Many scientists are studying our solar system for conditions in places other than Earth that are suitable for life. But other scientists are looking beyond our solar system for planets circling stars other than the Sun. Before scientists can determine the potential for life on any Earthlike planets they find, they must overcome unique challenges: these small extrasolar planets (planets outside our solar system) are many light years away from Earth, they produce no light of their own, and they are hard to spot in the glare of their parent stars. Because extrasolar planets are so difficult to observe directly, astronomers instead look first for minute changes in the motion or brightness of the parent star caused by its orbiting companion. To do this, scientists use a variety of methods and instruments.

In this video, we learn about NASA’s Kepler mission. To fulfill the mission’s scientific objective—to explore the structure and diversity of extrasolar planetary systems—NASA is using a specially designed 0.95-meter diameter telescope called a photometer, which is akin to the light meter in a camera. The mission and the telescope were named after Johannes Kepler, the German astronomer who developed the laws of planetary motion and made advances in the field of optics. Both are critical to the operation of the current mission.

The Kepler telescope continuously monitors the brightness of 150,000 stars in a very small patch of sky. If a planet crosses in front of the star—or “transits”—the star’s brightness dims a tiny amount. Kepler’s sensitive instruments can detect this periodic dip in brightness. The amount of dimming depends on the size of the planet in relation to the size of the star. From the period and level of darkening seen during the transits, the orbit and size of the planet can be calculated.

To date, Kepler has discovered more than 2,000 Earth-sized planet candidates. Many of these have been confirmed by two or more other telescopes. In 2011, Kepler discovered its first rocky planet, the smallest planet ever discovered outside our solar system at the time. It also found a solar system with six planets in a space no greater than the distance between Venus and the Sun. This discovery redefined our scientific understanding of how small a solar system can be. Perhaps the most significant find to date has been the discovery of Kepler-22b, the first planet found in the habitable zone of another Sun-like star. Planets in a habitable zone are far enough away from their host stars that they have just the right temperature for liquid water, and perhaps life, to exist on their surfaces.

As scientists continue to survey the cosmos, some interesting data continue to emerge. Counting only currently observed planets orbiting close to their stars, smaller planets vastly outnumber larger ones. Fully 80 percent of these planets are smaller than three Earth diameters. Applying this estimate to our galaxy of roughly 200 billion stars, more than 46 billion Earth-sized planets may exist in habitable zones around distant stars in the Milky Way alone.

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