How the Solar System was born

Everything in the Solar System—the Sun, the eight planets, moons, asteroids, comets, and the matter that makes up your body—came from the same cloud of gas and dust that collapsed approximately 4.57 billion years ago.

Any theory regarding the origin of the system must explain three observed characteristics:

1. Why all planets lie almost on the same plane and orbit the Sun in the same direction.
2. Why rocky planets are located in the inner region while gas giants are in the outer region.
3. Why the Sun accounts for 99.86% of the system's mass but retains only about 1% of its angular momentum.

The third point is the "killer" that invalidates most theories.

Discarded theories

The Nebular Hypothesis (Kant 1755, Laplace 1796). The Solar System began as a slowly rotating nebula. Gravity caused it to contract, and the conservation of angular momentum caused it to rotate faster and flatten into a disk. Rings of matter broke off from this disk, with each ring condensing into a planet.

This hypothesis neatly explains the first two characteristics—the shared plane and direction of rotation are direct consequences of a rotating disk. However, it cannot explain the third point, which is why people searched for alternatives for over a century.

The Collision Theory (Chamberlin and Moulton, early 20th century) and the Tidal Theory (Jeans and Jeffreys, 1918) proposed the same idea: a passing star pulled material out of the Sun, and that material condensed into planets.

Both failed. They also could not solve the angular momentum problem, and worse: gas pulled out of a star would be so hot that it would dissipate into space rather than condense into planets. They also implied that planetary systems must be extremely rare, as stars rarely pass close to each other—a notion definitively disproven by the discovery of thousands of exoplanets.

The current model

Astronomers have returned to the original ideas of Kant and Laplace, but with the addition of mechanisms that were unknown in the 18th century.

Collapse and disk formation. A molecular cloud is compressed—possibly by a shockwave from a nearby supernova—and begins to collapse. Conservation of angular momentum causes it to rotate faster and flatten into a protoplanetary disk, with the majority of the mass accumulating at the center to form the Sun.

This is no longer speculation: the ALMA and James Webb telescopes have imaged such disks existing around young stars, including empty gaps swept clean by forming planets.

Accretion, not ring separation. Planets do not break off into individual rings as Laplace imagined. Dust particles collide and stick together, growing into kilometer-sized planetesimals, which then collide and merge over millions of years. This is a slow and chaotic process, not a clean separation.

The snow line explains distribution. There is a distance from the Sun—called the snow line, located somewhere within the current asteroid belt—inside of which temperatures are too high for water, methane, and ammonia to condense.

• Inside the snow line: only rock and metal can solidify. There is little material, so planetary cores grow slowly; by the time they are large enough, the gas in the disk has been cleared away by solar winds. The result: four small, rocky planets.
• Outside the snow line: ice also solidifies, providing much more solid material. Cores grow rapidly, exceeding a threshold of about 10 times the mass of the Earth while the disk is still full of gas, and begin to pull in gas on a massive scale. The result: four giant planets.

The angular momentum problem

Returning to the third point: If the entire system was born from a rotating cloud, angular momentum should be distributed according to mass—meaning the Sun should hold nearly all of it. In reality, Jupiter alone holds more angular momentum than the Sun.

The current model solves this with three concurrent mechanisms:

Magnetic braking. The magnetic field of the early Sun grabbed onto the surrounding ionized gas disk. The disk further out rotated more slowly, causing magnetic field lines to bend and generate a drag force that slowed the Sun's rotation, transferring angular momentum outward into the disk. This is also the mechanism that explains why Jupiter does not rotate fast enough to tear itself apart.

Solar wind. The stream of charged particles escaping the Sun carries away angular momentum. Because these particles are guided by the magnetic field far into space before finally escaping, each particle carries away much more momentum relative to its mass.

Planetary migration. This is the newest and most unexpected part of the theory. Supercomputer simulations show that planets do not stay where they were born. The exchange of momentum with the gas disk and with the countless remaining planetesimals caused the giant planets to shift significantly—some models suggest that Jupiter may have migrated inward as close as the orbit of Mars before moving back out. The system may have once had more planets than it does now, with some having been ejected entirely.

Exoplanets confirm and challenge

To date, over 6,000 exoplanets have been confirmed. They confirm the framework of the model: protoplanetary disks are real, accretion is real, and planetary systems are common rather than rare.

However, they also present challenges. Many systems do not resemble our Solar System at all: Hot Jupiters, which are as large as Jupiter but orbit their parent stars in a matter of days; super-Earths—the most common type of planet in the galaxy, yet missing from our Solar System; and unusually elliptical orbits.

This implies that while the general mechanisms are correct, the specific results depend heavily on initial conditions and the migration history of each individual system.

The Solar System was once considered the template for all planetary systems. Thirty years of exoplanet observation have shown that it is not the template, but rather one result among many possible outcomes.

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Adapted from the article The Birth of the Solar System by Dang Vu Tuan Son, published by the Vietnam Astronomy Society (VACA). Original content copyright belongs to VACA.

Further reading

• Why planets rotate on their axes