As the highly successful Kepler spacecraft ends its 9-year mission, humanity can look back at an instrument that greatly expanded our knowledge of planetary systems around other stars and discovered many Earth-sized and smaller planets, some of them in the habitable zone where liquid water, and therefore life as we know it, could exist.
The Kepler mission was conceived as a Discovery Program mission at the turn of the century and was approved as the tenth mission in the program in December 2001.
At that time only 80 exoplanets had been discovered since Gamma Cephei Ab was first found in 1989 (but confirmed in 2002), and the exoplanets being discovered with the available technology at that time were large “hot Jupiter” gas giants orbiting close to their stars, completely unsuitable for life as we know it.
The Kepler telescope was initially set to launch in 2006, but ended up being pushed back to 2009 as the spacecraft design and development proceeded and the Discovery program worked through issues like the loss of the CONTOUR spacecraft and budget difficulties. While Kepler’s development proceeded, exoplanet discovery methods and technologies improved and the exoplanet count at the time of Kepler’s launch in March 2009 was up to 300, with some Neptune-sized planets being discovered.
The Kepler spacecraft itself was designed around a 55 inch Schmidt telescope which would act as a photometer to measure the brightness of stars and any variations in that brightness that could be caused by transiting planets or other phenomena like flares or starspots. A focal plane array with 42 CCD detectors was installed as the instrument that would detect minute variations in starlight from the field that the telescope would be assigned to observe.
The spacecraft would stay pointed at a single region of space spanning the constellations Cygnus and Lyra, which was selected because the field was rich in stars but also fit the constraints for the spacecraft’s optics and sunshade as the field needed to be 55 degrees above or below the Sun. The field has 100,000 stars and is richer than the field in the Southern Hemisphere.
The stars Kepler would observe would be between 600 – 3000 light years away from Earth (very few stars in field closer than 600 LY and stars greater than 3000 LY would be too faint for the required observations), and Kepler would need to observe this field for at least three years to be able to confirm Earth-like planets orbiting Sun-like stars at the same distance our planet is from the Sun. (Three transits are required to confirm that the planet exists and there are no other artifacts that are causing the dips in starlight).
The mission would take a statistical sample of stars to figure out how common planets – and specifically, Earth-like planets – are in the universe.
The Kepler telescope launched on the early morning of March 7, 2009 aboard a Delta II medium-lift launch vehicle, the same vehicle that launched many other notable space science missions like Mars Global Surveyor, the two MER rovers, and Deep Impact.
After a nominal launch, the spacecraft was soon propelled on its way to an Earth-trailing orbit which would keep pushing the spacecraft farther away from Earth. This orbit would enable Kepler to keep itself trained on the specific point in the sky that had been chosen for its studies.
On April 7, 2009 the telescope cover was jettisoned and Kepler achieved first light. The Kepler control center in Boulder, Colorado, managed by Ball Aerospace and the Laboratory for Atmospheric and Space Physics (University of Colorado), conducted commissioning activities for the spacecraft, and the first light image was taken on April 8th. This first light image was 95 megapixels, which at the time was the highest resolution image taken in space. After refining the telescope’s focus, Kepler began its groundbreaking scientific mission on May 13, 2009.
Kepler’s first planetary discoveries were announced on January 4, 2010, a batch of five “hot Jupiters”. As Kepler kept observing, the discoveries mounted. On December 5, 2011, Kepler-22b’s discovery electrified the world, as it was the first “super-Earth” discovered orbiting in the habitable zone of a Sun-like star.
In 2014, after analysis of downloaded data, Kepler-186f became the first planet the size of Earth to be discovered in the habitable zone of another star, which in this case was an M-class dwarf (red) star. The following year, continuing analysis turned up Kepler-452b, only 60% larger in diameter than Earth and likely rocky, was discovered orbiting in the habitable zone of a G-class (yellow, Sun-like) star.
The Kepler primary mission had been set for 3.5 years but additional time was required to confirm certain Earth-like planets due to issues with noise in the detectors, and so it was extended. In July 2012, one of Kepler’s four reaction wheels (gyros) stopped operating, and was on three gyros, the minimum required for steady observation of the Cygnus-Lyra star field.
On May 14, 2013, the Kepler mission was threatened when the spacecraft went into safe mode. A second gyro had failed and turned out not to be recoverable, so the primary mission was over. The Kepler science teams worked on ways to recover some use of the telescope on two gyros while exoplanet scientists, including citizen scientists using the website Planet Hunters, continued to sift through the existing Kepler light curves to find new exoplanets.
A new mission called “K2” was drawn up and proposed, and in August 2013 the Kepler spacecraft passed a critical test showing that the K2 mission would work, as Kepler successfully remained steadily pointed using the pressure of sunlight in place of the failed gyro, along with using the two working gyros. The spacecraft would stay pointed at a part of the sky for 83 days before needing to change orientation to avoid sunlight getting into the optics, and would observe a different part of the sky.
The K2 mission started operations in May 2014, and was expected to discover more planets, mainly those orbiting more closely to their stars, but also phenomena such as binary stars, supernovae, and stellar structure. Notable K2 discoveries include the K2-138 system, which is a multi-planet system discovered by crowdsourced observation of K2 data, as well as the K2-72 system which has four rocky planets, two of which might be in the habitable zone.
The K2 mission was expected to last until 2017 or 2018, and has met that projection. Earlier this year there were indications that the Kepler spacecraft was running out of the fuel necessary to keep the spacecraft properly pointed.
During the summer and fall of 2018, the Kepler spacecraft went into safe mode several times, but the spacecraft controllers were able to successfully download the remaining data from K2 campaign 19 and to start observations for Campaign 20 before Kepler entered safe mode for the last time on October 19th.
On October 30, 2018, the Kepler spacecraft end of mission was declared, with the spacecraft finally running out of fuel.
Although the Kepler spacecraft has reached its end of mission, the Kepler and K2 missions have helped us begin find our place in the cosmos. Before the Kepler mission launched, 340 exoplanets, most of them “hot Jupiters” were known. Humanity has now discovered 3,826 exoplanets as of October 26, 2018, and 633 multi-planet systems, with Kepler discovering 2,327 exoplanets, and the K2 mission discovering 354 exoplanets.
The Kepler and K2 discoveries are not over. There are 2,426 “candidate planets” found by Kepler that still need to be confirmed, and another 473 candidates for K2. During the Kepler/K2 missions, up to 30 roughly Earth-sized planets have been discovered orbiting in the habitable zone of stars where liquid water could exist.
Although a recent study has cast some doubt on the number of exoplanets Kepler has found, Kepler has confirmed that exoplanets are a common part of the universe, with at least one planet for every star in the Milky Way Galaxy.
The Kepler spacecraft, though it has now passed into history, has made its mark as one of the most successful space science missions ever, and its discoveries will be an important part of astronomy from now on.