Physics 320
Astrophysics I:  Lecture #20
Prof. Dale E. Gary

Pluto, Kuiper Belt

A. Kuiper Belt Objects KBOs

Recently, increasing numbers of asteroid-like objects have been discovered in the outskirts of the solar system, beyond the orbit of Neptune. This region is known as the Kuiper Belt.  These objects are classed in a separate category from asteroids, and are simply called Kuiper Belt Objects (KBOs).  So we should restrict the term asteroid to be rocky bodies with orbits having semi-major axes between Mars and Jupiter, the so-called Asteroid Belt, or perhaps some other objects that have a dynamical relationship to these--i.e. they may have once been part of the asteroid belt but have been ejected into a different orbit.

Note that there is a huge gap (from inside Jupiter's orbit to outside Neptune's orbit) where there are few if any objects between the Asteroid Belt and the Kuiper Belt.  So these two distributions are well separated, and the composition of the bodies is also very different. Here is the distribution of KBOs and comets in the outer solar system. Note that this plot is for TODAY! The orbits are tracked daily, and can be animated. This is to be compared with the distribution of main-belt asteroids. Again, an animated version is available.

We will talk about asteroids in the next lecture, but note that the scheme of asteroid designations: (numbered in order of discovery, followed by name): e.g. 1 Ceres, 951 Gaspra, is shared by the Kuiper Belt Objects, e.g. 2060 Chiron, 7066 Nessus.  Both asteroids and KBOs can be referred to under the common name Minor Planet, with the larger ones (large enough to be spherical) termed Dwarf Planet.

Here is an artist's conception of the largest known KBOs, showing their relative sizes and known moons. Note that Pluto, once considered a planet, is now known to be one of the KBOs, and is officially designated a dwarf planet. Some people, Americans in particular, are upset about the "demotion."

Below is a figure showing the relative sizes of some of the objects of the solar system, including Pluto at the lower right:

Interestingly, KBOs show evidence of resonances, where they cluster around at least the 3:2 resonance instead of avoiding it.  What planet are they in resonance with?

One explanation that has been put forth is that objects in this 3:2 resonance never come near Neptune at their closest approach (see figure below) and play with Neptune/Pluto orbit.  [Note, turn on Neptune and Pluto orbits, change time step to 3 months or more, and run the clock forward and backward to see the planets move through one complete Pluto orbit.]

This shows two consecutive perihelions for Pluto.  In the first, Neptune is far from Pluto.
After one Pluto orbit, Neptune has orbited 1.5 times and is again far from Pluto.  On the
next orbit, Neptune and Pluto will again be in the configuration on the left, so Pluto never
comes near Neptune.  An object in any other orbital relationship would eventually
(perhaps after thousands or millions of orbits) have the object come near Neptune and
be disrupted.  It is likely that the Oort cloud is populated mostly by objects that were
ejected into the far reaches of the solar system by Uranus and Neptune during the early
history of the solar system.

The dynamical families of KBOs are:

B. Pluto

Pluto is interesting because of its history as the first-discovered KBO, and its designation as a planet. It also has an interesting prehistory (the time before it was discovered). Starting in 1906, Percival Lowell began a concerted effort to find "Planet X," a hypothetical planet beyond Neptune. The location to search was based on calculations of the orbit of Neptune that seemed to suggest a perturbation by some unknown body. Recall that Neptune was discovered after similar calculations of the perturbations of Uranus, so the idea was plausible. After a 10-year search, Percival Lowell came up empty and died without discovering Planet X. However, in 1930 his protege Clyde Tombaugh discovered Pluto near the location for Planet X. This discovery made huge news world-wide, and a school girl in Britain suggested the name Pluto, which Tombaugh liked because its first two letters, PL, are the initials of Percival Lowell. As the only planet discovered by an American, Pluto quickly became part of the American psyche, helped along by Walt Disney's creation of Mickey Mouse's lovable pet dog Pluto.

Unfortunately, it was quickly determined that Pluto was much too small to be Planet X--it could not be the source of the apparent perturbations of Neptune. In the end, it has been determined that these perturbations were simply errors in the original mass for Neptune, and with the precise value determined during the flyby of Voyager 2 past Neptune, the evidence for a planet X has disappeared. Nevertheless, Pluto was considered the ninth planet until the discovery of other KBOs caused murmurs of discontent among some scientists. The issue came to a head when Eris was discovered in 2005 by Mike Brown. Because it appeared to be larger than Pluto, Eris was informally called the 10th planet, but this motivated the International Astronomical Union to face the issue of Pluto and KBOs. The problem is that it has been predicted that there may be hundreds of Pluto-sized KBOs out there, most still undiscovered. We could either have hundreds of planets, or Pluto could be demoted to be a member of a different club, the so-called dwarf planets. In 2006, in a rather controversial vote on the issue, an IAU resolution did create a definition for the term planet that did not include Pluto, and so demoted Pluto to dwarf planet status. There are deficiencies in the definition that make it still controversial--for one thing, the definition only applies to planets around our own Sun, not other stars. There are currently 5 known dwarf planets, Eris, Pluto, Ceres (an asteroid), Haumea, and Makemake.

Note that this same sequence of events occurred in the 19th century after Ceres, the largest main-belt asteroid, was discovered on Jan 1, 1801 by Giuseppe Piazzi. It, too, was thought to be a planet until its small size was determined and the discovery was quickly followed by several more asteroids (Pallas, Juno, Vesta) in a similar orbit. Eventually it was decided that Ceres was just the largest of a new class of bodies called asteroids (which we'll talk about next lecture). So the case of Pluto is highly parallel, except that the time between Pluto's discovery in 1930 and the discovery of other KBOs was longer.

Pluto's demotion is also complicated by the fact that it occurred after the launch of the New Horizons mission to Pluto, which arrived and flew through the system in 2015. I call Pluto a system, because it is known by now to have at least 4 moons (the largest, Charon, is so large than the Pluto-Charon system has been called a "double-planet" system). Below are some images documenting the discoveries of the moons, one after the other. Check out these videos from New Horizons:

The best groundbased photo of the "double-planet" Pluto and Charon, and an early HST image is shown at left. The middle photo shows HST's discovery of two additional moons, Nix and Hydra. Then on the right is the most recent HST image showing yet a fourth moon, so far designated P4. With so many moons, there is a concern that there may be a debris field that is dangerous to the enroute New Horizons spacecraft.

Here is a diagram of the New Horizons mission's path to Pluto and the Kuiper Belt.