PHY 113 Lecture Notes - Lecture 15: Asteroid Belt, Axial Tilt, Htc Sense

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9 Jun 2018

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Chapter 15 Exoplanets
15.1 Modeling Planet Formation
Any model for solar system formation must explain why: 1. Planets are relatively isolated in
2. Planetary orbits are nearly circular
3. Planetary orbits all lie in (nearly) the same plane
4. Direction of orbital motion is the same as direction of Sun’s rotation
5. Direction of most planets’ rotation is also the same as the Sun’s
6. Most moons’ orbits are also in the same sense
7. Solar system is highly differentiated
8. Asteroids are very old, and not like either inner or outer planets
9. Kuiper belt, asteroid-sized icy bodies beyond the orbit of Neptune
10. Oort cloud is similar to Kuiper belt in composition, but farther out and with random orbits
The solar system is not a random assemblage of bits and pieces, but has a single origin.
Planetary condensation theory, first discussed in Chapter 6, seems to work well.
Lots of room for variation; there are also irregularities (Uranus’s axial tilt, Venus’s retrograde
rotation, etc.) that must be allowed by the model.
15.2 Terrestrial and Jovian Planets
Terrestrial (rocky) planets formed near Sun. Due to high temperature, lighter elements got
blown away, so gas giants couldn’t form there.
15.2 Terrestrial and Jovian Planets
Most stars go through a very active “T Tauri” phase, and have strong solar winds early on.
Possible problem for Jovian planet formation. There isn’t enough time to build up the cores
by gradual accretion
Jovian planets possible formation scenarios:
Once they were large enough, may have captured gas from the contracting nebula
Or may not have formed from accretion at all, but directly from instabilities in the outer, cool
regions of the nebula
Detailed information about the cores of Jovian planets should help us distinguish between the
two possibilities. The two scenarios predict different amounts of heavy elements in the core.
Also possible: The jovian planets may have formed farther from the Sun and “migrated”
15.3 Interplanetary Debris
How does the Asteroid belt fit in?
• Orbits mostly between Mars and Jupiter
• Jupiter’s gravity kept them from condensing into a planet, or accreting onto an existing one
• As a result, they are made up of fragments left over from the initial formation of the solar
Icy planetesimals far from the Sun were ejected into distant orbits by gravitational
interactions with the Jovian planets, into the Kuiper belt and the Oort cloud.
Some were left with extremely eccentric orbits and appear in the inner solar system as
Kuiper belt objects have been detected from Earth; a few are as large as, or larger than, Pluto,
and their composition appears similar.
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