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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 43:
Icy Worlds of the Outer Solar System

Key Ideas:

Neptune's giant moon
Young surface with cryovolcanism & geysers

Trans-Neptunian Dwarf Planets
Pluto, Eris, & Makemake
Very similar to Triton in their properties

Trans-Neptunian Objects:
Family of icy bodies orbiting beyond Neptune
Kuiper Belt Objects
Plutinos (3:2 Resonance with Neptune)
Scattered Disk Objects

Triton: Neptune's icy moon

Triton is the only giant moon of Neptune: Cold, icy surface: Triton has a geologically young surface with very few impact craters.

Cryovolcanoes & Geyers

The smooth plains are repaved by cryovolcanic flows: Geysers of N2 gas from the interior: Help feed the thin N2 atmosphere of Triton.

Origin of Triton

Triton is in a circular retrograde orbit around Neptune.

A collision model has been proposed to explain this:

Tides from Neptune melted the interior and eased it into its present circular orbit.

This tidal heating gives Triton is current-day geologic activity, and explains its young surface.


Pluto was discovered in 1930 by Clyde Tombaugh of the Lowell Observatory who was carrying out a search for a putative "Planet X" beyond Neptune.

It was immediately declared the "9th Planet"

Did not fit the pattern of the other 8 planets, but so far as anyone knew at the time, it was also unique in its orbit, and so it was still considered a planet.

Pluto is now officially designated a Dwarf Planet, and is the prototype of the "plutoid" subclass of trans-Neptunian dwarf planets.

Frozen Pluto

Pluto is a small, icy world:

Icy Surface & Thin Atmosphere:

All based on studies from the Earth, since no spacecraft have visited Pluto yet.

Pluto's Moons

Pluto has three moons:


Pluto & Charon Rotate & Revolve synchronously:

Nix & Hydra

Two small outer moons discovered in 2005 using the Hubble Space Telescope.

Eris: The largest Dwarf Planet

Discovered in August of 2005 by Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale).

The provisional name was 2003UB313 (the discovery team code-named it "Xena"), and it was considered for a time a candidate "10th planet". Its discovery ignited the 2006 debate over the definition of "planet", and was eventually designated as a dwarf planet and given the name Eris, appropriately the goddess of discord in Greek mythology.

Eris is larger than Pluto:

Orbits beyond Neptune: It also has a Pluto-like composition as revealed by spectroscopy, and it has 1 small moon, named Dysnomia. We know Eris' mass because we can measure the orbit of its moon Dysnomia (otherwise we'd only have rough estimates).

It is currently the largest Trans-Neptunian Object known, and bigger than Pluto.

The Icy Worlds

Pluto and Eris are now recognized as the largest members of a class of distant icy worlds: Most are found beyond the orbit of Neptune, and hence are refered to generically as Trans-Neptunian Objects or TNOs for short.

Trans-Neptunian Objects

A general class of icy bodies that orbit the Sun in the space beyond the orbit of Neptune:

Roughly divided into three sub-classes:

These classes are distinguished by the properties of their orbits.

Kuiper Belt Objects (KBOs)

Most Trans-Neptunian objects are in the Kuiper Belt:

First KBO was discovered in 1992:

Total Mass of the Kuiper Belt is estimated to be about 0.03MEarth, or about 2.5x the mass of the Moon.

Dynamical Classes

Trans-Neptunian Objects are distinguished by their orbits into dynamical classes: The latter, Scattered Disk Objects, may have once been part of the main Kuiper Belt, but have been scattered into high, elliptical orbits by gravitational interactions with Neptune, and are not really KBOs proper.

Resonant Objects: Plutinos & Twotinos

An important subset of the KBOs are the Resonant Objects, which are divided into two basic groups:
In 3:2 resonant orbits with Neptune (2 orbits for every 3 orbits of Neptune).
This resonance defines the inner edge of the Kuiper Belt.
Pluto is the largest Plutino

In 2:1 resonant orbits with Neptune (1 orbit for every 2 of Neptune)
This resonance is thought to defined the outer edge of the Kuiper Belt.
Currently, resonant objects, primarily Plutinos, comprise about 25% of known Trans-Neptunian objects.

The existance of the resonant objects is evidence of an outward migration of Neptune during the late phases of the formation of the Solar System. Current estimates are that during the last 10-100Myr (or so) of the last stages of the Solar System's formation, Neptune moved outward as much as 5 AU to its present orbit. As Neptune moved slowly outward, its gravity swept icy planetesimals and larger objects, like Pluto, into the 3:2 and 2:1 mean motion resonances, where they get trapped and migrate outward in lock-step with Neptune. Pluto would have started out in a circular orbit, but as Neptune swept it into a resonance and then outwards, orbital dynamics theory predicts that its eccentricity and inclination get "pumped up", giving it the large eccentricity we see today. This is analogous to what we saw in the previous lecture where Jupiter migrated inwards and swept up asteroids into resonances during the same late stages of Solar System formation.

Leftover Raw Materials

The icy Trans-Neptunian Objects are thought to be the leftover primordial material from the formation of the solar system. Understanding their properties is therefore of great interest to understanding the origin of the Solar System.

These objects will be the target of the New Horizons mission launched on 2006 January 19. So far the spacecraft is performing well, and is on-course for a Pluto Pluto fly-by in July 2015, followed by an extended mission to explore various Kuiper Belt Objects from 2016-2020.

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Updated: 2008 July 22
Copyright Richard W. Pogge, All Rights Reserved.