So Long, and Thanks for All the Planets: NASA Retires the Kepler Telescope

After nearly ten years and 2,681 planets, it’s time to say good-bye to the Kepler space telescope. NASA has announced that the mission has ended. Unlike more open-ended missions like Curiosity or Opportunity, Kepler always had a definitive shelf-life. In order to detect the minute variations around distant stars that serve as evidence those stars have planets, Kepler needed to be farther away from Earth than a telescope like the Hubble. The space telescope relied on hydrazine fuel to keep its position in order to conduct scientific operations, and it’s much too far away from Earth to be refueled or serviced. But it leaves behind a legacy of incredible accomplishments and a new telescope, TESS, is already continuing the good work Kepler began.

Kepler drove a sustained doubling of our previous rate of planet discovery, with two enormous surges in 2014 and 2016 that presumably reflect the analysis of data collected in previous missions (NASA says it will be busy analyzing previous data from Kepler for years to come). It wasn’t that we had no idea if exoplanets existed before Kepler — we’d detected them before. But what we didn’t know very well at all was how common planets might be. That might seem silly, given that science fiction has presumed the existence of alien worlds and races since long before Star Trek was a gleam in Gene Roddenberry’s eye. The science fiction greats like Asimov and Heinlein filled their galaxies with worlds beyond counting on the supposition that other planets must exist, but they didn’t actually know if they did or not.

“When we started conceiving this mission 35 years ago we didn’t know of a single planet outside our solar system,” said the Kepler mission’s founding principal investigator, William Borucki, now retired from NASA’s Ames Research Center in California’s Silicon Valley. “Now that we know planets are everywhere, Kepler has set us on a new course that’s full of promise for future generations to explore our galaxy.”

Borucki would know — he spent decades fighting for the mission that would become Kepler, at a time when a space-based telescope out of range from direct service missions was as much a flight of fancy as warp drive. At the time Kepler launched, the small group of planets that had been found were primarily hot Jupiters — gas giants orbiting very close to their parent stars. One of Kepler’s most important contributions to our understanding of the cosmos isn’t just that it found planets, but that it could detect planets in more scenarios than we had previously been able to find. The fact that it found so many is one reason scientists are now more confident that planets are a common feature around stars, even if Kepler struggled to find worlds that were strictly Earth-sized. Kepler relied on picking up minuscule variations in a star’s luminosity as a planet passes in front of it. The larger the planet in relation to its host star, the larger the variation.

Kepler mostly found planets much larger than our own, but it detected a solid number of near-Earth or Earth-like planets and set the stage for the Transiting Exoplanet Survey Satellite, or TESS. TESS is already in operation and will survey an area of the sky 400x larger than what Kepler covered. Scientists expect that it could detect up to 20,000 exoplanets over its lifetime, potentially dwarfing Kepler’s contribution.

But as they say, you always remember your first — and it was Kepler that first showed us that the number of planets scattered across the stars is, in fact, enormous. It was Kepler that gave us our first glimpse of a handful of stars and worlds that might actually harbor life. It was Kepler that found many planets orbiting in the habitable zone of their stars, and while this does not prove the existence of life, it proves that rocky planets can form at an appropriate distance from their host stars. Through nine tumultuous years on Earth, Kepler kept churning out new data, surviving reaction wheel failures, and pushing back the boundaries on our understanding of the heavens, clarifying whether the scientific facts we found confirmed or challenged our own ideas about how the universe works.

Well done, we say. Well done, thou good and faithful servant. And a sincere thank-you to the NASA scientists who rescued the mission after its initial reactor wheel failure and who have tirelessly worked to analyze the data in all the years since. All of us who love space exploration without actually doing any of it owe a debt to those of you who do.

Or, for those who prefer the slightly cheekier version: So long, and thanks for all the planets.

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