7.1 Planetary Overview

By the end of this section, you will be able to:
- Describe how the objects in our solar system are identified, explored, and characterized.
- Describe the types of small bodies in our solar system, their locations, and how they formed.
- Model the solar system with distances from everyday life to better comprehend distances in space.

The solar system consists of:
- The Sun
- Multiple smaller objects:
- The planets
- Their moons and rings
- Debris including asteroids, comets, and dust
Formation timeline:
- Objects formed together with the Sun approximately 4.5 billion years ago from an enormous cloud of gas and dust.
- Central part of the cloud evolved into the Sun; outer material condensed into planets and other bodies.

Characteristics of the Sun:
- A star brighter than approximately 80% of the stars in the galaxy.
- Diameter: roughly 1.4 million kilometers.
- Made of incandescent gas with an interior temperature of millions of degrees.
The Sun will be further discussed in following chapters as the best-studied star.

Mass of Members of the Solar System (Table 7.1):
- Sun: 99.80%
- Jupiter: 0.10%
- Comets: 0.0005–0.03% (estimate)
- All other planets and dwarf planets: 0.04%
- Moons and rings: 0.00005%
- Asteroids: 0.000002% (estimate)
- Cosmic dust: 0.0000001% (estimate)
Most material concentrated in Jupiter, which holds more mass than all remaining planets combined.
Mass determination methods include:
- Kepler’s laws of planetary motion
- Newton’s law of gravity
- Tracking gravitational effects on spacecraft.

Six planets were known before the invention of the telescope (Mercury, Venus, Mars, Jupiter, Saturn).
Uranus and Neptune were discovered post-telescope invention.
All eight planets revolve in the same direction around the Sun, orbiting in roughly the same plane, similar to cars on concentric tracks in a racecourse.

Newly discovered small worlds beyond Neptune, first identified in 1930 (Pluto).
Noteworthy TNOs include:
- Eris: Comparable in size to Pluto, has at least one moon.
- Dwarf planets and largest asteroids: Eris and Ceres.
Over 2600 TNOs discovered; Arrokoth explored by New Horizons spacecraft.

Each planet and dwarf planet rotates around an axis, most in the same direction as their revolution around the Sun.
- Exceptions include:
- Venus: retrograde (backward) rotation
- Uranus and Pluto: significant axial tilt.

Inner (Terrestrial) Planets:
- Mercury, Venus, Earth, Mars (and Moon counted within)
- Composition: primarily rock and metal, solid surfaces, geological features (craters, mountains, volcanoes).
Outer (Jovian) Planets:
- Jupiter, Saturn, Uranus, Neptune
- Composition: light ices, liquids, and gases; lack solid surfaces, resemble vast spherical oceans.
- Size comparison: About 1,300 Earths could fit inside Jupiter.

Pluto: First distant icy world discovered beyond Neptune; visited by New Horizons in 2015.
Additional dwarf planets include Eris, Haumea, Makemake, and Ceres.

Name	Distance from Sun (AU)	Revolution Period (y)	Diameter (km)	Mass (1023 kg)	Density (g/cm³)
Mercury	0.39	0.24	4,878	3.3	5.4
Venus	0.72	0.62	12,120	48.7	5.2
Earth	1.00	1.00	12,756	59.8	5.5
Mars	1.52	1.88	6,787	6.4	3.9
Jupiter	5.20	11.86	142,984	18,991	1.3
Saturn	9.54	29.46	120,536	5,686	0.7
Uranus	19.18	84.07	51,118	866	1.3
Neptune	30.06	164.82	49,660	1,030	1.6

Density Formula: Density = Mass / Volume
Volume of a sphere: $V = \frac{4}{3}\pi r^3$
Demonstration using Mimas (a moon of Saturn) with:
- Radius: approximately 200 km
- Density calculations revealing its composition (mostly ice).
Check Your Learning: Calculate density of Earth vs Mimas, showing that Earth's density is 4 to 5 times greater.

Most planets have moons; Mercury and Venus are the exceptions.
More than 210 known moons, with largest comparable to small planets.
Moons of interest include:
- Galilean moons of Jupiter (Io, Europa, Ganymede, Callisto).
- Titan (Saturn), Triton (Neptune).

Asteroids, rocky objects mostly found between Mars and Jupiter, remnants of the early solar system.
- Some moons, like Mars' moons, may be captured asteroids.

Comets consist predominantly of ice and orbit in colder, distant regions.
- Remnants of solar system formation exploring the outer reaches of the system.

Countless grains of rock, termed cosmic dust, are scattered throughout the solar system, producing meteors upon entering Earth's atmosphere.

Carl Sagan, a prominent astronomer known for:
- Public advocacy for space exploration and understanding of planetary science.
- Pioneering research on Venus's greenhouse effect and Mars's seasonal changes.
- Involvement in several robotic missions to explore the solar system.
Established The Planetary Society to enhance public support for exploration.
Sagan's significant works include "Cosmos," "Pale Blue Dot," and "Contact."

His television series "Cosmos" reached millions, making science accessible to a broad audience.
Neil deGrasse Tyson, a modern advocate and communicator of astronomy, cites Sagan as his inspiration.

Understanding vast distances in space via scale models is essential for visualizing the solar system.
Example of a scale factor of 1 billion:
- Diameter of Earth: 1.3 cm (size of a grape).
- Distance from Sun: 150 meters (1 city block size).
Distances of other planets and their scaled measurements were also included, emphasizing the immense size of the solar system.

Naming conventions include:
- Planets and moons after gods and heroes (mostly Greek/Roman origin).
- Dwarf planets and their moons draw from various mythologies.
Asteroids named after people or significant moments in astronomy.
Features named according to specific themes, such as:
- Craters on Venus named for women.
- Features on Io named for fire/thunder gods.

Recent debates concerning the definition of a planet owing to the discovery of varied planetary systems.
The necessity for naming lies in exploring and understanding newly discovered worlds beyond Earth.