Kepler’s Odyssey

Nidhi Shakhapur

5 min readOct 22, 2023
Artist’s conception of the Kepler space telescope observing planets transiting a distant star
Artist’s conception of the Kepler space telescope observing planets transiting a distant star | Credit: NASA Ames

As celestial voyagers, we’ve been buzzing with excitement over the recent exploits of the James Webb Telescope — a marvel of engineering, that is capturing the images of the celestial bodies beyond our solar system. With each photograph it takes, this telescope deepens our understanding of the cosmos. In the history of space exploration, a predecessor to this remarkable instrument exists — Kepler, a telescope that embarked on its journey in 2009, with the objective of finding the exoplanets capable of supporting life and was finally successful in discovering over 1,200 exoplanets to date.

Kepler, a prominent German mathematician, formulated the famous “Kepler’s Law of Planetary Motion”, which determined how planets revolve around stars. In essence, the name “Kepler” symbolizes the telescope’s mission to observe and understand the motion of planets, particularly exoplanets, as they orbit distant stars. Just as Johannes Kepler’s laws revolutionized our understanding of celestial motion, the Kepler Space Telescope revolutionized our understanding of exoplanetary systems and the prevalence of planets beyond our solar system.


Kepler’s Primary Mirror

Diagram of the primary mirror on the Kepler Telescope
(Left hand side) The primary mirror was placed in the front end of the Kepler telescope. (Right hand side) The primary mirror being placed inside Kepler. | Credits: Michael Richmond, NASA, Ball Aerospace

What’s the main purpose of a mirror? Reflection, isn’t it? But not all mirrors serve the same purpose. Convex mirrors converge the light rays while concave ones diverge them. Similarly, the primary mirror in the telescope gathers light from a specific field of view in space and directs it towards the telescope’s camera array. This allowed Kepler to capture the faint light emitted by stars and to observe any changes in brightness caused by the transit of exoplanets.

Kepler’s Focal Plane Array

Location and structure of focal plane array
(Left hand side) Location of the Focal Plane array in the Kepler Telescope. (Right Hand Side) The structure of the focal plane array | Credits: NASA, Ball Aerospace

The focal plane array (FPA) refers to the array of detectors or sensors located at the focal plane of an optical system, which is the telescope situated in the Kepler satellite. The focal plane array consists of a grid of charge-coupled devices (CCDs). CCDs are specialized electronic devices that are sensitive to light and can convert incoming photons (particles of light) into electrical signals. Each individual CCD element in the focal plane array acts as a pixel, much like to the pixels in a digital camera’s sensor. It captures the presence of an exoplanet with the change in the intensity of light falling on the sensors.

Reaction Wheels

Diagram of the Kepler spacecraft shows the location of two of the four reaction wheels that control the pointing accuracy of the vehicle.
The Highlighted part shows the reaction wheels in the Kepler satellite | Credit: Ball Aerospace

Kepler used a set of four reaction wheels to maintain its precise pointing and stability. By adjusting the rotation speeds of these wheels, the telescope could accurately focus on specific target stars for extended periods. The mission required a minimum of three sets of wheels to function at a time.

An interesting fact about Kepler’s architecture is that it was initially set to have a lifespan of 3.5 years, but it was extended by two years, due to its exceptional performance and groundbreaking discoveries. The longevity was partly attributed to the effectiveness of the solar panel’s power generation capabilities, enabling it to continue far beyond its original mission timeline.

Data Handling and Transmission System

To understand how Kepler transferred the data, it is important to know the principle on which Kepler worked.

Kepler’s operation is based on an interesting principle — the transit properties of planets. This principle held the key to Kepler’s discovery process, propelling it forward in its exoplanetary quest.

As a planet orbits its host star, there are instances when it crosses between the star and the observer’s line of sight. This transit causes a temporary dimming of the star’s brightness as the planet partially blocks the starlight. Going by simple terms, imagine you’re watching a bright streetlight from a distance. Now, imagine a small ball moving around the light. Sometimes, this ball passes in front of the light, blocking some of its brightness from your view. This is similar to what happens with planets and stars. This phenomenon is known as transit. Kepler’s precise instruments were adept at detecting these subtle changes, such as the CCDs in the focal plane array, that captured the exoplanet presence with a slight dip in the intensity of light falling on it, thereby allowing scientists to further research on the potential exoplanet candidates and confirm their presence.

The pictorial and graphical demonstration of Transit of a planet
The pictorial and graphical demonstration of Transit of a planet | Credit: NASA Ames

The collected data is stored on board the spacecraft in a dedicated data storage system. Although it is a large storage system, the data has to be periodically transmitted to Earth to allow for the storage of the new data.

Kepler used its high-gain antenna to establish communication with ground-based receiving stations. The antennas transmitted the data in packets that were received by ground control for further analysis by the scientists.

However, transmitting data from Kepler to Earth was not without its challenges. The vast distances involved meant that data transmission rates were not as high as we are accustomed to on Earth. Engineers had to carefully plan and prioritize the data to ensure that the most relevant and valuable information was sent first.

But there were a few false positives too that were identified by the scientists when they received the data packets from Kepler. Whilst Kepler failed to classify it as ‘not a planet’, scientists calculated it to prove so.


Kepler, broadened our cosmic understanding by confirming the presence of thousands of distant worlds outside our solar system. These newfound celestial domains showcased a previously unimaginable diversity, ranging from rocky planets to colossal gas giants. Among its revelations were “Earth-like” exoplanets nestled in the habitable zone of their stars, offering potential havens for liquid water and sparking discussions about extraterrestrial life. Kepler also uncovered planetary systems resembling our own, with planets of differing sizes and orbits. Surprisingly, it revealed “hot Jupiters” orbiting closely to their stars and “mini-Neptunes” occupying a size spectrum between Earth and Neptune. The telescope even peered into binary star systems, illustrating the vast range of planetary environments. Before concluding its mission, Kepler astounded the scientific community by capturing the faint glow of a supernova in a distant galaxy, highlighting its adaptability in studying diverse cosmic phenomena.

In the realm of space exploration, the Kepler Space Telescope stands as an inspiration of innovation and discovery. The remarkable use of the transit method has paved a new way of discovering the exoplanets in the cosmos. Its legacy will continue to inspire and guide our quest to unravel the mysteries of the cosmos.




The official blog of SEDS-VIT, Indian Headquarters of the Global NPO, Students for the Exploration and Development of Space.