Rotational Axis Orientation of Planets
Name: Bryan J.
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:
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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Update: June 2012