7.4 Origin of the Solar System

Solar System Formation

Our solar system formed from a spinning disk of gas and dust around the young Sun. The fact that planets all orbit in the same direction, in nearly the same plane, with rocky worlds close in and gas giants farther out, points to a shared origin from that disk. Understanding how this happened also helps explain the surprising variety of planetary systems discovered around other stars.

Key Planetary Characteristics for Formation Models

Any good model of solar system formation needs to explain several patterns we observe today.

Orbital characteristics provide some of the strongest clues. All planets orbit the Sun in the same direction (prograde), which tells us they formed from a common rotating disk. Their orbits are nearly circular (low eccentricity), suggesting a relatively smooth formation process. And they all orbit close to the same flat plane (the ecliptic), consistent with formation from a flattened disk.

Compositional characteristics reveal a clear trend with distance from the Sun:

• The inner (terrestrial) planets are rocky and dense, built from high-temperature materials like iron and silicates that could survive close to the hot young Sun.
• The outer (Jovian) planets are gaseous and much less dense, having accumulated lighter elements like hydrogen and helium that only remained stable farther from the Sun where temperatures were lower.
• The asteroid belt sits between these two zones, marking the transition in composition and formation conditions.

Rotational characteristics also carry information. Most planets spin in the same direction as their orbit (prograde), a trait inherited from the rotating protoplanetary disk. However, their rotational axes are tilted at various angles relative to their orbital planes (this tilt is called obliquity), which resulted from collisions and gravitational interactions during formation.

Satellite systems come in two flavors that tell different stories:

• Regular satellites orbit in the same direction as their planet's rotation and formed from material orbiting the planet, much like mini solar systems.
• Irregular satellites have inclined, eccentric, or retrograde orbits. These were likely captured by the planet's gravity rather than forming in place.

Formation and Evolution Processes

The formation of the solar system follows a sequence of physical processes, each building on the last.

1. Gravitational collapse starts everything. A region of a molecular cloud contracts under its own gravity, forming a dense central mass (the protosun) surrounded by a swirling envelope of gas and dust.
2. Angular momentum conservation shapes the disk. As the cloud contracts, it spins faster (like a figure skater pulling in their arms), and the material flattens into a thin disk called the protoplanetary disk.
3. The condensation sequence determines what forms where. Closer to the protosun, only metals and silicates can condense because temperatures are too high for ices and gases to solidify. Farther out, where it's cooler, water ice, ammonia, and methane can also condense. This is why rocky planets formed close in and gas/ice giants formed farther out.
4. Planetary migration can alter orbital positions after formation. Gravitational interactions with the remaining disk material or with other planets can push planets inward or outward from where they originally formed.
5. Orbital resonances influence long-term stability. When the orbital periods of planets or moons form simple integer ratios (like 2:1), they exert repeated gravitational tugs on each other, which can either stabilize or destabilize their orbits over time.
6. Differentiation creates layered interiors. As young planets heat up from collisions and radioactive decay, denser materials (like iron) sink to form a core while lighter materials rise to form mantles and crusts.

Role of Collisions in the Early Solar System

Collisions were not just common in the early solar system; they were the primary mechanism by which planets grew.

Planetesimal formation begins small and scales up:

1. Dust grains in the protoplanetary disk collide and stick together, gradually building up planetesimals (kilometer-sized rocky or icy bodies).
2. Once planetesimals reach a certain size, their gravity helps them attract more material, and they grow into protoplanets (roughly Moon- to Mars-sized objects).

Terrestrial planet formation continues through even larger collisions:

1. Protoplanets collide and merge over tens of millions of years, eventually forming the four terrestrial planets (Mercury, Venus, Earth, Mars).
2. One of these giant impacts likely created Earth's Moon. The giant impact hypothesis proposes that a Mars-sized body (sometimes called Theia) struck the early Earth, and debris from the collision coalesced into the Moon.

The Late Heavy Bombardment was a period of intense asteroid and comet impacts on the inner planets, roughly 4.1 to 3.8 billion years ago. The Nice model suggests this bombardment was triggered by the migration of the outer giant planets, which gravitationally scattered small bodies inward.

The asteroid belt bears the scars of ongoing collisions. Collisions between asteroids produce smaller fragments and dust, and Jupiter's powerful gravity prevents the remaining material from ever merging into a single planet. The Kirkwood gaps (empty zones in the belt) are regions where Jupiter's gravitational resonances destabilize asteroid orbits.

The Kuiper Belt and Oort Cloud are distant reservoirs of icy objects beyond Neptune's orbit, home to bodies like Pluto and Eris. Their structure has been shaped by both collisions and gravitational interactions with the giant planets over billions of years.

Extrasolar Systems and Formation Understanding

Discoveries of planets around other stars have revealed that our solar system's tidy arrangement is far from universal. This diversity has forced astronomers to revise and expand their formation models.

• Hot Jupiters are gas giants orbiting extremely close to their host stars. 51 Pegasi b, the first exoplanet discovered around a Sun-like star, is a classic example. Gas giants can't form that close to a star (there isn't enough material, and temperatures are too high for gas accretion), so they must have migrated inward after forming farther out.
• Super-Earths are planets larger than Earth but smaller than Neptune, like Kepler-10b. Nothing like them exists in our solar system, yet they turn out to be one of the most common planet types in the galaxy.
• Eccentric orbits show up in planets like HD 80606b, which follows a highly elliptical path around its star. These orbits point to past gravitational interactions, such as close encounters with other planets or companion stars.

Planetary migration is now understood as a key process, not an exception. Gravitational interactions with the protoplanetary disk or with other planets can drive planets inward or outward. The Grand Tack model, for instance, proposes that Jupiter migrated inward and then back outward early in our solar system's history, sculpting the inner solar system in the process.

Two main mechanisms explain how gas giants form:

• Core accretion is the dominant model for our solar system. A solid core of rock and ice forms first (over millions of years), and once it's massive enough (roughly 10 Earth masses), it rapidly pulls in gas from the surrounding disk. This explains the internal structure of Jupiter and Saturn, which have dense cores surrounded by thick gas envelopes.
• Disk instability offers an alternative pathway. In this scenario, gravitational instabilities in a massive protoplanetary disk cause clumps of gas to collapse directly into giant planets, bypassing the slow core-building stage. This may explain some of the giant planets observed far from their stars through direct imaging, such as the four planets in the HR 8799 system.