Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 29: Overview of the Solar System

Key Ideas:

The Solar System contains:: The Sun; Terrestial Planets; Jovian Planets; Giant Moons; Dwarf Planets; Smaller rocky and icy bodies (asteroids, comets, etc.)

The View from Afar

What observations can we make of the planets without leaving Earth?: Orbits: direction, ellipticity, tilt (or inclination), distance; Rotation: speed, tilt; Number and sizes of moons; Masses and radii (and thus density); Reflectivity, presence of craters; Spectroscopy (temperature, chemical composition)

The Neighbors

Sun: A star (middle-aged, average-sized)
Terrestrial Planets: Rocky planets: Mercury, Venus, Earth, Mars
Jovian Planets: Gas/Ice giants: Jupiter, Saturn, Uranus, Neptune
The Rest: Dwarf Planets: Pluto, Eris, Ceres; Giant Moons; Icy bodies: Icy moons, Kuiper Belt Objects, Comets; Rocky bodies: Asteroids

Basic Properties of the Planets

Location:: Terrestrial in the inner solar system: 0.4 - 1.5 AU; Jovian in the outer solar system: 5 - 30 AU
All orbit in the same direction and same plane:: Orbit counterclockwise, in the same sense as the rotation of the Sun

These facts provide clues to the formation of the Solar System

The Sun

A middle-aged, average star:: Mostly Hydrogen and Helium; 99.8% of the entire Solar System; About 4.6 Gyr old
Shines because it is hot:: Surface temperature ~6000 K; Mostly emits UV, visible, and infrared light
Kept hot by nuclear fusion in its core: Builds Helium from Hydrogen fusion; Can shine for ~12 Gyr

The Sun contains nearly all of the mass in the Solar System
The abundance of the Sun is nearly equal to that of the entire Solar System

Terrestrial Planets

Mercury, Venus, Earth, and Mars: "Earth-like," rocky planets; Largest is the Earth; Only found in the inner solar system (0.4 to 1.5 AU)
Rocky Planets:: Solid surfaces; Mostly silicates and iron; High density: 3.9-5.5 g/cm³ (rock and metal); Earth, Venus, and Mars have atmospheres

Jovian Planets

Jupiter, Saturn, Uranus, and Neptune: Largest planets: at least 15 times the mass of the Earth; Jupiter is the largest at 318 Earth masses; Only found in the outer solar system (5 to 30 AU)
Gas Giants:: No solid surfaces (mostly atmosphere); Mostly Hydrogen and Helium; Rocky/icy inner cores; Low density: 0.7 to 1.7 g/cm³ (compare to water at 1 g/cm³)

The Giant Moons

Natural satellites orbiting planets
Giant Moons:: Earth: The Moon; Jupiter: Io, Europa, Ganymede, and Callisto; Saturn: Titan; Neptune: Triton
Many smaller moons, both rocky and icy
Many dwarf planets have small moons
Only Mercury and Venus have no moons

Getting Goofy Over Pluto

The discovery of additional, 'pluto-sized' objects in the outer solar system led to a recent reconsideration of the definition of a planet
In August 2006 the International Astronomical Union adopted the new category of 'dwarf planet'
A Dwarf Planet is defined to be a celestial body that:: Is in orbit around the Sun; Must be in hydrostatic equilibrium due to its mass (must be round); Has not cleared the neighborhood around its orbit; Is not a satellite

The Dwarf Planets

Five official (so far):: Pluto: Formerly the ninth planet, now the second-largest dwarf planet; Ceres: Largest member of the asteroid belt; Eris: Largest dwarf planet, whose discovery started the controversy; Haumea; Makemake

Trans-Neptunian Objects

Any object that orbits at the distance of Neptune or beyond
Three regions: Kuiper Belt from 30-50 AU (e.g. Pluto); Scattered Disk out to 100s of AU (e.g. Eris, Quaoar); Oort Cloud postulated to exist 50,000 to 100,000 AU from the Sun (e.g. Sedna???)
More than 1000 objects orbit beyond Neptune. Only a small number (e.g. Pluto and Charon) were known prior to 1992.

The Leftovers (Minor Bodies)

Asteroids:: Made of rock and metal (density of 2-3 g/cm³); Sizes: 500 km (Ceres) to large boulders
Meteoroids:: Bits of rock and metal; Sizes: grains of sand to boulders
Comets:: Composite of rock and ice ("dirty snowballs"); Long tails of gas and dust are swept off them when they pass near the Sun

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.