On March 7, 2009, the Kepler Space Telescope took off from Cape Canaveral aboard a Delta II rocket. For almost a decade, the space telescope expanded our understanding of the universe before it finally went dark for good last October. But it’s worth revisiting the spacecraft one last time, on its 10th anniversary.
Of the major launches and long-term missions in my own lifetime, Kepler’s achievements are unparalleled, in my own opinion. This is not to downplay the tremendous long-term mission of the Hubble Space Telescope, the earlier Galileo and Voyager missions, Cassini, Juno, or New Horizons. Each of these probes transformed our knowledge of our own solar system or the larger cosmos. But Kepler provided knowledge of literally thousands of planets we hadn’t detected before, in a staggering variety of shapes, sizes, and other variations.
Prior to Kepler’s launch, there were some 340 confirmed exoplanets. Today, we know of ~3,800, ~2,700 of which were Kepler contributions. There are still scientists today sorting through the data the probe sent back, checking candidates and confirming orbits with follow-up observations. In fact, the very first candidate Kepler ever detected was finally confirmed to be an exoplanet this week.
Kepler’s raw performance was better than any Earth telescope, but didn’t quite meet target design goals due to higher-than-expected noise levels from stars themselves. As a result, Kepler had to observe more transits of potential planets in order to confirm a target. The telescope worked by monitoring specific stars for the minute drop in brightness it could measure when a planet passed in front of them. Imagine trying to calculate the drop in room brightness when a fly passes between you and the room’s light bulb and you’ve got an idea of just how difficult a task this was.
Because larger planets cover more of their stars during transit, Kepler’s planetary detections tended to run to the large end of the scale. Future telescopes, like the already-operational TESS (Transiting Exoplanet Survey Satellite) should have better luck with smaller rocky planets, closer to their host stars.
Kepler launches in 2009. Image credit: NASA
Kepler’s explicit scientific goals were to characterize and categorize the types of planets that form around stars; discover whether Earth-like planets were present; if those planets orbited in the habitable zone of their host stars; discover additional facts about the size, location, mass, and speed of the already-known “Hot Jupiters;” and to determine the general properties of stars that have planets in the first place as opposed to those that don’t.
While Kepler’s mission has been taken over by the more effective and powerful TESS (which is itself intended to act as a spotter for further investigations by the James Webb Space Telescope, in certain instances), Kepler’s nearly decade-long mission gave us the first evidence that rocky worlds weren’t just theoretically likely to exist based on the logical projections of astronomers, but that they factually existed in significant numbers.
The best coda I can think of for the Kepler Space Telescope is this: The work it did will continue. TESS is expected to find 20,000 exoplanets, nearly an order of magnitude more than Kepler ever did. If the James Webb Space Telescope launches successfully, it will offer another. But either way, the vital work that Kepler did is already going to continue, pushing back the frontiers of human understanding in the process.