Bad Astronomy: Misconceptions and Misuses Revealed, from ...

IN THE BEGINNING 195
it’s natural—and makes good scientific sense—that some don’t
anymore.
All planets should spin in the same direction, but Venus, Uranus,
and Pluto all rotate backwards.
According to the collapsing-cloud theory, the planets should all
spin in the same direction in which they orbit the Sun because the
initial disk spun that way. Anything forming in that disk should
spin in the same direction. However, Venus rotates backwards, and
Uranus rotates on its side! How can the disk theory explain that?
Actually, the answer is simple: it doesn’t. The disk theory concerns
only how the planets formed and not necessarily how they
look today. A lot can happen in 4.55 billion years. In this case, collisions
happen.
We know for a fact that cosmic collisions occur. We had repeated
graphic examples of this in July 1994, when the comet
Shoemaker-Levy 9 broke into dozens of pieces and slammed repeatedly
into Jupiter, releasing more energy than could humankind’s
entire nuclear arsenal. Had the comet hit Earth instead of Jupiter,
it would have been a catastrophe of, well, biblical proportions.
Humanity, along with 95% of the land animals on Earth, would
almost certainly have been wiped out.
And even this collision is small potatoes. In the early past, when
the disk was forming into planets, gravitational interactions would
have been common. Two planets forming too closely together
would affect each others’ orbits, and the smaller one might actually
get flung into a wildly different orbit. This orbit could have sent it
on a collision course with another planet. An off-center, grazing
collision could physically tilt a planet, changing the axis of rotation,
in much the same way that poking a spinning top off-center
causes the axis to wobble.
In the case of Uranus, a large collision is what most likely
knocked it on its side. For poor Venus, whatever collided with it
knocked it almost completely heels-over-head. To us it looks like
Venus is upside-down and spinning backwards.