Lecture 11 – The Solar System

Chapter 6: The Solar System

6.1: An Inventory of the Solar System

Ancient astronomers were familiar with:
- Stars
- Sun
- Moon
- 5 Planets: Mercury, Venus, Mars, Jupiter, Saturn
- Comets and Meteors
Knowledge of the Solar System remained relatively unchanged for a significant portion of human history.
In the early 1600s, the advent of optical telescopes marked a turning point.
Galileo's refractor telescope, though simple, provided influential observations:
- Discovery of 4 moons orbiting Jupiter.
- Observations of Venus' phases, which aligned with the heliocentric model predictions.
Telescopes significantly broadened our understanding of the Solar System.
By the mid-1800s, telescope technology had greatly advanced in both size and quality.
In 1781, William Herschel discovered Uranus.
The existence of Neptune was mathematically predicted to account for irregularities in Uranus' orbit, and it was discovered in 1846.
- This discovery was considered a major triumph for Newton's theory of gravity.
The asteroid Ceres was discovered in 1801.
- Diameter: 945 km
- Most asteroids reside in the Asteroid Belt, located between the orbits of Mars and Jupiter.
- Asteroids are classified as Minor Planets.
In 1930, Pluto was discovered.
- Initially considered a planet but later reclassified as a dwarf planet in 2006.
- Pluto orbits within the Kuiper Belt:
  - A disc-shaped region in the outer Solar System, extending from Neptune's orbit (at 30 AU) to approximately 50 AU from the Sun.
  - Similar to the asteroid belt but significantly larger.
- Numerous other Kuiper Belt Objects have since been identified.
Since the late 20th century, spacecraft have been employed to study Solar System objects.
- Example: The Mars rover Spirit (2004-2010) analyzed the chemistry and mineralogy of soil and rocks.
- The Perseverance rover, which landed on Feb. 18, 2021, is tasked with searching for signs of ancient life and collecting rock and regolith samples for potential return to Earth.
Current Inventory of the Solar System:
- 1 star (the Sun)
- 8 planets
- 205 moons (and counting)
- 7 large asteroids and millions of smaller ones
- 5 dwarf planets with 8 moons
- More than 100 Kuiper belt objects exceeding 300 km in diameter
- Comets and meteoroids
Studying the objects within our Solar System aids in understanding:
- The origin of the Solar System
- Planetary formation and evolution
- Earth's history and geology
- Conditions conducive to the formation of life
Over 4000 extrasolar planets have been discovered.
Understanding the formation of our Solar System provides insights into the formation of other Solar Systems (and vice versa).

6.2: Measuring the Planets

The sidereal orbital period of each planet can be determined through repeated observation of its position relative to the stars.
- This determination must account for Earth's motion around the Sun.
The distance of each planet from the Sun can be calculated using Kepler's 3rd Law:
- $P^2 (in \, Earth \, years) = a^3 (in \, AU)$
- Radar ranging of Venus was used to calibrate the Astronomical Unit (AU): 1 AU = $1.5 × 10^8$ km.
The masses of all planets are known.
- For planets with observable moons, Newton's Laws can be applied.
- For Mercury and Venus, masses can be calculated by observing their gravitational perturbation on other bodies.
  - Modern methods use the orbits of satellites around these planets (refer to "More Precisely 2-2" in the textbook for details).
- $M = \frac{r v^2}{G}$
The size of each planet (diameter or radius) can be calculated from its angular size and distance from Earth:
- $\frac{diameter}{2 \pi × distance} = \frac{angular \, diameter}{360^o}$
The number of satellites orbiting each planet can be counted.
- Easier for terrestrial planets.
- More challenging for gas giant planets and Kuiper Belt objects due to their greater distance.
Rotation period can be determined from observations.
- More complex than observing surface feature movement, especially for gas giant planets.
- A negative rotation period indicates the planet spins in the opposite direction compared to Earth.
Density can be calculated from radius and mass:
- $density = \frac{mass}{volume}$
- $volume = \frac{4}{3} \pi radius^3$
- For Earth: mass = $6 × 10^{24}$ kg, radius = 6000 km, density = 5500 kg/m3.
Density provides insights into a planet's composition:
- Water: 1000 kg/m3
- Rocks: 2000 – 3000 kg/m3
- Iron: 8000 kg/m3
- Earth (5500 kg/m3): primarily rock and iron.
- Saturn (700 kg/m3): contains significant amounts of gases.

6.3: The Overall Layout of the Solar System

Rocky Planets (Terrestrial Planets) orbit closer to the Sun: Mercury, Venus, Earth, Mars.
Gas Giants (Jovian Planets) orbit farther from the Sun: Jupiter, Saturn, Uranus, Neptune.
The main asteroid belt is located between the Rocky Planets and Gas Giants.
Kuiper Belt Objects are icy bodies orbiting at the edge of the Solar System.
The Oort Cloud (the origin of comets) extends to nearly 1 light-year from the Sun.
The solar system spans approximately 100 AU across, roughly the diameter of the Kuiper belt.
- Planets are relatively close to the Sun.
  - Neptune: a few light-hours away.
  - Nearest star: 4 light-years away.
Planet orbits are elliptical but nearly circular.
- Mercury has the most eccentric orbit.
The orbits of planets are not evenly spaced.
- The distance between adjacent orbits increases with distance from the Sun.
All planets orbit the Sun counterclockwise, except Venus and Uranus, as viewed from above Earth's North Pole.
The Solar System is very flat.
- All planets orbit in nearly the same plane.
- Mercury's orbit has the greatest inclination (7o) relative to Earth's orbit.
Planetary Alignment: Because the planet's orbits are close to being in a plane, it is possible for them to appear in a straight line as viewed from Earth.

6.4: Terrestrial and Jovian Planets

Relative sizes of the planets and our Sun, drawn to scale:
- Terrestrial planets are small.
- Jovian Planets are large.
Comparative Planetology: Goal is to explain these differences.
Terrestrial Planets: Mercury, Venus, Earth, Mars
- Physical and chemical properties similar to Earth.
- Small, dense, rocky bodies.
- Close to the Sun.
- Rotate slowly.
- Weak magnetic fields.
- Few moons.
- No rings.
All have atmospheres, but they are very different
- only Earth has oxygen in atmosphere and liquid water on surface
Surface conditions vary as well
- Mercury is heavily cratered, while Earth has large oceans
Differences among Terrestrial Planets:
- Only Earth and Mars have moons.
  - Earth has a relatively large moon for its size.
  - Mars has 2 small “potato-shaped” moons (likely captured asteroids).
  - Mercury and Venus have no moons.
- Rotation:
  - Earth and Mars spin at about the same rate, resulting in similar day lengths.
  - Venus and Mercury have very slow rotation rates (taking months to rotate once).
  - Mercury: 59 days
  - Venus: 243 days and spins in the opposite direction
- Magnetic Fields:
  - Only Earth and Mercury possess magnetic fields.
  - Venus and Mars do not
- Composition:
  - Earth and Venus exhibit similar composition.
  - Mercury's density is higher, suggesting a greater proportion of nickel or iron.
  - Mars' density is lower, indicating a deficiency in heavy elements.
Jovian Planets: Jupiter, Saturn, Uranus, Neptune
- Physical and chemical properties similar to Jupiter (Jove).
- Much larger than terrestrial planets
- Large, low-density, gaseous bodies.
- Each Jovian has a dense “terrestrial” inner core that makes up more and more of each Jovian planet as we move outward from the Sun
Compare with density of water: 1000 kg/m3
- Saturn would float in water!
- Far from the Sun.
- Rotate quickly.
- Strong magnetic fields.
- Many moons, all very different from our own.
- All have rings.

Next Lecture

Chapter 6: The Solar System
6.5: Interplanetary Matter
6.6: How Did the Solar System Form?
6.7: Jovian Planets and Planetary Debris