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Module 2 - Planetary Formation and Differentiation

Asteroid collision

Planets, including those like our own Earth, form from epic collisions between asteroids and even bigger bodies, called proto-planets. Sometimes the colliding bodies are ground to dust, and sometimes they stick together to ultimately form larger, mature planets. This artist's conception shows one such smash-up, the evidence for which was collected by NASA's Spitzer Space Telescope. Spitzer's infrared vision detected a huge eruption around the star NGC 2547-ID8 between August 2012 and 2013. Scientists think the dust was kicked up by a massive collision between two large asteroids. They say the smashup took place in the star's "terrestrial zone," the region around stars where rocky planets like Earth take shape. NGC 2547-ID8 is a sun-like star located about 1,200 light-years from Earth in the constellation Vela. It is about 35 million years old, the same age our young sun was when its rocky planets were finally assembled via massive collisions -- including the giant impact on proto-Earth that led to the formation of the moon. The recent impact witnessed by Spitzer may be a sign of similar terrestrial planet building. Near-real-time studies like these help astronomers understand how the chaotic process works. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

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Processes in the protoplanetary disk The dust particles of the solar nebula condensed to form a protoplanetary disk, a rotating disk of dense gas and dust surrounding a newly formed star. An example proto-planetary disk that was imaged by the Hubble Space Telescope is shown below. As the disk spins, chondrules and CAIs condense and dust and particles concentrate along rings.

Planets are thought to form out of the dust that collide and stick to form larger and larger bodies. The growth of small bodies (less than 1 km) relies on random, turbulent motions in the gas to cause collisions. Bodies larger than 1 km, called planetesimals, can attract each other by gravity, which is a force where two bodies of mass are directly drawn to eachother. Even larger bodies (100 to 1000 km), called protoplanets have enough mass, not only to keep together by gravity, but to also change the path of approaching rocks and are able to grow much faster.

The time early in our Solar System’s history when planetesimals and protoplanets formed was active and energetic.
The energy came in multiple forms:
1. Initial energy of the Solar Nebula: After the supernova explosion, the solar nebula was very hot. Innermost portions of protoplanetary disks are thought to have reached temperatures as high as 1000 degrees Kelvin. The protoplanetary disk cooled slowly with time.
2. Decay of radioactive isotopes: Early in the Solar System’s history there was a higher abundance of short-lived radioactive isotopes. These isotopes are atoms with large masses and short half-lives (seconds to 1000s years). Radioactive decay of short-lived isotopes to more stable isotopes (lower masses, longer or infinite half-lives) releases particles, energy (heat) and radiation. The type of energy and radiation that is released by radioactive decay is the same type that is released by a nuclear bomb explosion. The difference is that natural radioactive decay is more dispersed and cumulative, rather than a sudden, violent explosion.
3. Collision of bodies, or impacts: Impacts are one of the major processes leading to the formation of planets. It is also a very energetic process that can cause wholesale melting of a body, bringing about planetary differentiation (separation into core, mantle, and crust). The next section will discuss this process in greater detail.