Solar System Debris, Dwarf Planets, and Comets

Debris Belts and Trans-Neptunian Regions

There are two primary debris belts in our solar system, divided by their composition and location relative to the frost line: - The Asteroid Belt: Located inside the frost line, primarily composed of rocky material. - The Kuiper Belt: Located beyond the orbit of Neptune, primarily composed of ice.
Trans-Neptunian Objects (TNOs): This is a categories including any object, including comets and dwarf planets, that exists beyond the orbit of Neptune.
The solar system's layout can be summarized by composition: the inner solar system is dominated by rock (asteroids), and the outer solar system is dominated by ice (Kuiper Belt/Oort Cloud).

The Reclassification and Physical Geography of Pluto

Classification: Pluto is no longer considered a major planet; it is classified as a dwarf planet. Reasons for this transition include its atypical orbit, its small size, and its lack of differentiation compared to major planets.
Historical Observation: Until the New Horizons spacecraft mission in $2015$ , the best images of Pluto were heavily pixelated. New Horizons provided the first high-resolution data in the last 5 to 10 years, placing Pluto on the "cutting edge" of modern planetary science.
Orbital and Physical Statistics: - Distance from Sun: Approximately $39.5\,AU$ . - Light Travel Time: Since light takes roughly $8\,\text{minutes}$ to reach Earth ( $1\,AU$ ), it takes light approximately $320\,\text{minutes}$ ( $8 \times 40$ ) to reach Pluto, which is over $3.5\,\text{hours}$ . - Revolution Period: It takes Pluto $250\,\text{years}$ to orbit the Sun, moving in a counterclockwise direction. - Rotation Period: $6.4\,\text{days}$ . - Eccentricity: $0.25$ , which is higher than any major planet in the solar system and contributed to its reclassification. - Axial Tilt: Pluto is tilted at $120^\circ$ , resulting in retrograde rotation.
Atmosphere and Composition: Pluto possesses a tenuous atmosphere primarily composed of nitrogen ( $N_2$ ). It is extremely cold due to its distance from the sun.

The Dynamics of the Pluto-Charon System and its Moons

Center of Mass (Barycenter): Pluto and its largest moon, Charon, do not follow the typical planet-satellite relationship. Charon is approximately one-third the size of Pluto. Instead of Charon orbiting Pluto, both objects orbit a common center of mass located in the space between them. This "weird little dance" is not typical of planets.
Synchronous Orbit: The two objects are in a synchronous orbit around each other as they move counterclockwise around the Sun.
Pluto's Five Moons: - All moons are named after themes associated with the underworld to match Pluto (the god of the underworld). - Resonance: The moons exist in a gravitational resonance with the Pluto-Charon system. - Styx: Orbits $3$ times for every $1$ orbit of the outer system ( $1:3$ resonance). - Nix: Orbits $4$ times for every $1$ orbit of the outer system ( $1:4$ resonance). - Kerberos (Cerberos): Orbits $5$ times for every $1$ orbit of the outer system ( $1:5$ resonance). - Hydra: One of the largest moons in the system, located further out and also orbiting the center of mass.

Nomenclature and Surface Features of Pluto and Charon

NASA allowed public participation in naming features on Pluto's surface, leading to a mix of scientific and cultural references.
Pluto Surface Features: - Often named after NASA missions: Viking, Voyager, Venera, Pioneer, Hayabusa. - Craters are often named after scientists, such as Clyde Tombaugh, who discovered Pluto. - The "Heart" of Pluto: A large, heart-shaped feature officially known as Tombaugh Regio. The speaker notes it also resembles the profile of Pluto the Disney dog. - Nerdy/Mythological references: Balrog (Lord of the Rings), Cthulhu, Tartarus.
Charon Surface Features: - While scientists named Pluto's features, the public influence on Charon led to heavy science fiction references. - Mordor: A dark region named after the land in Lord of the Rings. - Science Fiction Authors/Directors: Clarke Mons (Arthur C. Clarke) and Stanley Kubrick mountain. - Star Trek: Spock, Kirk, Uhura, and Sulu craters. - Chasms: Named TARDIS (Doctor Who), Macross, Serenity (Firefly), Argo, and Nostromo (Alien). - Star Wars: Vader Crater and Leia Organa Crater. - Alien: Ripley Craters within the Nostromo chasm.

The Kuiper Belt and Trans-Neptunian Populations

Structure: A relatively flat disc of icy objects discovered in $1992$ . It contains over $1,000$ identified individual objects.
Planet X Theory: The concentration of masses in certain areas of the belt suggests the potential existence of a ninth major planet (Planet X) influencing the belt gravitationally.
Classification of Kuiper Belt Objects (KBOs): - Plutinos: Objects that exist in a $2:3$ resonance with Neptune (meaning they orbit twice for every three Neptune orbits). These are concentrated around $40\,AU$ . - Classical KBOs (Cubewanos): Objects concentrated in a peak around $44\,AU$ to $45\,AU$ with no currently discovered resonance with Neptune. - Scattered Disc: A region where objects are spread out; it is the primary source region for short-period comets.

Profiles of Notable Dwarf Planets

Eris: Larger than Pluto and located in the scattered disc. It was originally nicknamed "Xena" and "Gabrielle" before being named after the goddess of discord. Eris's discovery was the primary catalyst for Pluto's reclassification.
Haumea: An egg-shaped object discovered at the Hawaii observatory. It was originally nicknamed the "Easter Bunny" due to its shape and moons appearing like "ears."
Makemake: Originally nicknamed "Santa Claus" due to its red color.
Sedna: One of the most distant objects found in the solar system that orbits the Sun.
Ceres: The largest object in the asteroid belt and is also classified as a dwarf planet.

Asteroids: The Inner Debris Belt

Location: Primarily located between the orbits of Mars ( $1.5\,AU$ ) and Jupiter ( $5.2\,AU$ ), specifically starting around $1.8\,AU$ .
Orbits: Generally circular with low eccentricity and periods under $5\,\text{years}$ .
Ceres: - The largest asteroid, nearly spherical and thought to be differentiated. - Features the Occator Crater, the largest on its surface, which suggests salt collections and organics. - Other craters include Urbara and Mutet (Egyptian names).
Trojan Asteroids: Specific asteroids that orbit within the orbital path of Jupiter, clustered "in front" and "behind" the planet.

Comets: Classification, Structure, and Origin

Structure: Comets are defined as small bodies that occasionally exhibit a tail. They are made of ice and dirt (often called "dirty snowballs"). - Nucleus: The solid center composed of water ice and carbon dioxide ice ( $CO_2$ ). The Giotto spacecraft captured the nucleus of Halley's Comet, revealing jets of material ejecting as ice turns into gas. - Coma: A hazy cloud of gas and dust surrounding the nucleus. - Dust Tail: Composed of debris and ice dust left behind on the comet's curved orbital path. - Ion Tail (Plasma Tail): Composed of particles pushed directly away from the Sun by solar wind. This tail always points away from the Sun. - Hydrogen Cloud: An invisible, hazy concentration of hydrogen protons surrounding the comet.
Tail Development: Comets do not develop tails until they get inside the orbit of Jupiter.
Classification by Orbital Period: - Jupiter-Family Comets: Orbital periods of $20\,\text{years}$ or less. - Short-Period Comets: Periods between $20$ and $200\,\text{years}$ (e.g., Halley’s Comet at $86\,\text{years}$ ). They originate in the Scattered Disc. - Long-Period Comets: Periods greater than $200\,\text{years}$ . They originate in the Oort Cloud.
The Oort Cloud: A spherical region surrounding the solar system at roughly $10^5\,AU$ ( $100,000\,AU$ ). It contains long-period comets that can enter the inner solar system from any angle.