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Planet Building Part 3 Growth of Protoplanets

Planet Building Part 3

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Planet Building Part 3. Growth of Protoplanets. Starter. Hubble images https://www.youtube.com/watch?v=1lPXy-WKn7k&list=PLiuUQ9asub3Ta8mqP5LNiOhOygRzue8kN https :// www.youtube.com/watch?v=PuY0GfGLFMU&list=PLiuUQ9asub3Ta8mqP5LNiOhOygRzue8kN - PowerPoint PPT Presentation

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Page 1: Planet Building Part 3

Planet Building Part 3

Growth of Protoplanets

Page 2: Planet Building Part 3

Starter

• Hubble images• https://www.youtube.com/watch?v=1lPXy-W

Kn7k&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

Page 3: Planet Building Part 3

Protoplanets• Protoplanets were formed from the coalescing of

planetesimals. – Proto- is a prefix that means first.

Page 4: Planet Building Part 3

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.

Page 5: Planet Building Part 3

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).

Page 6: Planet Building Part 3

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/2027.4

2/62894/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.

Page 7: Planet Building Part 3

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.

Page 8: Planet Building Part 3

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.

Page 9: Planet Building Part 3

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.

Page 10: Planet Building Part 3

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.

Page 11: Planet Building Part 3

Jovian Planet Formation – Gravitational Collapse

Page 13: Planet Building Part 3

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.1. All the planetesimals had the same chemical

composition.2. The protoplanets that formed were made of

homogenous composition throughout. 3. The heat generated at the planet’s core was at

least in part due to the decay of short-lived radioactive elements.

Page 14: Planet Building Part 3

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.

Page 15: Planet Building Part 3

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.

Page 16: Planet Building Part 3

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.

Page 17: Planet Building Part 3

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.

Page 18: Planet Building Part 3

Hubble – Once More

• Hubble at 20 years old.• https://www.youtube.com/watch?v=pE9VUgT

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