Why Are Planets Round? Discover scientific theory that explains the fact

The study of the solar system began as early as the 17th century. Until now, scientists are still discovering new space objects and expanding our understanding of the power of the cosmos.

How does it all work?

It's impossible to give a short answer to this question. The solar system was created by the gravitational contraction of a gas-dust cloud about 4.57 billion years ago. It consists of natural space objects that orbit around the Sun as the center.

The system weighs about 1.0014 M☉. At the same time, the heaviest of all the compounds is the Sun. Due to the fact that it concentrates the vast majority of the mass of the system (about 99.866%), it plays an important role. Its gravity holds the planets and other entities together.

Planets are dense space bodies that are usually close to the form of a sphere and they look round to an unaided human eye. Forming planets from a gas-dust cloud occurred as follows. First, the cloud compacted in its central part and, thanks to thermonuclear reactions, the star Sun erupted. In the rest of the cloud, the nebula particles clumped together into dense units that later became all the planets. Near the Sun, temperatures were higher, so the light gases quickly evaporated from the surface and the planets became rocky. The farther away the site is from the Sun, the lower the temperature was, so the distant planets are covered on top with thick layers of light gases such as hydrogen, helium, and others.

All of the celestial planets we can observe are of different sizes, composed of different elements, and located in many different corners of the galaxy and its bulge. But they do have one thing in common: the planets look perfectly round in shape. The sight of the many orbs around leads us to wonder, and we involuntarily ask, "Why shouldn't the stars really be small dots in the sky? Or why shouldn't there be at least one non-circular planet? Well, let one, just one, be cubic or pyramidal. Why is that impossible?" Here's why. There is a force in the entire universe that turns worlds into smooth spheres. The whole point is that there is some force that "polishes'' objects, turning them into spheres. It is the force of gravitation, that is, the force of gravity, or, more precisely, the force of gravitation. Thus, according to the gravity`s intensity the following planets` forms are distinguished:

  • giant planets are ellipsoids - somewhat deformed spheres;
  • planets with solid surface or geoids have slightly irregular geometric shape;
  • small planets usually have an irregular shape.

In essence, gravity is a force that mutually attracts one object to another, such as a ball falling to the ground. It is such "balls"-planets-that it holds in constant orbits.

Why are planets round in shape?

In our universe, all matter heavier than hydrogen and helium forms stars. Over time, due to increasing internal forces and its pressure, they simply rupture and all matter is ejected into space. And from this moment begins the formation of the planets. The particles of matter thrown into space begin to attract each other forming larger compounds. And the larger this formation becomes, the more it attracts debris.

Why are planets round in shape

As the planets grow, gravity turns them into a ball, they become round. The shape of a sphere is the ideal shape, both mathematically and physically. So why a sphere? Every point on the surface of a sphere is the same distance from its center. Gravity pulls everything toward the center which means that all points on the surface of the sphere will have the same gravity. The heavier the body is, the greater is its gravitational force, that is, the greater is its gravity.

The planets in our star system are held together by self gravity. From the time they formed, they gradually gained volume and weight:

  • attracting small particles of cosmic dust;
  • and as they did so, their gravity increased;
  • more and more massive objects were caught in the gravitational pull.

A striking example of this are the smaller asteroids falling to Earth. It turns out that the force of gravity, which corresponds to the center of the planet, tends to compress it into one whole - a ball. All those objects that fall on it, gradually spread out over its entire surface. Thus, over billions of years, all universe`s objects take on a spherical shape.

Gravity tends to hold objects together, such as the eight planets of the solar system, which were formed by the collision of small particles of the world's dust about 4.6 billion years ago. As the planets grew, so did the force of attraction between their parts. They attracted more matter from space, and their weight grew. Meteorites falling to Earth are a good example of this process.

As a planet grows larger, gravity tends to turn it into a ball. The bigger it grows, the stronger its gravity becomes. More and more pieces of matter are added to the planet and spread across its surface. The result of this process is a round entity.

Let's try to mentally conduct an experiment in which we observe how a large object will attract a smaller one to itself, with the objects having arbitrary shapes. Since the objects` weight does not change, the force of attraction will be greater when the distance between the centers of mass is smaller. In other words, each new "addition" will tend to take as close to the center of mass as possible. This will eventually lead to the formation of a spherical shape. Consequently, since the gravitational pull is directed toward the center of the attracting unit, all the clots that arise during contraction must have a spherical shape.

Although gravity works and forms almost perfect spheres, there are still protrusions on their surface. From space, the Earth looks like an almost perfect white-blue sphere. But as you get closer, the high mountains protruding above the surface of the Earth become noticeable. At an even closer distance, buildings and people become visible.

planet earth from space

It is the force of gravity that not only determines the trajectories of celestial bodies in space, but also affects the shape of the celestial bodies themselves. However, since the value of the gravitational constant is very small, the effect of pull becomes noticeable only when its source is either very close or very vast units.

Observations don`t contradict this. For example, celestial entities of relatively low weight (cometary nuclei and asteroids) have irregular shapes such as elongated and completely asymmetric asteroids (e.g., Eros) or cometary nuclei (e.g., the nuclei of Comet Hale-Bopp and Halley's Comet).

For solids, the minimum weight that can provide hydrostatic equilibrium and the corresponding spherical shape is 5-1020 kg, and the diameter is 800 km. All asteroids and comet nuclei are significantly smaller than the above limits both in weight and size, so a weak gravitational force is not able to "smooth out" possible irregularities. Asteroid Ceres with a diameter of more than 900 km turned out to be spherical, and on the basis of this was transferred to the status of a dwarf planet.

At large scale, significant deviations from hydrostatic equilibrium are not observed. This is related to the characteristics of the ground`s strength. On Earth, the fluidity limit of typical rocks corresponds to the height of a pillar of stone about 10 km high, so no tectonic processes can create mountains of higher height on our planet. At that height rocks begin to spread under the action of their own gravity.

As a result, deviations of heights from the Earth's average level on the surface don`t exceed 10 km. The highest peak, Everest, is below 9 km above sea level, so our planet is almost spherical.

On Mars, gravity is much lower. Respectively, the maximum height of Martian mountains may be almost three times higher than on Earth which is observed in practice. Deep canyons and other lowered landforms are smoothed over time by landslides, rolling and sliding of material from the edges under the action of gravity.

This, according to experts from the National Aeronautics and Space Administration (NASA), explains why Olympus, the highest peak on Mars, is 24,000 meters high. That's almost three times higher than Everest. This summit of Mars was named Olympus because, according to ancient Greek mythology, Olympus was the high mountain on which the gods beyond the reach of mortal men lived. On a planet larger than Mars or Earth, where the force of gravity is ten times that of Earth, the landscape would be flatter, the animals smaller and stockier. A giraffe with its long neck would be very uncomfortable on such a planet. Sometimes the gravitational force of a cosmic entity can change the shape of another nearby one. For example, scientists believe that one blue supergiant star revolves around its invisible neighbor, a black hole. A black hole (sometimes formed from an extinguished star) is a unit with such high gravity that no light is emitted from its surface, which cannot overcome the force of gravity.

Gases leaking from its surface are attracted by the black hole and fall on its surface. The rotating black dwarf pulls the stellar wind with it. This flow of particles pulls the matter of the star with it, and its silhouette changes, becoming more elongated. On the other hand, small lightweight space units often do not even remotely resemble a ball. Their gravity is clearly insufficient to turn them into spherical bodies. For example, some asteroids are shaped like mountains. Phobos, the satellite of Mars, looks like a round potato.

Are they really perfectly round?

The ancient Greeks were the first to talk about the Earth being shaped in a perfectly spherical form. The scientist Eratosthenes in the third century BC calculated that the radius of the planet should be 6287 kilometers. Amazingly, he was wrong by only 84 kilometers to the lesser side!

However, in the middle of the seventeenth century, people of science began to doubt the Earth and the other planets round form. This idea has pushed them to an amazing event, which occurred to the French astronomer Jean Richet. In 1672 he traveled from Paris to Cayenne, the capital of French Guiana (an overseas department of France in northeastern South America). The purpose of the trip was to observe Mars. Richet took with him an astronomical clock with a second pendulum. But in South America the wonders began: the most accurate device began to lag daily by 2 minutes 28 seconds. To achieve the correct time Jean had to shorten the pendulum by 3mm. Upon his return to Paris, however, the watch began to lag.

Scientists were at a loss, until the familiar Englishman Isaac Newton proposed the correct solution. He mathematically calculated that such errors in time measurement could only occur if the Earth is not a sphere, but an ellipsoid flattened near the poles. Later, Newton was proved right: it turned out that the Earth's polar radius is 21.3 kilometers shorter than the equatorial radius and is 6356.8 kilometers.

Isaac Newton

The fact is that the gravitational forces in different places on the planet deviate markedly from the average value. This occurs because the density of the Earth's crust is not uniform, and the shape of the Earth is not a perfect sphere. The centrifugal force resulting from the Earth's rotation also interferes with the calculations.

In reality, additional influences (rotation and star attraction) act on the planets, causing the shrinking body to become more or less flattened near the poles. These principles begin to work only when the body is big enough.

It is because the planets and stars spin on their axis that they are flattened at the poles. Because of this, every rotating celestial body is not a perfect spheroid and has different equatorial and polar radii. The flattening appears because of the centrifugal force resulting from the fact that the entity spins, and depends on its density (the lower the density, the greater the flattening). Therefore, gas huge planets, which have low density, are more flattened than Earth group planets, which have a solid rocky shell.

Since the Sun's rotational speed at the equator is very low, its flattening is too insignificant to be measured.

The shape of the Earth is also only slightly different from a sphere making it an oblate spheroid. The diameter at equator is 12,756 kilometers and the radius at pole is 6,356 kilometers, such a difference (also called equatorial bulge) is not very noticeable (flattening is 0.3%). But the gas giants have a much larger flatness. Because of the high spinning speed around its own axis and the lack of a solid surface.

  • Jupiter has an equatorial radius of 71,492 kilometers and a polar radius of 66,854 kilometers. Jupiter holds a record not only in size, but also in speed - a full revolution of it takes only 10 hours. Jupiter's disk, when viewed through a telescope, is already noticeably flattened near the pole, this figure is 6.5%.
  • Saturn has an equatorial radius of 60,268 kilometers and a polar radius of 54,364 kilometers, its flatness is 9.8%.

In the solar system, planets and satellites orbiting at a slower speed, such as Mercury and Venus, are slightly flattened.

Rocky clusters, too, have issues with rotation speed:

  • Saturn's “moon” Japet spinning around it has a so-called "Japet equatorial wall" which may have formed because of its high rotation rate in the past.
  • The dwarf planet Haumea probably got a very elongated shape because of its very high rotation rate. If it were to spin on higher rates, the dwarf planet would have been torn to pieces.

Small entities (moons, comets and asteroids) do not have enough weight to acquire a spherical shape. Especially if they have a high rate of rotation around their own axis. Such entities can have very different shapes and sizes.

In general, however, the universe is an object that requires a very long study. Mankind has had telescopes in its arsenal relatively recently. For humans, this time seems long, but as compared to other processes it is only a brief moment. Therefore, it can be considered that the science and investigation of space has only just begun. Therefore, humanity will have to be patient and continue its observations for centuries and even millennia. We need the study of outer space first and foremost in order to have a broader view of the world.