Kepler is the plucky little spacecraft that taught us that planets are everywhere. It captured the imagination of farmers, citizen-scientists, and astronomers alike. This is the story of Kepler: a celebration of turning the theories of planetary science upside-down.

Kepler is a Planet-Hunting Powerhouse

Being depicted in a corn maze is an honour Kepler shares with only one other space telescope, Hubble. Image credit The MAiZE Inc./Space Farm 7

Since the satellite launched in 2009, NASA's Kepler telescope has discovered 961 confirmed exoplanets, with 3,845 candidates awaiting verification. After five years of new observations, new worlds, and new theories, Kepler's field of view has been realigned to compensate for equipment failures. Crippled, but functional, this year is the start of a new mission of discovery.

To mark Kepler's 5th anniversary and the transition from its original mission to the new secondary mission, we're bringing you a round-up of everything Kepler. This is the story of a half -decade of planetary science theories demolished, revised, remodeled and demolished again, and a telescope that just won't give up on making new discoveries.

Dr. Alexander Wolszczan identified the first exoplanet in 1994, when he found planets blocking the radio pulses of their home star, an inhospitable pulsar. The next exoplanets were discovered using radial velocity, the pull of a massive planet on its star. By the end of the century, several dozen worlds had been found. Then in 2006, the French launched CoRoT, the first dedicated planet-hunter. CoRoT successfully located a few dozen exoplanets and confirmed others. By 2009, CoRoT was sitting pretty on a respectable stack of carefully-characterized planets outside our solar system. And then came Kepler...

2009: Launch, First Light, and learning to hunt for planets

Kepler is a Planet-Hunting Powerhouse

Rocket launches at night are inherently dramatic.

A night launch at Cape Canaveral on March 6, 2009 sent Kepler into orbit to hunt for exoplanets. After orbital adjustments, camera tests, and settling down into just the right field of view, Kepler's First Light of the Cygnus-Lyra region in April was captured without a hitch.

Kepler is a Planet-Hunting Powerhouse

This sky is Kepler's now.

Kepler was positioned to look at exactly the same patch of sky all the time. It wouldn't capture beautiful photographs like Hubble, but instead collect extremely detailed light curves of the same stars, hour after hour, day after day, for year after year. Scientists were hunting for the telltale dip in brightness of a planet transiting its star. As a planet crosses in front of its star at regular intervals, the star's brightness is reduced by a tiny fraction. Any star with periodic dimming is tagged as a candidate system. Follow-up observations weed out false positives before scientists confirm discovery of a new exoplanet.

In the official mission documentation, astronomers expected that Kepler would find about 50 Earth-sized planets in one-year orbits, and maybe 185 slightly-larger-than Earth-sized planets. The expectations were similar for giant planets, originally anticipating the discovery of 135 inner-orbit giants, and 30 outer-orbit planets. The hunt was on: find 400 exoplanets before funding ran out in November 2012 to justify this fancy new space-telescope.

2010: The Year of Hot Jupiters

The first five finds unique to Kepler were announced in January 2010. All five were gas giants in close orbits, promptly earning the moniker "hot Jupiters." These were disappointing worlds for scifi fodder (aside from Cloud City in Star Wars, we have a distinct fondness for exoplanets with a surface we can see), but more importantly, they demolished our exiting theories about planetary formation. The established model, "heavy stuff at the center, light stuff at the edges" that our system exhibits so cleanly with its inner terrestrial planets and outer gas planets didn't make sense in a universe that had to include "hot Jupiters". Suddenly, we had a handful of huge, low-density gas giants nestled in against their parent stars, in orbits that made no sense in existing theory.

Desperate to restore peace and order to planetary science, theories immediately started popping up to declare hot Jupiters were a momentary abnormality, quickly destroyed by their suns or pushed into more reasonable orbits. Their parent stars consume them in a feeding frenzy of hydrogen gas. Stellar winds blow the gas right off them, leaving denuded hard cores in orbit. The intense heat and gravity of such a close orbit would tear them apart and burn them to a crisp, possibly at the same time. Whatever the mechanics, hot Jupiters must die.

Kepler is a Planet-Hunting Powerhouse

The first five exoplanets found by Kepler were large, toasty, and way too close to their stars to satisfy dreams of aliens.

Within months, a system was observed that exhibited rhythmic variation in its periodic dimming. Clever brains thought clever thoughts, and tried modeling what sort of timing variations a multi-planet system would produce. They paired observed variations with modeled planetary transits, and the first multi-planet system, two planets orbiting a single star, was confirmed.

2011: From rocky planets to an explosion of candidates

The first discovery of the new year was a more conventional rocky planet but this one came with a twist too. It was even closer to its star than Mercury is to ours, and the modeled surface temperatures are comically high. Even the most enthusiastic astrobiologist wasn't going to theorize about life on a planet with surface temperatures so hot even rock was charred.

A few candidates were eliminated by astronomers as exoplanets, but embraced by astrophysicists for giving insight into stellar formation. One of the false-positives was a triplet-system of three stars. A massive red giant and two tiny red dwarfs twirling around each other in a complex dance cycle of eclipses, producing entrancing brightness variations. Even Kepler's rejected not-really-planets made for spectacular astronomical discoveries.

By May 2011, the research team was beginning to go into shock. Kepler had racked up a list of more than 1,200 candidate planets far exceeding the initial 400 projected discoveries. Multi-planet systems were turning out to be far more common than anyone thought.

Francois Fressin of the Harvard-Smithsonian Center for Astrophysics called a time-out to introduce a new processing technique. Blender, a light-curve computer simulator, was applied to candidate systems to try to explain away the brightness variations as the interaction of multiple stars. Only if no combination of stars worked did he invoke exoplanets. He tested the idea with the Kepler-10 multi-planet family, recruiting NASA's Spitzer Space Telescope to stare at the system. When his simulated light curves matched both Kepler's visible light and Spitzer's infrared observations, the multi-planet system was confirmed and Blender joined the arsenal of planet-hunting techniques.

But the relentless Kepler was just getting warmed up. Next, Kepler spied an exoplanet circling a single star of a binary system and then a planet of perpetual night, a planet whose surface absorbs 99% of all light that hits it. "TrES-2b is considerably less reflective than black acrylic paint, so it's truly an alien world," astronomer David Kipping explained in the press release.

As more data poured in, researchers experimented with validating instead of verifying candidates. In verification, confirming an exoplanet meant gathering a secondary set of observations to confirm its existence. With validation, astronomers construct a theoretical model of the planetary system, then confirming that the observed dynamics match the model. The technique was first tested on a system featuring multiple gas giants squeezed into a ridiculously tiny orbital radius, the cosmic equivalent of a clown-car bursting with fake noses and over-sized shoes. Once again, experiment matched theory, and another tool joined the collection of planet-hunting techniques.

The itch to do something really clever was scratched by the appearance of Kepler-19, in the constellation Lyra. This time Kepler had found a planet where the orbital dynamics were off. Very precisely 5 minutes off, give then take, each orbit. Off, in just the same manner that it would be off if another planet was yanking on it. Just like that, Kepler had discovered the Invisible World. Finding an invisible planet is a slick move. Without ever directly seeing the planet transit its star, another exoplanet was added to the list based on its invisible gravitational tug on the other planet in its system

The following month, Kepler started spotting Earth-sized planets. The first one was a bit bigger (1.6 x Earth radius), a bit heavier (<10 x Earth mass), and a whole lot closer to its slightly bigger sun (0.04 x average Earth-Sun orbital distance). Then it spotted a multi-planet system that had a rocky planet even smaller than Earth. It looked like that rocky planet might even be tectonically active, eliciting the appearance of sprouting volcanoes in the artistic representations. By the time Kepler's first annual report rolled around, everyone involved was a bit googly-eyed. Planets had turned out to be shockingly easy to find but their behavior was not at all what we expected.

2012: Circumbinary Planet Systems: weird, weirder, weirdest

By January 2012, the science team was compelled to invent new vocabulary — circumbinary planet systems — to describe systems where planets orbited more than one star. Circumbinary systems opened up a whole new realm of comparative planetology, with such strange possibilities that you can almost hear the giddy excitement even though the sterile language of a press release:

"These planets can have really crazy climates that no other type of planet could have," said Dr. Jerome Orosz, a co-author from San Diego State University. "It would be like cycling through all four seasons many times per year, with huge temperature changes."

[co-author Dr. William] Welsh adds, "The effects of these climate swings on the atmospheric dynamics, and ultimately on the evolution of life on habitable circumbinary planets, is a fascinating topic that we are just beginning to explore."

Kepler is a Planet-Hunting Powerhouse

Kepler-961, the most squee-worthy exoplanetary system of tiny adorableness.

Before long, Kepler challenged its press officers to harness their inner cuteness-addict to describe what even the press release acknowledged was "the cutest planet system yet," a tiny trio of planets orbiting a wee little red dwarf. Each planet was smaller than Earth, one was even bordering on Mars-sized small. The system offered a poetic analogy: set side-by-side, Jupiter and its moons and Kepler-961 and its planets were roughly the same sizes and distances apart. Kepler-961 was the Jupiter that succeeded as a star; its planets, the Galilean moons promoted. Of course, the artist's conception for all this adorableness took advantage of the red star-glow to coat everything in pale pink light.

The weird systems kept piling up. A pair of planets in separate orbits had close approaches every three months, coming within five times the Earth-Moon distance in some version of Upside-Down that somehow stumbled into reality. Another circumbinary system, then a multi-planet circumbinary system. Just when we started to feel like maybe our solar system was the anomaly by being so normal compared to the exoplanetary insanity reaching out in all directions, Kepler found our solar-system twin. Finally, here was a system aligned just like ours. Sure, the distribution of rocky and gaseous planets within our system might still be a fluke, but at least we weren't alone in our rationality.

Citizen-scientists started combing through the Kepler data as part of the Planet Hunter project. In what was becoming par for the course, their first confirmed planet was extraordinary. The amateur Planet Hunters found the first ever quad circumbinary system: a planet circling not one, not two, but four stars! In the accompanying press release, astronomer Meg Schwamb acknowledges,

"The discovery of these systems is forcing us to go back to the drawing board to understand how such planets can assemble and evolve in these dynamically challenging environments."

It's a beautifully understated acceptance of the iterative process of scientific discovery. No bafflement. No confused outrage. No irrationally clinging to planetary formation theories that look ever more naively simplistic. It's a simple statement of, "Whoops, guess we're going to need to come up with a new idea for what's going on!" that encompasses everything I love about science. Thankfully, soon after admitting that we had no idea what was going on anymore, Kepler's budget was extended for several more years of planet-hunting. With a bit of luck, at some point we'd hit a saturation point of accepting incredible, undreamed-of planetary arrangements, and start putting the theoretical pieces back together again

In July 2012, the first hiccup to the mission hit: one of four gyroscope-like reaction wheels used to stabilize and aim Kepler crapped out. Like most spacecraft, Kepler was built with redundancy in mind, so the other three were sufficient to keep the mission running smoothly. Engineers fruitlessly brainstormed ways to restore the malfunctioning reaction wheel while Kepler resumed hunting for planets. Meanwhile, geeks everywhere lost an afternoon playing with an interactive adaptation of the XKCD graphic of all 786 confirmed planets, and then another one watching the "What if…" video of all 2,299 exoplanet candidates orbiting a single star.

By October 2012, second-generation papers were coming out. These ones took the observations beyond, "Look, it's a planet!" and dove into the data to see what else could be teased out. The detailed data from observing Kepler-1b for years was analyses for ellipsoidal variations and Doppler beaming — how Kepler-1b moved and how brightness varied with planetary transits. The combined analysis results in mass ratios between the planet and its star. The radius of the star can be calculated from astroseismology, changes in brightness from internal waves within the star. This drops the number of unknowns to the point that relative planet size is used to calculate planetary radius. Read that again: by looking at infinitesimally small changes in brightness in a star, we can calculate the radius of the planets that orbit it.

But the papers don't end with orbital dynamics. Daily rotation of the planet led to thinking about diurnal temperature differences. Run temperature gradients as inputs into atmospheric models, and suddenly a whole new field of exometeorology emerged, looking at potential weather patterns on alien worlds. That is a seriously impressive amount of information to pull out of simple brightness curve, even when the data is extraordinarily detailed and covers multiple years.

2013: Even more candidates, telescope-tragedy, and rebirth into Second Light

During the annual discovery-roundup in January 2013, the theme was "Planets, planets, everywhere!" In every patch of sky covered by Kepler's field of view, exoplanets of all sizes had been observed. We were finally pounding it into our own thick heads: planets aren't rare. The odds of finding an Earth-like planet were skyrocketing due to the sheer number of planets in the universe. The press statement for the preceding year concludes:

"The analysis of increasingly longer time periods of Kepler data uncovers smaller planets in longer period orbits— orbital periods similar to Earth's," said Steve Howell, Kepler mission project scientist at Ames. "It is no longer a question of will we find a true Earth analogue, but a question of when."

Kepler is a Planet-Hunting Powerhouse

Planets, planets, everywhere, of every shape and size. Hot ones, cold ones, right before your eyes...

Statistical analysis that February reinforced the point: Earth-sized planets in the habitable zone of cool stars are overwhelmingly common, and we've probably got one within 68 light-years from Earth. From the press release:

"We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted," said Harvard astronomer and lead author Courtney Dressing

Even better, those planets could be circling long-lived, cool red dwarf stars. That opens up the possibility that life around one of these alien stars could have a several-billion year head start on our puny 4.6 billion year history. So all that scifi with ultra-evolved alien species, the ones that consider Earthlings the upstart teenagers? Not necessarily far off the mark.

Kepler is a Planet-Hunting Powerhouse

Exoplanets keep getting smaller every year.

As the length of time we have been observing a system keeps getting longer and longer and longer, the planets we can detect in that system keep getting smaller and smaller and smaller. Soon, Kepler would need to deal with the Pluto debate on its discoveries. At what point would a candidate exoplanet really be a dwarf-exoplanet? (Or is that an exo-dwarf planet?) The Pluto-killers at the International Astronomical Union (IAU) place the boundary between planet and dwarf planet at something that has enough mass to be round, and able to clear its orbit of any other planetary bodies. At a whisper bigger than our moon, Kepler-37b probably clears out its orbit, so it's a planet. Or, at least, it is for now until kill-joys at IAU scatter some asteroids in its path to make it prove it can clear as effectively as the big kids.

Not even 3.5 years after the phrase "hot Jupiter" was coined, the theories of their imminent and ongoing doom were contradicted. In the evocatively-titled Stars Don't Eat Their Young Migrating Planets, astrophysicists conclude that stars don't cannibalize hot Jupiters as a matter of course. Instead, tidal forces circularize the planet's orbit. The stable radius varies by planetary mass, each planet drifting outwards until attaining a more reasonable distance from their star.

In May 2013, tragedy hit the telescope when a second reaction wheel died. By August, engineers resigned themselves to being unable to restore Kepler to its former fully-functioning glory. In October, the shutdown downed the Kepler website and all non-essential functions. For a telescope tumbling alone in space, the future couldn't look much more grim.

Despite the echoing silence from Kepler, ever more discoveries of strange new worlds were being extracted from the accumulated data. A spitfire world races through an entire year's orbit in just 8.5 hours. Another world wobbles within its orbit as if its had a few too many drinks before stumbling home. One system has an ordinary planetary plane, except for one planet pried into a suspiciously high angle. A Goldilocks system is uncovered, where a trio of Earth-like planets are all within the habitable zone just right for the development of life. Even the false positives were advancing science, with stellar astrophysicists developing a new technique to link brightness variations to the surface gravity of stars.

By the time the shutdown ended, those very clever brains had come up with a very clever solution to the tragically tumbling telescope. With only two reaction wheels, they couldn't keep Kepler stabled and pointed at its original star field, so they stopped trying. Instead, they borrowed the sun to act as a third stabilizer. They reoriented Kepler so that solar pressure was balanced across the spacecraft's surface. This marked the end of Kepler's primary mission, and the long-term observation of the Cygnus-Lyra star field, but the beginning of it's new mission: (K2) to observe the Sagittarius star field. Kepler was stabilized and captured its Second Light. Now, it was up to the team to practice maintaining the delicate balance of unstable equilibrium for hours, days, and weeks at a time.

2014: Exoplanet, Confirmed!

The New Year brought unwelcome news that along with reduced mobility, Kepler was also slowly going blind. The first of 25 science modules had been lost years before; now a second died to a random system failure. Undeterred, Kepler soldiered on through the testing phases for K2 with a few more blind spots but still a functional scientific machine.

The real news for 2014, the news that brought Kepler into the headlines and spawned endless jokes, is that a new data processing technique moved 715 planets from "candidates" to "confirmed" overnight. I'm surprisingly grateful NASA didn't pitch the press release as, "Find 715 planets with this one weird trick!" The "weird trick" is verification by multiplicity. If one false positive is rare, then several simultaneous false positives in the same system is downright implausible.

Imagine a system with five candidates around a single star. Those candidates can be either five real exoplanets, or five false positive stars. In this NASA animation, watch the planets stay in neat, orderly, stable orbits, and watch the stars spiral, swirl and shoot off into chaos. Every candidate confirmed with verification by multiplicity must be in a multi-planet system: all 715 planets are in just 305 systems.

This batch-verification brings total discoveries up to 961 confirmed exoplanets, more than double the original expectation of 400. All data collected in the original mission before the reaction wheel breakdown contains an additional 3,845 exoplanet candidates awaiting verification. While designed as a planet-hunter, Kepler also served stellar astrophysicists by identifying 2,165 eclipsing binary systems. Kepler produced a unique, high-quality dataset for a single field of view covering multiple years: it's a dataset of incalculable scientific value. Already, researchers are contemplating how to mine the data for evidence of moons outside our solar system.

The sheer number and variety of planetary systems uncovered has taken planetary science from cocky confidence through uncomfortable confusion and back out into awe-struck curiosity. We started this hunt with a perfect theory that explained our own solar system, only to watch it be shredded by the first batch of planets we uncovered. Each time we tried to patch the theory back up, another exoplanet would tear out an exception. Finally, we were left staring at the data, acknowledging the depths of what we don't know, and starting to put the pieces together in a whole new order.

In just five years, Kepler has exceeded all expectations. It delivered a steady stream of new types of planets, and planetary systems. We've gone from a handful of verified exoplanets, to a diversity of worlds that defies imagination. In the ongoing tradition of our robotic explorers doing far more than we anticipate of them, at five years and partially crippled, Kepler is just settling into starting a whole new mission.

Here's to you, Kepler. You're a planet-hunting beast that has transformed the very foundations of how we think about planets, moons, and alien stars. You made astounding contributions to science, the depths of which we will not fully process for years to come. Even with failing parts and damaged equipment, you're still contributing to scientific discovery. I salute you, Kepler. May the planets be plentiful and the stars full of data as you settle into your new quest.

Images credits NASA/Ames/JPL-Caltech unless otherwise stated. Check out the Five Years in Space gallery for photographs, images, and animations highlighting Kepler's history. You can learn about some the 728 exoplanets discovered by everything that isn't Kepler on the European Southern Observatory, the Hubble Space Telescope, or the Spitzer Space Telescope sites. Finally, here's a tribute Kepler's exoplanets.

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