Definition
An exoplanet is a planet that orbits a star other than the Sun. Like Earth or Jupiter, it makes no light of its own and only travels in the glow of its star. But it lives outside our solar system, so we mostly spot it through tiny changes in starlight.
Why can't you see the moth next to the streetlight?
You've probably seen a streetlight glowing far down a dark road. All you notice is the bright lamp. The moths circling it are almost impossible to see.
The stars in the night sky are streetlights too. Each one is a giant ball of fire that burns and shines on its own. The planets circling them are like moths. They make no light themselves. They only bounce back a little bit of starlight.
On top of that, stars are unimaginably far away. So the faint light a planet reflects gets lost in the glare of the star beside it. It's the same problem as trying to find a moth right next to a streetlight.
A planet that orbits a star other than the Sun is called an exoplanet. People suspected they existed for a long time. But we only began actually finding them fairly recently.
How to spot an invisible moth
Even if you can't see the moth, there's a way to tell it's there. When it flies in front of the lamp, the light dims, just a tiny bit.
Astronomers use a very similar trick. They watch a star's brightness for a long time. If the light dips a little and then returns, again and again at regular intervals, a planet is probably passing in front of it. The time between dips is how long the planet takes to go once around its star.
Another method is to look for a wobble. Imagine a moth dangling from a thread and tugging on the lamp. The lamp would shake a little. In the same way, a planet tugs on its star.
So the star doesn't sit perfectly still. It moves in a small circle. We can read this wobble in tiny shifts in the color of its light.
A bit more precisely
Most exoplanets we've found were not seen directly. It's closer to this: we saw the lamp flicker and wobble, and figured out that a moth must be there.
So what we know is mostly size, weight, and how long the planet takes to circle its star. Depending on the method, we often know only the size, or only the weight. In many cases we still can't say what is on the surface. Lately, scientists have started studying the air of some planets by looking at how starlight changes as it passes through it.
A planet at a distance where water could stay liquid is often called a candidate for a "second Earth." But being at the right distance is not the same as having life. "Candidate" really means it's worth a closer look.
Thousands of exoplanets have been confirmed so far. The biggest news from these discoveries is that planets are not rare.
Common misconceptions
We confirm exoplanets by taking direct pictures of them with telescopes.
Most are found indirectly, from a slight dimming of starlight or a tiny wobble in the star. Direct images of planets are very rare.
An exoplanet at a distance where water could exist must be a world with life.
The right distance is only one of many conditions. For most of these planets, we still don't know whether they have air or what their surface is like.
Where you meet it
Finding small dips in brightness that repeat at regular intervals on a graph of a star's long-term brightness
Reading a star's wobble from its light turning slightly redder and then slightly bluer, over and over
In one sentence
An exoplanet is a planet orbiting a star other than the Sun, and we find it by reading signals like slight dimming or wobbling of starlight.