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How do you tell whether a planet trillions of miles away is Earth-like? You look at its orbit and the starlight reflecting off its surface and atmosphere, which can reveal whether it has oceans, oxygen, or ozone.
This is hard to do. “You can't just point your telescope at a star out there and look for its planets,” says John Mather, senior astrophysicist at the NASA Goddard Space Flight Center. “It's flooded with glare.” Any Earth-like planet will almost certainly be found orbiting near a host star. And compared to a star, the light spectrum reflected off a planet is incredibly dim—10 billion times fainter than the star, to be exact. “You’re looking for something ridiculously faint next to something that’s wonking bright,” says NASA research astrophysicist Aki Roberge. Hunting for an exoplanet with just a telescope, even a really big one, is as useless as looking for a firefly with a spotlight shining in your face.
But NASA has a few solutions in the works. One is called a high-contrast coronagraph—a complex instrument that suppresses light inside a telescope and will be a feature of the Nancy Grace Roman Space Telescope, which is expected to launch in 2027. The star shade, a younger technology, throws shade another way. Star shades are uncrewed probes that fly far out in front of a telescope to block the light. In scaled simulation testing on the ground, star shades provide incredible imaging abilities, though they haven’t been tried in space yet.
NASA has asked scientists to ramp up these starlight-suppression technologies. Future missions might pair them with large ground-based telescopes or a yet-to-be-designed telescope planned for launch in the 2040s; it will replace the Hubble Deep Space Telescope and be charged with discovering, and then inspecting, 25 or so Earth-like exoplanets. The two star-blocking tools offer overlapping technologies, but some scientists believe they could work together. “It’s a very vigorous debate,” says Matt Bolcar, the optical systems engineering lead on the Roman telescope and one proposed Hubble replacement mission, the Large UV/Optical/IR Surveyor, or LUVOIR. “And I’m sure it’s going to be going on for the next several years.”
Light streaming forth from a star (and the tiny, dim planet next to it) moves in waves. Looked at directly by a powerful telescope, those waves are one massive, glaring blob of starlight. For every photon of planet light, a telescope sees 10 billion photons of starlight. To see the planet next to a star, you have to knock down that starlight by a factor of 10 billion, without losing the scarce photons from the planet’s own faint light. That’s called a 1 x 10-10 suppression or contrast. At 10-10, a starlight-suppressed telescope can read the light of most Earth-like exoplanets, even at 100 trillion miles away.
Coronagraphs, which are inside a telescope, block the glare of a far-off sun using a set of specially designed “masks” and a pair of deformable mirrors. First, the mirrors “clean up” the beam of light. Then the masks (which, Bolcar says, place “a little dot right over the image of a star”) reject the sunlight, and an instrument in the back of the telescope collects the image. Ideally, the sunlight is blocked, but not the light from the orbiting exoplanet.
In the lab, high-contrast coronagraphs have approached 10-10 contrast, but they still need improvement; in space they’ll require an incredibly stable telescope. Lower-contrast coronagraphs have been working in space for decades. Hubble has a low-contrast coronagraph, and the James Webb Space Telescope’s coronagraph will hit around 10-5 suppression thanks in part to its very own integrated sunshade, which it is currently deploying. Future versions, like the one slated to be used on the Roman telescope, are intended to spot exoplanets at around 10-8 contrast, two factors of brightness and clarity lower than what is currently called for in the Hubble replacement mission.
The star shade is a less proven option, but it has a big potential upside. “Star shades can open up a whole new way of investigating exoplanets—for potentially much less than a brand-new space telescope such as JWST,” or the James Webb Space Telescope, Paul Byrne, a planetary geologist at North Carolina State University, told WIRED by email. “The ability to directly image an exoplanet, and perhaps even to gain information about its surface (brightness, evidence for oceans, etc.) would go a very long way toward turning specks of light, or squiggles on a graph, into real worlds in their own right.”
In 1962, astrophysicist Lyman Spitzer described a method in which “a large occulting disk” could be placed far in front of a telescope to reduce the glare from a star and make it easier to see nearby planets. Today, scientific advancements have allowed astrophysicists to envision a star shade about 25 to 75 meters in diameter, which would fly some 50,000 miles in front of a telescope and unfold like origami into a circular “sunflower” shape—a central circle surrounded by petals. (Spitzer described such petals as “sharp spikes” that could be used to make the shadow behind the shade “much blacker.”)
The telescope sits right on the edge of the sunflower’s shadow, where the petals bend and diffract the few photons of light that get through. Obscuring and diffracting light waves works kind of like blocking moving water. “Imagine putting a wall-like obscuration in the middle of a stream,” says Manan Arya, a technologist with the Advanced Deployable Structures group at NASA’s Jet Propulsion Laboratory. “The water is not going to infinitely diverge and create a long dry spot in the stream bed. The water is going to bend around that obstacle, creating ripples. Some of those ripples will add up into bigger waves, way downstream of that wall I’ve put in the stream. A star shade is a perfectly-shaped wall in a river that, far downstream, creates a tiny patch of dry land.”
Flying tens of thousands of miles ahead of its main craft, a star shade positioned directly between a star and a telescope would create a shadow (or a “dry spot”) in this stream of light that blocks out nearly all the light from the star, yet captures the faint light reflecting off any exoplanets orbiting it. A telescope sitting directly on this spot, which is about a meter wider than the telescope, would see not a blob of starlight but a donut of blackness (the star shade’s shadow) surrounded by faint light (from the exozodiacal dust surrounding the star) and one or several bright dots orbiting the star—exoplanets at 10-10 contrast.
To prove that star shades provide this level of contrast, a team led by Anthony Harness, a postdoctoral research associate in mechanical and aerospace engineering at Princeton University, built an Earth-based proof of concept by creating a 1-inch scale version inside an 80-meter tube in a hallway. The tube blocked out ambient light, simulating the darkness of space. At one end they put a giant laser; at the other end a simple set of lenses acting as a telescope. In between, they placed a 1-inch model of a star shade, cut out of a silicon wafer. Reading the laser light that slipped past the star shade into a telescope-like camera at the back of the tube revealed that the star shade model worked, producing 10-10 suppression.
A star shade can achieve this level of contrast because it loses very little planet light. “In a coronagraph, both the star light and planet light enter the telescope, and then the job for the coronagraph is to separate the two,” Harness wrote to WIRED in an email. “That process of separating the star light from the planet light results in some of the planet light being lost. Losing planet light is bad because the planets are extremely faint, and we need to collect every photon that we can to provide a large enough signal to detect the planet and produce its spectrum.”
Unlike a coronagraph, a star shade separates the two before the light enters the telescope. The sunlight is all but blocked by the star shade, but the exoplanet’s light gets through. “This high throughput is why the star shade could do a better job at spectrally characterizing the planet—because producing spectra involves spreading out the light by its wavelength and requires more light than simply detecting the presence of a planet,” Harness wrote.
“Star shades are doing contrast just a little better than coronagraphs at the moment,” says Phil Willems, manager of the S5 Starshade Technology Development Activity with NASA’s Exoplanet Exploration Program (ExEP). “Because of the simplicity of star shades, we can get to that 10-10 contrast, and we can do it for a whole bunch of different wavelengths at the same time, which is a bit of a challenge for coronagraphs because they have to be much more complicated while operating inside a telescope. In short, simply showing that you can reach 10-10 suppression indicates that star shade technology needs to be taken seriously as a technique.”
NASA officials are currently funding star shade technology at Technology Readiness Level (TRL) 5, which means building scaled flight-sized replicas and full-scale components on Earth in order to demonstrate that they work. The next level, TRL 6, would require scaled flight-sized star shades to be tested in space-like conditions; NASA likes to have its tech at at least this level before a mission enters formulation.
Part of NASA’s interest in starlight suppression technology comes from the need to replace the aging Hubble. The recently released results of the Astro2020 Decadal Survey, which steers the direction of American astrophysics research, also prioritized the hunt for Earth-like exoplanets, calling for an estimated $11 billion spacecraft to be launched in the 2040s with that as its main mission. The Astro2020 report specifically calls for the craft to observe at the same wavelengths as Hubble and carry at least a 6-meter telescope and a high-contrast coronagraph instrument to spy on at least 100 suns and their planets, before using deeper imaging techniques on the 25 “most exciting” exoplanets in the hopes of discovering biosignatures.
The report tapped two mission proposals as starting points for such a craft: LUVOIR and HabEx (Habitable Exoplanet Observatory). Of the two, the LUVOIR project’s proposal is closest to the design specs required by the Astro2020 survey, in that it was designed with a coronagraph alone and a large 8-meter telescope. (The larger aperture of its telescope would have required a massive star shade, far beyond present feasibility.) “It’s true that if you could make a star shade work with LUVOIR, you could probably get better quality spectra of the planets,” says Roberge, a study scientist for the LUVOIR proposal. “But we judged that the coronagraph was absolutely necessary, and we got good enough spectra with that alone.” The LUVOIR team estimates their design will spot somewhere in the ballpark of 28 exoplanets.
The HabEx team has proposed a 4-meter telescope paired with a coronagraph and a star shade 52 meters in diameter. (“Having both a belt and suspenders is good,” says Bertrand Mennesson, a NASA JPL principal scientist and the HabEx co-chair.) Beyond providing the potential for 10-10 suppression, a star shade could image a wide bandwidth of light spectra, checking for ozone, oxygen, and water vapor wavelengths in a single image. (LUVOIR’s coronagraph would need to take many images to capture the entire light spectrum for clues of those features.) It might also allow imaging of an exoplanet at a smaller separation from its host star, helping to catch planets that are “hiding” closer in orbit to their suns.
Yet a star shade, which must fly separately from the telescope, poses some challenges that a coronagraph doesn’t. The need for a separate power source would limit the uses of the craft to around 100 observations or so before it would need to be scrapped or refueled. It would also require the two crafts to engage in a delicate, coordinated flight.
And then, of course, there is the matter of it unfurling like origami. Arya and others have been working on that task, crafting several large-scale test star shades made from blanket-like Kapton polymer sheets and an unfolding carbon fiber frame. (The “blanket” is made of many layers of Kapton so that any holes punched in the shade by micro-meteorite strikes won’t compromise its shadow.) It’s not easy. The edge of a star shade’s petals must be extremely sharp to reflect as little sunlight as possible into the telescope, and any perturbations could affect exoplanet imaging. “We are creating an optical precision structure that must fold and unfold robotically, and that presents a lot of challenges,” Arya says. “We’re approaching these problems stepwise, and there’s still a list of things yet to be done to prove out this technology.”
Perhaps because the task at hand is so difficult, some astrophysicists believe a coronagraph plus a star shade could be the perfect one-two punch. “I really see the benefit of a hybrid system,” says Mennesson. Repointing from star to star, a coronagraph could image a wide number of potentially habitable exoplanets, then a star shade could provide a high-resolution look with a wide bandwidth and throughput of each planet’s light—great for a deep characterization of its habitability. The HabEx and LUVOIR teams have worked closely together, and any future teams will likely draw from their members.
Star shades may also be useful for more than deep space missions. NASA has given Mather’s team funding to study using an orbiting star shade to spot exoplanets from Earth. ORCAS, or Orbiting Configurable Artificial Star, would be the first hybrid ground-space observatory, using a laser beacon in space to help focus a terrestrial telescope, therefore cutting out the distortion caused by looking through the atmosphere. The next step in the proposal would see a 100-meter “RemoteOcculter” star shade in near-Earth orbit, where it would cast its shadow onto the telescope. “The orbiting star shade is much harder, but it could be the ultimate exoplanet observing system,” Mather wrote in an email. “Using it, we could see an Earth orbiting a nearby star in a one-minute exposure, and in an hour we could know if it has water and oxygen like ours.”
A decision on which of these projects will go forward is still many years out. Direction for HabEx and LUVOIR might come during a NASA Town Hall at the American Astronomical Society meeting on January 11, and the ORCAS and RemoteOcculter mission proposals are still being studied. But the James Webb Space Telescope, which launched in December, will soon be beaming back images made with the help of its lower-contrast star shade. That telescope will become fully operational in mid-2022, and it is expected to be the new leader in the exoplanet hunt—until even more powerful shade-throwers come along.
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