Today’s Astronomy Picture of the Day is a great visualization of what is involved in studying exoplanets—planets circling other stars. The picture (shown to the right in small form) has a large number of stars with tiny shadows representing the passage of an exoplanet across the star’s disc. The Sun, with shadows for Jupiter and Earth, is included for reference.
The picture is constructed from data from the Kepler exoplanet-hunting mission, and uses the transiting of exoplanets across their star’s disc to measure not only the orbital properties—how large is the orbit, how long does it take to orbit, etc.—but the physical size of the planet, a piece of data that isn’t available to most other methods. Despite the picture, Kepler isn’t seeing the actual disc of the star or the real shadow of the planet; instead, Kepler is measuring the amount of light the start produces with and without the planet in our line of sight. The tiny eclipse may not be very large in terms of the star’s total light output, but it’s measurable.
(Click on the figure to see it in a larger version.) The cartoon shows five different points in the orbit of an exoplanet as it moves across the disc of its host star. When no eclipse is occurring, we get all the star’s light unimpeded; when the planet is partly blocking the star, we see a gradual decrease in intensity until the planet is fully in between us and its host. Unlike the Moon eclipsing the Sun, we’ll never see an exoplanet fully blocking its star’s light—the only reason this happens on Earth is because the Moon is so close to us. Given the vast distances between us and the nearest exoplanet system, even the largest planet is going to be a tiny speck, too small to see unless we’re very fortunate.
Transits don’t tend to stand alone for characterizing exoplanets; though I won’t elaborate in this post, generally another method is used in combination. Several things have to work in concert for transits to even be visible: we have to see the orbit of the planet “edge on” (think of how a plate appears differently if you see it from the top or from the side). If the planet’s orbit is too tilted, we’ll never see it cross its host star’s disc. Also, the amount of light blocked has to be enough that we can detect the difference, so as with any other exoplanet detection method, we will tend to see larger planets—or at least planets that are large in comparison with their host star.
By correlating the depth of the eclipse (how much light is blocked) with the time it takes for the planet to move fully over the star’s disc, we can determine the relative sizes of both planet and star. By timing between eclipses, we can determine how long it takes the planet to complete one orbit and (using other methods) determine how large the orbit is. Altogether, this is a very powerful and effective way to locate planets when we have little hope of imaging them directly within the next decade or so.