Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 31:
The Family of the Sun

Key Ideas:

• The Sun
• Terrestrial Planets
• Jovian Planets
• Dwarf Planets
• Giant Moons
• Trans-Neptunian Objects
• Asteroids, Comets, & Meteoroids

The planets all lie in nearly the same plane and orbit in the same general direction.

The Golden Age of Planetary Exploration

• Landed on the Moon, Venus, Mars, Titan, & asteroid 433 Eros
• Returned rocks from the Moon (~382 kg)
• Probed the Atmospheres of Venus, Mars, & Jupiter.
• Flown spacecraft past all planets
• Mapped Venus and parts of Titan with Radar
• Flown by asteroids & comets, with one asteroid landing.

Much of what we know about the Solar System has been learned in the last 35 years of planetary exploration.

The next few years will see a number of new missions to visit Mars, Saturn, Comets and Asteroids.

The Family of the Sun

The Terrestrial Planets:

• Rocky Planets: Mercury, Venus, Earth & Mars

The Jovian Planets:

• Gas Giants: Jupiter, Saturn, Uranus & Neptune

Dwarf Planets:

• Rocky & Icy Bodies: Pluto, Eris, & Ceres

Small Solar System Bodies:

• Rocky: Asteroids & Meteoroids
• Icy: Kuiper Belt Objects (KBOs) & Comets

Contents of the Solar System by Mass Plot of the Mass (in Earth Masses) vs. orbital Semi-Major Axis (in AU) for all of the major constituents of the Solar System other than the Sun. Click on the image to view full size. Based on an original plot by Scott Gaudi (OSU).

The Eight Planets, in order from the Sun:

First 6 were known from antiquity, and are all visible to the naked eye as viewed from the Earth.

The last two were discovered using telescopes:

Uranus

Discovered by William Herschel (UK) in 1781.

Neptune

Predicted mathematically using orbital deviations of Uranus and Newtonian Gravity by Urbain LeVerrier (France) and John Couch Adams (UK)

Found at predicted location by Johann Galle (Germany) in 1846.

Basic Properties of the Planets

• Terrestrial Planets are found in the inner solar system: 0.4-1.5AU
• Jovian Planets are found in the outer solar system: 5-30 AU

All orbit in the same direction & plane:

• Orbit counterclockwise, in the same sense as the rotation of the Sun.
• All orbit very near the Ecliptic plane.

These facts provide us with important clues to the formation of the Solar System.

The Sun

• Mostly Hydrogen & Helium
• Contains 99.8% the mass of the Solar System
• About 4.6 Gyr old

The Sun shines because it is hot:

• Surface (photosphere) is ~6000 K
• Radiates mostly Visible light plus UV & IR

Kept hot by nuclear fusion in its core

• Builds Helium from Hydrogen nuclei, liberating nuclear binding energy.

Terrestrial Planets

"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/cc (rock & metal)
• Earth, Venus & Mars have atmospheres

The Jovian Planets

• Largest Planets: at least 15 times mass of Earth.
• Jupiter is the largest (318 Earth Masses)
• Found only in the Outer Solar System (between 5 and 30 AU)

Gas Giants ("Jupiter-like"):

• No Solid Surfaces (mostly atmosphere)
• Mostly Hydrogen & Helium
• Rocky/icy inner cores
• Low density: 0.7 to 1.7 g/cc (water is 1 g/cc)

Dwarf Planets

1. They must orbit the Sun, and not be satellites of another, larger body

2. They are shaped by self-gravity: which means that they have sufficiently large masses that their self-gravity overcomes internal rigid-body forces and shapes them into spheroids in "hydrostatic equilibrium". Usually means it is larger than about 800km in diameter, but it depends on the material it is made of (rock, ice, or a mix).

3. Their gravity is insufficient to have cleared the neighborhood around their orbit.

There are currently three recognized Dwarf Planets:

• Ceres: first and largest of the Asteroids, discovered in 1801 by Guiseppe Piazzi.
• Pluto: first large Trans-Neptunian Object, discovered in 1930 by Clyde Tombaugh.
• Eris: largest known Trans-Neptunian Object, discovered in 2005 by Mike Brown, Chad Trujillo, and David Rabinowitz.

The Giant Moons

The largest of these are the "Giant Moons":

• Earth: The Moon
• Jupiter: Io, Europa, Ganymede, & Callisto (the 4 Galilean moons discovered by Galileo in 1610)
• Saturn: Titan
• Neptune: Triton

Many smaller moons, both rocky & icy, are found throughout the solar system orbiting all planets except for Mercury & Venus. Only these two planets have no moons.

Trans-Neptunian Objects

• Found only in the outer Solar System beyond 30 AU
• Densities of 1.2 to 2 g/cc, indicating they are made mostly of ices.

Examples:

• Kuiper-Belt Objects (30-50 AU)
• Charon, Pluto's large moon
• Sedna & Quaor: distant, large icy bodies

The Leftovers: Small Solar System Bodies

Asteroids:

• Made of rock & metals (density 2-3 g/cc)
• Sizes: Range from few 100km down to the size of large boulders
• Largest are in the Dwarf Planet range (e.g., Ceres)

Meteoroids:

• Bits of rock and metal (iron)
• Sizes: grains of sand up to small boulders

Comets:

• Composite rock & ice "dirty snowballs"
• Develop longs tails of gas & dust are swept off them when they pass near the Sun.

All of these are the leftover material from the original formation of the Solar System.