There are objects in our solar system coming with a great velocity and are of any random shape. These objects lose material (sublimate) as they come near the sun. This melting process creates the tails of these objects which we see in the sky famously called halos. After 100 or more orbits around the sun they disappear, yet fresh objects keep on coming.
What are these objects in our solar system? Where do they come from? There are so many other questions too related to these like – Why was Pluto once considered as a planet and why was it abandoned? What is Oort Cloud? Is there a ninth planet in our solar system?
To answer such questions let’s have a journey to 4.5 billion years in the past, before the formation of the solar system. Covered with the flat disc of gases around the sun, inner planets were warmer, smaller and rocky, while the outer planets were colder, huge and gaseous. In the outer part of the system, the water came in the form of ice mixed with dust and some other stuffs like methane and ammonia. The discs collided and stuck together, growing bigger in the inner part. Jupiter was one such newly born big planet. The outer planets had a lot of gravity.
The icy objects were scattered by the gravitation of the gas giants (specially Jupiter). The icy objects either could be flung into the planet or kicked out to different orbits either they could then have flung inward towards the sun or have flung off out into the deep space. Trillions of such ice balls floated around. After the icy objects were scattered, they were distributed in three main belts. First is Kuiper Belt (fig - 1) which starts just after Neptune and extends from 4.5 to 7.5 billion km from the sun. Pluto is part of this belt only. Like other planets it was considered as a planet earlier, but later it was found as part of Kuiper belt.
Fig. 1 - Kuiper Belt
The second region is called the scattered disc (fig - 2). This is composed of the ice balls sent by Neptune to these tilted orbits. This overlaps the Kuiper belt in its inner edge and extends out to 150 billion km from the sun. Finally, outside those two belts, there is a spherical cloud of icy objects which starts 300 billion km from the sun .e. 2000 Astronomical Units (1 AU = distance between earth and sun) and perhaps the width of this cloud is 1 light year. This huge spherical cloud is known as Oort Cloud.
Fig. 2 - Scattered Disk
Oort Cloud (fig - 3) is named after the Dutch astronomer Jan Oort, who proposed theory in 1950. When we began this article mentioning objects in the sky, you might have guessed that we were talking about comets. Now these comets come in two varieties – first with orbital periods of less than 200 years and second with longer periods. Jan Oort was trying to find out where these long period comets come from, which follows widely elongated elliptical orbits (short period comets come from other two belts, mainly from the scattered disc). Oort calculated that these long period comets with the biggest and most extreme orbit must be coming from the reservoir of comets, icy cloud on the edge of the solar system. These comets are projected into the inner solar system when the gravitation field of a passing star or of the Milky-way Galaxy pushes them inwards.
Fig. 3 - Oort Cloud
It should be made clear that up till now, the status of the Oort Cloud is totally hypothetical. The reason that it is still hypothetical is that direct evidences that could confirm its existence still eludes us. An estimate says that at its current speed of about a million miles a day, NASA’s Voyager 1 spacecraft (which is the fastest and farthest of the interplanetary space probes currently exiting) it will take more than 300 years to reach the Oort cloud if it is really there and it will take another almost 30,000 years to reach the other side of the cloud. The reason we believe in this hypothesis is that defining Oort Cloud answers a lot of important questions related to space which were earlier undefined.
The Oort Cloud may have around two trillion icy objects which are made up of ice, dusts, ammonia, methane, carbon dioxide etc. The Oort Cloud is thought to be comprised of two regions - a spherical outer Oort Cloud (20,000 to 50,000 AU) and a disc shaped inner Oort Cloud known as Hills Cloud (2,000 to 20,000 AU). The outer cloud may have trillions of objects larger than 1 km in diameter and billions of objects of 20 km in diameter.
Most of the studies are done based on the data collected from the past seen comets. They orbit the sun on a sphere with no preferred orientation and enter our solar system from random directions. Their orbital periods could be more than 1000 years long. They swing around the sun nearly with the escape velocity and their orbits are close to be parabolic. On the basis of known information about Haley’s comet, which is assumed to be an outer Oort Cloud object and has a mass of nearly 5 earths, predictions of mass of the outer icy objects are made. The mass of inner Oort Cloud object is still unpredictable. Hale Bopp is another outer Oort Cloud comet.
In the past decades, two more objects were found and they may be inner Oort Cloud objects. Sedna (fig - 4), discovered in 2003, at the farthest point in its orbit goes 140 billion km away from the sun and the other is VP-113.
Fig. 4 - Location and Dimension of Sedna
Both of them follow elliptical orbits. However, the nature of these elliptical orbits suggests some new things. One theory suggests that sun has stolen comets from outer discs of the other stars which were forming in the same nebula as the sun. Another explanation is that there could be another giant planet beyond Neptune which lurks in this area causing perturbation in the motion of some inner Oort Cloud objects in the vicinity and putting them in similar elongated elliptical orbits.
All in all, the hypothesis of Oort Cloud has made the scientists to answer some of the elusive questions, but who knows, it may or may not be true. Whatever may be, but it has thrown light on the 4.5 billion years long journey of our solar system this far and reveals the possible reasons of where the long period comets come from.
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