Why Celestial Bodies are Spherical?

Peering into the cosmic depths via Earth-bound and space telescopes, we have the privilege of observing the universe's farthest corners. Regardless of the distance or extraterrestrial peculiarity, one universal truth seems to stand out: the prevalence of spherical shapes in space. But, what is the secret behind this celestial sphericity? The answer lies in gravity.

Astrophysicist Anjali Tripathi, a member of NASA's Exoplanet Exploration Program at the Jet Propulsion Laboratory in California, sheds light on this phenomenon. She highlights the prevalence of spherical formations in the cosmos, attributing their shape to the self-gravitational pull these celestial bodies exert on themselves.

The story of the universe's birth begins about 13.8 billion years ago, with the Big Bang. From its aftermath, particles emerged, ultimately forming the cosmos we know today. These particles, circulating within vast, donut-shaped clouds of dust and gas, collided. Gentle collisions, as NASA explains, led to an increase in mass and consequently, gravity.

With each collision, the object's mass and gravity grew, attracting more matter until it eventually shaped itself into a planet, moon, or asteroid.

"The gravitational pull draws matter towards the center of gravity," explains Bruno Merín, astronomer and head of the European Space Agency's ESAC Science Data Center in Madrid. The process is akin to water flowing through a sinkhole; every piece of matter strives to be as close to the center of gravity as possible.

Celestial bodies shuffle matter around until equilibrium is achieved - a state where every point on the object is as close to the center as possible. In space, this unique balance is best achieved by a spherical shape.

Planets like Mercury and Venus, which are rocky and rotate slowly, are near-perfect spheres. Ice planets and moons also typically adopt a spherical shape due to the even distribution of their ice layers.

However, 'spherical' does not imply a flawless sphere. Gas giants like Jupiter and Saturn bulge at the equator due to their rapid rotation. Earth, too, possesses a slight bulge, less than 1 percent, due to the centrifugal force produced by its rotation. Thus, Earth is oblate, or a slightly flattened sphere.

While spheres abound in the universe, not every celestial body conforms to this shape. Small asteroids, comets, and meteorites take on various forms, their shapes altered by collisions and interstellar rotation. Mars' moon Phobos, for example, resembles a potato. Only about 20 of the nearly 300 known moons in our solar system are round; the smaller ones are more irregular in shape because their small mass doesn't generate enough gravity to even out their form.