Why Are Planets Round?

Why Are Planets Round
Planets are round because of gravity, which pulls matter inward from all directions. But, they are very slightly flattened due to rotation.

Gravity is at the heart of understanding why planets are round. Gravity pulls everything towards the center of mass. When a planet forms, it starts as a collection of gas, dust, and rock. As these materials gather, gravity pulls them towards the center. This process, known as accretion, compresses and heats matter. This is where the denser materials, like metals, sink towards the center while the lighter materials, like silicates, rise to the surface. This differentiation contributes to a more uniform distribution of mass, which aids in creating a spherical shape. Eventually, matter forms a spherical shape, as gravity pulls equally from all directions, smoothing out irregularities.

Planets Are Round by Definition

Being round is actually in the definition of being a planet. More specifically, all planets exist in hydrostatic equilibrium:

According to the International Astronomical Union (IAU), which is the globally recognized authority for naming and defining celestial bodies, a celestial body must meet the following criteria to be classified as a planet in a solar system:

  1. Orbit around a Star.
  2. Sufficient Mass for Hydrostatic Equilibrium: The body must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round (spherical) shape. This state is hydrostatic equilibrium.
  3. Clearing the Neighborhood Around Its Orbit: The body must have cleared the neighborhood around its orbit, meaning it is gravitationally dominant and has removed most other objects in its orbital path.

Pluto is round. It orbits the Sun and has hydrostatic equilibrium. But, it’s a dwarf planet and not a planet because it did not clear its orbit of debris.

Why Planets Aren’t Perfectly Round

While planets are spherical, they are not perfectly round. They are slightly flattened, bulging at the equator. Two main factors cause this deviation from perfect roundness:

  1. Rotational Flattening: As a planet rotates, centrifugal force causes it to bulge at the equator and flatten at the poles. This effect is more pronounced in planets with rapid rotation.
  2. Tidal Forces: Gravitational interactions with other celestial bodies, like moons or a parent star, cause elongation and tidal bulges.

The Most and Least Round Planets in the Solar System

Overall, planets are round. But, none are perfect spheres.

  • The Most Round Planets: Venus and Mercury are the most spherical planets in the solar system, with very minor flattening due to rotation.
  • The Least Round Planets: Saturn and Jupiter are the least round planets. Saturn, in particular, has a noticeable equatorial bulge.

Overall, the most round planets are the terrestrial planets (Mercury, Venus, Earth, Mars). Their relatively slow rotation rates contribute to their roundness, but they have irregularities due to factors like topographical and gravitational variations. The ice planets (Neptune and Uranus) are also relatively spherical. They rotate faster than Venus but are less affected by rotational flattening compared to the gas giants like Jupiter and Saturn.

How Round Is the Earth?

The Earth is not a perfect sphere. Due to its rotation, it has an equatorial bulge. The diameter at the equator is about 43 kilometers larger than the pole-to-pole diameter. Technically, Earth is an oblate spheroid.

Why Are Some Planets More Round Than Others?

The key factors that determine how round a planet is are its size, composition, and rotational speed.

  1. Size and Gravity: Larger planets have stronger gravity, which pulls them into a more spherical shape. This effect is significant for both gas giants and terrestrial planets.
  2. Composition: Gas giants are mostly of gases and are more fluid in nature compared to the solid terrestrial planets. This fluidity contributes to variations in shape, especially when combined with rapid rotation.
  3. Rotational Speed: Perhaps the most crucial factor is how fast a planet rotates. Faster rotation leads to a more pronounced equatorial bulge due to centrifugal force. Gas giants like Jupiter and Saturn rotate very quickly, causing them to have significant equatorial bulging.

In the solar system, Venus, a terrestrial planet, is one of the most spherical due to its slow rotation. Meanwhile, gas giants like Saturn have a noticeable flattening at the poles due to their rapid rotation.

Simple Demonstration of Planetary Bulging

A simple demonstration illustrating why planets bulge slightly is spinning a soft, malleable ball (like one made of dough or clay) on a turntable. When the ball spins slowly, it keeps its round shape. But, increasing the speed causes it to starts flattening at the poles and bulging at the equator due to the centrifugal force.

Is the Sun Round?

The Sun, like planets, is also round due to gravity. However, being a fluid mass, its shape slightly distorts due to its magnetic field and solar wind. While it rotates pretty quickly, gravity overcomes the effects of centrifugal force, so the Sun is nearly spherical.

Are All Stars Round?

Most stars are round for the same reasons as planets and the Sun. However, some stars have significant bulges. An example is the rapidly rotating star Regulus, which has a significant equatorial bulge due to its high rotational speed of around bout 700,000 miles per hour at its equator. In contrast, the Sun rotation is around 4,500 miles per hour.

Are Moons Round?

Many moons around planets are round, but not all. The roundness of moons, like that of planets, largely depends on their size and the resulting gravitational forces.

Rounds Moons

Moons that are round are usually in a state of hydrostatic equilibrium, where their gravitational force is strong enough to shape them into a nearly spherical form. This typically occurs in larger moons. Some examples include:

  1. Earth’s Moon: It’s large enough to have achieved hydrostatic equilibrium, making it round.
  2. Ganymede (of Jupiter): The largest moon in the solar system, Ganymede is round due to its substantial size and mass.
  3. Titan (of Saturn): Titan is another large moon that has achieved a spherical shape.

Non-Round Moons

Smaller moons often do not have enough mass for their gravity to overcome the rigidity of their material, resulting in non-spherical shapes. Examples include:

  1. Deimos (of Mars): It is small and irregularly shaped, more like an asteroid.
  2. Hyperion (of Saturn): Known for its unusual, sponge-like appearance, Hyperion is too small to be round.

Reasons for Roundness (or Lack of It)

Whether a moon is round or not primarily depends on its size and mass, which dictate whether it has sufficient gravity to overcome the structural rigidity of its material and assume a spherical shape. This is why larger moons are generally round, while smaller moons often have irregular shapes.

  • Size and Mass: Larger moons, with greater mass, have stronger gravity that pulls the moon into a spherical shape. This process is the same as that which shapes planets.
  • Composition: The composition of a moon affects its ability to become round. Moons made of ice, for example, reach hydrostatic equilibrium more easily than those made of rock, because ice is less rigid and more susceptible to reshaping under the influence of gravity.
  • Geological Activity: In some cases, geological activity contributes to a moon becoming round. For example, if a moon has active volcanism or tectonics, it can reshape its surface over time, helping it achieve a more spherical form.

Are Asteroids and Dwarf Planets Round?

In general, dwarf planets are round. Most asteroids are not round. Comets and other smaller bodies are not round.

There is no exact size guideline for a celestial body to be round, as the threshold for achieving hydrostatic equilibrium (and thus becoming round) depends on several factors, including composition, temperature, and internal structure.

  1. Density and Composition: Bodies with lower densities and more ductile materials (like ice) achieve hydrostatic equilibrium at smaller sizes than those composed of denser and more rigid materials (like rock).
  2. Typical Sizes in Our Solar System:
    • For rocky bodies (like asteroids or moons), the transition to roundness typically occurs at diameters of approximately 400 to 600 kilometers. For example, Ceres, the largest asteroid in the asteroid belt and classified as a dwarf planet, is round and has a diameter of about 940 km.
    • For icy bodies, like those found in the Kuiper Belt, this transition occurs at smaller sizes, possibly around 200 to 400 kilometers in diameter.
  3. Temperature and Internal Structure: The internal temperature of a celestial body affects its ability to flow and, therefore, its ability to achieve a spherical shape. Warmer, more pliable internal structures allow a body to become round at a smaller size compared to colder, more rigid bodies.


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