other stars, called exoplanets, but in a rather unusual way.
Roman will be able to find many of these exotic worlds without seeing the planets themselves. Instead, it will watch for distant stars that temporarily become brighter.
The reason this strategy works is one of the stranger consequences of Einstein’s theory of gravity: Gravity can bend light. Under the right circumstances, a star – or even a planet – can behave like a natural magnifying glass. The phenomenon is called gravitational microlensing.
As an exoplanet scientist, I study how to detect and learn about worlds far beyond our solar system. One challenge in this field is that no single observing technique can reveal every type of planet. Microlensing is exciting because it functions as a tool that can help astronomers find planets that are difficult to find in other ways.
How can gravity act like a magnifying glass?
Imagine you are looking at a very distant star. Somewhere between you and that star, another star happens to pass almost directly across your line of sight. The two stars may actually be thousands of light-years apart, but from your viewpoint, they appear to line up.
According to Einstein’s theory of general relativity, an object’s mass curves the space around it. Light travelling through that curved space also follows a curved path. So when the foreground star passes almost exactly in front of the more distant one, its gravity bends some of the background star’s light toward you.
The result is surprising: Instead of hiding the distant star, the star in the foreground can make it appear brighter.
It is like looking through a magnifying glass, except there is no glass. The lens is gravity itself.
Because stars are constantly moving through our galaxy, the two will align only temporarily. The background star gradually brightens and then fades as the two stars move out of alignment.
Now, add a planet.
If the foreground star has a planet orbiting it, the planet’s gravity also affects the light from the distant star. It can produce an additional short disturbance in the otherwise smooth brightening pattern. Astronomers measure these changes in brightness and can use that small deviation to infer that a planet is there.
Remarkably, this technique can work even if the planet itself is far too faint and distant to see directly.
Roman will watch hundreds of millions of stars
There is a catch to microlensing: Astronomers cannot simply choose a star and wait for it to lens another one.
The necessary alignment happens by chance, and once an event is over, exactly that same alignment will generally never occur again.
The solution is therefore to watch a huge number of stars, over and over again. This is exactly what Roman will do.
As part of its Galactic Bulge Time-Domain Survey, Roman will repeatedly observe six fields of view toward the crowded centre of the Milky Way. It will monitor hundreds of millions of stars, observing its main survey fields roughly once every 12 minutes during six intensive observing seasons.
Roman does not know which two stars will happen to line up. Instead, it will watch so many stars so frequently that large numbers of these rare alignments should occur in its field of view.
NASA expects the survey to discover more than 1,000 planets on relatively wide orbits through microlensing. That category is particularly valuable because most exoplanets known today orbit close to their stars; Roman will probe the colder, more distant regions of planetary systems that remain much less explored.
Roman is particularly well suited to this task because it combines the sharp infrared vision of a space telescope with an unusually wide field of view. That combination allows it to repeatedly image huge numbers of stars, even in extremely crowded regions toward the center of our galaxy.
Hunting for planets
Roman provides a strong example of how different planet-hunting techniques complement each other. While the telescope’s Galactic Bulge survey is designed to primarily search for microlensing events, Roman will also repeatedly measure the brightness of hundreds of millions of stars along the way. Those observations should reveal around 100,000 transiting planets.
This is a very different way of finding a planet: A transit occurs when a planet passes in front of its star from our point of view, blocking a small fraction of the star’s light and causing it to dim slightly.
The two populations – planets discovered through microlensing and transiting planets – should look very different. Transits favor planets orbiting close to their stars, particularly large planets that block more starlight and complete their orbits frequently. Microlensing, in contrast, is particularly sensitive to colder worlds on wider orbits, including planets resembling those in our outer solar system.
So Roman will effectively use the same sequence of images in two opposite ways. A star that periodically gets dimmer may reveal a transiting planet, while a star that temporarily gets brighter may reveal a gravitational lens.
But there is more. Microlensing can reveal planets that do not orbit a star at all. These so-called free-floating or rogue planets may have been ejected from the planetary systems where they formed and now travel through the galaxy alone. Because microlensing relies on gravity rather than light from the planet or its host star, even these dark, isolated worlds can reveal themselves when they pass in front of a distant background star.
Building a census of planetary systems
Ultimately, Roman serves as an example of why using several planet-hunting techniques matters. The thousands of exoplanets scientists have discovered so far are not an unbiased sample of all the planets in our galaxy. What they find depends partly on what telescopes are best able to detect.
Roman’s microlensing survey will fill in an important part of that picture by measuring the population of planets on wider orbits that previous surveys have explored much less completely. Combined with Roman’s enormous sample of transiting planets and discoveries from other telescopes, the observations will help astronomers answer one of the most fundamental questions in exoplanet science: What kinds of planetary systems does nature actually make?
Roman will do that using one of nature’s own telescopes: gravity itself.
- is a Research Associate Professor of Physics, American University
- This article first appeared in The Conversation
