Transit
Searching for Shadows
4291 planets discovered
When a planet passes directly between its star and an observer, it dims the star’s light by a measureable amount
A solar eclipse is one of the coolest astronomical events you'll ever experience. It happens when the moon passes directly in front of the sun, blocking its light.
This is similar to how the transit method finds exoplanets. When a planet passes directly between an observer and the star it orbits, it blocks some of that star's light. For a brief period of time, that star actually gets dimmer. It's a tiny change, but it's enough to clue astronomers into the presence of an exoplanet around a distant star.
The graph you see being drawn on the left side of the animation is what astronomers call a 'light curve'. It's a chart of the level of light being observed from the star. When a planet passes in front of the star and blocks some of its light, the light curve indicates this drop in brightness.
​Sensing size
The size and length of a transit can tell us a lot about the planet that's causing the transit. Bigger planets block more light, so they create deeper light curves. You can see that in the animation above, when you click 'different planet sizes and distances'.
​Also, the farther away a planet is, the longer it takes to orbit and pass in front of its star. So the longer a transit event lasts, the farther away that planet is from its star.
Mixed signals
When you click on 'multiple planets' you'll see that light curves get complicated when more planets are transiting a star. The combined light curves can give us the same information as a single one, it just takes more work from astronomers to pick out each planet in the data.
Atmospheric ambitions
The transit method isn't just useful for finding planets, it can also give us information about the composition of a planet's atmosphere or its temperature.
When an exoplanet passes in front of its star, some of the starlight passes through its atmosphere. Scientists can analyze the colors of this light in order to get valuable clues about its composition. Using this method, they've found everything from methane to water vapor on other planets.
Information goldmine
The transit method has been spectacularly successful at finding new exoplanets. NASA's Kepler mission, which hunted for planets using the transit method from 2009 - 2013, found thousands of possible exoplanet discoveries and gave astronomers valuable information about the distribution of exoplanets in the galaxy.