An exoplanet is a planet outside of our solar system. Most of them have been discovered to orbit other stars, but there are also rogue planets, which orbit the galactic center in the dark, untethered to any star. There are actually more planets than stars in the galaxy. They are made of many different mixtures of elements, just like the planets in our solar system. There have been planets found that are entirely covered in lava. Some are covered in ice. Some gaseous. Some rocky and holding water, like Earth. Some are smaller than Earth and others, larger than Jupiter.
The vast majority of exoplanets have been discovered within a few thousand light years of our solar system. That is as far as current telescopes can probe. The closest known exoplanet to Earth, Proxima Centauri b, is about four light years away from us. Most exoplanets are found through imaging the dimming of a star as the planet passes between it’s star and our imaging technology, though some have been discovered through direct observation with a telescope. Other discovery methods use radial velocity, and microlensing techniques. The exoplanets are known as candidates until they become confirmed, or witnessed through three telescopes. Generally scientists can learn a planet’s diameter and mass, the energy that radiates from it as light from its star strikes the surface, and its temperature. The level of light can be estimated, as well as the color of the sky.
Exoplanets are designated into five broad categories: Neptune-like, Gas Giant, Super Earth, Terrestrial, or Unknown. There have been 1,467 Neptune-like exoplanets discovered so far, 1,357 Gas Giants, 1,331 Super Earths, 163 Terrestrial, and 6 unknown. There have been 4,324 total exoplanets discovered so far, though in the next ten years, that number is expected to rise to tens of thousands.