When a Planet Makes a Shadow: Spotting Worlds Around Other Stars

Right now, there are more than 5,500 planets discovered outside our solar system — and most were found because they made a tiny shadow. When a planet drifts in front of its star, the star looks just a little bit dimmer, like holding your hand in front of a flashlight. Scientists call this a transit, and it is one of the coolest tricks astronomers use to find faraway worlds.

• What a transit is and why stars flicker • How scientists use the transit method to spot planets • What the Kepler Space Telescope discovered • Key words: transit, exoplanet, brightness, orbit

Stars shine very brightly all by themselves. But sometimes a planet moves between us and the star — right across the front of it. When that happens, the planet blocks a tiny bit of the star's light, so the star looks a little dimmer. Imagine watching a firefly from far away. Now imagine a moth flies in front of it for a second. The firefly seems to blink! That blink is just like a transit. Astronomers use special telescopes to watch thousands of stars at once, looking for these tiny blinks.

Here is a great way to picture it. Hold a flashlight and shine it at the wall. Now put your hand in front — the bright circle on the wall gets smaller and darker. That is exactly what a planet does to its star's light. The bigger the planet, the more light it blocks. Scientists can even figure out the size of the planet just by measuring how much dimmer the star gets! A tiny dip means a small planet; a bigger dip means a big planet.

From 2009 to 2018, NASA sent a special telescope into space called Kepler. It stared at the same patch of sky — about 150,000 stars — without blinking for years and years. Kepler found more than 2,600 confirmed exoplanets! Some were giant gas balls like Jupiter, some were rocky like Earth, and some were in the habitable zone — just the right distance from their star where liquid water might exist. One exciting example is Kepler-22b, discovered in 2011, which sits in its star's habitable zone.