Planet Building Part 2

1. Planet Building Part 2 Planetesimals and Protoplanets

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

3. The three processes • The three processes of planet building: • Condensation • Accretion • 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. • http://planets.ucf.edu/research/planetesimal-formation (grad students needed!)

4. Artists Rendition of Planetesimals in the Nebular Cloud

5. 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.

6. 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).

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. Planetary Accretion

14. 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.”