EXCERPTS:
The Space Telescope That Could Find a Second Earth
What will it take to capture images of a distant world capable of harboring life?
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For all the excitement surrounding the search for distant exoplanets in recent years, the 4,000-plus planets confirmed so far have been unseen actors on the cosmic stage. Except for a handful of very large bodies imaged by ground-based telescopes, virtually all exoplanets have been detected only when they briefly dim the light coming from their host stars or when their gravity causes the star to wobble in a distinctive way. Observing these patterns and using a few other methods, scientists can determine an exoplanet’s orbit, radius, mass, and sometimes density—but not much else. The planets remain, in the words of one researcher in the field, “small black shadows.”
Scientists want much more. They’d like to know in detail the chemical makeup of the planets’ atmospheres, whether liquid water might be present on their surfaces, and, ultimately, whether these worlds might be hospitable to life.
Answering those questions will require space telescopes that don’t yet exist. To determine what kinds of telescopes, NASA commissioned two major studies that have taken large teams of (mostly volunteer) scientists and engineers four years to complete. The results are now under review by the National Academy of Sciences, as part of its Decadal Survey for Astronomy and Astrophysics that will recommend government funding priorities for the 2030s. Past and current NASA mega-projects, from the Hubble Space Telescope launched in 1990 to the James Webb Space Telescope, which is scheduled for launch this year, have all gone through this same vetting process. Sometime this spring, the Decadal Survey is expected to wrap up its deliberations and make recommendations.
That puts four proposals in the running to become NASA’s next “Great Observatory” in space: an X-ray telescope called Lynx; the Origins Space Telescope for studying the early universe; and two telescopes devoted mostly, but not exclusively, to exoplanets. One is called HabEx, for Habitable Exoplanet Observatory. The other—the most ambitious, most complex, most expensive, and most revolutionary of all these concepts—is called LUVOIR, for Large UV/Optical/IR Surveyor.
If scientists really want to find out if life exists on distant Earth-like planets, they’ll eventually need LUVOIR—or something very much like it.
The LUVOIR concept study (published in 2019 as an
illustrated, 426-page report) calls for a massive telescope mirror as big as 15 meters, almost 50 feet, in diameter. That would be six times larger than the Hubble Space Telescope mirror and more than twice the size of the 6.5-meter Webb telescope. The most ambitious version of LUVOIR would have a mirror as big as a carnival Ferris wheel.
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Direct imaging of such a faint target also requires certain technical innovations, most critically a powerful coronagraph—a screen to block out the blinding light of the planet’s host star. Inside the coronagraph instrument, an advanced imaging camera is needed to detect small, rocky planets like our own. Then a highly sensitive spectrograph is required to identify elements like oxygen or methane in a planet’s atmosphere that might suggest the presence of life.
The LUVOIR study team concluded in its final report that the deluxe version of the observatory—as opposed to a scaled-down option that reduces the mirror size almost by half—
could identify and study 54 potentially Earth-like planets over a five-year observing period, along with hundreds of larger planets.
This estimate comes from matching the telescope’s technical specifications against the number of small, rocky exoplanets predicted to exist in our celestial neighborhood based on data from NASA’s Kepler survey mission of the last decade. Key to reaching the 54-planet goal will be LUVOIR’s internal coronagraph and the large size of the telescope itself, which has 40 times the light-gathering power of Hubble and can capture images much more quickly.
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Had LUVOIR been proposed a decade ago, it would have been something of a fishing expedition. Today scientists are much more confident it will produce solid results. “What’s new and exciting is that, for the first time, we have major proposals to the Decadal [Survey] in the context of knowing for sure that there are Earth-size planets that get Earth-type energy from their suns in our relatively near neighborhood,” says Shawn Domagal-Goldman, a NASA astrobiologist who is the deputy study scientist for LUVOIR. “This is the first generation of concepts to incorporate that understanding and make it a central part of their science case.”
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“When you compare the planet brightness in two wavelengths, you get a color. That’s basically how your eye works,” says Aki Roberge, LUVOIR study scientist and an astrophysicist at NASA’s Goddard Space Flight Center. “So when we say we’ll measure the colors of planets, we really do mean the reflected blue, green, red that you could see with your eyes. In addition, we’ll measure colors outside of the range of light your eye can see.”
The planets will be “unresolved” in LUVOIR images, Roberge cautions. With only a few pixels total, they’ll still appear as fuzzy dots without surface features. “However,” she says, “there are ways to pull out information on surface features without actually resolving the planet. For most habitable planet candidates, we’ll be able to see their brightness vary as they rotate and find the lengths of the planets’ days. And if you look at the variation in more than one wavelength, you can probably decompose the variation into total fractions of land, oceans, and clouds. Seeing seasonal changes in planet brightness or color is probably possible too.”
Scientists will be able to read the contents of exoplanet atmospheres as never before with the
Extreme Coronagraph for Living Planetary Systems (ECLIPS), which will be able to detect compounds such as oxygen, ozone, water vapor, carbon dioxide, and methane that could be biosignatures. Neither Hubble nor JWST can do that for Earth-like planets orbiting sun-like stars, so LUVOIR represents an important new capability for astrobiology.
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That diversity may be key to understanding planetary habitability, she believes. “Let’s say we look at a bunch of planets we assume to be similar—maybe five rocky planets that are the same Earth size and same temperature—and we see very different atmospheres. This tells us a lot more than by looking at one planet alone.”
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That difference has major scientific implications. The larger the number of habitable planet candidates studied, the better we’ll determine whether our own planet is common or rare, while increasing the chances of finding extraterrestrial life. This is a major reason why LUVOIR-A’s ability to find so many small, faint exoplanets has such appeal. Still, it would surprise no one if Congress and NASA—should LUVOIR get the nod—were to decide on the economy version.
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For now, LUVOIR-A is about the biggest space telescope we can build. Its 15.1-meter mirror is the maximum size able to fit inside the next generation of heavy-lift rockets, whether it’s NASA’s Space Launch System or the SpaceX Starship. Folded up inside the rocket at launch, the observatory would be sent out into cold, dark space, away from Earth’s warmth and brightness, to a gravitationally stable spot called Lagrange Point 2.
The notion of sending up LUVOIR-A in pieces, to be assembled later in space, has been presented to the Decadal Survey, although the panel traditionally makes recommendations based on science return rather than how a project should be executed. With in-space assembly, NASA could in theory put together a mirror larger—maybe much larger—than 15 meters. Back in the mid-2010s, then-NASA science chief (and three-time Hubble repairman) John Grunsfeld proposed that astronauts working in deep space could reasonably do the job. NASA commissioned a study of in-space assembly of large observatories in 2018, but the conclusion was that Grunsfeld’s astronaut construction crew was not feasible now and that robotic assembly would be better.
Nick Siegler, chief technologist for NASA’s Exoplanet Exploration Program at the Jet Propulsion Laboratory and a leader of the in-space assembly study, said its 2019 report concluded that in-space assembly will be necessary as observatories get ever larger. It would even lower mission risk. Complex, autonomous self-deployments would no longer be necessary, and since the pieces go up on different rockets, the possibility of one failed launch killing the entire project is eliminated. Siegler says in-space assembly by robots could cost about as much as a single launch in a heavy-lift rocket, possibly less if you factor in servicing. The in-space assembly team has sent a white paper to the Decadal Survey panel, in case LUVOIR gets its stamp of approval.
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What will it take to capture images of a distant world capable of harboring life?
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