The Kepler spacecraft consisted of a large telescope, equipped only for observing subtle signs of planets. The telescope detected transits, or slight eclipses of light from the star when a planet passes in front. Some of these light changes are so slight that the difference in brightness is equal to that of a fly on a windshield, but Kepler detected them all the same.
The Kepler spacecraft was launched on March 7, 2009 from Cape Canaveral, Florida. It escaped Earth's orbit and settled into its orbit around the Sun, which causes Kepler to follow Earth around its orbit. Kepler went through a commissioning phase and began observation on May 13, 2009. Then, the first information was transmitted to Earth in June. NASA will sorts through the thousands of images to find signs of exosolar planets. In September, Kepler verified the existence of an exosolar planet. The planet's orbital period is just over two days, so Kepler took under a week to detect its transit three times.
On January 12, 2010, the first five new planets were discovered from an analysis of the results obtained in November of the previous year. On August 26, 2010, three additional planets orbiting the same star were announced. Another major discovery occurred in early 2011, when a system of six planets was announced, along with the smallest extrasolar planet yet discovered. Planet discoveries continued to trickle in as the year went on, including another planet in May 2011.
Another interesting discovery was that of a planet, named Kepler-16b, orbiting a binary star system. Discovered in September 2011, it is the first definitively confirmed circumbinary (circum = around, binary = two [stars]) planet. Also, in November 2011, Kepler-21b was discovered. It is a rocky planet only 60% massive than Earth. Unfortunately, it is so close to its parent star that it orbits in less than three days.
On January 26, 2012, 26 new planet discoveries were released, including two more instances of circumbinary planets, and numerous systems containing two or more planets, leading to higher estimates of planets per star in the Milky Way. In response to this unexpected bounty of planets, NASA extended the mission of Kepler through 2016 in April 2012. This allowed the confirmation of orbiting bodies with longer periods of revolution. Other significant discoveries of 2012 include the first known occurrence of two planets orbiting two stars and also the discovery of a planet that orbits one of the two stars in a binary system.
By 2013, Kepler had discovered over 100 planets. In April of that year, planets in the habitable zone of two stars were discovered, and they were also Earth-like in size, being less than twice the size of the Earth. Unfortunately, a failure of the orienting mechanism of the telescope on the spacecraft in May halted observation. The spacecraft was then put into hibernation while NASA planned maneuvers to restore Kepler's mobility. Over the next several months, tests revealed that the failure could not be corrected.
Despite these difficulties, Kepler turned to other observations with its remaining capabilities, including the study of supernovae and small solar system bodies. Meanwhile, analysis of the data that Kepler had already provided continued to reveal many new extrasolar planets. Using a data analysis method called verification by multiplicity, many planets in multiple-planet systems were verified in early 2014, culminating in an announcement on 2014 that an astounding 715 new planets had been confirmed! Several of these planets were also small (smaller than Neptune) and a few were Earth-sized and in their stars' habitable zones.
In April 2014, data analysis revealed the most Earth-like planet yet known: an Earth-sized body orbiting in the habitable zone of its star, a red dwarf. Known as Kepler-186f, this planet is about 492 light-years away, and is potentially habitable.
On May 16, 2014, NASA approved a new mission for the Kepler telescope itself, after scientists developed a method to keep the spacecraft sufficiently steady with only two reaction wheels to observe an area of the sky continuously for over 80 days, enough to detect transiting planets. Using the radiation pressure from the Sun as a counterforce, the telescope can balance the force from the remaining orientation mechanism. The image below illustrates the so-called K2 mission.
Instead of fixing the gaze of the telescope at a small area of sky for a number of years, as in the first scientific campaign, the K2 mission explored several different fields of view, spending a few months on each phase or "campaign". The new mission detected its first confirmed exoplanets in January 2015. That same month, the number of confirmed planets from the Kepler mission surpassed 1000 with the further analysis of previous data.
In July 2015, one of Kepler's more notable planet candidates was confirmed. Known as Kepler-452b, the planet was the most similar to Earth of any yet discovered: it is 60% larger than the Earth in diameter (and therefore has a good chance of being rocky), orbits a sun-like star in an orbit only 5% larger than Earth's, and has an orbital period of 385 Earth days, very close to our own. In addition, the planet is estimated to have existed for 6 billion years, even longer than the Earth, giving it a better chance of harboring life.
Mission operations continued normally until April 7, 2016, at which time it was discovered that the spacecraft had entered emergency mode. NASA immediately made efforts to return the telescope to normal operations in order to make the scheduled maneuver. These effort were successful and the spacecraft was able to resume normal operations on April 22. It was then able to begin Campaign 9 (abbreviated C9) of its mission. This involved using gravitational microlensing to detect planets farther away from their host stars. This works as follows: when a planet passes in front of its star, the mass of the planet causes starlight to bend (very slightly) around it, causing a temporary increase in brightness, as illustrated in a graphic below from Kepler's NASA website.
The scale of the bending is exaggerated here for illustration. Note that for planets close to their host stars, Kepler looked for a decrease in brightness that would indicate starlight being blocked. However, for sufficiently massive and distant planets, the gravitational microlensing effect is larger, leading to a net increase in brightness.
Meanwhile, continuing data analysis continued to yield new planet confirmations from among Kepler's earlier candidates. On May 10, 2016, NASA announced that an additional 1,284 planets had been confirmed, more than doubling the total that Kepler had verified. Among these, almost 550 were of a size that they could be rocky, and nine of these were in the habitable zones of their parent stars.
In June 2016, the mission was officially extended through the anticipated end of Kepler's fuel resources. Over the next few years, the K2 mission confirmed the existence of another few hundred exoplanets. Low on fuel, the spacecraft was still operating in April 2018, when its successor, the Transiting Exoplanet Survey Satellite (TESS) was launched by NASA. Built on the experience gained through the Kepler mission, TESS and Kepler shared the same detection method, but with TESS having a much larger field of view. In May, the K2 mission began its 18th observing campaign, focusing on observing star clusters. Finally, at the end of October 2018, Kepler had exhausted the remainder of its fuel resources. The telescope was officially retired on November 15, 2018, nearly a decade after its launch. During its service, Kepler discovered a remarkable 2662 exoplanets, well over half of the total known at that time. Its success moved exoplanet astronomy forward by leaps and bounds and paved the way for future missions to answer the many remaining questions concerning worlds outside our own Solar System.
Sources: http://www.nasa.gov/mission_pages/kepler/main/, http://keplerscience.arc.nasa.gov/K2/, http://www.theverge.com/2016/4/8/11395796/nasa-kepler-spacecraft-mission-emergency-mode,https://keplerscience.arc.nasa.gov/k2-mission-officially-extended-through-end-of-mission.html, https://tess.gsfc.nasa.gov
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