The Outer Planets

The Kuiper Belt: Beyond Neptune

• Introduction
• Breaking Up: The Roche Limit
• Rings, Shepherd Moons, and the Roche Limit
• Kuiper Belt Objects
• Discussion Questions
• Optional Website Reading

Introduction

Breaking Up: The Roche Limit

The rings of the planet Saturn were first discerned around 1655 by the Dutch astronomer and physicist Christiaan Huygens, who used a telescope to resolve the oval shape of the planet into two parts: the more usual planetary sphere, and what appeared to be a flat ring surrounding it in its equatorial plane. Over time, as the planet processed (wobbled) on its axis, the rings displayed themselves at different angles.

About 20 years later, Giovanni Cassini discerned a gap within the rings. For nearly 2 centuries astronomers debated whether these rings were solid ringlets are composed of individual particles. In 1859, James clerk Maxwell (the same Maxwell who discovered the laws of electromagnetism) demonstrated that the rings would not be solid because they would become unstable and break apart.

Exploration in the 20th century with both the Earth-based and satellite-based telescopes has confirmed Maxwell's theory: the rings of Saturn are made of small particles shepherded into distinct orbits by the gravitational influences of the planet and its many moons. The gaps represent areas where opposing gravitational forces "sweep" the area clean.

The Voyager 2 spacecraft flyby of Jupiter led to the unexpected discovery that Jupiter also has rings. Jupiter's rings are shepherded inside and out by small moons, two of which, Metis and Adrastea, are actually on the planet-side within the rings -- and within Jupiter's Roche limit. One theory is that the inner rings of Jupiter system are composed of material being pulled off of the two moons by tidal forces from the planet below.

The discovery of Jupiter's rings let earth-based observers to look for rings on the two remaining gas giants. Although they were not successful in observing rings for Neptune from Earth, when astronomers tried to use the planetary occultation of start SAO 158687 by Uranus to study the planet's atmosphere, they also observed the periodic dimming of the star five times as the planet approached and passed the star. Using the data for the times of SAO 158687's disappearances, the astronomers were able to calculate the distance of the rings from the surface of Uranus. Subsequent observations by the Hubble orbiting telescope and the Keck ground-based telescope led to the discovery of a very complex ring system around Uranus.

during the Voyager twos fly by an oath Neptune, astronomers deliberately sought for evidence of The ring system for Neptune and found .

Rings, shepherd moons, and the Roche limit

Obviously, ring systems are "normal" for large planets orbiting in the outer reaches of the solar system. The question that arises is how do they form? Are they the result of material left out of the collapse into the planetesimal, or are they captured later? Or do both situations occur (most likely!)?

Observations of the moons Prometheus and Pandora orbiting just inside and outside of Saturn's F rings give us some idea of the forces at play. Notice how ripples form on both the inside and outside rings as the two moons pass by. These shockwaves through the rings are the result of a three-way gravitational pull on each particle in the ring: pulls towards each of the two moons and pulls towards the center of Jupiter's mass.

Gravitational forces that act unevenly on a single body are called tidal forces. A single massive body can exert tidal forces on a smaller orbiting body by attracting the near side of the orbiting body more strongly than the far side. The result is a net force inside the body that distorts it if it is pliable enough. If it is rigid, and the force difference is sufficiently large, tidal forces will pull the orbiting body apart. Since the force due to gravity from the planet depends on the mass of the planet and the distance of the orbiting body from the planet (F = GMm/r², the tidal force difference depends on planetary mass and distance from the planet as well. Within a particular distance (different for each planetary mass), any object within the limiting distance or Roche limit will be pulled apart by tidal forces. Outside this limit, the orbiting body will be able to stay together — unless there are other orbiting bodies (other moons) to create a tug-of-war of tidal forces.

We can calculate the Roche limit as a function of primary planetary mass and the orbiting bodies distance from its center.

The interplay of gravitational tidal forces between multiple moons, debris in rings, and their primary planets gives us a current-day vision of the interplay of forces acting during the formation of the solar system, between the planets, the remaining materials in the condensing stellar cloud, and the sun itself. By studying the formation of ring systems, we get a better idea of how planetary systems can form. One of our conclusions is that the formation of planets around stars is a normal part of stellar formation. Our planetary system is not an exception to the rule. If our theories of planetary formation and ring formation are correct, then most stars will have planetary systems.

Kuiper Belt Objects

The objects of the Kuiper belt appear to share common composition. They are planetesimals made up of rocks and ice. In contrast to the asteroids of the internal solar system, what are primarily rocks.

Pluto and Charon	Eris	Haumea
Naked Eye Observation
None of these objects are visible to the naked eye.
Exploration
While Pioneer 1, and both Voyager spacecraft have passed through the Kuiper belt, few pictures of Kuiper objects were sent back to Earth. The project New Horizons, launched in 2006, reached Pluto in July, 2015. Depending on its orbit at that time, New Horizons may be given instructions to pass by other worlds of the Kuiper belt.
Orbital and Rotational Characteristics
Computer simulations starting with a more or less even distribution of asteroid like objects beyond Jupiter result in ejection of these objects to orbits beyond that of Neptune as a result of perturbations in the orbits of Jupiter, Saturn, Uranus, and Neptune. These perturbations resulted in a circular orbits, but many with high inclinations, for the objects of the Kuiper belt.
Eccentricity: 0.2488 Orbital inclination: 17.16° Rotation: 6.4d (retrograde!)	Eccentricity: 0.44177 Orbital inclination: 44.187 Rotation: 8hrs?	Eccentricity: 0.1887 Orbital inclination: 28.19 Rotation: 3.912 hrs
Magnetosphere
These worlds are, so far as we can tell, geologically "dead", and without a liquid metal layer, cannot produce magnetic fields. Measurements by New Horizons may confirm this assumption, or surprise us.
Atmosphere
No Kuiper belt objects appear to be large enough to hold atmospheres.
Surface
Because these dwarf planets have only been observed using Earth-based telescopes, we have very little information about their surface structures. At the moment, most astronomers assumed that they are similar to Triton: geologically dead, and heavily cratered.
Core
If our assumption that these objects are geologically dead is correct, then they are likely to be primarily conglomerations of rock and ice, with a concentration of rock at the core, overlaid by layers of rock mixed with ices. Because these materials are less rigid than the more rocky structures of the terrestrial planets, the high rotational velocity of Haumea has forced it into an ellipsoid shape.
Planetary formation
Several theories about the origin of the quicker projects have been proposed. One is that the objects formed closer to the center of the solar system, and were ejected to their current locations during interaction with the gravitational fields of the gas giants. Other theories assume that the objects formed where they are from leftover materials that fail to condense to the center of the solar system.

Discussion Questions

• How does using the physics of gravity help us understand the formation (or lack of formation) of planets and rings, and the location of clusters of moons?
• What aspects of the formation of the solar system are controlled by the Roche limit?
• Why is the composition of objects in the Kuiper belt different from the composition of objects closer to the Sun?

Optional Web Reading

• Check out the NASA page on the New Horizon's mission for the latest information on exploration of the Kuiper Belt.

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