Ten years ago, you soared into space and slipped beyond Earth's gravitational grasp, leaving a wake of fire in Florida's nighttime sky. Twinkling above you were the stars you would mine for alien worlds. Below you spun a world on the verge of a scientific revolution.
You, Kepler, are one of the most transformative spacecraft humans have ever made. But you're gone now, and though we've both known for a few years that the end of your star-studded journey was drawing near, it still feels wrong.
For much of your time aloft, you seemed invincible: a planet-hunting probe that bravely competed in the cosmos’s most challenging staring contest, in which your unblinking eye caught the flickering light from hundreds of thousands of stars. In doing so, you not only revealed that our galaxy is stuffed with planets—you helped me connect with who I am.
I am just one Earthling among billions. But 59 Earth-orbits ago, my dad devised a formula that could calculate how many extraterrestrial civilizations are detectable in our galaxy. It was a question he’d been thinking about ever since he was a kid growing up in Depression-era Chicago, wondering whether there were other planets out there, and if any were like Earth.
So, Dad thought it made sense to define the things we’d need to know if we wanted to calculate the probability of finding smart, chatty aliens.
Fast Facts: Kepler
Launch Date: March 7, 2009
Launch Vehicle: United Launch Alliance Delta II
Entered Service: May 12, 2009
End of Mission: November 15, 2018
Launch Mass: 2,320 pounds (1,052 kilograms)
Power Source: 1,100-watt solar panel array
One variable in this “Drake equation” is the fraction of stars that have planets; another is the average number of habitable worlds in a given stellar system. When my dad wrote it nearly six decades ago, no one had any idea what the values of those two variables were. After all, until the 1990s, we hadn’t even spotted a single planet orbiting a star other than our sun.
Thanks to you, we now mostly know those crucial values. On average, at least one world circles each star in the sky, and roughly one-fifth of stars probably host a rocky, Earth-size planet in an orbit where the temperature is right for liquid water to trickle, pool, and wash over its surface. As well, some scientists think there’s an average of one habitable world per planetary system—perhaps even more, if you consider moons and other solid objects as being life-friendly. And why not? In our neighborhood, icy moons are among the best places to look for life beyond Earth!
These numbers are revolutionary. They’re telling us that if life emerges on other planets as it did here, then there are literally billions of surfaces for life-forms to fasten themselves onto and make their own. Countless Earths are hiding in our galaxy’s starfields, gravitationally tethered to stars both like and very unlike our sun. And if evolving life is sculpted by its environments, as it was here, then many of those distant worlds might not only host life as we know it—but also life as we don’t know it.
That you provided these insights over just a few years of tending our starry skies is remarkable. And as you were finding new worlds, I was moving into a new world of my own: science journalism.
The first time I reported on you was early 2011, when your team announced roughly 1,200 new planet candidates and a six-world system called Kepler-11. NASA had called a national press conference at the agency’s Ames Research Center, and I—an intern at the San Jose Mercury News—was assigned to report the story. “No offense, but why is the Merc sending an intern to cover this? This is a major story about complicated science," one of NASA’s spokespeople grumbled.
Fresh from defending my Ph.D. in genetics, I had gleaned most of my astronomical knowledge from prints on our walls, nights observing the stars back in elementary school, and an introductory astronomy course in college. I had so many questions. Should I refer to your celestial haul as “exoplanets” or “extrasolar planets”? How precisely could I describe the sizes and orbits of the worlds you’d found? And how the hell did you go planet-hunting at all? Your methods were mystifying to me, like a language I was hearing for the first time.
So I got to work. I nervously scheduled interviews with scientists, talked my way into an early copy of the embargoed research paper, and strategically booked a hotel room so I could beat the Bay Area traffic to Ames. Afterward, I hurried back to the newsroom and dropped everything I knew into a story, while my editors wondered what was taking so long.
It ended up running on A1. Mission accomplished.
Eight years later, I’ve written dozens of stories about you, and now I can more easily explain the whats and hows of your incredible mission. In small patches of sky, you’ve spotted hints of more than 2,600 exoplanets as they traipse across the faces of their distant stars and temporarily block a sliver of starlight, which you register as a brief dip in brightness. Those fleeting smudges offer clues to a planet’s size and orbit, which scientists can use to figure out what faraway worlds must be like. And the worlds you have uncovered—fiery lava planets, puffy mini-Neptunes, some orbiting two stars, some snuggled so close together that each rises in the other’s sky, one maybe dragging a giant moon around—constitute the greatest garden of unearthly delights imaginable.
How wonderful it is to exist in a time of such tantalizing strangeness. How lucky I feel to be sharing your discoveries.
But like all good adventure tales, your story had its share of obstacles. By 2013, you had lost several essential motors that let you hold still as you peered at distant stars; the cosmos would win your staring contest after all. But, though disappointed, your people weren’t deterred. They figured out how to harness the pressure of sunlight itself to keep you stable as you aimed your raptor’s eye at many patches of sky. Reborn as “K2,” you found even more planets, planetary debris in our own neighborhood, and fistfuls of exotic, exploding stars.
And then, your fuel started running low. Your team on Earth fought their hardest to keep you going—to wring as much data as they could from your sputtering gaze—but eventually, they instructed you to rest, 388 years to the day after the death of Johannes Kepler, the astronomer whose name you bear.
It happens to the best of us; we all need a break, especially those as vigilant as you.
Now, fittingly, among all the worlds you’ve spotted, you will keep closest to the planet that loves you best. We both will loop around the sun together for billions of years, until our home star eventually balloons into a red giant and devours us both—an exquisite flash, perhaps, in an alien astronomer’s telescope.
But know that even now, your legacy is grand and decisive. When I first showed my dad an image of your field of view—populated by all the alien planets you’d found by 2011—he responded with a sharp inhale. “So many planets….” he said, astounded.
Over the years, the view got more and more crowded.
And now, just imagine what today’s children might be wondering about a cosmos so packed with possibilities, thanks to you.