Kepler Space Telescope – Know Everything About It

Ask most people to name a space telescope, and they’ll probably tell you Hubble. While that telescope’s name is certainly worthy of its fame, there’s another that, arguably, has done as much to broaden our cosmological horizons as its more famous sibling: the Kepler space telescope.

The tradition of naming cutting-edge space equipment after legendary astronomers has a rich history, and the Kepler Space Telescope proudly continues this tradition. Johannes Kepler, a renowned German astronomer from the late 16th to the early 17th century, is the namesake of this modern marvel. He is best known for introducing the groundbreaking laws of planetary motion. These principles have empowered astronomers to precisely forecast the positions of planets throughout time.

Fast forward to four centuries later, when NASA, in the early 1990s, envisioned a new line of telescopes. Their main objective? To explore the vast universe for planets revolving around distant stars. However, it wasn’t a straightforward journey. It took nearly a decade, encompassing four rejected proposals, before the dream materialized. In December 2001, the Kepler telescope received the green light.

Considering its primary goal was to unearth potentially habitable planets similar to Earth and given the foundational reliance on Kepler’s laws of planetary motion for such discoveries, naming the telescope in his honor felt inherently fitting.

The astronomer Johannes Kepler.
The astronomer Johannes Kepler

When was the Kepler Telescope First Launched?

The Kepler telescope itself consisted of a primary mirror 1.4 meters (55 inches) in diameter, the largest of its kind at its launch. Also, on-board were 42 CCD cameras, each with a resolution of 2,200 x 1,024 pixels, giving a total resolution of nearly 95 megapixels. These cameras were meticulously engineered to discern minute brightness fluctuations in stars, signaling potential planetary transits.

It was finally launched on March 7th, 2009, aboard a Delta II rocket from Cape Canaveral, Florida. At the time, some 350 exoplanets were known to exist, and it was hoped the telescope would discover thousands more during the course of its three and a half year mission. Scientists also hoped to discover dozens of Earth-sized worlds, but only time would tell if the telescope would be successful.

Continuously pointed toward the same area of sky, around the constellations of Cygnus and Lyra, the telescope studied approximately 150,000 stars in an area containing 4.5 million stars over 115 square degrees (or roughly 0.25% of the entire night sky). Hold your fist out at arm’s length, and it would cover about the same area!

Kepler’s Discovery

The Kepler Space Telescope. Image credit: NASA
The Kepler Space Telescope. Image credit: NASA

After a month in orbit, its cover was discarded, and the first test images were taken on April 8th. The mission began properly on May 12th, and scientists would spend the next two months calibrating its systems by studying five previously known exoplanets.

One standout from this period was the in-depth ten-day study of HAT-P-7b, commonly referred to as a “hot Jupiter.” This mesmerizing exoplanet, located a staggering 1,000 light-years from us, completes an orbit around its star in a fleeting 2.2 days. Impressively, Kepler’s advanced systems were capable of discerning the atmospheric composition of HAT-P-7b and observing the variations in its luminosity as it journeyed around its star.

HAT-P-7b wasn’t the only exoplanet studied during that initial two month period. Others included Kepler-1b and TrES-2b, the darkest known exoplanets. Kepler discovered this gas giant reflects less than 1% of the light it receives, making it darker than acrylic black paint and giving rise to the planet’s nickname: the Coal Planet.

What Were Some of the Kepler Space Telescope’s First Discoveries?

An artist's concept of the Kepler-16 system, the discovery of the first planet orbiting a multiple star. Image credit: NASA/JPL-Caltech
An artist’s concept of the Kepler-16 system, the discovery of the first planet orbiting a multiple star. Image credit: NASA/JPL-Caltech
  • January 4th, 2010: During a meeting of the American Astronomical Society in Washington, scientists announced the first exoplanets discovered by the Kepler space telescope. In all, there are five new worlds, called Kepler-4b, 5b, 6b, 7b, and 8b. While 4b was akin in size to Neptune, its counterparts echoed the grandeur of Jupiter.
  • February 2nd, 2011: Astronomers announce the discovery of six planets orbiting a single star some 2,150 light-years away. What makes this system so special is that five of the planets have orbits smaller than Mercury’s, making it one of the most compact systems known. The star, Kepler-11, is Sun-like so the worlds are too hot to support life.
  • September 15th, 2011: A nostalgic moment for Star Wars enthusiasts occurred on September 15th, 2011. The unveiling of the first exoplanet to orbit the common center of mass of a double star system – reminiscent of the iconic Tatooine – thrilled fans. However, in stark contrast to the fictional world, Kepler-16b is characterized as a frigid, Saturn-esque gas giant by most planetary scientists and is much too distant from its host stars to host habitable moons either.
  • December 20th, 2011: Astronomers announce the discovery of the first Earth-sized planets orbiting a Sun-like star. The star, Kepler-20, lies some 929 light-years away and has five exoplanets in orbit. One, Kepler-20f, was the first exoplanet discovered to be smaller than the Earth. Unfortunately, neither of those two worlds lay within the habitable zone.

How Did Kepler Telescope’s Primary Mission Come to an End?

By the end of 2011, there were 2,326 exoplanet candidates. Of those, 207 were Earth-sized, 680 were about twice the size of the Earth, 1,181 were the size of Neptune, 203 were Jupiter-sized and 55 were thought to be larger than Jupiter.

The implications were staggering. Based on the data, scientists estimated there could be as many as fifty billion planets in the Milky Way, including about two billion Earth-sized worlds with at least half a billion in the habitable zone. Of those, some 30,000 were thought to lie within a thousand light-years of the Earth.

Other discoveries include:

  • May 18th, 2012 – The discovery of a super-Mercury is announced, but this planet is not like any other. Some 1,500 light-years from Earth, this planet is slowly disintegrating as it orbits its parent star and could be gone in as little as 200 million years.
  • June 21st, 2012 – A celestial pair of planetary twins is announced. The two worlds, Kepler-36b and 36c are only 1.2 million miles apart at their closest – about 20 times closer than any two planets within our own solar system. The planets would be easily visible as spheres within each others’ skies.

Despite these successes, the Kepler telescope suffered the first of several setbacks in July 2012 when one of its four reaction wheels failed. Used to keep the telescope pointing in the right direction, the telescope required three of the four wheels to be functioning and, fortunately, the telescope was able to continue its work.

  • August 26th, 2012 – The discoveries continued with the announcement of Kepler-47, the first multi-planet system orbiting a binary star system some 5,000 light-years away.
  • February 20th, 2013 – The discovery of Kepler-37b is announced. The smallest known exoplanet, it’s only slightly larger than our own Moon. It orbits its parent star at less than one-third of Mercury’s distance from the Sun. At that distance, it seems highly unlikely to support any kind of life.
An artist's concept of Kepler-47, the first discovery of multiple planets orbiting two sun. Image credit: NASA/JPL-Caltech/T. Pyle
An artist’s concept of Kepler-47, the first discovery of multiple planets orbiting two sun. Image credit: NASA/JPL-Caltech/T. Pyle

The K2 Mission

On May 15th, 2013. NASA holds a press conference to announce the failure of a second reaction wheel. With only two of the four wheels functioning, the program risks cancellation.

Many believed that the Kepler Space Telescope’s days of discovery might be drawing to a close. However, the scientific community and the ingenious engineers at NASA were not ready to let Kepler’s capabilities go to waste. Harnessing the remaining operational capabilities of the telescope, they initiated a new phase known as the K2 mission, giving Kepler a renewed purpose.

To address the issue, engineers made use of the sun’s radiation pressure as a form of stabilization. The concept, quite ingenious in its simplicity, was to balance the telescope against the consistent push of photons from the sun, effectively turning this external force into a ‘virtual’ third reaction wheel. This is the same sort of concept that a solar sail would use, albeit applied over a much smaller surface area.

By orienting the telescope’s solar panels directly towards the sun and aligning its field of view almost perpendicular to the sun’s direction, Kepler achieved a delicate equilibrium. This alignment created a stable observation mode, albeit with a limitation. The field of view had to be repositioned roughly every three months to ensure a consistent balance against the sun’s radiation as the telescope orbited around our star. This is why K2’s observations shifted along the ecliptic plane, allowing for varying 80-day campaigns focusing on different patches of the sky.

  • September 30th, 2013 – The first cloud map of an exoplanet, Kepler-7b, based upon data from Kepler, is made public.
  • October 15th, 2013 – Using data from Kepler, civilian volunteers and astronomers from Yale lead to the discovery of exoplanet PH1, the first world found to be orbiting a quadruple star system.
  • October 30th, 2013 – The discovery of Kepler-78b is announced. The first Earth-sized planet confirmed to be rocky and with a density the same as Earth’s, it whizzes about its parent star in just 8.5 hours. It is totally inhospitable to life as we know it.

What Were Some of the Highlights of K2, Kepler’s “Second Light” Mission?

On November 26th, 2013, K2, also known as “Second Light”, the second major Kepler mission, is unveiled. One of K2’s most significant achievements was the continued discovery of exoplanets. While the original Kepler mission focused on a single field of stars, K2’s shifting observation strategy allowed it to discover planets around a diverse range of star types, ages, and environments. By the time the K2 mission ended, hundreds more candidate exoplanets had been identified, adding substantially to the legacy of the Kepler telescope.

In addition to planet-hunting, K2 contributed significantly to stellar astrophysics. By observing stars throughout their life cycles, from young stellar clusters to ancient populations, the mission expanded our understanding of stellar evolution. Moreover, K2 observed numerous objects in the Solar System, including asteroids, comets, and even Neptune, providing insights into the dynamic processes of our own planetary system.

April 17th, 2014One of the most momentous discoveries from the Kepler mission was Kepler-186f, an exoplanet that lies within the habitable zone of its host star, Kepler-186. While its discovery was announced in 2014, it was found with original Kepler mission data from 2011. Situated about 500 light-years from Earth in the constellation Cygnus, Kepler-186f is notable because it’s roughly Earth-sized and orbits its star within the range where liquid water might exist on its surface. This makes it one of the most promising candidates in the search for life beyond our solar system.

Located roughly 1,100 light-years away in the constellation Lyra, Kepler-442b orbits the star Kepler-442. This exoplanet is about one-third larger than Earth and is also situated within its star’s habitable zone. Given its size and position, Kepler-442b has often been highlighted as one of the best candidates for a potentially habitable Earth-like planet. Its star, being a K-type dwarf, is cooler and less massive than our Sun, yet offers a stable environment that could potentially support life.

Often dubbed Earth’s “cousin”, Kepler-452b orbits its star, Kepler-452, in the habitable zone, where conditions might be right for life. Located about 1,400 light-years away from us, this planet is about 60% larger than Earth and orbits a star similar to our Sun in size and temperature. Kepler-452b’s discovery sparked discussions about the potential of finding Earth-like planets in habitable zones.

Among the most enigmatic of Kepler’s discoveries was Tabby’s Star (KIC 8462852), named after the lead astronomer, Tabetha Boyajian, who studied its mysterious light fluctuations. The star exhibited erratic dimming, with light levels dropping by significant percentages, a phenomenon not observed in any other star. Multiple hypotheses were proposed to explain this unusual behavior, from a swarm of comets to the speculative idea of an ‘alien megastructure’. Although no definitive explanation has been accepted, the star’s odd behavior has made it a favorite subject of both scientific inquiry and popular speculation.

An artist's concept of the possible cometary dust orbiting Tabby's Star. Image credit: NASA/JPL-Caltech
An artist’s concept of the possible cometary dust orbiting Tabby’s Star. Image credit: NASA/JPL-Caltech


From 2016 on, Kepler was involved in a large number of projects—called campaigns – that took its focus away from the hunt for exoplanets. These campaigns included studying over 36,000 galaxies, the center of our own Milky Way galaxy, the planets Mars and Neptune, comets and asteroids within our own solar system, star-forming regions in Taurus, supernovae, variable stars, the Praesepe, and M67 star clusters, and the red dwarf star Wolf 359, as made famous by the television series Star Trek: The Next Generation.

Campaign 19 was to be Kepler’s last. On October 30th, 2018, NASA announced that the telescope had run out of fuel and that, after nine years, its mission would come to an end. The telescope was deactivated with a “goodnight” command on November 15th, the anniversary of Johannes Kepler’s death in 1630.

It’s hard to underestimate the role the Kepler space telescope has played in the history of astronomy. As of the time of writing, the telescope is credited with 2,343 confirmed exoplanet discoveries, with another 2,421 yet to be confirmed. When the mission was first proposed, back in the early 1990s, none were known to exist at all.

And if the astronomer Johannes Kepler could know about his namesake and the worlds it had discovered, what would he think? When he was born, people still believed that everything in the universe orbited the Earth. When he died, the idea that the planets orbited the Sun instead was just beginning to take hold. Both the astronomer and the telescope existed at a time when our view of the universe was evolving, and they both played an enormous role in expanding it.

For a more complete timeline and list of discoveries, click here.

Leave a Comment