The image below is today's Astronomy Picture of the Day. It's not a compilation of photographs,but a representation of stars that have planets orbiting them.
The Kepler mission was launched in 2009,with the goal of discovering Earth like extrasolar planets (i.e. planets that revolve around other stars besides our sun). To do this, Kepler looked at hundreds of thousands of stars and watched to see if they got any brighter or dimmer. If the brightness of a star dropped, it might be due to a planet crossing in front of it. We experience this all the time in our own solar system. When Venus appears to cross in front of the sun from Earth's perspective, the Sun looks dimmer by about 0.2%. It's not something we notice with our eyes, but telescopes can easily measure this subtle change in brightness. Using this method, Kepler found 1,235 potential exoplanets! The image above represents each host star (yellow/orange) and the planet crossing in front of it (black circles). Based on the data Kepler receives, we can figure out where the planet crosses the star (top, middle, bottom, etc.) and about how big it is. Telescopes today do not have the power to directly image most exoplanets, but Kepler's method proves incredibly successful. If you look closely at the image, you might even see some multi-planet systems!
Illustration Credit: Jason Rowe, Kepler Mission
Go to APOD.NASA.gov to see a higher resolution image
