The Oort Cloud

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The Oort Cloud

In 1932 the Estonian astronomer Ernst Opik (1893-1985) proposed that long-period comets came from an orbiting cloud at the outer limits of our Solar System. The Dutch astronomer Jan Oort independently gave new life to this theory in 1950 in an effort to resolve a paradox.

Over the course of our Solar System’s history the orbits of comets have become unstable and ultimately dynamics dictate that a comet must either crash into the Sun or a planet or, alternatively, be rudely evicted from our Solar System altogether by planetary gravitational perturbations. Furthermore, their volatile composition means that, as they repeatedly migrate towards our Sun, radiation eventually boils the volatiles away until the comet either fragments or forms an insulating crust that shields it from additional outgassing.

Taking everything into account, Oort reasoned that a comet could not have formed while in its current orbit. Instead, it must have inhabited a frigid outer reservoir of comet nuclei for almost its entire existence.

The Oort Cloud

Estimates have placed the outermost edge of the Oort cloud between 100,000 and 200,000 AU. The region itself can be subdivided into a spherical outer Oort cloud of 20,000 to 50,000 AU, and a torus-shaped inner Oort cloud at 2,000 to 20,000 AU. The outermost region of this vast cloud is only weakly bound gravitationally to our Star and it is the original home of the long-period comets that invade the inner Solar System. The inner Oort cloud, known as the Hills cloud, is named in honor of Dr. Jack G. Hills, a retired Laboratory Fellow of the Los Alamos National Lab (New Mexico), who proposed its existence in 1981. Models predict that the inner cloud should host tens or hundreds of times more cometary nuclei than the outer halo–and it is a possible source of new comets that resupply the thin and delicate outer cloud, as the latter’s numbers gradually diminish. The Hills cloud does, indeed, explain the continued existence of the Oort cloud over a time span of billions of years.

The Oort cloud itself is believed to be a lingering relic of the original protoplanetary accretion disc that formed around our newborn Sun. The most widely accepted theory suggests that the Oort cloud’s numerous icy inhabitants first coalesced closer to our brilliant, hot, and fiery baby Sun as part of the same process that created both the eight major planets, as well as the minor planets. However, a gravitational dance with youthful gas-giants like Jupiter hurled these objects into extremely long elliptical or parabolic orbits. Indeed, recent research conducted by NASA scientists indicates that our Sun’s sibling stars (stars that were born in the same stellar cluster as our Sun) eventually drifted apart, and went their separate ways, when they were still young. In addition, many–possibly even the majority– of icy Oort cloud denizens did not form close to our Star. Supercomputer simulations of the evolution of the Oort cloud from the birth of our Solar System to the present indicate that the cloud’s mass peaked approximately 800 million years after its formation, as the rate of accretion and collision slowed down, and depletion started to overtake supply. mkvking

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