Planet Building Part 2 Planetesimals and Protoplanets

Planetesimals There are three processes that take place which bring solid bits of matter – rocks, metal, and ice – into larger bodies called planetesimals. – Planetesimals coming together is what eventually builds planets. Planetesimals were, seemingly, small, irregular bodies (ice or rock) and scarred by craters from collisions with other planetesimals.

The three processes The three processes of planet building: 1.Condensation 2.Accretion 3.Gravitational collapse (covered later). Incidentally, thanks to computing power, we have been able to build models that show how planets form from planetesimals. We are still working on how the planetesimals themselves formed. – formation (grad students needed!) formation

Artists Rendition of Planetesimals in the Nebular Cloud

It began with the growth of dust grains According to the solar nebula theory, planetary development in the solar nebula began with the growth of dust grains. They grew from microscopic size – first through condensation and then through accretion.

Growth by Condensation A particle grows by condensation when it adds matter one atom or molecule at a time from a surrounding gas. – An example we all know, snowflakes grow by condensation in the Earth’s atmosphere. 3D image of snowflakes formed via condensation (from NSF website).

Condensation In the solar nebula, dust grains were continuously bombarded by atoms of gas – some of which stuck to the grains. Microscopic grains capturing a layer of gas molecules on its surface increases its mass by a larger fraction than, say, a gigantic boulder capturing a single layer of molecules. Condensation increases the mass of small grains rapidly.

Accretion Accretion is our second process. It involves the sticking together of solid particles. – An example of accretion is what happens during a with snow and rain drops. A large, fluffy snow flake is actually many snowflakes that have accreted. – Accretion is also very common in geology and can be witnessed in geologically active places.

Accretion In the solar nebula, the dust grains would have been separated by mere centimeters and thus collided frequently – accreting into larger particles. When the particles reached sizes >a few centimeters, they would have tended to “concentrate” within the plane of the solar nebula.

Accretion Smaller dust particles would not have fallen into the plane because turbulence would have kept them “stirred up.” – Larger objects, however, would have overcome the turbulence and settled into the moving plane.

Accretion The larger particles that had fallen into the nebular plane, which was less 0.01 AU thick, would have continued to accrete. – In fact accretion would have speeded up due to the concentration of the larger particles in such a “small” space. The accretion process led to the creation of planetesimals >0.6 miles in diameter.

Accretion Forces Computer models have shown that the rotating disk of particles should have been gravitationally unstable. It would have been disturbed by spiral density waves, which would have resembled the much larger spiral density waves we see operating in spiral galaxies. The spiral density waves could have helped the particles to coalesce faster – into objects up to 100km in diameter.

Planetary Accretion

Nebula filled with particles Through condensation and accretion, the SNT explains how the nebula became filled with trillions of solid particles – sizes between pebbles to tiny planets. As the largest of the particles exceeded 100 km (60 mi) in diameter, a new stage was reached resulting in “protoplanets.”