Two Palm-Sized Mirrors Poised to Revolutionize Exoplanet Science on NASA’s Roman Telescope
Two palm-sized mirrors, small enough to rest comfortably in the cupped palm of one hand, are set to reshape how we understand the cosmos. NASA’s Nancy Grace Roman Space Telescope is quickly approaching its launch window, and while the observatory’s primary instrument is designed to advance nearly every branch of astrophysics, a special experimental tool is tucked away inside its hull: a cutting-edge coronagraph that will attempt the first-ever direct capture of starlight reflected off an exoplanet’s surface.
This is an ambitious test project NASA hopes will clear a path for future space telescopes, which could one day capture clear images of an Earth-like planet orbiting a Sun-like star. That milestone remains far beyond the capabilities of modern engineering, which is why researchers are testing the core technology on Roman first. “For the first time, we’ll be testing these tools in actual space conditions, which will let us see exactly what work still needs to be done to refine the technology,” says Vanessa Bailey, an astrophysicist at NASA’s Jet Propulsion Laboratory and instrument scientist for the Roman coronagraph.
At its core, a coronagraph is simply a purpose-built scientific sunshade: it blocks the overwhelming glare of a bright star to reveal much fainter objects that would otherwise be completely lost in the star’s light. Similar instruments have flown in space before—both Hubble and James Webb carry their own basic coronagraphs. But Roman’s model is exponentially more sophisticated than its predecessors, thanks largely to adaptive optics: a technology that deforms the telescope’s mirror to cancel out unwanted light distortion that obscures faint targets.
Even with this innovation, the challenge Roman’s coronagraph faces is monumental. NASA compares the task to photographing a firefly sitting next to a powerful floodlight—from 2,500 miles away, on the opposite side of the United States. “Even a tiny speck of stray starlight that lands in the wrong spot can ruin an entire section of your image,” explains Margaret Turnbull, an exoplanet scientist at the non-profit SETI Institute in California who leads a Roman coronagraph science team.
Adaptive optics is already standard on the world’s most advanced ground-based telescopes, including Chile’s Very Large Telescope and Hawaii’s twin Keck Observatory. For ground observatories, adaptive optics counteracts interference from Earth’s thick, shifting atmosphere, which blurs incoming starlight, allowing for far sharper images. Space telescopes have not traditionally needed adaptive optics, since they sit above atmospheric distortion entirely—but no space telescope has ever attempted to image old, cool exoplanets that are only visible via reflected starlight, until now.
The key to Roman’s success is its two palm-sized deformable mirrors. Each mirror is fitted with roughly 2,300 tiny actuators that expand when a small electric jolt is applied, reshaping the mirror at an infinitesimal scale to cancel out stray light interference. This is the first time NASA has ever flown active deformable mirrors in space.
The mirrors cannot deliver clear images on their own, however. The full system relies on ultra-sensitive detectors that amplify the signal from individual photons—a critical requirement, since the instrument will capture only a tiny handful of photons from any given distant exoplanet. At the heart of the coronagraph sit its star-blocking masks: a set of exquisitely precise tools that Bruce Macintosh, an astronomer leading University of California Observatories and a member of the Roman coronagraph science team, calls “beautiful, complicated shapes” unlike anything currently in orbit. “Hubble’s masks were just brute force—literally a small chunk of metal placed in the star’s way to block its light,” Macintosh notes.
Throughout the coronagraph’s mission, engineers will collect massive volumes of test data to assess how well the system performs in space, and flag any issues to address before the technology is used on future missions. Scientists will first test the instrument by attempting to spot a handful of already confirmed exoplanets. “If we go through the full process and don’t see them, we’ll know something is wrong with the coronagraph—because those planets are definitely there,” Turnbull says. The instrument will also target stars wrapped in clouds of dust and debris, collecting observations that could reveal gaps in those clouds carved by as-yet-undetected planets, and help astronomers understand how typical the debris structure of our own Solar System really is.
Every insight from Roman’s coronagraph will inform the design of future telescopes built to hunt for smaller and smaller exoplanets. This includes NASA’s proposed Habitable Worlds Observatory, which the agency hopes to launch in the 2040s and will require a coronagraph up to 100 times more powerful than Roman’s. A similarly sensitive adaptive optics coronagraph, comparable in performance to Roman’s, is also planned for the proposed Lazuli Space Observatory, a project announced in January by the research institution founded by former Google CEO Eric Schmidt.
While imaging habitable alien worlds is the most tantalizing use case for this technology, future coronagraphs could also help astronomers study binary star systems and faint objects hidden near bright quasars. Researchers emphasize that Roman’s mission is a critical first step toward this future, and any scientific discoveries it makes along the way will help accelerate that progress. “The best way to prove a new technology works is to use it to do meaningful, interesting science,” says Julie McEnery, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland and Roman’s senior project scientist.
