Planets

Planets are heavenly bodies orbiting around a star or stellar remnant (Dead body of a star – Blackhole, Neutron star) in an elliptical orbit as our Earth rotates around the Sun – this happens due to the phenomenon of gravity – and has cleared its orbit of other debris. Planets possess a spherical shape, though not a perfect sphere.

1.0Types of Planets

In our solar system, Planets are mainly classified into three types based on the materials and temperature of the planets:  

• Terrestrial Planets: These planets are mainly composed of rocks, hence also known as rocky planets. They are characteristically smaller and closer to the sun with a solid surface. For Example, Mercury, Venus, Earth, and Mars. Features of Terrestrial Planets include the presence of a thin atmosphere, high temperatures due to closeness to the sun, and a few moons. These planets also have distinct surface features, such as mountains, craters, and valleys.  
• Gas Giants (Jovian Planets): Examples of these planets are Jupiter and Saturn, which are known as gas giants due to the absence of a solid surface. Their atmospheric composition mainly consists of Hydrogen and Helium. The Jovian planets consist of the biggest planet in the solar system, “Jupiter.” The characteristic features of Jovian planets are the presence of many moons and a ring system; for example, Jupiter has more than 79 known moons. 
• Ice Giants: More massive than terrestrial planets but less massive than gas giants. They contain more volatile substances like water, ammonia, and methane that freeze at low temperatures. Examples include Uranus and Neptune. Uranus is the coldest planet in our solar system, with temperatures averaging -224°C.

2.0Formation of Planets

Planets are formed with a process known as accretion, in which the dust and gas particles in the protoplanetary disk – a rotating disk-like structure of gas and dust surrounding a young star – collide and stick with each other under the influence of gravity, forming larger bodies. 

The formation of our solar system planets also involves the same process of accretion where the dust and gas from early solar nebulae condensate to form planets. As this process continues, they first form planetesimals, then after eventual collision and merger, form their final product, “The planet.” 

3.0Criteria to be Classified as a Planet

Planets possess some special characteristics that are defined and managed by the International Astronomical Union (IAU). In 2006, IAU redefined the then-existing criteria that an astronomical object must meet to be known as a planet. The New criteria are: 

1. It Must Orbit the Sun: The Astronomical object must revolve in a fixed orbit around the sun and not just float independently through space.  The Moon is not a planet because it doesn’t revolve around the sun but rather a planet. This property shows that a given star is a part of a star system like our solar system.
2. It must be massive enough for its gravity to pull it into a nearly round shape: To be a planet, an astronomical object must have enough mass of its own to collapse into a nearly spherical shape. This is the most significant characteristic of the planet, which differentiates it from other irregular celestial shapes such as asteroids and dwarf planets. 
3. It must have cleared its orbit of other debris: The Celestial body must have cleared its orbital zone of other objects with the help of gravity. That is to say, it must be the gravitationally dominant member in its region of space, either pulling in smaller objects toward itself or flinging them out of orbit. 

Why are dwarf planets excluded from the list of planets?

Dwarf planets are somewhat like other planets but lack some of the characteristics that a planet must possess; hence, they are not included in the list of planets. The third criterion specified by IAU is the main reason why dwarf planets were eliminated from the definition of a planet. Dwarf planets pass the first two criteria, i.e., orbiting the Sun and being roughly spherical, but they fail to pass the third criterion, namely, the clearing of their orbit. This, by definition, means an object that has developed enough gravitational influence so it can absorb, deflect, or eject other objects smaller than itself within its orbital zone.

Before 2006, Pluto was considered the 9th and the last planet of our solar system, but after the new criteria set by IAU in 2006, Pluto was put into the list of dwarf planets due to its undefined orbit.

4.0Planets In Our Solar System

Our solar system consists of 8 planets revolving around the sun in different orbits due to the effect of gravitation of the sun, their moons, dwarf planets, asteroids, comets, and other small celestial bodies. The eight solar system planets in order are:

1. Mercury: Nearest planet to the sun.
2. Venus: The hottest planet in the solar system. It is also the nearest planet to Earth. 
3. Earth: The only planet that has life on it. 
4. Mars: It is also known as the red planet due to its red surface. 
5. Jupiter: It is the largest planet in the solar system.
6. Saturn: Characterized by the presence of its beautiful rings.
7. Uranus: The Coldest planet of the solar system. 
8. Neptune: The farthest planet from the sun and also the last planet of our solar system.