Earlier this year, news broke that the Kepler planet hunting telescope was in trouble. While in orbit, Kepler relies on precision hardware called reaction wheels to keep itself pointed in a single direction. It launched with four, and it needs three to aim properly. One of the reaction wheels failed in 2012, and a second showed signs of failure that led its controllers to put it into a fuel-saving mode that precluded further science. On August 8, engineers attempted to restart the two failed reaction wheels, but they were unable to get them to work. Kepler's mission is officially over.

But what a mission it was. According to NASA, the first half of the data it collected contained over 3,600 planetary candidates, 135 of which have been confirmed. Analysis is proceeding on the second half of the data now, but the first 3,600 were enough to change our perception of the Milky Way. We can now infer that it probably has at least as many planets as stars and that the most common types are small, rocky ones. Planets that are able to have liquid water on their surface are also likely to be common.

The downside of Kepler's end is that a longer mission would identify planets further from their host stars. To identify a candidate, Kepler needs to see it pass between Earth and the star the exoplanet orbits three times. The further out a planet is, the longer one orbit takes, so the longer it will need to do three passes. As such, the existing data is heavily biased toward planets that orbit very close to their host stars; this also means that most of the potentially habitable planets we've spotted are orbiting dwarf stars, which are dim enough that water can remain liquid close in.

In other words, we simply haven't looked long enough to detect planets in a habitable zone around energetic stars. We have a much better picture of the diversity of exoplanets, but it's far from a complete one.

The telescope and imaging equipment on Kepler are still functioning well, so NASA is now inviting researchers to submit proposals to use them for additional science missions that can work even with the telescope's limited pointing capabilities. Those proposals will be evaluated later this year, at which point NASA will determine if any of them will have a scientific return that's worth the cost of the continued operations. If not, Kepler will be retired from service.