Astronomers have been finding exoplanets out in the cosmos for 25 years, and if we’ve learned anything about all those planets, it’s that a lot of different, weird kinds exist. They are big and hot and close to their stars. They are smaller than Earth. They are gassy and Jupiter-y. They are rocky and terrestrial. They are so cold even the most extreme earthly organisms would freeze to death. They are so hot they could melt glass. They rain glass. They are by themselves. They have neighbors. They are far away. They are right next door.
And over the years, astronomers have found more and more planets that are increasingly “like” Earth — at least in terms of their size, their distance from their stars, and potentially their compositions and characters.
On Aug. 24, 2016, astronomers announced a potentially habitable, likely rocky planet orbiting the star nearest us, Proxima Centauri. Certain corners of the internet freaked out, dubbing it an “Earth-like planet” and calling for interstellar travel. Proxima b, as the world is known, is among the smallest known exoplanets, mass-wise, and it’s as close to Earth as one can get. But it’s not substantially smaller than many others, and it’s not guaranteed to be any more Earth-like, either. Proxima b fell from the public consciousness and the front page within weeks, just one more among 3,565 other known exoplanets.
Because big announcements like this happen regularly now, every year or so, it’s easy to just say “cool” and move on. Readers are used to seeing news stories about the next-closest-to-Earth-sized planet, the maybe-could-be-Earth’s-twin planet, the no-really-this-time-it’s-like-super-close-to-maybe-being-like-how-Earth-is planet. And with that escalation, exoplanets have begun to seem very normal, even possibly boring.
Astronomy fans have begun using the term “exoplanet fatigue” to describe the mindset that comes with yet another announcement of otherworldly, and potentially worldly, worlds. When every near-Earth-sized planet gets hype, and thousands of others are announced at a time, it’s easy to feel like we should just put planets in the same been-there-done-that category as stars: Discovering more is just adding to a pile no one cares about.
But we should resist that urge toward apathy. Exoplanets haven’t finished changing our worldview, our universe-view, our view of life itself, scientists say. Their work is just beginning. They hardly know anything. After all, it wasn’t that long ago that these worlds were little more than science fiction.
The Reality of Exoplanets
For a long time, astronomers thought planets were hard to make, perhaps requiring “two stars to pass close enough to each other to pull out material in a disc,” says Jill Tarter, who worked on some of the earliest exoplanet telescope plans and is considered a pioneer in searching for extraterrestrial intelligence. Planets emerged from that two-star-spun disk. But how often do two stars come that close to each other? Not often.
The current standard scientific canon suggests that stars, and planets, form from a shrinking cloud of gas. After the gas collapses into a dense enough clump to start its path toward stardom, its gravity flattens the remaining gas into a disk. Flecks of dust and molecules of gas smack into each other and stick together, giving them more mass, and hence more gravity, which attracts more dust and gas to them. This process snowballs, and eventually the small clumps grow into small planets, big planets, asteroids and comets. But this idea didn’t mature until the 1980s, and even then most scientists continued to believe that conditions had to be just right to make planets, which they thought were uncommon.
But people began searching for them anyway, and then the discoveries started to trickle in. In 1992, astronomers Alexander Wolszczan and Dale Frail found two planets around a pulsar, the husk of a star left over after it explodes as a supernova. Three years later, astronomers Michel Mayor and Didier Queloz discovered a planet about half the mass of Jupiter, whirling around a sunlike star in a roughly four-day dance. Planets kept popping up, as people used ground-based telescopes to detect the stretching and shrinking of a star’s light waves — the result of the tug of a planet’s gravity. Scientists’ ideas about the abundance of other worlds began to change; maybe it wasn’t so hard to make a planet after all.
But there was an even better way of looking, first detailed in 1971 and revised by Bill Borucki, formerly of the NASA Ames Research Center in Mountain View, Calif., in the mid-1980s. A telescope could stare at a star and wait for it to dim — just a little — when a planet passed in front of it and blocked some of its shine. This is called a transit, and Borucki was convinced that it would work on a large scale. He wanted to build an orbiting telescope that would watch a wide swath of space, and all the stars within, at once. He began proposing it officially in the early 1990s and tried four times until, in 2000 (fifth time’s the charm), NASA approved it.
With its launch nine years later, the Kepler space telescope was born. The underlying hope, of course, was biology-based: to find a planet truly like ours, where life could survive, or even thrive. And, along the way, scientists would be thrilled to learn more about planetary dynamics and demographics.
When the first results came back, Kepler mission instrument scientist Doug Caldwell took his first look at the data on a known planet. “It was so clear, and it looked like a fake computer model,” he says. “We were amazed. It really worked!”
Kepler’s impressive work has revolutionized the field. It gave us so many planets — and enough rocky, Earth-ish ones — that we now find these once-extraordinary worlds commonplace.
As Kepler stayed in space longer, it gathered enough data to detect smaller planets, farther from their stars. At first, Caldwell’s team confirmed planets individually, pointing a ground-based telescope directly at a given star system. But soon, Kepler had amassed entirely too many candidates — the team had to find another way to confirm them.
And that, says Caldwell, caused another shift in the field. Astronomers decided they didn’t need to know each candidate planet was a true planet: They could just be 99 percent sure. They began confirming the existence of other worlds in batches, using a statistical validation technique that matches transits against models to see how likely it is that they probably come from a planet. “If you pick any individual one, it might not be a planet,” he says. “Chances are it is, but it might not be. But if you take the whole set of them and you want to try to understand properties of them, you can make very good conclusions based on that because you know that most of those — 99 percent — are really going to be planets.”
This idea works partly because planets are so common — so easy to make — that, chances are, the scientists aren’t misinterpreting the signals. Astronomers estimate, based on past observations, that the number of Earth-sized planets in our galaxy approximately equals the number of stars, roughly 100 billion.
Suddenly, scientists could do demographic studies on the planets, just like pollsters do with census data. What percentage of people with incomes under $45,000 live in one-person households? What fraction of planets within 100 million miles of their star are more than twice the mass of Earth? That catalyzed another shift in scientists’ thinking, from the quest for Earth’s twin to the analysis of what its many and varied siblings are like. It went from “we’re going to find Earth,” says Caldwell, to “we’re going to find lots of things that could be like Earth and try to understand how their properties vary around different stars.”
Some solar systems mirror our own, with a neat set of planets lined up in a flat plane like a posed portrait, small ones mostly close to the sun and big ones farther out. Others have hot Jupiters, big planets that live very close to their stars; still others have planets in wonky orbits at weird angles to each other. Yet others have mini-Neptunes and super-Earths, varieties that don’t show up at all in our own family photo.
Even 25 years after finding the first exoplanets, and thousands of discoveries later, we still don’t have an answer to the questions that spurred the Kepler mission in the first place. How did solar systems get to be the way they are? And how often does a livable planet like Earth — really like Earth — come to be?
But Wait, There’s More
That remaining uncertainty and potential don’t always come through in headlines or TV reports, though, which focus more on excitement over the latest find. Take the coverage of Proxima b: Many press releases and breathless news stories splashed the words “habitable” and “Earth-like,” adjectives that have also appeared in dozens of discovery articles in the past.
To be clear: Humans currently know of no certainly habitable or even just Earth-like planets. But when scientists and the media throw these terms around, they suggest that astronomers have already found everything they’ve been looking for in a planet. People think we’ve already found an Earth twin. No wonder they lose interest.
The first problem is that scientists’ phrase “in the habitable zone” sometimes gets shortened — by scientists, the press and people’s minds — to simply “habitable.” Scientists say the former and mean “could host liquid water,” but that gets morphed into the latter and, effectively, “could host life.”
“Those words have different meanings in English, which is what the public is actually going to read,” says Rory Barnes, an astrobiologist at the University of Washington. “ ‘Oh, it’s in the habitable zone, ergo it’s habitable,’ and it makes perfect sense to do that.”
On top of that, the habitable zone means different things to different people. Determining the exact boundary — this side of the imaginary line can host liquid water, this side can’t — depends on many factors beyond just the hike from the planet to the star. The planet’s internal composition and its atmosphere, as well as the star’s stability and intensity, all play a role.
To reflect that complexity, Barnes has developed a metric called the habitability index for transiting exoplanets, which comes a little closer to telling whether they’d be habitable, in the true English-dictionary sense of the word. The traditional habitable zone is binary: Yes or no, a planet is in it or it’s not. But the habitability index gives the probability that a planet actually has liquid water, after taking into account the surface temperature of the planet. He hopes that scientists can use the index in the future to decide which planets next-generation telescopes should pay most attention to. Those telescopes will be able to tell not if a planet could be Earth-like, but if it actually is another Earth.
If the public knew how close we could be to finding an actually habitable planet — and that we hadn’t really found another Earth yet — that’d surely spike their interest.
The Meyers-Briggs Inventory of Planets
“ ‘Earth-like’ is probably even more fraught with problems than ‘habitable zone,’ because what does that mean?” says Caldwell. “To my mind, if something is Earth-like, there’s trees, there’s water, and [similar] things. That’s certainly not what we’re talking about because we have no idea.”
But it turns out we soon might. The personality details of planets — details beyond their superficial attributes of size, weight and neighborhood — are starting to come into view. Some of the next generation of telescopes plan to zero in on Proxima b, a less-extreme zoom than what’s required for other similarly sized planets that live farther away.
The coming studies with next-generation telescopes like the James Webb Space Telescope and the Transiting Exoplanet Survey Satellite aren’t just about finding new worlds; they are about exploring them, via their atmospheres.
Scientists are interested in biosignatures, or combinations of molecules indicating the presence of life as it breathes, eats, photosynthesizes or otherwise interacts chemically with its environment. Biological processes like these leave chemical concentrations out of their natural equilibrium, telling scientists that something — or someone — must be altering them. On Earth, for instance, the atmosphere contains oxygen and ozone with relatively little methane, which indicates photosynthesis is happening.
So far, scientists have only been able to see the spectra from a few planets’ atmospheres, as they need bigger and better telescopes, with special equipment to block starlight, to really get to know a planet. Maybe we should wait for that before giving in to boredom with exoplanets. After all, a pile of census data doesn’t mean humans are boring and mundane because we know so many of them exist in so many forms. It’s the individuals that really spark interest, that tell you why the population matters; we should give exoplanets the same chance.
Vive la Révolution Scientifique
The scientists get it: So far, there’s only been so much to get excited about. “Maybe a certain amount of fatigue in the public is natural and fine,” says Aki Roberge, an astronomer at the NASA Goddard Space Flight Center in Greenbelt, Md., “as long as when the time comes that we really do discover something crazy-amazing, we’re still able to get people to pay attention.”
That crazy-amazing thing is an actually Earth-like, actually habitable, perhaps actually inhabited planet. And it’s still in the future. Tarter calls the 21st century “the century of biology on Earth — and beyond.”
Roberge elaborates on the same idea. “I do believe we’re standing on the verge of a scientific revolution,” she says. “But it’s not in astronomy, per se. It’s actually in biology. And the discovery of life on other worlds — of an independent line of life — would be as revolutionary as the realization that the sun was a star or that those moving lights in the sky are planets like the Earth.” Or, perhaps, that Earths are as common as stars throughout the cosmos.
It could be in a couple of decades, Roberge says, or 100 years, or more. There’s no way to know. But she imagines that just as Newton’s laws of gravitation govern how planets interact with each other (and how you interact with the ground), a parallel set of laws governs how life arises or doesn’t, and then survives (or doesn’t). “Maybe life is rare,” she says. “Maybe it isn’t. But I think that the habitable conditions that Earth-like life could tolerate — I don’t think those are rare.”
The only way to know is to keep looking, to keep amassing more planets (and announcing them), to start probing their atmospheres from afar. With tomorrow’s telescopes, that revolution will come, and it will be glorious. Now that’s something to get excited about.