The formation of planets is a complex, multi-stage process governed by the fundamental forces of nature. If we imagine a hypothetical universe where gravity does not exist, the physical mechanisms that assemble matter into stars and planets would cease to function. The entire cosmic architecture, from gas clouds to aggregated dust grains, depends on the attractive influence of mass. Without this force, the systematic growth from scattered space dust to large, spherical worlds with stable orbital paths becomes impossible. This thought experiment reveals how every step in planetary development relies entirely on gravitational physics.
The Collapse of the Nebula and Star Formation
The story of any planetary system begins with the formation of its central star, a process initiated solely by gravity. Stars originate within vast, cold reservoirs of gas and dust called molecular clouds or nebulae. Regions slightly denser than their surroundings attract nearby matter through their minute gravitational pull. Once the self-gravity exceeds the outward pressure, the cloud begins an irreversible gravitational collapse. This concentrates mass at the center, causing the material to heat up and eventually form a dense protostar. Without gravity, the molecular cloud would remain static and dispersed, preventing the establishment of a central star and a protoplanetary disk.
From Dust Grains to Initial Clumps
Even if a diffuse cloud existed, the earliest phase of solid growth would quickly stall without gravity. Planet formation starts with micrometer-sized dust grains that aggregate into larger clumps using short-range, non-gravitational forces like van der Waals or electrostatic forces. These forces can only build aggregates up to about a meter in size before they become too weak to hold the object together. The transition from these meter-sized objects to kilometer-sized bodies, known as planetesimals, is a major hurdle. Without gravity, collisions between these small bodies would result in bouncing or fragmentation, leaving the universe filled with dispersed, meter-scale rubble.
The Failure of Core Accretion
The growth of planetesimals into planetary cores is driven by core accretion, a process entirely dependent on gravity, where a body reaching a few kilometers uses its own gravity as the dominant force to efficiently sweep up smaller, surrounding material. This phase, known as “runaway growth,” means the largest bodies increase their mass faster than their neighbors due to their stronger gravitational field. Without gravity, a planetary core cannot form the deep potential well required to attract and capture incoming solids and gas. Gas giant formation requires the solid core to reach a critical mass before it can capture a massive atmosphere. An absence of gravity means no object could ever accumulate enough mass, resulting in a universe with no terrestrial or giant planets.
Defining Planetary Structure and Orbits
The final definition of a planet is tied to its ability to manifest its own gravity. A body must have sufficient self-gravity to overcome its material strength and achieve hydrostatic equilibrium, forcing the object into a nearly round, spheroidal shape. Without gravity, even if a massive object were assembled, it would not pull itself into a sphere, remaining an irregularly shaped aggregate. A planetary system is defined by its orbital mechanics, which result from gravity dictating the curved paths of objects around a central mass. Without gravity, objects would travel in a straight line at a constant velocity, meaning there would be no stable, elliptical orbits or predictable motion.