Stages of Star System Formation

• Starting point: A cloud of interstellar gas and dust, the "solar nebula"; Most of it (98%) is hydrogen and helium, but it includes atoms and dust grains of heavier material, formed in previous generations of stars.
• Onset of formation: The nebula is already thicker than the average interstellar region, and possibly part of a chaotic region of starbirth; Because of some disturbance that compresses it, such as a supernova explosion, it starts a gradual process of collapse.

• Contraction: The cloud starts collapsing under its own gravity; over 100,000 years, it shrinks down to 100 AU, heats up (thermal energy), and compresses in the center.
• Accretion disk: The matter around the center spins up and flattens into a disk, while heat vaporizes the dust.
• Protostar: Forms in the center, when the core becomes opaque; later will become the Sun. (The gas orbiting the protostar in some cases may start to compress under its own gravity, producing a double star.)
• Condensation: The disk radiates away its energy and cools off; some gas condenses into tiny dust grains of metal, rock and, far enough from the forming star, outside the "snow line", ice (differentiation).
• Planetesimals: Dust grains stick to each other (ice helps) and sweep their paths, forming larger particles; this accretion goes on until the particles become the size of boulders or small asteroids, which attract matter with their gravity.

• Protoplanets: The larger particles' growth accelerates, and they accumulate all of the solid matter close to their own orbit. In 100,000 to 20,000,000 yr, the protoplanets' size is large asteroid/lunar size in the inner solar system, and several times the Earth's size in the outer solar system (lower temperature).
• Question: What is the relative importance of gravitational instabilities and core accretion in the formation process for gas giants?
• Solar wind: After about 1,000,000 yr, it sweeps away the leftover gas. If a protoplanet is already large enough, its gravity pulls in the surrounding gas, and it becomes a gas giant (leftover gas and dust around it condenses into moons); if not, it remains a rocky or icy body.
• Fragmentation: In 10 to 100 million years, while larger planetesimals become more massive, smaller ones break into smaller pieces when they collide, end up as meteorites on larger objects, and/or their orbits are altered.

Result

• Overview: After a billion years of clean-up and meteoritic bombardment, you end up with ten or so planets, in stable orbits; The protostar turned into a star when the core became hot enough.
• Catastrophes: Needed to explain specific isolated features and exceptions. The planets, their surfaces and atmospheres may be heavily modified by the last, big collision they experience.
• Examples: For Earth, our Moon and the presence of water, brought by comets (the original Earth could not have retained it); Also, composition of Mercury, Venus' rotation, Uranus' tilt.
• Debris: Some planetesimals remain in the asteroid belt (a would-be planet, if not for Jupiter) and the Kuiper belt; others are thrown outwards by "gravity assist" during close encounters (Oort cloud); Some dust remains in a dust disk in the plane of the solar system; we see it from the zodiacal light it scatters..
• How big is it? Pluto's orbit at 40 AU, Kuiper Belt between 30 and 100 AU or so, the Oort Cloud extends out to 50,000-100,000; The nearest star is at about 300,000.

What Evidence Do We Have?

• Earth and Moon rocks: They can be dated using their radioactive elements; The oldest ones are about 4.5 billion years old.
• Meteorites: The oldest objects in our solar system are 4.57-Gyr old, mm-sized grains found in some meteorites; Some even give us evidence that a star exploded in our neighborhood around the time the solar system formed, and the Sun may have been part of a cluster.
• Exploration and experiments: Spacecraft have been sent to observe asteroids made of "primitive rock" (like NEAR, with asteroid Eros, and Hayabusa, to asteroid Itokawa) and comets (Rosetta), and collect samples of solar wind (Genesis); Conditions have been recreated in a Space Shuttle flight.
• Solar neighborhood: Its configuration also shows evidence for some kind of past explosion; For example, we seem to be inside a bubble with walls about 70 light years away; Further away, we can see other (proto)planetary systems where the process is happening right now.

Additional Twists

• Jovian planets: Probably formed in stages; How could the ice giants Uranus and Neptune form in a region so far from the Sun, with very little material? They probably formed closer to the Sun and then "migrated", pushed outward by Jupiter..., and in the process moved other debris farther out, to what is now the Kuiper belt.
• Terrestrial planets: The outer planets might have played an important role in their formation; Was there a fifth terrestrial planet once, beyond Mars?
• Gamma ray bursts: Are they related to the formation of the solar system?