As of November 2023, NASA has confirmed 5,539 planets outside our Solar System. There are many possibilities for how exoplanets behave, such as their revolution around their host star. However, some limitations can be attributed to various factors when it comes to the planets' sizes.
Rocky Versus Gaseous Planets
In the Solar System, planets are categorized into minor, rocky, dense planets resembling the Earth and large, gaseous planets similar to Jupiter. Based on what astrophysicists have detected so far, all planets fall into these two classifications.
Looking at the data from planet-hunting missions like the Kepler mission or the Transiting Exoplanet System Satellite, experts noticed a gap in the sizes of the planets. This means that not many planets achieve the requirements for a "super-Earth," a type of planet that is about twice the Earth's radius and a mass that is 5 to 10 times greater.
The differences between the two planet categories and the reason for the super-Earth gap are attributed to a planet's atmosphere, especially when the celestial object was starting to form.
When a star is born, a huge ball of gas comes together and starts to spin, collapsing while igniting a fusion reaction within its core. Since this process is not perfect, there is a lot of extra gas and dust left over after the formation of the star. This additional material rotates around the star until it forms a stellar disk, a ring-shaped collection of rocks, dust, and gas.
As these motions and commotion happen, the dust grains hit each other and create pebbles that grow into larger boulders, eventually forming a planet. The planet grows in size and increases its mass and gravity, which allows it to capture the accumulated rocks, dust, and gas. A lot of gas can be found within the stellar disk, but there is rocky material since only a limited amount was made during star formation.
The Trouble With Super-Earth Planets
If a planet remains small with a radius less than 1.5 times that of the Earth, its gravity will not be strong enough to hold onto a huge amount of atmosphere. Such is the case with Neptune and Jupiter. If it continues to grow larger, however, it will capture more and more gas, eventually forming an atmosphere that causes it to enlarge to the size of Neptune or Jupiter.
This means a planet stays small and rocky or becomes a large, gaseous celestial object. A super-Earth might be formed in the middle range, although it could be very difficult. Once a planet has enough mass and gravitational pull, it requires the exact right circumstances to prevent the avalanche of gas from piling onto the planet and puffing it up. This state is also called "unstable equilibrium," where a cosmic body departs further from the original position once it gets slightly displaced.
Another factor to consider is the planet's position from its host star. Once a planet is formed, it does not always stay in the same orbit, as it can sometimes move or transfer toward its host star. As it gets closer to the star, its atmosphere heats up, triggering the atoms and molecules to move quickly and escape the planet's gravitational pull.
From this assumption, it can be theorized that some of the small rocky planets are the cores of bigger ones that have been stripped of their atmosphere. Although there are no super huge rocky planets or very small fluffy planets, there is still a huge amount of planet diversity in composition, geometries, and sizes.
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