Comets are enigmatic celestial objects that have captivated mankind through history. Their long, bright tails often dazzle as they approach the sun, but they can be a destructive force, pulverizing planetary surfaces throughout the evolution of the solar system.
But where do these interplanetary interlopers come from? As it turns out, that's far from an easy question to answer.
The concept of a cloud from which long-period comets originate was first discussed in 1932 by Estonian astronomer Ernst Öpic and independently revisited by Dutch astronomer Jan Oort 18 years later.
Oort drew the conclusion that a distant cloud must exist because new comets are often observed and, given that the sun causes much of a comet's material to 'boil' off into space, they would either disintegrate or harden very quickly and thus prevent further outgassing.
This alone suggests comets have a finite lifetime and must have existed somewhere, waiting until some chance event throws them toward the inner solar system and their slow demise.
He also noted that due to their orbits, which seemingly change slowly over time, they should eventually hit something or be ejected from the solar system through gravitational interactions with planets. For these reasons, Oort believed there must be some large 'reservoir' where comets exist for most of their lives.
The possible origin of the cloud too is subject to conjecture but many believe it could be the remains of the protoplanetary disk out of which the planets formed.
About 4.6 billion years ago, the young hot sun was surrounded by a vast disk of material that slowly formed into the planets. Much of the remaining material is now thought to have been ejected to the outer reaches of the solar system through gravitational interactions Jupiter and Saturn.
Since the idea was formulated, we have still have yet to confirm the existence of the so-called "Oort Cloud" and so it remains purely hypothetical.
If it does exist -- and there's a lot of evidence to suggest it does -- it is thought that it would be roughly spherical in shape with an outer sphere extending from about 2,000 Astronomical Units (1 AU is the average distance between the Earth and the sun) and 50,000 AU from the sun and an inner disc known as the Hills Cloud. (This is not to be confused with the Kuiper Belt which refers to a region much closer to the sun.)
The Oort Cloud is believed to be composed of billions, possibly trillions of icy planetesimals which ultimately, become cometary nuclei if dislodged from their orbit and sent in toward the inner solar system.
So what strange force could possibly eject comets from their deep freeze in the Oort Cloud?
One of the most popular theories is that the gravity from the Milky Way itself is responsible for sending comets in our direction. Just as the moon exerts a pull on the Earth, making its oceans (and to a lesser degree the land) bulge toward the moon, so the pull of gravity from our galaxy is thought to make the Oort Cloud bulge, dislodging the comets.
Another possible cause for ejecting comets is the gravitational effect from stars that pass close to our system as they orbit around the Milky Way.
Unfortunately, no man-made spacecraft has reached the distance of the Oort Cloud and space observatories aren't powerful enough to prove its existence, so for now, it will remain a topic for debate. One thing is for certain though, long period comets will continue to unexpectedly appear out of the depths of space, like a whale from the depths of the oceans. Quite where they come from though will remain a mystery.