The Kepler spacecraft has been with us long enough (it launched in 2009) and has revealed so much about the stars in our galaxy that its retirement — Kepler lacks the fuel for further science operations — is cause for reflection. The end of great missions always gives us pause as we consider their goals and their accomplishments, and offer up our gratitude to the many people who made the mission happen. Let’s try to back up and see things in their totality.

Image: An artist’s conception of Kepler at work. Credit: NASA/Ames/Dan Rutter.

Kepler’s job was essentially statistical, an attempt to look at as many stars as possible in a particular field of stars, so that we could gain insights into the distribution of planets there, and thus deduce something about likely conditions galaxy-wide. We didn’t know in 2009 that there was statistically at least one planet around every star, nor did we know just how diverse the worlds Kepler found, more than 2,600 of them, would be. Moreover, Kepler pushed the bounds of the possible with an ingenious extended mission.

After the mission was extended in 2012, engineers had to work around gyroscope failures to use solar photon pressure to keep the spacecraft pointed. The so-called ‘K2’ mission left the fixed view to move around the sky, studying 19 different patches of it and thus producing entirely different datasets. As with any mission, we’ve accumulated such extensive information that papers from Kepler data will be coming out for decades to come. Using the transit method to detect the passage of planets across the face of their star, Kepler’s planetary survey, originally studying over 156,000 stars, would move on through K2 to cover more than 500,000.

We have a great deal to learn as we carry Kepler discoveries forward, but it now seems likely that somewhere between 20 and 50 percent of all visible stars have rocky planets in the habitable zone of their stars. ‘Super Earths’ turned out to be the most common size of planet Kepler found, worlds between Earth and Neptune in size. Unless we find a hitherto undiscovered world deep in our outer system, we have no examples of such planets around Sol.

Interestingly, we’ve also learned how common compact planetary systems can be. Some of the Kepler systems bear more resemblance to Jupiter and its moons than planets and their host stars. KOI-961 is a case in point. About 130 light years away in Cygnus, the star is an M-dwarf about 70 percent larger than Jupiter, orbited by three small and likely rocky planets, all three of which orbit their host in less than two days.

Image: This illustration depicts NASA’s exoplanet hunter, the Kepler space telescope. The agency announced on Oct. 30, 2018, that Kepler has run out of fuel and is being retired within its current and safe orbit, away from Earth. Kepler leaves a legacy of more than 2,600 exoplanet discoveries. Credit: NASA/Wendy Stenzel/Daniel Rutter.

Another Kepler find: Kepler-11, approximately 2000 light years away, with six planets, the outermost of which is twice as close to its star as the Earth is to the Sun. In fact, the five inner planets are closer to their star than Mercury is to the Sun. The list goes on: Kepler-32 is an M-dwarf with five transiting worlds orbiting within a distance that is just one-third the size of Mercury’s orbit. We now explore how such systems form and examine multi-planet systems like TRAPPIST-1 (not a Kepler discovery) that offer planets in the habitable zone.

Kepler also discovered its share of unusual exoplanets like Kepler-10b, a world with a year that lasts less than an Earth day and a density implying it is made of iron and rock, a ‘lava world.’ K2 found a planetary object around the white dwarf WD 1145+017. Kepler-16b is the first planet discovered around a double-star system, reminiscent of Tatooine in Star Wars. Kepler-444 has five planets that are 11 billion years old. And need I mention Boyajian’s Star, whose oddly fluctuating lightcurve is indicative most likely of dust clouds but was intensely studied in terms of possible alien engineering.

“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.”

Image: William Borucki, principal investigator for NASA’s Kepler mission, in his office at NASA’s Ames Research Center at Moffett Field, California. June 2015. Credit: NASA Ames.

Kepler kept sending us data even as it ran low on fuel, with the latest, from Campaign 19, marking a kind of handover to TESS, the Transiting Exoplanet Survey Satellite, which launched back in April. All told, we pulled in 678 GB of science data in Kepler’s 9.6 years in space, leading thus far to 2,946 published scientific papers, with many more to come. Last night at a favorite restaurant my wife and friends joined me in a toast to the Kepler team, and the mission’s indefatigable PI William Borucki. An outstanding accomplishment all around.

To wrap this up, here’s Kepler’s ‘first light’ image from 2009.

Image: This image from NASA’s Kepler mission shows the telescope’s “first light” — a full field of view of an expansive star-rich patch of sky in the constellations Cygnus and Lyra stretching across 100 square degrees, or the equivalent of two side-by-side dips of the Big Dipper. Credit: NASA.