There are already thousands of exoplanets found, but scientists are able to categorize them into 3 main groups. Here are the 3 types of exoplanets and how to classify planets.
The Three Types of Planets
Planets can be generally classified into three groups. Those are terrestrial planets, Neptunian planets, and gas giants. Here’s how to determine which group a planet belongs to.
Terrestrial planets are planets that are similar to Earth — not in the existence of water and life, but the density of the planets. These planets are mainly composed of solids and liquids from the top down. As a result, their densities are generally high, a few times the density of water. Their sizes are typically small, with a diameter ranging from about 30% that of Earth to a little more than 2 times that of Earth.
These planets are of particular interest. Having liquid water on its surface is essential if it wants to host life as we know it.
In the intermediate group, we encounter Neptunian planets. They are gaseous planets with rocky cores, like Uranus and Neptune. They are mainly made of hydrogen and helium, but they contain heavier substances like ammonia, water, and methane, so they are often called ice giants. The diameter of a Neptunian planet can be as small as 2 times that of Earth, but the upper limit should still be lower than the size of most gas giants.
Now, we encounter the group of planets whose members are the largest: the gas giants. They are mainly composed of hydrogen and helium and have solid cores. However, instead of tiny pieces of rock undergoing accretion, gas giants start as gas in their formations. When the region’s gravitational pull gets strong enough, the gases are compressed enough so that the planet becomes a solid core with a heavy layer of gas above it, but isn’t massive enough to sustain nuclear fusion.
More Information About the Classifications
After we explain how planets are classified, we’ll talk about some intriguing facts about these classifications. For instance, what type of planets is the most common? According to data from NASA, the number of planets in the three categories are very close. Terrestrial planets are the most common, with 1,555 confirmed exoplanets of this type, while 1,532 Neptunian exoplanets and 1,439 gas giant exoplanets have been discovered to date. Remember that in NASA’s dataset, super-Earths and terrestrial planets are in separate categories, but they are classified as one group in this article.
Moreover, one of the reasons our Solar System is quite unusual among the planetary systems in our galaxy is because it lacks an entire range of planet sizes. As you know, the Solar System contains four terrestrial planets, two Neptunian planets, and two gas giants. However, the terrestrial planets are pretty small, and the ice giants are already larger than many planets of the same type. In fact, the diameter of the smallest ice giant, Neptune, is a little less than 4 times that of Earth! This is quite unusual as most of the terrestrial planets discovered to date are larger than Earth, and there are also many ice giants smaller than Neptune. But these common types of planets seem to be missing from the Solar System.
In this article, we’ve explained the 3 types of exoplanets existing in the Universe, and how to classify them. If you want to learn more, you can read the articles mentioned in the references below.
References and Credits
- (2021, March 22). Gas Giant. Retrieved October 19, 2021, from https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/gas-giant/
- (2021, March 22). Neptune-like. Retrieved October 19, 2021, from https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/neptune-like/
- (2021, March 22). Terrestrial. Retrieved October 19, 2021, from https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/terrestrial/
- (2020, March 2). The Different Kinds of Exoplanets You Meet in the Milky Way. Retrieved October 19, 2021, from https://www.planetary.org/articles/the-different-kinds-of-exoplanets-you-meet-in-the-milky-way