Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 39: The Outer Solar System

Key Ideas:

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; Scattered KBOs have long, elliptical orbits
First KBOs discovered in 1992: Many hundreds now known; ~70,000 objects > 100km across; Largest are >1000 km

Plutoinos ("little Plutos")

KBOs in a 3:2 orbital resonance with Neptune:: Complete 2 orbits for every 3 Neptune orbits; Comprise ~25% of Trans-Neptunian Objects
Orbits are similar to Pluto's: About the same period and semimajor axis; Different eccentricities, tilts, and orientations

Leftover Raw Materials

Trans-Neptunian Objects are the icy planetesimals leftover from the formation of the Solar System
New Horizon's Mission: Launched January 2007; July 14, 2015 Pluto fly-by; Explore Kuiper Belt 2016-2020

Frozen Pluto

Pluto is a cold, icy world:: 1/1600th the sunlight received by Earth; Temperature of 35-45 K; Density is ~2g/cc with a rocky core and icy mantle
Thin atmosphere:: Surface covered with frozen N₂ mixed with CH₄ and traces of CO and H₂O.; Thin N₂ atmosphere
Moons:: Pluto and Charon are tidally locked in a 1:1 tidal resonance; Hydra and Nix are two new, smaller moons of Pluto discovered in 2005

How big is Eris?

Too small to measure angular size
Instead, the size is estimated based on reflectivity: Perfect mirror? Albedo = 100% and Size = 2210km; Fresh snow? Albedo = 90% and Size = 2330km; Antarctica? Albedo = 80% and Size = 2475km; Pluto-like? Albedo = 60% and Size = 2860km; Charon? Albedo = 38% and Size = 3550km
The lower the albedo, the large Eris must be to reflect the same (observed) amount of light

Comets

Small chunks of ices and dust
As they approach the Sun:: The ices sublimate into gas; Gas and dust are swept by sunlight into a luminous tail
Many faint comets per year: Bright, naked-eye comets every 10 years or so; Comet McNaught is the "Great Comet of 2007"
Comets are "dirty snowballs" that have fallen in from the outer solar system

Structure of Comets

Nucleus:: Dirty snowball of ices and dust a few kilometers across; Contains >99% the mass of the comet; Source of the gas and dust in the coma and tails; Very low density ~0.2 g/cc; Dark and heavily cratered
Coma:: Low-density cloud of gas and dust sublimed off the nucleus; >100,000 km in size

Comet Tails

Dust Tail:: Dust particles swept back by sunlight pressure; 1-10 million km long; White color is reflected sunlight
Ion Tail:: Ionized atoms and molecules swept back by the solar wind; 100 million km long; Blue color is from CO⁺ emission lines

Comet Sample Return

STARDUST - NASA Mission to return material from a comet: Launched February 7, 1999; Collected samples from the tail of "Wild 2"; Returned to Earth on January 15, 2006; Discovered many organic compounds and silicates

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