We've already learned about the Kepler Space telescope and how it searches for planets. The basic idea, just to refresh your memory, is to watch a star for a long period of time and see if it changes in brightness periodically. If it does, then the dimming of the star might be due to a planet crossing in front of, or transiting, the star and blocking some of its light! When astronomers do this sort of thing they construct what's called a "light curve". A light curve is just a graph with star brightness on the vertical axis and time on the horizontal axis. Each time you measure the star's brightness you put a dot on the graph corresponding to the time you took the measurement and how bright the star is. A real example of some light curves constructed from Kepler data are shown below.
The star appears to be at normal brightness, or 1 on the graph, most of the time. But once in a while we see a tiny drop in brightness (less than 0.1%). This drop in brightness occurs once every few days and lasts for a few hours. Not only can light curves reveal the existence of a planet, but they can also tell us the size of the planet, how far away it is from its host star, and where it sits in orbit! As you can see from the cartoon in the image, smaller planets block out less light, so we see a smaller dip in the light curve. Also, planets that cross in front of the star near the top or bottom rather than the middle have a thinner dip in the light curve. If we know how big the star is (and often we do) and we know the time between light curve dips, we can calculate the period of the planet (i.e. how long it takes for the planet to revolve around the host star one time) and from that get the distance between the star and the planet.
The coolest thing about light curves and planet hunting is that you don't need a big, fancy and expensive telescope to do it. The very first discovery of an exoplanet was made using a digital camera and a 4inch wide backyard telescope! With just basic equipment and some astronomy knowledge you can make light curves and hunt for planets right in your own backyard.
Image Credit: NASA/Kepler
