Kepler: The First Step Toward Reaching Earth-like Worlds

The Kepler Telescope is the very first mission capable of finding Earth-sized planets within our own galaxy.

The Kepler Mission is named after Johannes Kepler, the early 17th century German scientist who pioneered the fields of optics and planetary motion. Now, 400 years later, the mission will become the first step in answering a profound, fundamental question pondered since ancient times: Are there other worlds like ours or are we alone?

Johannes Kepler

Appropriately, the mission launches in the International Year of Astronomy (IYA), that celebrates not only Galileo Galilei's ground-breaking work in telescopic observational astronomy, but also the year that Johannes Kepler's paradigm-setting work, Astronomia Nova, was published, suggesting that the Sun rotates on its axis.

The mission will spend three-and-a-half years surveying more than 100,000 sun-like stars in the Cygnus-Lyra region of our Milky Way galaxy. Kepler will survey the stars in the habitable zone, a distance not too close to and not too far away from the star in order to measure the spot that is "just right" (known as the 'Goldilocks Zone') matching ideal conditions for life we have here on Earth. It is expected to find hundreds of planets the size of Earth and larger at various distances from their stars. If Earth-size planets are common in the habitable zone, Kepler could find dozens; if those planets are rare, Kepler might find none.

Kepler will detect planets indirectly, using the "transit" method. A transit occurs each time a planet crosses the line-of-sight between the planet's parent star that it is orbiting and the observer. When this happens, the planet blocks some of the light from its star, resulting in a periodic dimming. This periodic signature is used to detect the planet and to determine its size and its orbit. Photometry measures the periodic dimming of the star caused by a planet passing in front of the star along the line of sight from the observer.

Each rectangle indicates the specific region of the sky covered by each CCD element of the Kepler photometer.

To achieve this resolution, Kepler will use the largest camera ever launched into space, a 95-megapixel array of Charged-Coupled Devices (CCDs). CCDs are the silicon light-sensitive chips that are used in today's TV cameras, camcorders and digital cameras. Kepler must monitor many thousands of stars simultaneously, since the chance of any one planet being aligned along the line-of-sight is only about 1/2 of a percent. The telescope can detect even the faintest winks, registering changes in brightness of only 20 parts per million.

The Kepler Mission demands extreme precision-detecting planets in this manner can be compared to staring at a headlight a great distance away- and measuring changes in brightness when something as small as a flea crosses in front of that headlight.

Over the last two decades, scientists have detected more than 300 extrasolar planets circling other stars in the Milky Way galaxy. Most of these planets have been about the size of Jupiter, lessening chances that they might harbor life.

Over the course of its planned 3 1/2-year mission, Kepler will search the skies for planets 30 to 600 times smaller than Jupiter - closer to Earth's size. After launch, Kepler will enter a 372.5-day orbit around the sun, trailing in Earth's wake.

By staring at one large patch of sky for the duration of its lifetime, Kepler will be able to watch planets periodically transit their stars over multiple cycles, allowing astronomers to confirm the presence of planets and use the Hubble and Spitzer space telescopes, along with ground-based telescopes, to characterize their atmospheres and orbits. Earth-size planets in habitable zones would theoretically take about a year to complete one orbit, so Kepler will monitor those stars for at least three years to confirm the planets' presence.

Additional Resources

• More on the Kepler Mission