Planet Building Part 3

1. Planet Building Part 3 Growth of Protoplanets

2. Starter • Hubble images • https://www.youtube.com/watch?v=1lPXy-WKn7k&list=PLiuUQ9asub3Ta8mqP5LNiOhOygRzue8kN • https://www.youtube.com/watch?v=PuY0GfGLFMU&list=PLiuUQ9asub3Ta8mqP5LNiOhOygRzue8kN • https://www.youtube.com/watch?v=s9b0md5oGWw&list=PLiuUQ9asub3Ta8mqP5LNiOhOygRzue8kN • https://www.youtube.com/watch?v=FfCSa-t8-Ak&list=PLiuUQ9asub3Ta8mqP5LNiOhOygRzue8kN

3. Protoplanets • Protoplanets were formed from the coalescing of planetesimals. • Proto- is a prefix that means first.

4. Protoplanets • The problem the SNT has to overcome is if planetesimals had collided at observed orbital velocities, it is unlikely that they would have “stuck” together. • Typical orbital velocity in the universe is about 22,000 mph. • A head-on collision at that speed would have caused vaporization, not accretion.

5. Protoplanets • The key seems to be that the planetesimals were moving in the same direction, so the collisions were “gentle.” • The planetesimals also would have possessed adhesive effects. • They would have had sticky coatings and electrostatic charges on their surfaces. • http://adsabs.harvard.edu/full/1997A%26A...319.1007S (journal article on protoplanet formation).

6. Protoplanets • This is a brief journal article from Nature on the subject of the formation of protoplanets from the planetary disk. • http://141.213.232.243/bitstream/handle/2027.42/62894/nature06087.pdf?sequence=1 • Note: if research is done with federal money, then it must be provided free on-line. You can find very interesting research, high quality peer-reviewed, on-line.

7. Gravity and Protoplanet Formation • The “gentle” collisions would have fragmented some of the surface rock but gravity could have held the two planetesimals and fragments together. • The fragments could have created a relatively soft soil layer on the surface of the larger planetesimals. • This would have created a layer capable of more easily capturing small planetesimals.

8. Gravity and Protoplanet Formation • Their stronger gravitational field (along with the soil-like layer) would have supported an increased growth rate. • They would have been able to trap and hold fragments. • Models indicate that the largest planetesimals would have grown quickly to protoplanetary dimensions, “sweeping up” more-and-more material.

9. Gravity and Protoplanet Formation • Protoplanets would have begun to grow by accumulating solid material (rock, metal, and ice) because of weak gravitational field – they could not capture and hold large amounts of gas. • This goes along with what we already know about gravitational fields and escape velocity. • The gas molecules would have been travelling at velocities faster than the escape velocities of the forming protoplanets.

10. Gravitational Collapse • Once a protoplanet reached a certain size, 15 Earth masses, it could grow very quickly through gravitational collapse – the rapid accumulation of large amounts of “in-falling” gas from the nebula. • https://www.youtube.com/watch?v=bLLWkx_MRfk • This is the explanation for the development of the Jovian planets compared to the terrestrial planets.

11. Jovian Planet Formation – Gravitational Collapse

12. Jovian Planet Formation – Gravitational Collapse http://www2.astro.psu.edu/users/niel/astro1/slideshows/class43/slides-43.html This slide comes from Penn State Astro Department.

13. A number of assumptions – an aside • The Solar Nebular Theory (SNT) contains a few of assumptions that are currently being explored through Hubble and other probes. • All the planetesimals had the same chemical composition. • The protoplanets that formed were made of homogenous composition throughout. • The heat generated at the planet’s core was at least in part due to the decay of short-lived radioactive elements.

14. Heat of Formation • Heat of formation, another source of heat, would have come from violent impacts of in-falling particles. • Their crashing into the protoplanets would have released large amounts of energy. • The heat generated from the decay of radioactive elements and heat of formation would have “melted” the protoplanet and allowed it to differentiate.

15. Differentiation • Differentiation – the separation of material according to density. • Once a planet melted, the heavy metals such as iron and nickel, plus elements chemically attracted to them, would have settled in the core. • Lighter silicates would have floated to the surface to form a low-density crust. • So, we went from homogenous planetesimals to differentiated planets.

16. Differentiation • Differentiation depended partly on the short-lived heat from radioactive elements. • Their rapid decay would have released searing heat – melting a planet’s interior. • Astronomers know that these radioactive elements were present because of data collected from meteorites (containing daughter isotopes of those radioactive elements). • Example: the decay of Aluminium-26 to Magnesium-26 – a half-life of “only” 0.74 million years.

17. Outgassing – Atmosphere Creation • Outgassing – the creation of a planetary atmosphere from a planet’s interior. • Earth’s outgassing, due to its SS position, cannot account for Earth’s large quantity of water. • The hypothesis for the origin of much of Earth’s water depends on Earth sweeping up icy, nutrient-rich, planetesimals.

18. Hubble – Once More • Hubble at 20 years old. • https://www.youtube.com/watch?v=pE9VUgTgWAs