Topic 4: The Solar System Reading: Formation of The Solar System, Chapter 6

For the rest of the quarter we will concentrate on trying to understand how the Solar System formed, how it has evolved since its formation, and its (our)place in the Universe. In this context, at some point, we will address the questions of whether the Solar System and life is unique or whether there are other examples of these things in the Universe.

The Solar System is located in the grouping of stars (galaxy) known as the Milky Way galaxy. The Milky Way is a just one of many billions of galaxies in the Universe. The Milky Way galaxy is a spiral (barred spiral?) galaxy which contains roughly 200 billion stars (e.g., see here for the external spiral galaxy M51 and an edge-on view of the Sombrero galaxy, M104). The visible material of the Milky Way is contained in a thin, rotating disk ( optical image and infrared image). The diameter of the disk is on the order 100,000 - 300,000 light years (1 light year = 6 trillion miles = 9.3 trillion kilometers) and has a thickness of several thousands of light years.

The appearance of the Milky Way is striking. In the center of the disk there is a bulge out of which extends several spiral arms. The Solar System is located in one of the arms, roughly one-half of the way from the center of the disk. The Solar System orbits the center of our Galaxy with a speed of roughly 220 kilometers per second or 800,000 kilometers per hour!

The average mass of a star in the Milky Way galaxy is 30 % that of the Sun. The Sun is thus slightly larger than average, but not really that far out of line. The Sun is just 1 out of the two-hundred billion stars which make up the Galaxy. Given this, it is not unreasonable to suspect that the Solar System is not unique. We return to this issue a little later.

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In this Topic, we will

• Describe the features of our Solar System that guide our basic model for the formation of the Solar System:
  • the regularities of the motions of the planets, the facts that the planets orbit the Sun nearly in the ecliptic plane, the planets orbit about the Sun in the same sense (direction), and that the planets have nearly circular orbits
  • the breakdown of the planets into three general types, the solid Earth-like planets (Terrestrial planets), the gaseous Jupiter-like planets (Jovian planets), and the rock/ice objects (dwarf planets, Kuiper belt objects, asteroids, comets)

• Present further properties of the planets dealing mainly with their current atmospheres. For the Terrestrial planets because: (i) the atmospheres are not their primary atmospheres (the atmospheres with which they were formed) but are secondary atmospheres (atmospheres acquired after their birth); (ii) their atmospheres only make-up a small part of the overall mass of the planet, their current atmospheres tell us more about how the planets have evolved since birth and not as much as to how they formed.

Let's start outlining the properties of the objects in our Solar System.

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GENERAL FEATURES OF THE SOLAR SYSTEM

The Solar System displays a seemingly incomprehensible amount of diversity. However, on closer inspection, we see many regularities and patterns in the Solar System.

We first concentrate on the broad brush aspects of the Solar System looking at the properties of the Solar System that are crucial for any model for the formation of the Solar System to explain and properties that help us to understand how the Solar System has evolved after its formation.

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I. DYNAMICAL REGULARITIES

I have already mentioned the dynamical regularities of the Solar System. In the following, we remind the student of these features of the motions of the planets.

The principal regularities are:

• All planetary orbits are roughly in the same plane -- less than 5^o tilt with the exception of Mercury.
• All planetary orbits are roughly circular -- small eccentricities, ε close to 0 except for Mercury.
• All planetary orbits are in the same sense -- CCW as viewed from the North Celestial Pole.
• Most planets rotate in the same sense as they orbit about the Sun and their rotation axes are roughly perpendicular to the ecliptic plane.

see Youtube for a video representation that at least illustrates the three major points

The properties in red are the most important, the one in green although important for models of Solar System formtion, it says more about how the Solar System evolved than about formation. The exceptions to the rule, lead to the idea that nearly catastrophic events play large roles in how the Solar System evolved. There are also some other vexing properties to be explained that also suggest that catastrophic events are important for the evolution of our Solar System, e.g., the Earth-Moon system.

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DYNAMICAL REGULARITIES

Planet	a(A.U.)	Porb(y)	ε	i(o)	Prot(d)	Obliquity(o)
Mercury	0.39	0.24	0.206	7	58.65d	2
Venus	0.72	0.62	0.007	3.4	243.01d	177.3
Earth	1	1	0.017	0	23h56m4.091s	23.5
Mars	1.52	1.88	0.093	1.8	24h37m22s	25.2
Jupiter	5.2	11.9	0.048	1.3	9h50m28s- 9h55m30s	3.1
Saturn	9.54	29.5	0.056	2.5	10h13m59s-10h38m25s	26.7
Uranus	19.2	84	0.046	0.8	17.24h	97.9
Neptune	30.1	164.8	0.010	1.8	16.11h	29.6
Pluto	39.53	248.5	0.248	17.1	6.387d	122

Any Successful Theory for the Origin of the Solar System Must Explain the Principal Dynamical Regularities Described Above

II. THREE TYPES (GROUPS) OF PLANETS

There are three classes of objects, the Earth-like planets, the Terrestrials (Mercury, Venus, Earth and Mars), the Jupiter-like planets, the Jovians (Jupiter, Saturn, Uranus and Neptune), and Dwarf planets (and other rock/ice objects in the asteroid and Kuiper belts). Click on the above picture to see a nice video on the scale of the Solar System: Youtube. Dwarf Planets and Some Kuiper Belt Objects. An interesting object is Sedna.

Sedna is a likely member of the Oort Cloud, the hypothesized reservoir for the comets of our Solar System. Sedna's furthest distance from the Sun (aphelion) is estimated as 937 AU with its closest (perihelion) only 76 AU. Here, AU is astronomical unit, the average distance of the Earth from the Sun. Sedna has an orbital period of around 11,000 years (apply Kepler's 3rd Law of planetary motion). Interestingly, the Voyager 2 spacecraft found what we consider the edge of our Solar System (the heliopause) at 121 AU (Nov 2018); Sedna ventures into interstellar space at the furthest points in its orbit.

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Properties of the Terrestrials, the Jovians, and the Rock/Ice Planets

	Terrestrial Planets: The four planets closest to the Sun, Mercury, Venus, Earth, and Mars, are considered Earth-like in nature, solid with higher densities than the Jovian planets, smaller sizes, and smaller masses than the Jovian planets. The Terrestrials have similar interior chemical make-ups; their masses are dominated by silicates, iron, nickel, and other heavier elements. The Terrestrial planets maintain steady atmospheres excluding Mercury which, at best, has a transient atmosphere. The Terrestrial planets show neither ring systems nor extensive moon systems. The Earth does have one large moon, the Moon, and Mars has two small moons, Phobos and Deimos.
	Jovian Planets: The next four planets moving away from the Sun, Jupiter, Saturn, Uranus, and Neptune, are considered Jupiter-like in nature, gaseous with larger sizes and larger masses than the Terrestrial planets. The Jovians do, however, admit a dichotomy in that Jupiter and Saturn have somewhat different appearances and properties than do Uranus and Neptune. For here, we consider them as a single group, the Jovians. The Jovians have similar chemical compositions, compositions more similar to the Sun (roughly 85-90 % hydrogen and 10-15 % helium) than to the Terrestrial planets. The Jovian planets all show ring systems and extensive moon systems.
	Rock/Ice Objects: Pluto and the other dwarf planets, some of the large moons of the Jovian planets, and objects in the Kuiper and asteroid (some, e.g., Ceres) belts. These objects are composed of combinations of rocky material (silicates) and ices. They are solid with densities in-between those of the Terrestrial and Jovian planets because of their compositions. They are smaller in size and mass than are the Terrestrials.

In tabular form, we have for the general properties of the planets:

General Planetary Properties I

Planet	a(A.U.)	M(ME)	R(RE)	ρ(g-cm-3)
Terrestrials
Mercury	0.39	0.055	0.382	5.4
Venus	0.72	0.815	0.949	5.2
Earth	1	1	1	5.5
Mars	1.52	0.107	0.532	3.9
Jovians
Jupiter	5.2	318	10.4-11.2	1.3
Saturn	9.54	95.2	9.45	0.7
Uranus	19.2	14.5	4.01	1.3
Neptune	30.1	17.2	3.88	1.7
Rock/Ice (selected)
Pluto	39.5	0.0025	0.18	2.03
Europa	5.2	0.008	0.40	3.04
Enceladus	9.54	0.000018	0.040	1.61

For the scale of things:

• Mass of the Earth = 6.0x10²⁴ kilograms
• Diameter of the Earth = 7,917.5 kilometers
• Mass of the Sun = 2.0x10³⁰ kilograms = 330,000 Earths
• Diameter of the Sun = 1,400,000 kilometers = 109 Earths

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What do the observed densities of the planets tell us about the chemical compostions of the interiors of the planets?

• densities larger than 3 g cm^-3 suggest interiors dominated by heavy elements (referred to as metals and rocky materials)
• densities around 1 gm cm^-3 suggest interiors dominated by hydrogen and helium, the simplest elements
• densities between 2 to 3 gm cm^-3 suggest interiors dominated by a mix of rock and ice (water, ammonia, carbon dioxide) materials.

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Other Notable Features of the Solar System

• the Jovian planets have extensive satellite systems
• the Jovian planets have ring systems
• the existence of the comets (Kuiper Belt, Oort Cloud)
• the existence of the asteroids (as an amusing note, consider the Titius-Bode relation).
• Adding up the masses of the planets (including the asteroids which are less than 0.001 Earths), we find that the Sun is more than 99 % of the mass of the Solar System.
• ...

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There are clear differences in the properties of the Terrestrial planets, the Jovian planets, and the Rock/Ice bodies. They differ in their distances from the Sun, their masses and radii, whether they have solid surfaces or are gaseous, and their interior chemical compositions (as inferred from their densities).

Any Successful Model for the Formation of the Solar System Must Explain these Planetary Properties Described Above

We now present the properties of the atmospheres of the Solar System planets. Although important, we do not use these features when we discuss the basic model for the formation of the Solar System. We use them when we discuss the evolution of the planets, in particular, the evolution of the subset of Terrestrial planets Venus, Earth, and Mars.

Planetary Properties II

Planet	Interior	Atmosphere	vesc(km-s-1)	Patm(bars)	Tatm(oF)	ρ(g-cm-3)
Terrestrials
Mercury	Fe,Ni,silicates	traces of H,He,Na,O	4	...	333oF (167oC)	5.4
Venus	silicates,Fe,Ni	96% CO2,4% N2	10	90	867oF (464oC)	5.2
Earth	silicates,Fe,Ni	78% N,21% O2	11	1.01	59oF (15oC)	5.5
Mars	silicates	95% CO2,2.6% N2,1.9% Ar	5	0.0065	-85oF (-65oC)	3.9
Jovians
Jupiter	H,He	86% H, 7.4% He	60	1.01	-166oF (-110oC)	1.3
Saturn	H,He	9% H,7.4% He	36	1.01	-220oF (-140oC)	0.7
Uranus	ices,H,He	84% H,14% He	21	1.01	-320oF (-195oC)	1.3
Neptune	ices,H,He	84% H,?% He	23	1.01	-330oF (-220oC)	1.7
Rock/Ice
Pluto	ices,silicates	trace CH4	1.3	~10-5	-375oF (-225oC)	2.03
Europa	silicates,ice	primarily O2, some water	0.22	~10-12	-276oF(-171oC)	3.04
Enceladus	silicates,ice	H2O,CO2,N,CH4 (plumes?)	0.23	trace,variable	-324oF(-198oC)	1.61

The Jovian planets are gaseous and so to define their atmospheric properties, the layer where the gas pressure is the same as at sea level on the Earth is chosen to define the temperature and pressure.

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The differences in the atmospheric pressures and escape velocities of the Terrestrial and Jovian planets yield further information about the planets.