Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 30: Overview of the Solar System

Key Ideas:

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

What can we learn about the Solar System without leaving Earth?

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

Three 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

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: Pluto, Eris, Ceres; Icy bodies: Icy moons, Kuiper Belt Objects, Comets; Rocky bodies: Giant moons, 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; ~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. Therefore the abundance of elements in the Sun is nearly equal to that of the entire Solar System.

Terrestrial Planets

Mercury, Venus, Earth, and Mars: "Earth-like," rocky planets; Earth is the largest terrestrial planet; 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; Their composition is very different from the Sun

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); Their composition is very similar to the Sun

The Giant Moons

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

Trans-Neptunian Objects

Any object that orbits at the distance of Neptune or beyond
Split into three distinct regions: Kuiper Belt Objects (KBOs) from 30-50 AU (e.g. Pluto); Scattered Disk Objects (SDOs) out to 100s of AU (e.g. Eris, Quaoar); Oort Cloud is postulated to exist 50,000 to 100,000 AU from the Sun (e.g. Sedna???)
Since 1992 more than 1000 objects have been discovered beyond the orbit of Neptune. Only a small number (such as Pluto and Charon) were known previously.

Discovery of Eris (2003 UB 313)

Announced in August 2005: Larger than Pluto (and bigger than the Moon); Highly inclined, very elliptical orbit; Aphelion at ~98 AU; Has a moon (Dysnomia)
Named for the Greek goddess of strife and discord. Apt choice, considering this object's impact on the astronomical community (and Pluto)

Atmospheres (or Not)

Jovian Planets:: Massive and cool enough to retain all elements, including light gasses
Terrestrial Planets and Moons:: Not always massive enough, or cool enough, to retain everything

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

The Golden Age of Exploration

The Solar System has been explored with robotic spacecraft and astronauts: Landed men on the Moon; Robotic landers on the Moon, Venus, Mars, Titan, and the asteroid 433 Eros; Returned rocks from the Moon; Probed the atmospheres of Venus, Mars, and Jupiter; Flown spacecraft past all planets, and one on the way to Pluto; Mapped Venus and parts of Titan with radar; Flown by asteroids, comets, and returned comet dust to Earth

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

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