Planets are round because of gravity. Gravity pulls equally from all sides of a planet, smoothing out any non-roundness. This makes the planet’s shape a sphere, which is a three-dimensional circle. 

Planets are formed from gas and dust particles that orbit a star. Gravity causes the particles to attract each other, forming clumps that get bigger until they collect all the material in their way. 

However, some smaller bodies in the solar system are not round because their gravity is not strong enough to smooth out their shape. For example, WASP-103b is the first exoplanet that isn’t a sphere.

The Short Answer:A planet is round because of gravity. A planet’s gravity pulls equally from all sides. Gravity pulls from the center to the edges like the spokes of a bicycle wheel. This makes the overall shape of a planet a sphere, which is a three-dimensional circle.

Galaxies are flat because they are made up of billions or trillions of units. The spinning forces of angular momentum try to eject material outwards, while gravitational forces try to draw it inwards. The natural equilibrium state between these forces is a disk. 

Planets are round because they are solid. Solid objects flatten out less than fluid ones. Although planets bulge out in the middle, they don’t flatten. 

Planets are not perfectly spherical because they spin. The spinning force acts against gravity and causes many planets to bulge out more around their equators. For example, Saturn and Jupiter are a bit thicker in the middle.

Galaxies are disk-shaped because of the balance between gravity and centrifugal forces. 

• Gravity: Pulls gas in. 
• Centrifugal forces: Push gas out from the spin. 

The balance between these forces creates a disk shape. 

Galaxies are also flat because of the conservation of angular momentum. When objects revolve around a center of gravity, they have angular momentum. Over time, a rotating body of stars can flatten into a disk shape. 

Some galaxies are spherical because they lack angular momentum.

The only known non-spherical planet is WASP-103b, an exoplanet in the constellation Hercules. It’s shaped like a rugby ball and is believed to be deformed by the tidal forces of its host star. The star is 1.7 times bigger and 200 degrees hotter than the Sun. 

Saturn is the least round planet because it’s less dense than water and spins rapidly. This causes it to bulge more at the equator than the poles. 

The Sun is almost perfectly round because of its immense gravity and very slow rotation

Most planets are oval. The Earth is actually an ellipsoid. It bulges at the equator by 1/298 of its diameter. Jupiter and Saturn bulge by 10%.

Other planets that are oval-shaped include: 

• Haumea: A dwarf planet that’s shaped like a pancake 
• WASP-12b: An egg-shaped planet that’s 1.8 times the radius of Jupiter

In theory, planets can be triangular, square, pentagonal, or even donut-shaped. 

Here are some other possible planet shapes: 

• Haumea This dwarf planet is shaped like a pancake. It’s located beyond Neptune’s orbit and weighs one-third of Pluto’s mass. Haumea rotates once every four hours. 
• WASP-12b This egg-shaped planet is a hot Jupiter that’s almost twice the size of Jupiter. It’s being slowly consumed by its star. 
• WASP-103b This oval-shaped planet was discovered using the European Space Agency’s CHEOPS space telescope. 

The roundest planets in our solar system are Mercury and Venus. Saturn and Jupiter are the least spherical because they bulge at their equators.

Planets are massive enough that their own gravity pulls them into a spherical shape. However, other factors can also affect a planet’s shape, such as its composition and rate of spin. 

Small planets can have any shape other than a sphere. For example, the moons of Mars, Phobos and Deimos, are not spherical

The minimum size for a celestial body to become a sphere depends on its composition: 

• Rock: 600 km in diameter 
• Ice: 400 km in diameter 
• Stronger material: Larger threshold 

The lower bound for the radius of a gravitationally rounded object is between 200–500 km. 

Objects more massive than 1021 kilograms are expected to be approximately spherical

No, a planet can’t be square because gravity pulls all of the planet’s matter toward the center equally. A square shape would require a weaker pull for some locations, like the corners. The corners would collapse under their own weight and the object would take on a spherical shape. 

A planet can’t be donut-shaped because there would be no place for the core. With the hole in the center, the center of gravity wouldn’t exist

Saturn is the least round planet in our solar system. It’s also the least dense planet, with a density lower than water. Saturn’s equatorial radius is 60,268 kilometers, and its polar radius is 54,364 kilometers.

Here are some other facts about Saturn: 

• Shape: Saturn is visibly squashy. 
• Rings: Saturn may have 500–1,000 rings. 
• Density: Saturn is less dense than water and would float in a large enough pool

Why is everything in the universe round?

It is one of the fundamental forces of our Universe. For objects larger than approximately one fifth the size of Earth, gravity (rather than electrostatic forces, for example) will be the dominant force determining their shape. As gravity pulls matter towards other matter, a sphere forms.( full article source google)

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