What Is An Exoplanet?

Sixty three light-years away in a constellation called Vulpecula lies a planet called HD 189733b. While its name may sound unremarkable, astronomers proved this week that HD 189733b is, indeed, special: It's deep-blue in color and covered in clouds of liquid glass.

HD 189733b is an exoplanet. Located outside of our solar system, it's just one of more than 900 similar planets we've identified. However, scientists estimate there are at least 100 billion exoplanets - and that's just in the Milky Way Galaxy alone. But what exactly is an exoplanet?