Unit 4: Our Solar System
Our Solar System: Origin, Structure, and Components
Origin
The origin of our Solar System is explained by the Modern Nebular Theory (also discussed in Unit I). It proposes that our solar system formed 4.6 billion years ago from the gravitational collapse of a giant cloud of gas and dust called a solar nebula. As the cloud collapsed, it began to spin, forming a flat disk with the hot, dense Proto-Sun at the center. Planets, moons, and asteroids formed from the material in this disk.
Structure and Components
The Solar System is structured with a central star (the Sun) and various bodies orbiting it.
- The Sun: Our star, a G-type main-sequence star. It contains ~99.8% of all mass in the Solar System.
- Planets: The eight main bodies orbiting the Sun.
- Inner Planets (Terrestrial): Mercury, Venus, Earth, Mars.
- Outer Planets (Jovian): Jupiter, Saturn, Uranus, Neptune.
- Satellites (Moons): Natural bodies that orbit planets (e.g., Earth's Moon, Jupiter's Galilean Moons).
- Dwarf Planets: Smaller bodies that are spherical but haven't cleared their orbital path (e.g., Pluto, Ceres).
- Small Bodies:
- Asteroid Belt: A region between Mars and Jupiter filled with rocky asteroids.
- Kuiper Belt: A region beyond Neptune filled with icy bodies (including Pluto).
- Oort Cloud: A theoretical, massive spherical cloud of comets at the outermost edge of the solar system.
Physical Properties of Planets and Satellites
Planet Types and Properties
| Property |
Terrestrial Planets (Inner) |
Jovian Planets (Outer) |
| Planets |
Mercury, Venus, Earth, Mars |
Jupiter, Saturn, Uranus, Neptune |
| Composition |
Small, dense, rocky (silicates, metals) |
Large, low-density, gas/ice (H, He, water, methane) |
| Atmosphere |
Thin (except Venus) |
Very thick and deep |
| Rings |
No |
Yes (all Jovian planets have rings) |
| Moons |
Few or none |
Many |
Satellites (Moons)
Satellites are highly diverse. Their physical properties depend on where they formed.
- Inner Solar System: Our Moon is rocky, large (relative to Earth), and has no atmosphere.
- Outer Solar System: Most moons are icy (composed of water ice, ammonia ice, etc.) mixed with rock. They can be geologically active due to tidal heating (e.g., Io, Europa).
Study of the Surface of Mars, Venus, and Mercury
Mercury
- Surface: Looks very much like our Moon. It is heavily cratered, indicating it is geologically "dead" and has a very old surface.
- Key Feature: Large cliffs called "scarps" that formed when the planet cooled and shrank.
- Atmosphere: Almost none (an exosphere). This results in the most extreme temperature swings in the solar system.
Venus
- Surface: The surface is completely hidden by a thick, permanent layer of sulfuric acid clouds. We have mapped it using radar (e.g., the Magellan mission).
- Key Features: The surface is dominated by volcanoes (thousands of them) and vast lava plains. It has very few impact craters, suggesting the *entire surface* was "repaved" by volcanism relatively recently.
- Atmosphere: Extremely thick (90x Earth's pressure), composed of 96% CO₂. This has caused a runaway greenhouse effect, making Venus the hottest planet in the solar system (hotter than Mercury).
Mars
- Surface: The "Red Planet" gets its color from iron oxide (rust). Its surface is a cold, dry desert.
- Key Features:
- Olympus Mons: The largest volcano in the solar system (a shield volcano).
- Valles Marineris: A canyon system as long as the entire United States.
- Evidence of Water: The surface is covered in features that show liquid water once flowed, such as dry river beds, deltas, and floodplains. Today, water exists as ice in the polar caps and underground.
- Atmosphere: Very thin (1% of Earth's), composed of 95% CO₂.
Jupiter's Moons
Jupiter has many moons (over 80), but the four largest are the Galilean Moons, discovered by Galileo Galilei in 1610. They are like a miniature solar system.
- Io:
- The innermost Galilean moon.
- The most volcanically active body in the entire solar system.
- It is constantly squeezed by Jupiter's massive gravity (tidal heating), melting its interior and causing sulfur volcanoes to erupt constantly.
- Europa:
- A bright, smooth surface made of water ice, covered in long, dark cracks.
- There is overwhelming evidence that a vast liquid water ocean exists *underneath* the ice shell.
- This makes Europa a primary target in the search for extraterrestrial life.
- Ganymede:
- The largest moon in the solar system (it is bigger than the planet Mercury).
- It is composed of a mix of rock and ice and is the only moon known to have its own magnetic field.
- Callisto:
- The outermost Galilean moon.
- Its surface is ancient and extremely heavily cratered, indicating it has not been geologically active for billions of years.
Preliminary Concept of Stellar System
Stellar System: A system consisting of a star (or multiple stars) and the various objects that orbit it, such as planets, moons, asteroids, and comets.
- Our Solar System is just one example of a stellar system.
- Binary Star System: A common type of stellar system where two stars orbit each other.
- Exoplanets: Planets that orbit *other* stars. In recent decades, astronomers have discovered thousands of other stellar systems in our galaxy, proving that planets are common in the universe.