An Introduction to Astronomy

Part VI: Overview and Origin of the Solar System

Lambert E. Murray, Ph.D.

Professor of Physics

The Sun is the Dominant Influence in our Solar System

The Sun’s gravity controls the motion of all objects in the solar system.

Heat and the Solar Wind from the Sun have had a controlling influence on the atmospheres and surface environment of the planets and their moons.

The Sun

The mass of the Sun is about 700 times the mass of all the rest of the solar system combined.

It is composed mostly of Hydrogen (71%) and Helium (27%), but also contains a small fraction of nearly all the other elements.

Principle Components of the Solar System

The solar system is composed of– Eight Planets and their respective moons:

The Terrestrial Planets The Jovian Planets

– Minor Planets (Dwarf Planets)– Asteroids– Comets

Planetary Orbits All planets orbit the Sun in the same direction as

the Sun rotates (counter-clockwise as viewed from above the ecliptic), in nealy circular orbits, and nearly all within 3 degrees of the ecliptic.

The rotation of most planets is also in a counter-clockwise direction, with the exception of Uranus (and Pluto), which are tilted on their sides, and Venus which is “flipped over”.

Most moons orbit their respective planets in the same direction – although the outer moons of Jupiter are an exception, and most orbit in the equatorial plane of the planet. (Our own Moon, an exception, actually orbits nearer the ecliptic than the equator.)

Orbital Motion of the Planets

The Terrestrial Planets

These planets are small, solid planets composed of rocky (silicate) material mixed with a few heavier elements and have a relatively thin, or no atmosphere.– All these planets have a density of about 5.5

g/cc, with the exception of Mars (3.94 g/cc).– How do we know the density of the planets?

The Jovian Planets These are large planets, with thick gaseous

atmospheres composed mostly of hydrogen and helium gases and hydrogen compounds. Their interior may be composed of frozen liquids and gases such as carbon dioxide, water ice, frozen ammonia and methane (icy compounds).

All these planets have a density of 1.6 g/cc or less, with Saturn being the least dense (0.69 g/cc)

Each of the Jovian planets has a noticeable ring and a large number of moons:– Jupiter ~ 60– Saturn ~ 30– Uranus ~ 20– Neptune ~ 10

Jupiter’s Retrograde Moons

Are these moons captured

asteroids?

What is Pluto? Is it Really a Planet?

It is spherical!

Observations The outer planets appear to have compositions

similar to the Sun, while the inner planets are rocky.– However, the composition of the inner planets are

consistent with the composition of the Sun if hydrogen and helium were removed!

The age of the rocks on Earth, and the Moon and the ages of meteorites appear to be similar (the older rocks of the Earth are about 4 billion years old.)

The age of our fusion fueled Sun is consistent with the age of these rocks implying that they were all formed at about the same time.

Planetary Debris

In addition to the sun, the planets, and the major moons, there are additional pieces that make up our solar system– Asteroids– Comets– Meteors

We will look at these more closely later.

Comets

Meteors

The Formation of the Solar System:The Solar Nebula Hypothesis

Gravitational collapse of an interstellar dust/gas cloud– Formation of a disk of gas and dust– Formation of a fusion fueled star

Heating and condensation within the nebular disk. Accretion and further heating of planetesimals. The end of accretion – formation of craters. Creation of atmospheres

– Outer planets simply “captured” their atmosphere– Inner planets atmosphere arise from volcanic eruptions

releasing gases trapped in the rocky structure of the planet

InterstellarDust/Gas

Cloud

Solar Wind Blowing Away Dust and Gas

A Question on Planet Formation The prevailing theory of planet formation is based

upon accretion of “debris” from the solar nebula. This theory requires a long time to build up a planet the size of Jupiter.

Most observed extra-solar planets are about the size of Jupiter, and many of these are believed to be formed in “stellar nurseries” where there is a large flux of high energy particles that would “blow away” much of the “debris” left from star formation, thus limiting the time for development of such a large planet if accretion is the dominant process. Perhaps there is more to the story?

End of Part VI