Planetary Formation: Why Planets Are Spherical and the Exceptions

Planets are usually spherical, but why is this the case? This article will explore the role of gravity and the conditions that lead to the formation of spherical planets. We'll also examine the exceptions to this rule and the shapes that some celestial bodies can take.

Gravity's Role in Planetary Formation

Gravity is a powerful force that plays a crucial role in the formation of planets. When a celestial body accumulates enough mass, its gravitational pull becomes strong enough to overcome the structural integrity of the material it's made of. This gravitational force pulls matter toward the center, resulting in a round shape. The concept of hydrostatic equilibrium further explains this process: a planet reaches a state where it is large enough for its gravity to balance the pressure from its material. In this state, the most stable shape is a sphere, as this shape evenly distributes gravitational force across the surface.

Exceptions and Variations

There are exceptions to the spherical shape, particularly for smaller celestial bodies. Smaller bodies like asteroids or some moons can have irregular shapes due to weaker gravitational forces that are not strong enough to mold them into a sphere. Their structural integrity allows them to maintain a more rigid, non-spherical form. Additionally, rapidly rotating planets can become slightly oblate, flattened at the poles and bulging at the equator, due to centrifugal forces. A notable example is Jupiter and Saturn, which exhibit this characteristic.

The minimum size required for a planetary body to achieve a nearly spherical shape is approximately 200 miles (322 kilometers) for a rocky body. Smaller celestial bodies tend to retain their irregular shapes due to insufficient gravitational force. Larger bodies that have achieved sufficient size and mass can and do form nearly or perfectly spherical shapes over time, with minor deviations allowed by the balance of gravitational force and structural integrity.

Types of Irregular Shapes

While the majority of planets and moons exhibit a predominantly spherical shape, there are several variations that have been observed:

Biconvex disc: This shape is characterized by two convex surfaces. Biconcave disc: This shape features two concave surfaces. Potato-shaped: This shape resembles a potato and is quite irregular. Spherical: The most common and expected shape, resembling a perfect sphere. Ellipsoidal: This shape is elongated and resembles an ellipse. Oblate spheroid: This shape is flattened at the poles and bulging at the equator, often associated with rapidly rotating planets.

Earth: A Nearly Oblate Spheroid

It's important to note that even though Earth is predominantly spherical, it is technically an oblate spheroid due to its rotation. The centrifugal force caused by Earth's rotation causes it to bulge at the equator and flatten at the poles slightly. This is why some sources state that Earth is an oblate spheroid.

The Maximum Height of Mountains

The height of mountains on a planet is limited by gravity and the soil's strength. Mount Everest, for example, comes very close to the maximum possible height on Earth, thanks to tectonic activity. However, Earth's tectonic activity continues to push it higher. Martian gravity, which is about 38% of Earth's, means that the height of mountains on Mars, like Olympus Mons, is much greater. Olympus Mons stands at approximately 27 kilometers (16.7 miles), the tallest mountain in our solar system. Due to the absence of tectonic activity on Mars, Olympus Mons has stopped growing and is slowly weathering over time.

The Impact of Composition and Size

The composition and size of asteroids are significant factors in their ability to form spherical shapes. Asteroids need to be at least 400 kilometers to 600 kilometers in diameter to have enough gravity to pull themselves into a nearly spherical shape. Smaller asteroids tend to retain their irregular shapes due to insufficient gravitational force.

In conclusion, while planets form into spherical shapes primarily due to gravitational forces, smaller celestial bodies or those with specific rotational dynamics can take on irregular shapes. Despite these exceptions, the force of gravity tends to pull any significant mass into a spherical form over time, demonstrating its power in shaping the universe around us.