Physics 202
Intro to Astronomy:  Lecture #12
Prof. Dale E. Gary

Minor Bodies of the Solar System: Pluto, Moons, and Rings


Pluto was discovered in 1930 by Clyde Tombaugh, after a decades long search in vain by Percival Lowell. It has been known as the 9th planet of the solar system since that time, but perhaps we were a bit too hasty. Since then a large number of other objects that are very similar to Pluto have been found, and it is now recognized that these are part of the Kuiper Belt group of objects. In fact, there is a group of objects called Plutinos that have the same orbital characteristics as Pluto: they cross Neptune's orbit, and have a 3:2 resonance with Neptune.


All four of the giant planets have rings, although none are easily visible except the rings of Saturn. The rings probably have several different origins. Jupiter's faint rings are due to dust from meteroid strikes on small moons nearby. Saturn's rings are due to the breakup of a fairly large body (about 250 km in diameter) due to tidal forces. The faint rings of Uranus and Neptune may also be due to tidal disruption, but if so the bodies are much smaller. The rings of Uranus and Neptune are kept narrow by the action of shepherd moons, which "herd" the ring particles and keep them in a narrow range of orbits.

Note that the rings are not long-lived phenomena. Saturn's rings may not last more than a few hundred million years before the material eventually falls onto the planet.

High-sensitivity images of Jupiter's ring system, including the "Gossamer Ring", overlaid with the orbital positions of
the moons responsible for the rings.  The main ring is due to particles from Metis and Adrastea, while the outer parts
of the Gossamer Ring are due to Amalthea and Thebe.  Modified from a figure from NASA's photojournal web page.


Surface Feature Comparison