Author: Emily Jiang
Editors: Emily Yu and Shirley Chen
Artist: Jenny Luo
Have you ever wondered if life may exist in the Universe? Studying exoplanets helps us gain clues to understand and find an answer to this question. But what exactly are exoplanets? Any planet that resides outside of our solar system is called an exoplanet. So far, thousands of exoplanets have been confirmed, and most have been discovered in the Milky Way. There are currently four types of exoplanets: Gas giant, terrestrial, Neptunian or Neptune-like, and super-Earth. Exoplanets have different compositions, sizes, and masses; they are so different that some are extremely hot while others are completely frozen or covered by water. Some orbit stars or even two suns at once, while others are rogues roaming the galaxy without suns. To determine the type of exoplanet, we need to know the size and mass of the planet.
Gas giants are massive planets mostly made up of gas (helium either-or hydrogen). These kinds of exoplanets are similar to Jupiter and Saturn in both size and the fact they don’t have a solid surface. Instead, there's a core with gas swirling above it. Each of the four categories is quite broad and there is lots of variety within each. When gas giants are closer to stars, they’re called “hot Jupiters,” and their temperatures can reach thousands of degrees. But how do hot Jupiters get so close to their parent star? Since it's hard for a planet to form close to the parent star at such hot temperatures, scientists hypothesize that hot Jupiters form farther away from their parent star and then migrate close to them due to the gravity of a disk of gas/dust or the gravity of other planets.
Neptunian planets are gas worlds close to the size of Neptune or Uranus. These planets usually have a core of rock or heavier metals with helium and hydrogen-dominated atmospheres. There are also mini-Neptune planets that are larger than Earth but smaller than Neptune. Water, ammonia, and methane are usually found frozen as ice in the outer solar system, and since Uranus and Neptune are composed of these three chemicals, they are called “ice giants”.
Super-Earths are bigger than Earth; they can be twice the size of Earth and ten times Earth’s mass but smaller than Neptune. They are lighter than ice giants and can be composed of rock or gas or a combination (potentially rocky surface). They aren’t called super-Earth because they are like Earth, but because of their size. These exoplanets are enveloped in mystery because there is nothing similar to them in our solar system. Within the range of three to ten times the mass of Earth, there is a wide diversity of planet composition, including snowball planets and water worlds to planets made up of mostly dense gas. Exoplanets at the maximum size of super-Earth can be called sub-Neptunes or mini-Neptunes.
Terrestrial planets are a rocky world. Within our solar system, Earth, Mars, Mercury, and Venus are terrestrial planets. Exoplanets at half the size of Earth or smaller and twice Earth’s radius are called terrestrial. These exoplanets are made up of silicate, rock, water, and/or carbon, but in bulk, they are mostly rock or iron with a liquid or solid surface. In addition, these planets might have a gaseous atmosphere. More investigation is needed to see if these exoplanets have signs of habitability with their atmospheres, oceans, etc. Super-Earths are the larger terrestrial planets.
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