Planet

The Dune Encyclopedia

This article or section refers to elements that appear exclusively in The Dune Encyclopedia.

A planet is a celestial body in orbit around a star, but neither a star nor a moon, and had cleared out the area around its orbit as it formed by causing all other smaller bodies in its orbit to accrete with it. As a consequence, it will not share its orbital region with any other bodies of significant size, except for moons or those collected later under its gravitational influence.

Depending on the tilt of the planet's axis, planets also have varied seasons with ranging temperatures and weather. Planets are extremely varied in terms of environmental conditions and landscape.

Formation

The chemical composition of a planet depends upon its distance from the main star. Some planets consist primarily of: silicates, hydrogen (quasi-stars), or sulfides, while some are mostly metallic.

The Neta class is defined specifically as comprising those planets with a composition of 60 ± 10% silicates (by mass), 30 ± 10% metals, 10 ± 5% sulfides and less than 5% hydrogen.

Ancient views

The ancient Terrans held a simplistic view of planetary genesis, still accepted later in most respects:

During or after stellar formation accretion of remaining smaller fragments (dust, gas and ices) are accreted and form a planetary body. As its mass grows, its interior warms through gravitational contraction with heat released by radioactive decay. The surfaces and interiors of Neta-class planets soon becomes molten. After about one eon the exterior cools sufficiently to form a solid crust. Convection in its molten interior causes breakup and formation of new crust (compare with scum floating on the surface of a bubbling pot of molten fanmetal) which in turn cools down. This results to a thick crust and stable to be a basically permanent feature about 1.5 eons after formation. The interior comes to a quasi-molten state.

The heat generated by radioactive decay continues to cause a slow convection and in turn on-going disturbances within the crustal material, such as volcanic eruptions, quakes and movement of portions of the crust. These "plate tectonics" were ascribed to heat energy from radioactivity.

However, of the four Neta-class planets in the Sol planetary system only one exhibits plate tectonics behavior.

Bnaceret who had summarized his contemporary understanding, wrote that heat radioactive decay is insufficient to cause plate movements; internal heat is not necessary, although in all cases internal heating results from action of the mechanism primarily responsible for plate motions.

Later theories

Two mechanisms are known to be the actual cause of the plate movement. In some cases both mechanisms play a major role.

• Convection currents result from severe internal heating which comes in part from radioactive decay but mostly from neutrinos emitted by the host star. Neutrinos have no mass or charge and most will pass through a planet without interaction, but the neutrinos absorbed cause the heating. Hence only when the neutrino flux is very large (produced by very few stars) the required severe internal heating occurs. Only a few planets (none of which exist in the Sol planetary system) have convection currents as their sole mechanism for plate tectonics.

• External gravitational torques are caused by two factors:

1. adjacent planet(s) with mass considerably larger and/or one or more moons with mass of an appreciable fraction of the host planet, and
2. orbits of the adjacent planet(s) and/or the moon(s) which lie appreciably outside the ecliptic plane of the planet, and are preferably eccentric.

The resulting gravitational torques generate large stresses and in turn crustal movement with associated tectonic phenomena. The planet is heated internally by friction, but this heating is secondary to the entire process. This mechanism is the dominant for most planets exhibiting plate tectonics behavior.