ASTR 1210 (O'Connell) Study Guide 20

Outer Plan Title

Jupiter with erupting Io in foreground
(New Horizons Mission image)

"And now for something completely different," as they used to say on Monty Python. The outer solar system is different indeed.

The outer solar system we can now explore (out to about 50 AU) is vast -- over 35,000 times the volume of the terrestrial realm out to Mars -- and sparsely populated.

The four 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, display 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. 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 dwarf planets," of which Pluto is the archetype.

A. History of Exploration Beyond Mars

Spacecraft images of the four Jovian planets (from left: Jupiter, Saturn, Uranus, Neptune),
scaled to their correct relative sizes (but not distances from the Sun).

B. Jovian Planets: Properties

The four Jovian planets share gross properties. Pluto is entirely different (see below).

Distant from Sun: 5-30 AU. (Pluto is at 39 AU.)

Large: The Jovians are 4-11 times larger in diameter than Earth. Jupiter's volume is 1300 times the Earth's. Masses are 15(U)-318(J) times Earth's. Jupiter 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 recent, high resolution video.


Visible Surfaces

Special Probes of Jupiter


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, shown to scale
(Io, Europa, Ganymede, and Callisto).

D. The Jovian Planet Satellites

In many ways, the satellites of the Jovian planets are more interesting than the planets themselves. The four largest satellites of Jupiter (the "Galilean satellites") were discovered by Galileo and were the first additions to the planetary inventory of the Solar System in recorded history. Galileo saw them only as points of light, and good information on their surfaces was not obtained until the spacecraft flyby missions of the 1970's and 80's.

The composite picture above shows spacecraft images of the four Galilean satellites (scaled to the correct relative sizes). It illustrates the outstanding feature of these four satellites: they are amazingly different from one another. Each is a unique world in its own right. They are typical of all Jovian satellites in that they are diverse, and they often have had violent histories.

Here are some of the other characteristics of the Jovian satellites: Important examples of satellites: (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. With a diameter of only 1475 miles, it is smaller by a factor of 2 than any of the terrestrial or Jovian 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 25 years, after wide-field digital detectors were installed on large telescopes, astronomers have identified many more similar bodies, some with sizes comparable to Pluto. These, including Pluto, 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 and protested the demotion.

Long before the discovery of Pluto, we had already identified many "minor planets" or "asteroids," small, rocky objects with orbits lying mostly between the orbits of Mars and Jupiter. Now, we know about many similar, but icy, objects beyond Neptune. Sensible designations for these types, above some threshold in size, are as "rock dwarf planets" and "ice dwarf planets," respectively.

The Pluto Flyby

New Horizons, the first mission to Pluto and the Kuiper Belt, was launched by NASA in 2006 and, having received a gravity assist from Jupiter, finally reached Pluto on 14 July 2015. (Yes, nine years later. The fact that New Horizons was launched with the brisk velocity of over 36,000 miles per hour gives you some appreciation for the scale of the solar system.) The spacecraft could not carry enough fuel to decelerate into orbit around Pluto (that would have made it much more massive and expensive), so it was always planned as a "flyby," with a closest approach of about 7800 miles. It took over 15 months for all the data collected during the brief flyby to be telemetered back to Earth.

The Arrokoth Flyby

Following the Pluto encounter, New Horizons was retargeted to fly by another KBO,
2014 MU69M, which was located in a special 2014 search for objects near to the spacecraft's outgoing trajectory.

New Horizons performed a highly successful flyby of this second KBO, now officially named "Arrokoth," on 1 January 2019. Data taken will be streaming back to Earth through mid-2020. But it was immediately clear that Arrokoth is a fascinating object. It is a binary, with two lobes joined at a narrow neck; the largest lobe is about 22km across. Both lobes are significantly flattened. The reddish color (also conspicuous on Pluto and Charon) is caused by tholins, organic compounds produced by solar UV irradiation of carbon-based materials like methane and carbon dioxide.

Arrokoth as imaged by New Horizons

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Last modified January 2024 by rwo

Cross section drawings copyright © Pearson Education. Text copyright © 1998-2024 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.