How Earth Began Orbiting the Sun

Q: How did Earth start rotating around the Sun, and how did the Moon begin rotating around the Earth? Will Earth ever escape the Sun's orbit? Was Earth in a different orbit long ago? Will it change in the future? Since the Sun is changing in size slightly, will Earth's orbit about the Sun change? Will Earth ever be pulled into the sun (given enough time)?
Terry, Albuquerque, NM

A: Earth started rotating about the Sun — and the Moon about Earth — because the elements that evolved into the Sun, Earth, and Moon were already spinning. Most astronomers believe the same process that forms stars, also creates planets and their moons.

Our solar system started as an interstellar cloud of gas and dust. The cloud (our beginning) was roughly spherical; it revolved around the center of the galaxy, and spun about its own center. All things in the Universe spin in some way, and the nebula cloud was no exception.

Under the force of gravity the cloud collapsed into a spinning disk, which eventually became the Sun and planets. Earth spins about the Sun because the primordial disk that evolved into the planets spun about a central mass, which gradually evolved into the Sun.

The Moon rotates about Earth as a result of its creation. A Mars-sized body slammed into Earth and ejected a plume of matter into orbit around Earth. The plume became a ring of rubble circling Earth, which coalesced into the Moon, which continued orbiting Earth.

Even an impact by a Mars-sized body failed to dislodge Earth from its orbit around the Sun, although the impact probably did change our axis of spin to the present 23.5 degrees to the plane of the solar system. To escape the Sun, Earth would require a speed of 11,000 mph (5 km/s) over its orbital speed of 67,000 mph (30 km/s). It's difficult to envision a force that could accelerate the mass of Earth to such a speed and therefore improbable Earth will ever escape the Sun.

Earth may have been in a slightly different orbit before the Mars-sized body hit it but is unlikely to change much in the future. Earth's orbit has been relatively stable for over four billion years with little sign of change in its predictable orbits for at least the next five billion years. Although the Sun is shrinking slightly, astronomers believe this is a temporary contraction and has negligible effect on Earth's orbit.

No, Earth will not fall into the Sun — not a chance because in five billion years the Sun will expand into a red giant. Instead of Earth falling into the Sun, the Sun will balloon out and perhaps engulf Earth, evaporating our world into a puff of silicate and iron smoke.

Further Surfing:

Space.com: Earth might have been a ringed planet, like Saturn

(Answered Sep. 27, 2002; updated on Aug. 28, 2007)