The Earth spins in a forward direction as a result of collisions with giant celestial bodies in the early days of the solar system, two astronomers reported Friday.
The other terrestrial, or solid, planets - Mercury, Venus and Mars - also spin as they do because of stochastic accretion, or the random coming together of a few large bodies, said Luke Dones, a visiting astronomer at the NASA Ames Research Center in California, and Prof. Scott Tremaine of the University of Toronto."The spins of all the terrestrial planets are entirely determined by these large impacts" coming within a few million years of when the solar system was formed 4.5 billion years ago, Tremaine said. Their conclusions were published in the weekly Science.
Dones said the Earth collided with a body possibly as large as Mars, causing its forward or prograde spin. By that astronomers mean it turns counterclockwise, as viewed from its north pole, the same direction the Earth orbits the sun, also from that view.
Dones said their study supports theories that the moon was formed in such a cataclysmic collision.
All the planets orbit in a forward direction, dating back to the origins of the solar system when it was a disc of dust and gas rotating in one direction. Two of the eight major planets, Venus and Uranus, have retrograde or backwards spins, and scientists have long sought a physical law to explain how that is decided.
The two scientists concluded that at least for the terrestrial planets, the spin is determined randomly, depending on the nature of primordial impacts.
Dones said it was "equally possible for a planet like Earth to form prograde or retrograde spin." Man may one day discover a solar system where most of the planets have retrograde spin, he said.
He noted that until the 1980s it was generally believed that the terrestrial planets formed in a smooth and orderly way when small bodies came together in a process called ordered accretion. Astronomers now tend to support the stochastic accretion theory.
Dones said their calculations determined "the only way to make the Earth and Mars spin as fast as they do is large impacts late in their formation process."