Carl Sagan once said that “we are made of star stuff, but if we are all children of the stars, then how were the stars themselves born?

With all of the advances of modern technology and astrophysics, there is still much mystery left in the universe. Steps in the formation of solar systems remain unexplained by current models or remain points of contention, but observations and advances in data analysis have provided a tremendous amount of insight.

The current model of solar system formation is known as the nebular hypothesis, which was devised as early as 1734. In its earliest proposition, the model explained the accretion process, which forms the disk shapes we observe when viewing solar systems. It has since been refined and remains the most widely accepted hypothesis, though it faces a number of challenges based on modern observations.

First breaths

Credit: NASA / T. A. Rector & B. A. Wolpa, NOAO, AURA

In the vacuum of space, enormous clouds of hydrogen extend for trillions of miles. Over the course of millions of years, these clouds collapse into dense cores that form what is called a solar-mass protostellar nebula. This initial collapse takes approximately 100,000 years. The core of the nebula compresses faster than the periphery and achieves angular momentum, leading to a spiral rotation. This rotation forces much of the gas outward from the core, forming a disk shape. During this phase, we can only observe a bright cloud in space.

A flash of light

Credit: NASA / NASAJPL / Caltech

Over time, the core gradually accumulates mass from the surrounding hydrogen gas, similar to water spiraling down a drain. Eventually, the surrounding gas becomes thin enough that we can observe a young stellar object (YSO). If this object is dense enough, the force of gravity at the core will create heat and pressure at the core of the protostar that fuels the fusion of hydrogen. Over the course of a million years, all of the hydrogen gas will be accumulated in the core, and the energy from fusion will create a star.

From the dust

Credit: ESA Hubble & NASA, G. Piotto et al.

Planets form a separate disk within the hydrogen envelope of protostars known as the protoplanetary disk. These are accumulations of cosmic dust that are attracted to the protostar by its gravitational pull. Within the planetary disk, particles of cosmic dust collide and stick to one another by atomic forces such as Van der Wall’s force. These clumps of cosmic dust form what are known as planetesimals.

The step from these boulder-sized objects to larger planets is not fully understood as destructive forces from inward velocity should prevent them from growing larger. One proposed theory referred to as “streaming instability” posits that this growth is facilitated by the interaction of gases and solids in the protoplanetary disk alongside concentration differences of particles.

Over the course of 10-100 million years, planetesimals collide and attract each other through gravitational force to produce protoplanets, roughly the size of earth’s moon. Further collisions between protoplanets continue to increase the size of these objects until they eventually form planets, which organize along the radius of the star, finally yielding solar systems.

In orbit

Credit: NASA / JPL / Caltech

The mechanism by which giant planets form remains to be explained by existing models and presents a number of problems to proposed hypotheses. Rocky planets are somewhat better understood as being formed by their position within the protoplanetary disk. Because of their proximity to the star, heat prevents water ice from coagulating on the planetesimals. The rest of the process occurs in stages of accretion whereby planetesimals continue to be attracted to one another. The size of these planets is dictated by their proximity to the star with the upper limit of their mass reaching that of Mars.