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Three planets and several moons revolve backwards (Talk.Origins)

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Response Article
This article (Three planets and several moons revolve backwards (Talk.Origins)) is a response to a rebuttal of a creationist claim published by Talk.Origins Archive under the title Index to Creationist Claims.

Claim CE260:

The hypothesis that the solar system formed from the collapse of a revolving nebula is contradicted by the fact that three planets and several moons revolve backwards.


CreationWiki response: It is interesting that Talk Origins ignores both moons with significant tilts to the their parent planet's equator, including Earth's own moon. (Talk.Origins quotes in blue)

1. The "backwards" planets and moons are in no way contrary to the nebular hypothesis. Part of the hypothesis is that the nebula of gas and dust would accrete into planetessimals. Catastrophic collisions between these would be part of planet building. Such collisions and other natural processes can account for the retrograde planets and moons.

The problem here is that catastrophic collisions tend to average out to little or no net effect on a planet's rotation or orbit over time, and one collision large enough to reverse a planet's spin would have to have more impacting energy than even a body in retrograde solar orbit could have unless it was large enough for the impact to destroy both planet and impacter.

The only moons that orbit retrograde are small asteroid-sized distant satellites of giant planets such as Jupiter and Saturn, plus Triton (Neptune's large moon) and Charon (Pluto's satellite). The small retrograde satellites of Jupiter and Saturn were probably asteroids captured by the giant planets long after formation of the solar system. It is actually easier to be captured into a retrograde orbit.

No disagreement here. However the presence of so many captured asteroids, particularly for Saturn, is most consistent with a system-wide disaster.

The Neptune system also contains one moon, Nereid, with a highly eccentric orbit. It appears that some sort of violent capture event may have taken place.

No disagreement here. However such a violent capture is most consistent with a system-wide disaster.

The Pluto-Charon system is orbiting approximately "on its side," technically retrograde, with tidally locked rotation. As these are small bodies in the outer solar system, and binaries are likely to have been formed through collisions or gravitational capture, this does not violate the nebular hypothesis.

Has this ever been modeled? At best these seem like unlikely events, particularly given Pluto's retrograde rotation.

Uranus is rotating more or less perpendicular to the plane of the ecliptic. This may be the result of an off-center collision between two protoplanets during formation.

The problem is that any collision powerful enough to do this would probably have shattered the protoplanets.

Venus is rotating retrograde but extremely slowly, with its axis almost exactly perpendicular to the plane of its orbit. The rotation of this planet may well have started out prograde, but solar and planetary tides acting on its dense atmosphere have been shown to be a likely cause of the present state of affairs. It is probably not a coincidence that at every inferior conjunction, Venus turns the same side toward Earth, as Earth is the planet that contributes most to tidal forces on Venus.

Has this ever been modeled? Admittedly this is their best one. It is at least plausible and not dependent on highly improbable planet shattering collisions.

2. Orbital motions account for 99.9% of the angular momentum of the solar system. A real evidential problem would be presented if some of the planets orbited the sun in the opposite direction to others, or in very different planes.

The universe is a big place and such a planet may yet be found, but more likely than not some implausible explanation will be proposed to save the nebular hypothesis.

However, all the planets orbit in the same direction, confirming the nebular hypothesis, and nearly in the same plane.

This fact was known when the nebular hypothesis was invented, so it confirms nothing about the nebular hypothesis.

A further confirmation comes from the composition of the giant planets, which are similar to the sun's composition of hydrogen and helium. Giant planets could hold on to all of their light elements, but small planets like Earth and Mars could not.

The sun is composed of 98.1% hydrogen and helium, that leaves 1.9% For the Earth to have started with the same composition it would have had to have started at 50 times its current mass, making it more than 2.5 times the mass of the gas giants Uranus and Neptune. So Earth would have had to have started out as a gas giant to have started with a sun like composition. The same can be said of Venus, while both Mars and Mercury would have had to start at more than twice Earth's mass. So this actually goes against the nebular hypothesis.

See Also