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Name: Bryan J.
Status: other
Grade: other
Location: N/A
Country: N/A
Date: 10/2/2005

If the planets formed out of a protoplanetary disk, and if their spin was imparted to them by the disk, why are most of the polar axes of the planets inclined to the plane of their orbits? In other words, why aren't all of the polar axes of the planets perfectly perpendicular to the plane of their orbits?

Few things in nature are "perfect". In the case of planet formation, remember that all the other matter in the disk are affecting all the others gravitationally. Also if you have a gyroscope (a 'model' for the protoplanetary disk) you will notice that it "precesses" about the "polar" axis. So deviations from perpendicularity are not surprising. The planet Uranus is almost rotates perpendicular than its plane of revolution. Presumably due to a "close encounter" eons ago, but I do not think anyone really is certain.

The web sites below provide a wealth of data about the planets and Sun for your interest and reading:

Vince Calder

Thanks for your question, Brian... There are two important issues to consider when attempting to rationalize why the rotational axes of the planets are not perpendicular to their orbital plane, nor to the celestial plane (which is defined as the plane perpendicular to the sun's rotational axis). First, a protoplanetary disk is, for obvious reasons, not a perfectly two-dimensional space, so planetary material is relegated to orbiting the sun (or primordial sun) in a volume of space and along a path which need not be in the same plane as that of other bodies of planetary material; this is, in fact, the case with the planets today, which orbit the sun along not only different paths, but completely different planes, each of which with its own unique inclination to the celestial plane. The uniqueness of these orbital planes means that the planets will experience different degrees of gravitational interaction with each other (not to mention natural satellites, comets, etc.), depending on when and where they are along their orbital path. So, you can imagine that the occasional tug here and there can cause a disturbance to the orientation of a planet's rotational axis.

Secondly, and perhaps even more importantly, we must consider the fact that planets form by the process of accretion, whereby masses of pre-planetary material catastrophically collide with one another until most of the material in the vicinity of their orbital path is accumulated into a single, large mass (i.e., a planet). Even after planetary formation is completed (which is, admittedly, something of an ambiguous point in the evolution of a planet), planets are subjected to impacts with potentially large bodies such as asteroids and comets. A significantly large impact may cause a planet to "tilt" on its rotational axis (you may want to read some articles about the formation of Earth's moon to illustrate this point). The rotational axis of Uranus, for example, is inclined by ~98 degrees to its orbital plane. Our understanding of celestial mechanics prevents us from expecting an undisturbed planet to develop a rotational axis inclined essentially parallel to its orbital path, so it's pretty safe to assume something must have collided with Uranus in the past to cause such an extreme tilt.

You may want to also perform an Internet search for information about Milankovitch cycles, which describe the way in which the orbital parameters of planets (including axial tilt) change regularly over time. Such changes are believed to bear heavily on climate, and are quite fascinating to read about (at least, from my viewpoint). I hope this explanation has helped.

Scott J. Badham
Department of Geology and Geophysics
University of Wyoming

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