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Kepler Overview

Kepler: NASA's first mission capable of finding Earth-size and smaller planets around other stars

Artist's rendering of Kepler's target region in the Milky Way. Credit: Jon LombergThe centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars. There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants. The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone→ of their stars where liquid water might exist on the surface of the planet.

The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone→ and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.

The Transit Method of Detecting Extrasolar Planets

Artist's rendering of Kepler's Transit Method of Detecting Extrasolar Planets. Credit: SETI InstituteWhen a planet passes in front of a star as viewed from Earth, the event is called a “transit”. On Earth, we can observe an occasional Venus or Mercury transit. These events are seen as a small black dot creeping across the Sun—Venus or Mercury blocks sunlight as the planet moves between the Sun and us. Kepler finds planets by looking for tiny dips in the brightness of a star when a planet crosses in front of it—we say the planet transits the star.

Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.

Kepler instrument

Engineering rendering of the Kepler spacecraft. Credit: NASAThe Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope — 105 square degrees, which is comparable to the area of your hand held at arm's length. It needs that large a field in order to observe the necessary large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for the life of the mission—3.5 or more years.

The photometer must be spacebased to obtain the photometric precision needed to reliably see an Earth-like transit and to avoid interruptions caused by day-night cycles, seasonal cycles and atmospheric perturbations, such as, extinction associated with ground-based observing.

Results from the Kepler mission will allow us to place our solar system within the context of planetary systems in the Galaxy.

Additional Links
› About Kepler→
› JPL's New Worlds Atlas→

Kepler Science

Kepler's targeted star field. Credit: Carter Roberts of the Eastbay Astronomical SocietyThe scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:

  • Determine the percentage of terrestrial and larger planets that are in or near the habitable zone of a wide variety of stars
  • Determine the distribution of sizes and shapes of the orbits of these planets
  • Estimate how many planets there are in multiple-star systems
  • Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets
  • Identify additional members of each discovered planetary system using other techniques
  • Determine the properties of those stars that harbor planetary systems.

Kepler's targeted star field graphic. Credit: NASA Target Field of View

Since transits only last a fraction of a day, all the stars must be monitored continuously, that is, their brightnesses must be measured at least once every few hours. The ability to continuously view the stars being monitored dictates that the field of view (FOV) must never be blocked at any time during the year. Therefore, to avoid the Sun the FOV must be out of the ecliptic plane. The secondary requirement is that the FOV have the largest possible number of stars. This leads to the selection of a region in the Cygnus and Lyra constellations of our Galaxy as shown.

Additional Links
› Kepler Science Basics→
› Kepler Discoveries→

Page Last Updated: December 8th, 2014
Page Editor: Jerry Colen