What is an Exoplanet?
Planets are large, round objects that orbit around a central Star, and have orbit paths free of cosmic debris. We know of 8 planets in our Solar System. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
But there's some really crazy planets outside of our Solar System, with their own orbits, qualities, and quirks to go with. We call these celestial bodies "Exoplanets".
Rocky, Gas, and Ice Exoplanets
Some Exoplanets are just like ones we have in our solar system. Rocky Exoplanets are like Mercury, Venus, Earth, or Mars. They're solid, mostly made of carbon and iron.
Some are gaseous Jovian Exoplanets, like Jupiter and Saturn. They have a solid core, but most of the planet is made of gasses like hydrogen and helium, with no solid surface.
Icy Exoplanets are like Uranus and Neptune, cold and covered in frozen material, likely made from methane, ammonia, or in some cases, water.
Odd Exoplanets
A few types of Exoplanets are similar to the aforementioned classifications, but with a small quirk that sets them apart.
Super-Earths are Rocky Exoplanets with a mass substantially greater than Earth's, while Mini-Neptunes are Icy Exoplanets a few times smaller than their more common-classified counterparts, yet larger than any Super Earth.
"Hot Jupiter" exoplanets are Jovian Exoplanets with a close orbit distance to their Stars, making them very hot, around 1,000 degrees Kelvin (around 1340 degrees Fahrenheit/730 degrees Celsius)! Meanwhile, a "Cold Jupiter" is the same, except it's orbit distance is so far, it registers exponentially more freezing temperatures than normal.
Habitable Exoplanets
Some Exoplanets are very rare. Rocky Exoplanets like Earth are already uncommon, but Exoplanets that are VERY close are exceptionally more rare.
They're called Habitable Exoplanets, classified due to their orbit path being fully in their stars' habitable zone, where the heat is just right for life to flourish. An atmosphere must also be present for air to stay on the Exoplanet, as well as it orbiting a calm star with not a lot of solar activity.
A popular example of a Habitable Exoplanet is Proxima Centauri B, an Exoplanet orbiting a Red Dwarf star within the Centaurus constellation. However, Habitable Exoplanets are rare for a reason, even Proxima Centauri B's local star activity is prone to flare-ups that could strip away any atmosphere it has.
Chthonian Planets
Gas Giants are very big. But they're not very dense, nor massive. The gasses that make up a Jovian Exoplanet aren't as solid as a rocky exterior, and this delicate nature can be disrupted by the planet getting too close to its Star.
In this scenario, the gasses of the Exoplanet are slowly stripped away due to the heat and energy from the star it's close to. Eventually, after a long long time, the mighty Jovian Giant is left as nothing but a solid core.
This hypothetical is known as a Chthonian Planet, and there's only a few instances in the observable Universe debated to fall into this category, or that are going to in the future.
Rogue Planets
But what happens when an Exoplanet has no Star to orbit? Does it stop being a "planet"? Nope! It just gets one more title to it's name.
A Rogue Planet is an Exoplanet that doesn't orbit any Star. They're still planets, however. Originally, Rogue Planets were ejected from their previous solar systems, due to unstable orbit, gravity, or other instabilities.
Rogue Planets drift along the void of outer space, not held by any major sort of gravity. They're entirely dark, and completely frigid, neither warmed nor illuminated by any nearby star.