7.1 - Components of the Solar System Textbook pp 215-223

The Solar System consists of the Sun and the bodies orbiting in its gravitational field: the eight planets, their moons, and swarms of asteroids and comets.

The Sun The Sun is a star, a ball of incandescent gas, whose light and heat are generated by nuclear fusion at its core. It has more than 700 times the mass of all other bodies put together.

The Sun is mostly hydrogen (about 71%) and helium (27%), but it also contains minute proportions of nearly all the other chemical elements in vaporized form.

The Planets Planets emit no visible light of their own, but shine by reflected sunlight. In order from Sun : Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Orbits Planets move around the Sun in very slight elliptical orbits, nearly all lying in the same plane. (Somewhat like a spinning pancake, spinning counter-clockwise.)

Most planets spin counterclockwise on their own rotational axis, fairly perpendicular to their orbital motion around the Sun. Venus and Uranus are exceptions to this rule..

Uranus has an extremely large tilt to its rotation axis; it lies nearly in its orbital plane.

Venus’ rotation axis has only a small tilt, but it spins backward, a motion technically called “retrograde rotation.” Despite this, Venus orbits the Sun in the same direction as the rest of the planets. Venus in true color. The surface is obscured by a thick blanket of clouds.

The flattened structure and orderly orbital and spin properties of planets in our Solar System are two of its most fundamental features.

Equally important is that the planets fall into two families called inner and outer planets, based on their size, composition and location in the Solar System.

Two Types of Planets The inner planets – Mercury, Venus, Earth and Mars – are small rock bodies with relatively thin or no atmospheres. They are also sometimes called the Terrestrial Planets, or the Rocky Planets.

The outer planets – Jupiter, Saturn, Uranus and Neptune - are much larger than the inner planets. They are gaseous and liquid, with deep, hydrogen rich atmospheres. The outer planets are also called the Gas Giants or the Jovian Planets.

What do we mean by “rock?” Rock means material composed of silicon and oxygen, with an admixture of other heavy elements such as aluminum, magnesium, sulfur and iron. Even though “rock” is rare in the Solar System as a whole, the Sun’s warmth has caused more abundant materials, such as water, hydrogen, methane and ammonia to condense much further away, and leave the SiO2.

What do we mean by “ice?” Ice means frozen liquids such as water ice, frozen carbon dioxide, frozen ammonia, frozen methane, etc. These frozen liquids usually do not exist naturally except when past the inner planets. (Exceptions are Earth’s frozen water ice and the frozen carbon dioxide of Mars.)

The planets and Sun to scale.

Dwarf Planets Dwarf planets are objects in the Solar System which do not fit one or more of the International Astronomical Union’s 2006 list for planets: orbit the Sun have enough mass to pull itself into a spherical shape have cleared out its orbit of other material.

Pluto orbits the Sun, is large enough to be spherical, but is not the only object in its orbit – instead, it is one of the larger objects in the Kuiper Belt. Eris, a dwarf planet slightly larger than Pluto, was discovered in 2005 and spawned the “Great Planet Debate.”

Satellites As the planets orbit the Sun, most are themselves orbited by satellites. As of February 2012… Saturn has at least 60 moons. Jupiter has at least 67 moons. Neptune has at least 13 moons. Uranus has at least 27 moons.

Mars has two moons. Earth has just the one moon. Only Mercury and Venus are moonless when it comes to the regular planets.

Even some of the dwarf planets have moons. Pluto has 3 and Eris have 1 moon each. Each planet and its moons resemble a miniature Solar System.

Asteroids and Comets The Solar System has many smaller planetary bodies as well. Asteroids are rocky or metallic bodies with diameters that range from a few meters up to about 1,000 km (1/10th the size of Earth. Ceres

Comets, on the other hand, are icy bodies about 10 km (about 6 miles) or less in diameter that grow huge tails of gas and dust as they near the Sun and are partially vaporized by its heat.

Most asteroids circle the Sun in the large gap between the orbits of Mars and Jupiter, a region called the asteroid belt. They are probably material that failed – perhaps as result of disturbance by Jupiter’s gravity – to aggregate into a planet.

The Ort Cloud

The Oort cloud, named for the Dutch astronomer who proposed its existence, is thought to be a spherical region that completely surrounds the Solar System, and extends from about 40,000 to 100,000 km from the Sun. Most comets are thought to originate within the Oort cloud, and travel most of their orbit there.

Some comets come from a disk-like swarm of icy objects that lies just beyond the orbit of Neptune and extends to about 50 AUs from the Sun, a region called the Kuiper Belt.

Together, the Oort cloud and Kuiper belt probably contain more than 1 trillion comet nuclei, thousands of larger objects and possibly several dozen additional dwarf planets.

Compositional Differences Between the Inner and Outer Planets The compositional differences are so important to our understanding of the history of the Solar System that we should look at them more closely.

Astronomers can deduce a planet’s composition by measuring its spectrum to gather information about its surface rocks. Spectra, however, gives no clue as to what lies inside a planet where light cannot penetrate.

Earthquake waves reveal what lies inside of Earth and could work for other planets. The quake detectors placed on Mars did not work properly, and imagine the difficulties of trying to place a detector on a Gas Giant!

Density as a Measure of a Planet’s Composition The average density of a planet can help us study the interior of planets. Once the planet’s average density is known, we can compare it with the density of abundant, candidate materials to find a likely match.

For example, we know that the average density of Earth (5 For example, we know that the average density of Earth (5.5 cubic cm) was intermediate between silicate rock (3 g/cm3) and iron (7.8 g/cm3). From this we can infer that Earth has an iron core beneath its rocky crust.

Density comparison has its drawbacks. There may be more than one substance which matches the observed density. The density of a given material can be affected by the planet’s gravitational force – it may make rock more dense than in and Earth-like gravity.

All terrestrial planets have an average density similar to the Earth’s (3.9 to 5.5 g/cm3).

Jovian planets, however, have a much smaller average density (0 Jovian planets, however, have a much smaller average density (0.71 to 1.67 g/c3), similar to that of ice.

After correcting for the above gravitational compression, we can conclude that all the inner planets contain large amounts of rock and iron and the iron has sunk to the core. Interior of Mars

Likewise, the outer planets probably have cores of iron and rock about the same size of the Earth beneath their deep atmosphere.

The composition of the planets not only underlines the differences between the two families of planets but also furnishes another clue to their origin: the planets and Sun were all made from the same material.

Jupiter and Saturn have a composition almost identical to that of the Sun…

… while the inner planets have a similar composition to the Sun if we were to remove its hydrogen and helium.