Digging the dark hole: Coronagraphy and the quest to find other worlds

When French astronomer Bernard Lyot invented the solar coronagraph in 1939, he discovered a simple method for observing the hot, gaseous outer layers of the Sun without having to wait around for a total solar eclipse. By artificially blocking or suppressing most of the sunlight coming into his telescope, he could gather the data of greatest interest for his research.

But decades passed before coronagraphy—valuable as it is—was used to study much more than our nearest star.

Fast forward to today—some three decades after the first discovery of an exoplanet—and a cadre of astronomers and optical engineers are taking the elegant simplicity of Lyot’s invention to new and exciting extremes. With advanced versions of the coronagraph—supplemented by adaptive optics, wavefront sensing and control, and powerful computational image processing—they are searching for worlds far beyond our own solar system. Their ultimate goal: Direct imaging of rocky exoplanets in the habitable zones—for life as we know it—around the stars they orbit.

Almost everything about coronagraphy and the search for exoplanets is an extreme, says Julien Girard, an exoplanet researcher at the US Space Telescope Science Institute (STSCI). “It’s extreme in terms of contrast. It’s extreme in terms of the technology. On the ground, we call the adaptive optics extreme adaptive optics because it’s basically adaptive optics on steroids. It’s like the Formula One of astronomy.”

“In the late 1990s, there was really an explosion of papers [on coronagraphy], especially triggered by the first discovery of exoplanets using radial velocities,” says Rémi Soummer, director of the Russell B. Makidon Optics Laboratory at STSCI. When an exoplanet orbits a star, astronomers discovered, the radial velocity of the star will appear as spectra that are first blue- then red-shifted as the planet makes its rounds, crossing the light paths streaming toward humanity’s eyes in the sky.

Stellar coronagraphy began with astronomers looking for dust disks around stars like Beta Pictoris. Photo credit: ESO/A.-M. Lagrange et al.

Once it was known that exoplanets could be found with methods like radial velocity, “the question became, can we make images of them?” says Soummer, who began his graduate studies in the late ‘90s. Some of his early work centered on how to make coronagraphs better, and he went on to work on coronagraphs for such projects as the European Southern Observatory’s (ESO) Very Large Telescope (VLT) as well as the Gemini Planet Imager, both in the Chilean Andes.

And though coronagraph technology has evolved for newer instruments, the concept remains the same. “The most basic coronagraphs are a physical mask to block starlight, just like you would put your thumb in front of the Sun so as not be blinded,” explains Girard. But the ability to see visible light from exoplanets is still not so easy. With today’s coronagraphs, such as the one onboard James Webb Space Telescope, the achievable on-sky contrast between a planet and the star it orbits is somewhere around 10^-5, in rare cases 10^–7. Dedicated future missions to hunt habitable exoplanets will need to supersede that by orders of magnitude.

Today’s coronagraphs “have a physical mask that is in the focal plane of the telescope to block the starlight,” says Girard. “And then we do what we call a reimaging of the pupil [where light enters the telescope]. And that’s where we put another mask, which we call a Lyot stop after Bernard Lyot.” That mask is slightly smaller than the pupil (usually the primary mirror), and without it the telescope would still refocus some of the scattered light in the focal plane—that is, there would still be some light contamination from the star. So, at the most basic level, a basic coronagraph has one mask in the focal plane, and a Lyot stop mask in the pupil plane.

Coronagraphs for exoplanet research are part of the optical instrumentation on some of the largest ground-based observatories like the VLT and, in Hawaii, the Keck Observatory telescopes. They will be part of the first, second, or third generation instruments for new ground-based observatories like the ESO’s Extremely Large Telescope scheduled to come online within about a decade.

More than 5,000 exoplanets have been discovered so far using a variety of methods, including direct imaging with and without coronagraphy.

And coronagraphy will continue to be important for both ground- and space-based astronomy for many years to come. “Everything we do in high-contrast imaging is done in a tiny field of view,” Girard says. “The patch of sky we’re looking at [from ground-based instruments] is just the star and a little bit around.” Because they are looking so far out in the universe, in a narrow field, at very specific objects, “We are not so much affected by light pollution or low-Earth orbit satellites, for example.”

What’s more, he says, “we need ground-based telescopes with 30 to 40 m mirrors [for wider angular resolution], and we are probably not going to launch large-enough telescopes for habitable planet imaging from space for a long time.” On the ground, there can be more telescopes for the same amount of money as one space telescope, and “with more and bulkier instruments you can access different techniques simultaneously, which is more difficult to do in space because you usually have to decide everything once, in advance, unless robotic servicing is an option.”

But coronagraphy with space telescopes is vital, too, and has a robust future in astronomers’ plans even decades from now. Both the Hubble (HST) and JWST have coronagraphs to look for exoplanets, and there will be a coronagraph testbed instrument on the National Aeronautics and Space Administration’s (NASA) Nancy Grace Roman Space Telescope scheduled for launch in 2027. Waiting in the wings, in the very early planning stages, is NASA’s Habitable Worlds Observatory (HWO), a space telescope that will be dedicated to the search for exoplanets.  It will require high-contrast imaging on the order of 10^–10 between the planet and the star it orbits. That’s a power not yet achievable with today’s instruments; however, researchers feel confident in the path they are forging.

The coronagraph instrument on the 2.4 m Roman Space Telescope, for example, will aim to do extremely high-contrast imaging of exoplanets in visible light as a demonstration test for the technology to be used on HWO.

“As a technology demonstrator, the Roman coronagraph was designed to reach science goals, and so those will be like 10 to the minus nine contrast rather than 10 to the minus six or seven that JWST, HST, or the ground-based telescopes are currently shooting for,” says John Krist, a research scientist on the Roman coronagraph team at NASA’s Jet Propulsion Laboratory (JPL) in California.

The focal plane mask that will be part of the coronagraph instrument onboard NASA’s Nancy Grace Roman Space Telescope. Photo credit: NASA/JPL-Caltech

For exoplanet imaging with the coronagraph, “We’re sensitive to wavefront errors on the order of tens of picometers,” Krist says. “By comparison, a hydrogen atom is 100 picometers in diameter, so we have to make corrections [to the mirrors] that are on the order of the diameter of an atom or even less.”

Astronomers sometimes refer to coronagraphy as “digging the dark hole.” Krist explains: “Starting with the light around the star, when you look through any instrument it’s going to have a diffraction pattern, it’s going to have scattered light from polishing errors on the mirror and the optics and all that stuff.” He says another way to think of these distortions is that they are like glare from the Sun through a windshield, “and you’re trying to look for a very faint planet in that sea of light.”

The trick is to create a region around the star where that sea of light is suppressed enough that the planet comes into view. With the coronagraph in place and wavefront control via the deformable mirrors, “You want to create a dark hole around the star. You keep digging ‘dirt’ until you get this dark, dark hole. That’s what we mean by digging the dark hole,” says Krist.

Rather than diffracting light everywhere equally around the image of the star, Krist explains, the coronagraph on Roman is set up to diffract the light in certain directions, and by doing so create two dark zones around a star.

On Roman, Krist says, “We first use the hybrid Lyot coronagraph to image the field around the star to find planets, since it can see over a 360-degree annulus around the star.” Once astronomers find a planet, they add a shaped-pupil coronagraph to the mix. It has a smaller field of view, but since they know where the planet is, it can be positioned correctly. The shaped-pupil coronagraph can be used to direct light reflected off the planet through a prism, which separates it into colors. This is where researchers say they will find the signs of biological life, if it exists, on a planet under observation.

Asked if the quest for exoplanet imaging has meant something of a renaissance for coronagraphy, Krist says that for stellar coronagraphy—solar coronagraphy being a separate field—“we started out looking for dust disks around stars, notably Beta Pictoris. And then we started looking for brown dwarfs. We went after the easy targets first, things that you couldn’t see without a coronagraph, faint stuff near bright stars, before getting to planets.”

But the real key for exoplanet imaging with coronagraphy, he says, has been the advent of deformable telescope mirrors for wavefront control.

“It really took until we had deformable mirror technology in the early 2000s, where we could combine a chronograph and deformable mirrors and get down to the contrast levels that you could see exoplanets.”

The big newsmaker event for future space-based coronagraphy would be detecting signs of biological life in the atmosphere of another planet, says Marie Ygouf, another Roman coronagraph project scientist at JPL. She is both an optical engineer who has worked on building extremely large telescope mirrors and an astronomer.

Finding habitable worlds, she says, will mean placing much larger and very expensive telescopes in space. “You really need to have a dedicated observatory that has been designed with exoplanet imaging in mind,” Ygouf says. Indeed, a telescope like the nascent HWO is a goal of the astronomy community set forth in its most recent decadal survey.

The Roman telescope is not dedicated to exoplanet imaging and was not designed with direct imaging of exoplanets in mind. “That limits a lot of what the Roman coronagraph instrument can do,” Ygouf says. The coronagraph will not be on the lookout for rocky planets in habitable zones. Rather, its goal is to image so-called gas giants, planets more like Jupiter than Earth or Mars.

But that’s not to downplay all that astronomers hope to achieve with the Roman coronagraph. “When you have an instrument that is able to take pictures of exoplanets, it would also be able to take pictures of the circumstellar environment. That means we would be able to detect disks around the star that are the components of planet building. We expect with Roman that we might be able to revolutionize the study of circumstellar environments,” Ygouf says.

But “the real reason people want to do coronagraphy is to see another Earth,” says Soummer. “That’s been the real motivation from the beginning. Can we see other Earths around other stars? Can we detect life? Can we find other terrestrial planets like ours?”

William G. Schulz is Managing Editor of Photonics Focus