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!
Comment -- The Milky Way galaxy has another component (in addition to the visible disk). There is a large spherical component to the Milky Way which is made up of nonluminous material ( Dark Matter). The Milky Way is dominated by this Dark Matter component. Perhaps 90 % of the Milky Way is nonluminous. This is also true for the Universe where as much as 95-96 % of the Universe may be nonluminous!!
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.

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.

Terrestrial, Jovian, and Dwarf planets (and other 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.

DYNAMICAL REGULARITIES

I have already mentioned the dynamical regularities of the Solar System.

The principal ones are:

All planetary orbits are roughly in the same plane -- low inclinations
All planetary orbits are roughly circular -- small eccentricities
All planetary orbits are in the same sense -- CCW as viewed from the North

Secondary regularities are:

Most planets rotate in the same sense as they orbit
The rotation axes of most planets are roughly perpendicular to the ecliptic

There are also some vexing properties which need to be explained, e.g.,

The Sun contains over 99 % of the mass of the Solar System, but contains less than 1 % of the angular momentum (why does the Sun rotate so slowly?)
The Earth-Moon system
The senses of rotation of Uranus, Pluto, and Venus

DYNAMICAL REGULARITIES

Any successful theory for the origin of the Solar System must explain these facts.

TYPES OF AND PROPERTIES OF THE TYPES OF THE PLANETS
The planets show regularities in that they can be divided into distinct classes:

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 are solid with atmospheres, excluding Mercury which, at best, has a transient atmosphere.

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 have fairly 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 are not solid, all with extensive atmospheres.

Rocky/Icy Objects:
Pluto and the other dwarf planets, Triton and some of the other large moons of the Jovian planets, and objects in the Kuiper and asteroid (some, {\it e.g.}, Ceres) belts. These objects combinations of rocky material (silicates). They are solid with densities in-between those of the Terrestrial and Jovian planets. They are smaller in size and mass than the Terrestrials.

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

Planetary Properties. I

For the scale of things:

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

By adding up the masses of the planets (and including the asteroids which are less than 0.001 Earths, we see that the Sun is more than 99 % of the mass of the Solar System.

Random Comments

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).
...

The regularities in the properties of the planets are tied to how far they are from the Sun. The Terrestrial planets are closer to the Sun than are the Jovian planets. In general, the Terrestrial planets are also smaller and more dense than the Jovian planets. Let's look at the density for a second (to see why this is an important observation).
What do the observed properties of the planets imply about the chemical compostions of the interiors of the planets and the atmospheres of the planets?

Planetary Properties. II

There are clear distinctions between the Terrestrials, Jovians, and Rocky/Icy Planets in their:

distances from the Sun
masses and radii
interior compositions (inferred from their densities)

There are differences in the atmospheric pressures and escape velocities of the Terrestrial and Jovian planets. What do these properties tell us about the planets?