The wild years that created the Oort Cloud

Back when the Sun had just formed and the planets were gathering up the leftover gas and dust, things were crazy. Collisions were frequent. Blocks of rock and ice were either destroyed, or they coalesced with other hunks and chunks and made asteroids, planets or comets. Closer to the Sun, rocky planets and asteroids were born.

Farther out, the gas giant planets developed. And frigid temperatures allowed huge chunks of ice to condense. Over time, the gravity of the massive planets kicked the comets every which way.

Two main regions of comets developed.

The Kuiper Belt, proposed by Gerard Kuiper in 1951, extends from Pluto's orbit out to roughly 500 AU, or 500 times the distance from Earth to the Sun. Comets that pass through the inner solar system and orbit the Sun at least once every 200 years, such as the famous Halley's Comet, come from the Kuiper Belt.

Oort Cloud comets do not all orbit in the plane along with most planets, asteroids and Kuiper Belt objects. Instead, the Oort Cloud is a sphere of objects. Therefore, comets can come our way from any direction, a danger often cited by those who study the hazards of an Earth impact.

Stern and Weissman say that regardless of whether the Oort Cloud is less massive than thought, comet collisions need to be considered in any attempt to describe how the Oort Cloud formed.