Origin of the Solar System

Recall from previous lecture that the dominant hypothesis for the formation of this and other solar systems (developed by Pierre-Simon Laplace) is the Solar Nebula Hypothesis - that the Sun and planets formed from a collapsing cloud of interstellar gas and dust. Now we retell the story from the point of view of the planets.

Formation of the Sun

Protoplanetary disks in the Orion Nebula from Space Telescope.org
General processes in the early Solar System: We start with the collapse of what your text refers to as the dense cloud and end with the formation of the planets. Between these end points, several processes occurred in a general sequence, but maybe with considerable overlap:

Dense cloud collapse into an accretion disk. Collisions of dust grains and gas in the collapsing dense cloud fragment tend to equalize the direction of motion of gasses and dust, giving the dense cloud fragment an overall sense of rotation. At this point, two forces prevent a particle from sinking toward the center:

Protoplanetary disk of HL Tauri in infrared from Wikipedia.
Dark circles represent the orbits of coalescing exoplanets.
Thus, a particle falling from above or below the plane of rotation only has to overcome pressure, whereas one falling along the plane of rotation must resist both pressure and centrifugal force. So, the rotating cloud tends to be flattened along its axis of rotation. Takes from 100,000 to 500,000 years. The result: a protoplanetary disk (right). Consequences of this process are that:

But now we have a big problem: Angular momentum

As the disk contracts, its rotation should speed up to conserve angular momentum. The region with the fastest rotation should be at the center - in our case, the Sun. Alas, we don't see this. The Sun rotates much more slowly than it should, and the Solar System's angular momentum is concentrated in the jovian planets. How's that?

Consider the steps in which mass get distributed in a forming solar system.

Disk dissipation, during which material in the disk is transported to the protostar. Friction within the disk causes material to fall toward the center and onto the forming protostar, reducing the density of the disk. Taking ~50,000 years.

Terminal accretion. The star enters T Tauri phase. Remaining disk material begins to accrete into planetesimals (taking 1 - 2 ma). Note - the T Tauri winds are driven by the heat of gravitational contraction, not fusion. They continue to blow as long as the lump of material at the center of the circumstellar disk (the protostar) continues to contract.

Gas dissipation, planetary accretion in inner Solar System ends, and residual nebula gas is removed by T Tauri winds.

T Tauri phase ends as internal temperature and pressure reach threshold for nuclear fusion. Gravitational contraction is now balanced by internal pressure and ceases.

The beautiful aspect of this:

The presence of a violent T Tauri phase largely explains the angular momentum problem. By shedding a significant amount of mass in the form of T Tauri winds, the protoSun transferred its angular momentum to the outer Solar System, where we see it today. To see an analogy, watch this video. (Focus on how the spacecraft's spin is reduced between 1:35 and 1:45.)

But is it real? (What evidence have we recently discussed indicating that material from the inner Solar System was transported to its outer reaches early in its history?)

Formation of the Planets

Condensation: As the dense cloud collapsed, a point was reached at which the cloud became opaque to visible light, leading to a kind of Solar System wide greenhouse effect. Radiation from the protoSun could not radiate into space, so the center of the cloud would begin to heat up. Evidence from meteorites suggests that within 2 to 3 AU of the protoSun, temperatures reached 2000 K and nearly all material was vaporized at this early stage.

Later, as disk material either accreted to the protoSun or was blown into the outer disk, the inner disk became more transparent, allowing radiation to escape - effectively, the blanket was kicked off of the inner Solar System. As it cooled, materials began to condense as a new generation of solid grains. Remember:

The solids condensing from the vapor of the inner proto-solar nebula appeared in a sequence:

Coagulation: As dust particles collided, they occasionally adhered to one another through electrostatic, mechanical, or magnetic forces. As with condensation, coagulation would proceed at different rates in different regions:

Thus, growth was faster in the inner Solar System because material was concentrated, but objects didn't get as big because there was ultimately less material to work with than farther out.

Accretion: Planetesimals and planetary embryo formation: T Tauri winds are estimated to have blown any object smaller than 10 m diameter out of the inner Solar System, so any object destined to become part of an inner planet would have been larger than this prior to the protoSun's T Tauri stage. Among them, continued coagulation would have eventually formed planetesimals, objects about 10 km across - big enough that their gravity could begin to attract smaller objects in the process of gravitational focusing.

If two planetesimals occupied adjacent orbits, the larger one would preferentially attract smaller objects gravitationally, starving the other of material with which to grow. Such objects eventually developed into planetary embryos (objects measuring 100s of km). Of course, as disk material became concentrated into a small number of planetary embryos, collisions became rare, but increasingly spectacular. But note: growth proceeded differently in different regions:

Satellite Systems

The primary satellite systems of the jovian planets would have formed at the same time and in roughly the same way with two caveats:

The View from Exoplanets

Planetary migration: The foregoing "classical" view assumes that the planets have always occupied their current orbits. In fact, there are good reasons to doubt this:

Key concepts and vocabulary:
  • Dense cloud collapse
  • Disk dissipation
  • Terminal accretion
  • Gas dissipation
  • The angular momentum problem
  • Sun's T Tauri phase during terminal accretion
  • Planet formation:
  • Satellite systems have no angular momentum problem
  • Planet migration evidence:
  • The Nice Model and Jupiter's Grand Tack
  • The five-planet Nice Model
  • Resolution of the Late Heavy Bombardment mystery