Imagine a mosquito flying in front of a car headlight. You couldn’t see the mosquito, but if you had a sensitive light detector you could measure a tiny drop in the brightness of the headlight as the mosquito flew by. This is what the Kepler spacecraft will do as it searches for planets orbiting in front of their host stars. The passage of a planet in front of its star is called a transit. This tells astronomers more than you might think! If you plot a line on a graph of the amount of light being emitted by the star, the passage of a planet in front of the star is signaled by a dip in the line: the deeper the dip, the bigger the planet or the closer it is to its star. How often the same dip occurs tells you how long the planet takes to orbit the star. Once you know this, you can tell how far the planet is from the star.

One of the greatest astronomers in history, Johannes Kepler (1571-1630) formulated the laws of planetary motion. He was the first person to predict that Mercury and Venus would transit the Sun. The Kepler spacecraft is named in his honor.

Using the transit technique, the Kepler spacecraft will search for Earth-sized planets in Earth-like orbits around at least 100,000 stars in the constellation Cygnus—still only a small portion of our Milky Way galaxy. Kepler is scheduled for launch in late 2007.