For as long as humanity has been looking up at the heavens, we’ve been pondering some of the biggest questions of all. It’s only over the past few hundred years that science has caught up to our vast imaginations, and has begun answering those questions for the first time in our civilization’s history. We know what the stars are: they’re much like our own Sun, except very far away. We know that the majority of them have planets, and that some of those planets are Earth-sized. We know that those worlds are composed of very similar ingredients to our own Solar System’s planets, and that they are governed by the same underlying laws of nature.
But are any of those worlds actually inhabited? Here, as the end of 2025 approaches, that’s still a great cosmic unknown. We don’t yet know whether we’re alone in the Universe or not, and if not, how common or rare life actually is.
Finding alien Earths requires seeing Earth-sized planets at Earth-like distances from Sun-like stars. A new discovery completes the roadmap.
To truly explore the possibility of life beyond Earth, advancements in coronagraph technology are essential.
How Coronagraphs Assist in Search for Alien Planets
What is a Coronagraph?
A coronagraph is a relatively simple piece of equipment, based on a simple concept that we experience naturally here on Earth: a total solar eclipse. Celestially, a total solar eclipse occurs when the Moon passes in front of the Sun relative to our line-of-sight here on Earth. However, there’s a catch: the Moon needs to appear large enough, in terms of angular size, to block out the entirety of the Sun’s disk.
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When this alignment properly occurs, and you stand in the eclipse shadow on Earth, the Sun’s light is entirely blocked, and you can see all sorts of sights that would otherwise be washed out: the solar corona, the stars that lie behind the Sun, satellites, and more.
A coronagraph is a telescope that is designed to block light coming from the solar disk in order to see the extremely faint emission from the region around the sun, called the corona. What makes a coronagraph different is that instead of blocking out the light from our close, nearby Sun, it places a small obstacle (usually a disk) in front of the telescope’s lens, blocking out the light from an ultra-distant star. By having the coronagraph block only the light from the disk of the star, all sorts of features that are present around the star, even if faint, can then be revealed.
The coronagraph was introduced in 1931 by the French astronomer Bernard Lyot; since then, coronagraphs have been used at many solar observatories.
During a total solar eclipse, the Moon acts as an occluding disk and any camera in the eclipse path may be operated as a coronagraph until the eclipse is over. More common is an arrangement where the sky is imaged onto an intermediate focal plane containing an opaque spot; this focal plane is reimaged onto a detector. Another arrangement is to image the sky onto a mirror with a small hole: the desired light is reflected and eventually reimaged, but the unwanted light from the star goes through the hole and does not reach the detector. Either way, the instrument design must take into account scattering and diffraction to make sure that as little unwanted light as possible reaches the final detector.
For many years, astronomers focused on instrumentation have been working to improve the capabilities of a coronagraph: particularly one that goes onto a space telescope. The ultimate goal - a goal put forth for the Habitable Worlds Observatory, slated to be the next flagship mission for NASA astrophysics after the already-completed Nancy Grace Roman observatory launches - is to not only achieve that vaunted brightness contrast of 10-10, but to get there at optical, visible light wavelengths.
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The longer the duration of a total solar eclipse, the darker the sky becomes, and the better the corona and background astronomical objects can be seen. The solar corona, as shown here, is imaged out to 25 solar radii during the 2006 total solar eclipse.
Solar corona imaged during the 2006 total solar eclipse.
Evolution of Coronagraph Technology
Although many telescopes had leveraged coronagraphs throughout the 20th century, there was a real revolution when we launched the Hubble Space Telescope equipped with a coronagraph. For the first time, incredible features could be seen: debris disks, protoplanetary disks, and even full-fledged planets around other stars.
As remarkable as Hubble’s coronagraph was, however, it was still severely limited. In terms of optical performance, what we often talk about in the science of coronagraphy is known as contrast. Contrast, in this context, is how faint an object can intrinsically be - relative to the bright object being blocked by the coronagraph - and still be detected, resolved, and measured by the instrument.
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For Hubble, it was a huge revolution. We discovered debris disks for the first time, we obtained our first direct images of exoplanets, and we could detect faint companions - including brown dwarfs - in places we didn’t even necessarily expect to find them. But we couldn’t see small planets, or planets close to their parent stars, or planets around faint, low-mass stars, for an important reason: Hubble’s coronagraphic contrast was only at about the 1-part-in-1000 (or 10-3) level, meaning that an object needed to be at least 0.1% intrinsically as bright as the star the coronagraph was blocking in order to be seen by Hubble.
With the launch of JWST, however, its next-generation coronagraph provided an enormous improvement. Instead of contrasts of 10-3, its coronagraph achieves contrasts that are improved by nearly a factor of 100 over Hubble’s: contrasts of more like 10-5. As you can see, above, this has enabled us to find features that are much more profound, faint, and require much greater sensitivity than anything Hubble is capable of detecting.
Instead of just protoplanetary and debris disks, it could see features in those disks, including rings, gaps, and belts. Instead of just brown dwarfs, it could find Jupiter-sized and even Neptune-sized planets around dim stars. It could find planets closer in to their parent star than Hubble could, and its groundbreaking discoveries are still rolling in even today.
But JWST’s coronagraph’s contrast, even though it can resolve features that are a factor of 100,000 fainter than the parent star that it blocks, doesn’t get us close to reaching our main goal: of directly imaging Earth-sized planets at Earth-like orbital distances from Sun-like stars, and of doing it at optical (i.e., visible light) wavelengths. JWST’s coronagraph is much more effective at imaging companion objects around stars in infrared wavelengths than it is at optical wavelengths, and even with a contrast of 1-part-in-100,000, it still has a long way to go to detect an Earth-like world, which would require a contrast of 10-10 or better.
This two-panel view of the debris disk around Vega shows Hubble’s (left) and JWST’s (right) views, respectively. JWST’s coronagraph is approximately 100 times more sensitive than Hubble’s.
This limits the maximum coronagraphic contrast you can achieve with simply a circular disk to about 10-6, even with optically ideal materials. Does this mean that the dream of detecting Earth-sized worlds at Earth-like distances from Sun-like stars with a coronagraph is impossible?
No, not necessarily. What it means is that we have to develop better coronagraphic technologies than what’s achievable with the “simple optical disk” configuration. There’s an incredible amount of science that goes into this - most of which is being conducted at federally funded US institutions like the Space Telescope Science Institute, NASA’s Goddard and Ames, the Jet Propulsion Laboratory and Caltech, plus the University of Arizona - but the general approach is to introduce a segmented coronagraph design.
The Nancy Grace Roman Space Telescope
There’s a roadmap for how we’re going to do this, and we’ve already come so far as we prepare for the launch of NASA’s next flagship mission: the Nancy Grace Roman Space Telescope.
NASA’s Nancy Grace Roman telescope has completed construction and is nearly ready for launch. Its instrument suite, including its coronagraph, represents the cutting-edge of instrumentation in astronomy, and should pave the way for even further improved technology aboard the future Habitable Worlds Observatory: enabling the direct imaging of potentially Earth-like planets.
The state-of-the-art coronagraph (the best ever, by far) on board the Nancy Grace Roman telescope should represent an enormous step in that direction: to contrasts of at least 10-7 but that could go as high as 10-9, which is an enormous leap over all prior observatories.
At least, that’s how the Roman coronagraph is expected to perform, based on laboratory tests. But how will it perform when it’s actually in space, attached to the telescope, and observing a target of interest in the wavelength range (i.e., the optical) of interest?
In order to find out, we need to have an appropriate target. In this scenario, such a system would enable Roman to achieve a 5-σ significance (the gold standard for “discovery”) detection of the secondary object in under 10 hours of imaging.
That’s where the new research comes in, and what makes it so exciting: for the first time, a naturally occurring astronomical system that actually meets all of these parameters has been discovered! The key to discovering it took many steps, many years of work, and - most importantly - a variety of powerful astronomical facilities in order for it to be possible.
The Nancy Grace Roman Space Telescope is a NASA observatory designed to investigate essential questions in the areas of dark energy, exoplanets, and infrared astrophysics. The mission is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland. It carries one science instrument called the Wide Field Instrument, and one technology demonstration called the Roman Coronagraph Instrument.
Designed and built by NASA’s Jet Propulsion Laboratory, the Roman Coronagraph will advance scientists’ ability to directly image planets and disks around other stars (exoplanets). Coronagraphs work by blocking light from a bright object, like a star, so that the observer can more easily see a faint object, like a planet. The Roman Coronagraph is designed to detect planets 100 million times fainter than their stars, or 100 to 1,000 times better than existing space-based coronagraphs.
The Roman Coronagraph will be capable of directly imaging reflected starlight from a planet akin to Jupiter in size, temperature, and distance from its parent star.
How the Roman Coronagraph Instrument Works
The Roman Coronagraph Instrument aboard NASA’s Nancy Grace Roman Space Telescope will improve scientists’ ability to directly image planets around other stars. As the most powerful coronagraph to ever fly in space, it will demonstrate new technologies that might be used by future missions like NASA’s proposed Habitable Worlds Observatory.
For the dark hole test, the team placed the coronagraph in a sealed chamber designed to simulate the cold, dark vacuum of space. Using lasers and special optics, they replicated the light from a star as it would look when observed by the Roman telescope. When the light reaches the coronagraph, the instrument uses small circular obscurations called masks to effectively block out the star, like a car visor blocking the Sun or the Moon blocking the Sun during a total solar eclipse.
Coronagraphs with masks are already flying in space, but they can’t detect an Earth-like exoplanet. From another star system, our home planet would appear approximately 10 billion times dimmer than the Sun, and the two are relatively close to one another. So trying to directly image Earth would be like trying to see a speck of bioluminescent algae next to a lighthouse from 3,000 miles (about 5,000 kilometers) away.
The Roman Coronagraph will demonstrate techniques that can remove more unwanted starlight than past space coronagraphs by using several movable components. These moving parts will make it the first “active” coronagraph to fly in space. Its main tools are two deformable mirrors, each only 2 inches (5 centimeters) in diameter and backed by more than 2,000 tiny pistons that move up and down.
The deformable mirrors also help correct for imperfections in the Roman telescope’s other optics. Although they are too small to affect Roman’s other highly precise measurements, the imperfections can send stray starlight into the dark hole.
“The active components, like deformable mirrors, are essential if you want to achieve the goals of a mission like the Habitable Worlds Observatory,” said JPL’s Ilya Poberezhskiy, the project systems engineer for the Roman Coronagraph. “The active nature of the Roman Coronagraph Instrument allows you to take ordinary optics to a different level.
This graphic shows a test of the Roman Coronagraph Instrument that engineers call “digging the dark hole.” At left, starlight leaks into the field of view when only fixed components are used. The middle and right images show more starlight being removed as the instrument’s moveable components are engaged.
Future Missions: Habitable Worlds Observatory
This capability is central to NASA’s next flagship astrophysics mission concept, the Habitable Worlds Observatory, which aims to image and characterize planets similar to Earth that orbit Sun-like stars.
These two designs represent artist concepts for the potential look and architecture of the upcoming, planned NASA astrophysics flagship mission of HWO: the Habitable Worlds Observatory. The Habitable Worlds Observatory will represent a truly generational leap, the same way Hubble or JWST did for NASA science. As the #1 recommended mission by the National Academy of Sciences’ 2020 decadal survey, it will be the first mission to directly image Earth-sized worlds at Earth-like distances around Sun-like stars, but only if we design, advance, fund, and build it, along with the full relevant suite of instruments.
Artist concepts for the potential look and architecture of the Habitable Worlds Observatory.
CPI-C on CSST: A New Frontier in Exoplanet Imaging
The Cool Planet Imaging Coronagraph (CPI-C) on the Chinese Space Station Survey Telescope (CSST) represents a significant step forward in the direct imaging of exoplanets. Proposed to directly image cool planets around nearby solar-type stars (within 40 pc), CPI-C aims to conduct high-contrast imaging surveys of exoplanets ranging in size from Neptune-like to Jupiter-like, located at separations of 0.5 to 5 AU from their host stars.
CPI-C will be the first space-based instrument capable of directly imaging the reflection light from the cool exoplanets in the visible wavelength enabling the measurement of key physical parameters such as the effective temperature, surface gravity, radius, mass, and other key parameters.
CPI-C employs a step-transmission apodization technique to suppress the diffraction noises from the telescope pupil and a precise phase correction technique to eliminate the speckle noises due to imperfections of the optical surfaces. The contrast requirement is better than 10−8 at an inner working angle (IWA) of 3−4 λ/D, in the visible wavelength from 600 nm to 900 nm.
CPI-C on CSST will fully take the advantage of the space environment, which is free from atmospheric turbulence and offers broader observational wavelength windows. By integrating extreme high-contrast imaging techniques, CPI-C is expected to overcome current limitations in observational contrast. To achieve the contrast on the order of 10−8 and beyond, it is essential to mitigate diffraction-induced photon noise originating from the telescope pupil, as well as speckle noise caused by imperfections in both the telescope optics and the coronagraph.
With total integrated exposure times ranging from 30 seconds to one hour or longer, a sufficient signal-to-noise ratio (S/N ≳ 5σ) can be achieved for planets with magnitudes between 20th and 25th orbiting stars with magnitudes between 0th and 5th in the V-band. Specifically, a 20th magnitude planet requires approximately 30 seconds of exposure, while fainter planets approaching 25th magnitude require exposures of one hour or longer.
Additionally, through repeated observations, the planet’s orbital parameters can be determined, allowing further constraints on its dynamical mass. Based on CPI-C’s detection capabilities, the mission is expected to detect planetary systems ranging from Jupiter-sized to Neptune-sized planets at separations of 0.5 to 5 AU from their host stars.
The Future of Exoplanet Detection
Developing the capabilities to directly image Earth-like planets will require intermediate steps like the Roman Coronagraph. What NASA learns from the Roman Coronagraph will help blaze a path for future missions designed to directly image Earth-size planets orbiting in the habitable zones of Sun-like stars.
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