We’re Gonna Need Another Space Telescope

And this one is going to search for other Earths, around other suns.

A picture of the Milky Way stretching across the night sky
Tinker Street / Brenton Little / Gallery Stock

For most of us, today is just Thursday. For astronomers, it’s practically a holy day. Today is an event that comes only once a decade, and it’s of cosmic importance—literally. Today, a special committee has revealed the priorities for the next decade of American astronomy, like a synod giving word from on high. These directives, divined from many deliberations within the astronomy community, have guided the country’s exploration efforts since the 1960s, giving rise to instruments as wondrous as the Hubble Space Telescope. Go forth, the committee says, and discover something new.

Like others before them, the new recommendations will eventually be subject to slightly less harmonious influences—Congress and the White House among them—but on this day, they’re pure: a crisp vision of what the future of scientific discovery could look like. If that vision crystallizes into reality, it could change human knowledge as profoundly as the moment Copernicus realized that Earth wasn’t the center of the universe.

The decadal survey, organized by the National Academies of Sciences, Engineering, and Medicine, recommends that NASA establish a program to produce several more major space-based observatories in the coming decades, and that the next one should look for exoplanets—planets beyond our solar system—resembling this little chunk of rock that we call home. In other words, the report recommends, with a sense of urgency not seen in earlier surveys, that we should focus on the search for other Earths.

Not for people to go and live on, of course. This isn’t a Muskian fever dream. The planets that this observatory would uncover are too far away for any spacecraft to reach, no matter how appealing they might look. But these distant worlds could help astronomers answer a big, existential question: Are we alone? In these places we could say, with confidence, life could arise—because it happened here, to us.

Scientists have looked for distant life in many ways over the decades, drafting telescopes to listen for mysterious radio transmissions and sending rovers to dig through the soil on Mars. But exoplanets are a fairly recent prospect. At the time of the last decadal report, astronomers had discovered a few hundred—an exciting start, certainly, but not enough to seriously propose a major observatory dedicated to their study, according to Jonathan Fortney, a planetary scientist at UC Santa Cruz and one of the members of the decadal committee. Fortney told me that 10 years ago such a mission would have seemed “a little bit pie in the sky.”

Now it seems obvious, even fated. The inventory of known exoplanets exceeds 4,500, thanks in large part to a spacecraft that launched in 2009. Telescope technology has advanced, too. About 160 of them are rocky like Earth, and astronomers feel more certain now that if they look—really look—they could find small, rocky worlds orbiting in the habitable zone of sunlike stars, a mystical region where conditions are not too hot or too cold.

That search will require powerful technology. Rocky planets as small as Earth are the most difficult exoplanets to detect because they become obscured in the glare of their suns. So the decadal survey recommends an observatory bigger than Hubble, which the astronomy community made a top priority in the 1970s. This new instrument, astronomers imagine, would suppress the bright light of a star and look right at the planets hidden in the glow. It would be capable of observing worlds that are 10 billion times fainter than their sun.

“You’re not going to see continents on the surface of the planets,” Bruce Macintosh, an astrophysicist at Stanford and a member of the committee, told me. But you’ll see the exoplanets as they are, no fancy NASA illustrations needed. “We’ll see distinct little dots,” Macintosh said. Some might be blue, others red or purple. If this future space telescope manages to find an Earth-like planet, it will look, at that distance, eerily familiar. We will see it as the Voyager spacecraft, on its way to the edge of the solar system, saw Earth more than 30 years ago: a fuzzy speck in the darkness, a pale blue dot, in Carl Sagan’s words. To those of us accustomed to the gleaming CGI of space movies, the image won’t look like much. But neither would Earth from that far away—and look at what we have here.

How could we check for actual life on one of those fuzzy specks? “If you block out the star, what you’re left with is the reflected light from the planet,” explains Laura Kreidberg, an astronomer who studies exoplanet atmospheres at the Max Planck Institute for Astronomy, in Germany, and who was not a member of the decadal committee. Astronomers can analyze that light to identify the distinct signatures of certain molecules and figure out the chemical composition of a planet’s atmosphere. “If you looked at the reflected light coming from Earth, you’d see evidence of oxygen, methane, water, and a bunch of other things that, together, are very, very indicative of the presence of life,” Kreidberg told me. Before astronomers can say “aliens,” they will have to rule out a host of other sources that could explain an intriguing atmosphere, such as volcanic activity, the effects of solar radiation, and quirks of chemistry.

Two teams of astronomers have already proposed two space-telescope concepts that align with the decadal survey’s recommendations, and the committee has recommended blending the expertise of both groups to create something even more ambitious. The report estimated that the observatory would cost $11 billion and launch in the first half of the 2040s. The decadal-report authors have provided some tips on how NASA can approach these and other projects so that they don’t “become too complex, large, and costly”—a polite dig at the James Webb Space Telescope, which has been behind schedule and over budget and, after 20 years of development, is finally crawling to the finish line to launch next month. The Webb telescope is capable of studying exoplanet atmospheres, but the mission is targeting a type of star that is smaller and dimmer than sunlike stars, and which astronomers believe may not be conducive for life, because they’re prone to flares that could scorch any nearby planets. The telescope could discover some mesmerizing atmospheres, but it is not designed to find true Earth analogues.

Let’s assume that the decadal survey’s vision for a brand-new space telescope becomes reality. Could the instrument really find us another Earth, with a mix of familiar elements brewing in its skies? The odds are good, given how many stars are sprinkled across the cosmos. Earths could turn out to be quite common, and we could find ourselves marveling at someone else’s pale blue dot. Not alone at all.

But the same vastness that contains countless mysterious planets also impedes our ability to find them. Astronomers may not find anything in their search area. That won’t mean that no other Earths are out there, just that they’re not in the small slice of the cosmos where our searchlight hit. And astronomers still don’t have a strong prediction for how many Earth analogues may be out there at all. The decadal report says that about 30 percent of stars should have a planet in their habitable zone, but Kreidberg says that a more accurate estimate would fall somewhere between 10 and 25 percent. Even if the telescope finds a maybe-Earth and provides enough data about its atmosphere, astronomers might spend years trying to come to a consensus about its contents. (Consider that astronomers are still arguing over whether phosphine in the clouds of Venus could be a sign of life, and that’s the planet right next door.) “If you put an ocean on there and let it sit for a while, I think it’s totally reasonable that many, if not most of them, might be inhabited,” Kreidberg said. “My biggest question is, if they’re inhabited, can we tell?”

It is entirely possible that humankind will live out its existence without finding even a hint of another inhabited planet. We’ll be left with our statistics and space telescopes, and the sense that the universe might be lonelier than we imagined. Such an outcome could serve as a constant reminder, as we continue to search for answers deep in the universe, of the cosmic value of our own planet. “It might emphasize that we really shouldn’t mess this one up, because the process that took to make it was pretty hard for the universe to do,” Macintosh said.