ASTR 1210 (O'Connell) Study Guide 20

Outer Plan Title

Jupiter with Io and Europa in foreground
(Voyager Mission image)

"And now for something completely different," as they used to say on Monty Python.

The large "Jovian" planets (Jupiter, Saturn, Uranus, and Neptune) are entirely unlike the terrestrial planets (see more discussion in Study Guide 11). They may have rocky cores, like larger versions of the Earth, at their centers, but these are enveloped in giant gaseous atmospheres. Only the outermost skins of these atmospheres can be studied directly. This is meteorology, instead of the geology/topography we discussed for the terrestrials. However, it can be just as extreme with respect to Earth-bound meteorology as are the canyons and mountains of Mars compared to those of Earth.

Another major distinction of the Jovians is the large number of satellites they possess. The satellites, observed at close range by spacecraft, exhibit an astonishing diversity of surface types and features. Unlike the three terrestrial planet satellites, the larger Jovian satellites are rich in water ice and exhibit many different phenomena as a consequence. In many ways, they are more interesting than their parent planets. They may even harbor biospheres. The ring systems, which are present around all 4 Jovians, are probably the remnants of distintegrated satellites.

Many examples of a third kind of planet have recently been discovered outside the orbit of Neptune. These are perhaps most aptly called the "ice dwarfs," of which Pluto is the archetype.

A. History of Discovery/Exploration

B. Jovian Planets (J,S,U,N): Properties

These four share gross properties. Pluto is entirely different (see below).

Distant from Sun: 5-30 AU. (Pluto is at 39 AU.) The outer solar system is vast (over 10,000 times the volume of the inner solar system out to Mars) and sparsely populated.

Large: Radii are 4-11 x Earth. Masses are 15(U)-318(J) x Earth. J. contains twice as much mass as all other planets combined. An animated timelapse image of Jupiter's rotation and surface features is shown at the right. Click on the image for a more current, high resolution HD video.


Visible Surfaces

Special Probes of Jupiter

Magnetic Fields:

Pseudo-color infrared image of Saturn

C. Ring Systems

Saturn's Rings Saturn has the brightest rings, but rings are present around all 4 Jovians

Spacecraft images of the four Galilean satellites of Jupiter
(Io, Europa, Ganymede, and Callisto).
Each is a unique world in its own right.

D. The Jovian Satellites

Numerous: 14-67

Diverse(!) characteristics; often violent histories

Larger moons are mixtures of rocky/icy materials

Smaller moons, e.g. Hyperion (Saturn), are irregular in shape

Interesting examples: (click on the names for additional illustrations)

Left: Enceladus; Center: water vapor plume from Enceladus; Right: possible internal structure of Enceladus

Artist's Concept of Huygens Probe Landing On Titan

E. Pluto and the Kuiper Belt

Pluto is entirely unlike the four large outer planets. It is smaller by a factor of 2 than any of the other 8 planets. It is a rocky/icy object rather than a gas giant. Its orbit is the most highly inclined to the ecliptic plane of any of the classical "9 planets."

When first discovered, Pluto was thought to be isolated at the edge of the Solar System. However, in the last 20 years, astronomers have discovered many more such bodies, some with sizes comparable to Pluto. These are all members of the "Kuiper Belt".

These discoveries, particularly that of Eris, precipitated the messy discussion at the International Astronomical Union in the summer of 2006. Astronomers held a debate over the meaning of the term "planet"---specifically whether or not Pluto and the other large KBO's should be placed in a separate category. In the end, the IAU voted to create a new category of "dwarf planet" for these objects but was then forced to add the asteroid Ceres for consistency. All this was handled very clumsily, and it generated needless controversy. It turns out many non-astronomers were fond of Planet Pluto.

Even before the discovery of Pluto, we had already known of many small, rocky objects in separate orbits around the Sun---the "asteroids." Now, we know about many similar, but icy, objects. Sensible designations for these types, above some threshold in size, are as "rock dwarf planets" and "ice dwarf planets."

New Horizons, the first mission to Pluto and the Kuiper Belt, was launched in 2006 and, having received a gravity assist from Jupiter, is now approaching Pluto with a planned flyby date of 14 July 2015. The spacecraft was originally intended to fly within 6500 miles of Pluto's surface, but the discovery of a total of 5 moons in orbit around Pluto has raised concerns about an invisible ring system or debris field near Pluto, and the trajectory may have to be adjusted. Following the flyby, New Horizons will be retargeted to approach other Kuiper Belt Objects, assuming good candidates can be found within its limited range of maneuver.

Images of Pluto obtained by New Horizons in April and May, 2015.
Quality will improve greatly toward encounter on 14 July 2015.

Reading for this lecture:

Reading for next lecture:

Web Links:

Previous Guide Guide Index Next Guide

Last modified June 2015 by rwo

Cross section drawing copyright © Pearson Education. Text copyright © 1998-2015 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1210 at the University of Virginia.