Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 40: Outer Solar System

Key Ideas:

Outer Dwarf Planets: Pluto and its moons; Eris and Dysnomia; New Dwarf Planets
Trans-Neptunian Objects: Kuiper Belt and Resonances; Scattered Disk; Oort Cloud

Frozen Pluto

Pluto is a cold, icy world: Temperature: 35-45K; Density is about 2 g/cc with a rocky core and an icy mantle; Surface covered with frozen N₂ mixed with methane and traces of CO₂ and water; Thin N₂ atmosphere
New Horizons mission to Pluto: Launched January 19, 2006; Pluto fly-by on July 14, 2015; Explore the Kuiper Belt from 2016-2020

Eris

New Dwarf Planet that ignited the Pluto controversy: Slightly larger than Pluto; One large moon Dysnomia
How big is Eris?: Too small to measure the angular size; Size estimates are based on reflectivity; If a perfect mirror, about as large as Pluto; If as reflective as Pluto, it is 25 percent larger; If as reflective as Charon, it is 50 percent larger

Remaining Dwarf Planets

What is a Dwarf Planet?

A Celestial Body that: 1. Is in orbit around the Sun; 2. Must be in hydrostatic equilibrium due to its mass (i.e. round); 3. Has not cleared the neighborhood around its orbit; 4. Is not a satellite

Trans-Neptunian Objects

Class of icy bodies that orbit beyond Neptune
Divided into various sub-classes:: Kuiper Belt Objects; Scattered Disk Objects; Oort Cloud; Distinguished by their orbits

Kuiper Belt Objects (KBOs)

Most Trans-Neptunian Objects are found in the Kuiper Belt: Flattened region 30-50 AU from the Sun; Split into those in and not in orbital resonance with Neptune
First KBO discovered in 1992: Many hundreds now known; Estimates are that there are about 70,000 objects larger than 100km across; Largest are greater than 1000 km

Plutinos and Twotinos

Plutinos (little Plutos) are KBOs in a 3:2 orbital resonance with Neptune: Complete 2 orbits for every 3 by Neptune; Comprise ~25% of Trans-Neptunian Objects; About the same period and semimajor axis as Pluto, but different eccentricities and orbital inclinations
Twotinos are similar, but in 2:1 orbital resonance with Neptune

Scattered Disk

Oort Cloud

Existence of long-period comets (>200 year) indicates a reservoir of icy bodies in the outer solar system
In 1950 Jan Oort noted there were many comets with a~20,000 AU and their orbital inclinations were spherically distributed (not in the ecliptic plane)
Up to billions of potential comet nuclei may be there
Possibily composed of the primordial building blocks flung to the outer solar system by the giant planets
Composition of long-period comets suggest primarily ices, although there may be some rocky bodies
May extend to 50,000 AU from the Sun (nearly a light year)
Total mass of the Oort cloud may be several times that of the Earth

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.