Representative illustration showing an exoplanet.
Credit: iStock Photo
The first steps to solve the millennia-old mystery of our true place in the universe happened, of all places, on a brisk and early Tuesday morning in the unremarkable conference room of a hotel in Washington, D.C. Here a team of legendary heroes assembled on Halloween—Gandalf and a Star Trek captain among them. Yet these were not just costumes donned by trick-or-treating scientists. They were a fitting metaphor for the 60 astronomers chosen to begin one of the grandest tasks imaginable, not just in space science but across the spectrum of human history: to design a telescope that can find, or refute, signs of life on planets orbiting other stars. Such a goal seems almost fanciful. Can we actually build a multibillion-dollar observatory with a good chance of discovering aliens on worlds beyond the solar system? The answer appears to be that we can, and if a growing list of pivotal decisions can be surmounted, we will. Life may be abundant in the universe or it may be incredibly rare—learning which is closer to the truth would be epochal. By this NASA-led project’s end, the aim is to “have enough observations to know either way,” says Courtney Dressing of the University of California, Berkeley.
Called the Habitable Worlds Observatory (HWO) and targeted for launch around 2040, this would be by far the most ambitious and sophisticated telescope yet built. But its primary goal is almost childishly simple—to hunt for life on 25 Earthlike worlds. “This is the first telescope ever built that will be able to really address, in a scientific way, how common life is beyond the solar system,” says Marc Postman of the Space Telescope Science Institute in Baltimore, Md. “It could be zero percent or 100 percent or somewhere in between. We really have no measurement at all.” The journey is in its infancy; if it were to be imagined as a 100-meter race to launch, we would be “putting on our shoes,” Dressing says. But the prize that awaits at the finish line is enrapturing, a cultural shift in our understanding of our place in the cosmos. “It could be a society-changing discovery,” Postman says.
HWO will usher in an age unlike any other, one where we truly know Earth’s place among the stars. The path ahead, however, is fraught with challenges, not least the immense technological and political hurdles toward building such a machine. Can we solve them all to take our first glimpses of other living worlds? The journey to find and study alien Earths will span generations—but with their latest meeting, HWO’s architects have now taken its most significant first step.
Super-Hubble
If alien life does exist, it has not made itself easily known. We have hunted for signals from intelligent civilizations, scoured the worlds of our solar system and tentatively probed some planetary atmospheres across interstellar gulfs, but a clear indicator of cosmic neighbors eludes us for now. To date astronomers have discovered more than 5,500 worlds orbiting other stars. The majority of these have tended to be worlds inhospitably heavy and hot. The handful close to Earth in mass and size push the boundaries of plausibility for harboring life as we know it; they reside in tight orbits around red dwarf stars much smaller than our sun. For a true test of life’s cosmic prospects, we need to find and study planets eminently like Earth orbiting stars like our sun. “This has been percolating in the community for a very long time,” Dressing says.
In 2021 the National Academies of Sciences, Engineering and Medicine instructed NASA to begin work on a machine to achieve this goal as part of its decadal survey, which gives the space agency its marching orders every 10 years. The decadal survey committee focused on two proposed telescopes up to the task—one called the Large UV/Optical/IR Surveyor (LUVOIR) and the other, Habitable Exoplanet Observatory (HabEx)—which its final report combined into a single idea. The report instructed NASA to build a telescope that could observe in infrared, optical and ultraviolet light and “search for biosignatures from a robust number of about 25 habitable-zone planets.” Through the telescope’s optics, each world would be at best a lone, delicate dot of light—this is seemingly meager, but it would be enough to study the chemistry of the planets’ atmospheres for signs of life via gases such as oxygen and methane at a total estimated cost of no more than $11 billion in 2020 dollars. Mark Clampin, the Astrophysics Division director at NASA Headquarters in Washington, D.C., later gave this hybrid telescope its current name. “The one name I thought really captured the spirit of what we’re doing is the Habitable Worlds Observatory,” Clampin says. “This is the mandate we were given.”
Construction of the telescope is years away. In September 2023, however, NASA selected a group of about 60 scientists to begin planning a high-level “parts list” for HWO and its key components. The teams, one called the Science, Technology, Architecture Review Team (START) and the other the Technical Assessment Group (TAG), are expected across the next year to hold formal public meetings every few months alongside smaller-scale, more frequent intragroup meetings as well as broader discussions with the wider astronomy community. “It’ll be a busy year,” says Megan Ansdell, HWO’s program scientist at NASA Headquarters.
A three-day event in Washington, D.C., that started on October 31 was the first of these planning meetings—the starting gun in the decadal-paced race to make HWO a reality. “I want to encourage you all to breathe.... Just breathe for a moment,” said John O’Meara of the W. M. Keck Observatory in Hawaii, co-chair of START, who aptly later donned the Gandalf costume, on the first day of the meeting. “It took a long time to get here. It’s going to take a long time to go to the next step.... We’re going to be working together for a long time.” While the total number of both virtual and in-person attendees at the jubilant meeting numbered around 200, “it’s going to take hundreds if not thousands of people to get this done right,” O’Meara said. “I don’t know when this observatory is going to launch. But I do know I promised my wife I would retire when it does.”
A key theme of the planning meeting was that despite HWO’s name, the observatory should offer more than glimpses of light from putative mirror Earths. The immensity of the optics required to image exo-Earth analogues would make HWO supremely useful for many other astronomical tasks, too—similar to its workhorse predecessors such as NASA’s Hubble and the James Webb Space Telescope (JWST). “Studying dark matter is a possibility, the interstellar medium, galaxies—pretty much every aspect of astrophysics,” says Lee Feinberg of NASA’s Goddard Space Flight Center in Maryland. “This will be a general-class observatory.” Making that message clear will be key, said Jane Rigby of NASA Goddard, JWST’s senior project scientist, in a talk on day two of the meeting. “We have a lot of work to do,” she said. “We should stop calling it ‘Habitable Worlds’ because that [name] tells the general astrophysics community, ‘This is not for you.’” Postman describes it simply: “This is really like a ‘super-Hubble,’” he says.
Pale Blue Dot
The vision of HWO coalescing in its planners’ heads looks like something between JWST and Hubble in design. The telescope’s main mirror will likely be divided into honeycomblike segments—like that of JWST—allowing it to be folded up into one of several large new rockets under development, such as SpaceX’s Starship or Blue Origin’s New Glenn. “We see segmented as the way to go,” Clampin says. The mirror’s size—which greatly influences HWO’s ultimate acuity—is as yet unfinalized but will at minimum match JWST’s 6.5 meters (21 feet) and could scale up to reach 9 meters (30 feet). Like JWST, the telescope may sport a vast deployable sunshield to block incoming light from our home star and will be stationed at a deep-space locale 1.5 million kilometers from Earth. Unlike JWST, however, which probes deeply into the infrared to see the faint thermal glow of ancient galaxies, HWO won’t require extreme cryogenic cooling to perform its observations. Instead of an unfurling sunshield, HWO’s mirror may be stored within a barrel-like tube, like Hubble’s. This shroud might solve one of the most worrisome issues faced by JWST: micrometeorite strikes have dinged and dented its large exposed mirror. “A lot of people are thinking that [shroud] looks good,” says Aki Roberge, associate director for technology and strategy in astrophysics at NASA Goddard.
Although its launch remains many years away, HWO’s key design features are already coming into focus. The telescope probably won’t be as big as the 15-meter LUVOIR concept illustrated here, but it will likely include a large segmented mirror and perhaps also a sprawling protective sunshield. HWO’s architecture could even continue evolving after launch; the telescope is designed to allow servicing missions for major upgrades and repairs. Credit: NASA GSFC
HWO’s greatest technical challenge—imaging an Earthlike planet—is really twofold: the telescope needs not only a method to remove the otherwise-overwhelming glare of a planet’s star but also a way to hold itself breathlessly still to keep a targeted world in its sight. JWST was designed to exhibit a targeting drift as scant as one twentieth of a micrometer—a micrometer is a millionth of a meter and a fraction of the width of a human hair. The telescope has exceeded those capabilities by a factor of 10, Feinberg says, meaning that it is stable to within a strand of human DNA. Incredibly, HWO will still need to be “maybe a factor of 1,000 better,” he says, with a stability of up to tens of picometers—a unit of measurement that is a trillionth of a meter, less than the radius of a hydrogen atom. HWO will not need to constantly be so steady, but it will need to use this ultrastability mode when it looks at other Earths. A set of deformable optics—some of the telescope’s mirrors will be able to flex ever so slightly to eradicate any errors—will be one of several crucial tools to achieve the feat, HWO’s planners say.
To record a single photon of reflected light from an alien twin of Earth, HWO first needs to filter out circa 10 billion photons from the planet’s sunlike star. A coronagraph—essentially a small precision-shaped disk in the telescope’s optics to cover the star yet allow planetary light to pass through—will likely be HWO’s main way to achieve this Herculean task. HWO’s notional coronagraph would be limited to a relatively small swath of wavelengths—tuned for optimal sensitivity to Earth-sized worlds orbiting in the habitable zone or “Goldilocks zone” of sunlike stars, the circumstellar region where temperatures may be neither too hot nor too cold for liquid water to exist. NASA’s Nancy Grace Roman Space Telescope, set for launch in 2027, will include a technological precursor for HWO’s coronagraph, albeit one that limits Roman to imaging planets larger than Jupiter. The performance of Roman’s coronagraph will provide crucial information for HWO’s grander aspirations. “The coronagraph [on Roman] is a technology demonstration,” Dressing says. “For HWO it’s a critical instrument.”
Another way to suppress starlight would be to use a giant, sunflower-shaped “starshade” formation-flying in space far ahead of HWO’s gaze to cast a deep, planet-revealing shadow across its optics. But a separate spacecraft is a much more complex and unwieldy starlight-suppression solution than a coronagraph and thus is unlikely to be part of HWO from the get-go. Instead most experts see a starshade as a possible post-launch add-on. “You can imagine launching HWO with a coronagraph, doing initial observations and then later launching a starshade,” Dressing says. That would allow planets to be seen further out from their stars and in more detail than with a coronagraph alone.
With either of these technologies HWO should be able to deliver pictures of potentially habitable alien worlds akin to the famous Pale Blue Dot image of Earth taken by the departing Voyager 1 spacecraft in 1990 at the request of famed astronomer Carl Sagan. Exactly which systems HWO would target remains undecided. There are about 500 sunlike stars within 100 light-years of Earth—which is about as far as HWO’s life-finding survey seems likely to see. In January 2023 Eric Mamajek, deputy program chief scientist of NASA’s Exoplanet Exploration Program in California, co-authored a list of the most promising stars to observe within this volume of space. “I suspect that most of the top 50 or so have a very high probability of making it to the final survey list,” he says.
Settling on a target list is complicated by the fact that only HWO may be able to detect Earths in habitable zones around these stars, meaning that it would act as both surveyor and scrutinizer; no other presently planned telescope comes anywhere close to having similar capabilities. This does raise the question of whether enough targets can be found in the years ahead to serve as HWO’s raison d’être, but for the time being most astronomers appear unconcerned. Proxy measurements can still winnow down HWO’s targets. “If there’s a Jupiter right in the middle of the Goldilocks zone, you probably don’t want to bother looking for an Earth there,” says Bruce Macintosh, director of University of California Observatories at the University of California, Santa Cruz. “But it’s not actually that critical to mission success to know this star has an Earth and this one doesn’t, because the best Earth detector will be the mission we’re building”—HWO.
Renaissance
Worries over where, exactly, to point the telescope are part of what may be the project’s biggest challenge of all: ensuring unflagging support for the decades required to see it through, both from the public and from Congress, which will ultimately supply HWO with funding. “We need champions at [NASA] Headquarters, in Congress, in public and in industry so that when things are going tough, they’re talking on our behalf,” said Matthew Bolcar of NASA Goddard in a talk on day two of the HWO planning meeting. Heavily discussed were lessons to be learned from JWST, which was plagued by embarrassing and potentially ruinous budget overruns and schedule slips. In an effort to avoid those same mistakes, HWO is the crown jewel of a new NASA program called the Great Observatory Mission and Technology Maturation Program (GOMAP), which will carefully manage the budget and progress of the agency’s future large space telescope projects.
JWST’s ultimate success in spite of its setbacks, however, may be cause for optimism. “In the decade before we launched.... I can’t count how many people were like, ‘This thing’s never going to work,’” Rigby said in her talk; but, she said, the observatory’s above-expectations performance shows that “this is a doable thing.” And, many of HWO’s planners eagerly note, it will have a major advantage over JWST in that it will be designed from the start to be serviceable, just like Hubble. This means robots or astronauts could visit the telescope to periodically give it new leases on life, making repairs and swapping out instruments “sort of IKEA-style,” Roberge says.
If the technology and science behind HWO can be finalized, funding for the telescope can be secured and support for the project can be maintained, the payoff is almost unfathomable. In its study of some two dozen Earths in our corner of the galaxy, HWO will tell us if any of these worlds could support life or perhaps still do today. In the most wildly optimistic scenarios it could even see signs of technological civilizations, such as the night lights from notional alien metropolises or clear indicators of industrial pollution in an alien atmosphere. “You might use this telescope to look for ‘technosignatures’—evidence for not just simple life like bacteria but advanced life capable of building machines, industry, electric power, all of that,” Postman says. Such a possibility may seem far-fetched but remains at the edge of technical feasibility—and the possibility of success in such searches will forever remain zero if they are never undertaken.
On the other hand, HWO may scrutinize its targets and find none to contain anything we recognize as a sign of life, primitive or otherwise. Such an outcome would be disappointing but no less useful. It would be the best evidence yet that Earth truly is special in a cosmic sense—a precious oasis in a seemingly lifeless pocket of the Milky Way. “You would have a good upper limit on how rare life is right now,” Postman says. A robust detection of a living world, Dressing speculates, could drastically change our very culture, spurring a “whole new renaissance of art and literature”—not to mention even greater investments in more far-seeing space telescopes. Conversely, a failure to find anything might seem depressing but wouldn’t really be a “failure” at all—we humans would once again find ourselves seemingly solitary atop some cosmic pinnacle in a place of profound privilege we’d do well to better nurture and respect. “Either of those outcomes would be very interesting from both a scientific and philosophical point of view,” Postman says. We are lacing up our shoes at the start of that race to the ultimate prize. A podium of unbeatable knowledge awaits.