Dear Maria, Elise and Julia,

We do not know exactly why the spin axis of the Earth is tilted by approximately 23 degrees from the normal direction of the plane of the Earth's orbit around the Sun. No planet has the same angle as that of the earth: Mercury, 0 degrees; Venus, 177 degrees; Mars, 24 degrees; Jupiter, 3 degrees; Saturn, 27 degrees; Uranus, 98 degrees; Neptune, 30 degrees; Pluto, 118 degrees. The angle larger than 90 degrees indicates that the spin direction is opposite to the orbital direction. In other words, if the planet moves clockwise around the sun, it spins counter-clockwise on its axis.

To understand how the spin of a planet is determined, you have to go back to the time when the solar system was born, four and a half billion years ago. As any gas cloud in this universe, the gas cloud from which our solar system was born was whirling or spinning around. When the gravitational force is larger than the gas pressure, the whirling cloud begins to collapse into a pancake shape, with the Sun spinning at the center. The spin of the Sun is determined by the whirling direction of the original gas.

The pancake contains clumps of dense gas and dust. Dust particles in the clumps collide and collect other dust and gas particles, and grow in size. As they grow, larger particles eat up smaller ones and grow even more until they form into planets. During this process of growth, the spin of each planet is determined by the captured particles. Imagine you are sitting on a stool, which may freely rotate, while extending your arms. Now your sister throws balls at you mostly to your right hand. You are set into a spinning motion by these balls. A planet gains spin by capturing the dust and particles, which collide with it. In this way the spin keeps changing until no more particles and dust are left in the neighborhood. So, the spin direction is determined by these dust and particles, which collide with the planet. We do not know the details of the condition around the clumps from which the planets are formed, and hence we cannot tell the final spin direction.

There is another complication, which may change the direction of the spin even after most of the collisions are done. It is possible that another clump smaller than the planet runs into the planet, and changes its spin direction. The possibility for such an encounter is small, but possible. As a matter of fact, the standard model for the birth of our Moon involves the collision of another object with the early Earth.

Thanks for your question,
Aubie and Dr. Hinata