While the universe remains elusive in numerous ways, theories and new discoveries keep coming that keep us wondering about the hidden treasures of the unknown, unexplored vastness beyond our solar system which is no more than a tiny drop in a limitless ocean! The Oort Could is one such thing that will keep eluding us for at least another decade or two until we develop the technology that will allow us to go beyond the Kuiper Belt and explore its existence. So, what really is Oort Cloud?
Probably the name is going to give you an impression that it is a cloud (as we really know on earth) but that’s not true. The Oort Cloud is a spherical mass that consists of trillions of icy planetesimals surrounding the Sun at a distance of 50,000 AU. That’s only a quick explanation but we are sure that you have other questions in mind. Let is learn 25 interesting facts about Oort Cloud and find out the answers to questions like: “Why is it there?”, “How far does it extend?”, “What really is present in the Oort Cloud?”, “Can we reach there?”, “What keeps it in its place?”, “Why is it so called?”, and many more! So, are you ready? Here is our collection of 25 interesting facts about the Oort Cloud:
1. Prior to 1950, scientists didn’t have the slightest of idea that something like that could even exist. In 1950 however, a Dutch astronomer known as Jan Oort proposed that a spherical cloud of icy planetesimals exist beyond our known solar system. The sphere was then named after Jan Oort and hence, known as Oort Cloud.
2. Some scientists believe that Oort Cloud starts at 2000 or 5000 Astronomical Units and ends somewhere at 50,000 Astronomical Units. An AU or Astronomical Unit is the distance between Sun and Earth.
3. Other scientists believe that the Oort Cloud extends all the way up to 100,000 to 200,000 AU.
4. The Solar System’s cosmographical boundary is defined by the outer limit of Oort Cloud. To simplify things, Sun’s gravitational dominance ends with Oort Cloud.
5. Scientists believe that the Oort Cloud actually consists of two parts – the inner Hills Cloud which is a disc-shaped Oort Cloud and the outer spherical Oort Cloud.
6. The objects in the cloud are icy and are mainly made of methane, ammonia and water.
7. Scientists theorize that the objects in the Oort Cloud were actually formed close to the Sun but the gravity of larger or giant planets pushed them out during the early stages of evolution of the Solar System.
8. Despite the fact that no direct observations have been made till date scientists believe that Halley-type and long-period comets actually originate from the Oort Cloud. They also believe that Jupiter-family comets and centaurs too originate from the cloud.
9. Scientists say that because Oort Cloud is only loosely bound to our Solar System, the gravitation pull of Milky Way and passing stars actually dislodge objects from Oort Cloud and send them towards the inner Solar System.
10. The first and original suggestion of the cloud actually came from Ernst Öpik, an Estonian astronomer in 1932. Later it was Jan Hendrik Oort who independently revived the idea in order to resolve a long standing paradox. Oort noticed that the orbits of the comets were very unstable and according to dynamics, comets were supposed to collide with a planet or the Sun. If that did not happen, they should have at least been ejected out from the Solar System because of planetary perturbations. Jan also noticed that the comets had a very volatile composition and as they approached the Sun, the volatile matter continued to boil and vaporize until an insulating crust was developed preventing any further outgassing or until the comets split up. This eventually led Oort to theorize that comets cannot really form while they are on their current orbits and that they should come from a reservoir in outer space.
11. There are two types of comets – the isotropic or long-period comets and the ecliptic or short-period comets. While the ecliptic versions have an ecliptic plane as the planets, the isotropic comets have large orbits, usually thousands of AU and they tend to appear from all directions in sky. This was noted by Jan Oort who also noted that long-period comets had aphelia as high as 20,000 AU. This led Oort to hypothesize that these isotropic comets come from a large spherical reservoir located at a distance of 20,000 AU.
12. Long-period comets originate in Oort Cloud while the short-period comets originate in the Kuiper Belt.
13. The entire Oort Cloud is divided into two parts. The outer part is spherical shaped and the inner part is doughnut shaped.
14. The doughnut shaped inner Oort Cloud is known as Hills Cloud because its existence was proposed in 1981 by J. G. Hills.
15. According to scientific estimates and models, the Hills Cloud has 10 to 100 times more cometary nuclei compared to the outer Oort Cloud.
16. It is assumed that the Hills Cloud is the reservoir that supplies cometary nuclei to the outer Oort Cloud as the outer halo’s supplies continue to be depleted over billions of years.
17. It is assumed that several trillions of cometary nuclei exist in the outer halo, which are larger than 1 kilometer in diameter.
18. It is also assumed that there are several billions of objects in the outer halo that have absolute magnitudes brighter than 11. Absolute magnitude is the measure of intrinsic brightness of celestial objects.
19. Objects within the Oort Cloud are separated by a distance of millions of kilometers.
20. The exact mass of the entire Oort Cloud is uncertain however, considering Halley’s Comet as a prototype it is roughly estimated that the total mass of the Oort Could is 3 x 1025 kg which is nearly 5 times the total mass of Earth.
21. While Oort Cloud is mostly composed of different types of ices like hydrogen cyanide, carbon monoxide, ethane, methane and water, it also consists of rocky objects. Scientists proposed the presence of rocky objects in Oort Cloud after the discovery of an asteroid named ‘1996 PW’. This asteroid has an orbit which is usually seen in isotropic comets.
22. Scientists believe that Oort Cloud is actually the remnant of the protoplanetary disc of Sun that was created 4.6 billion years ago.
23. It is widely believed that the objects of the Oort Cloud were once very close the Sun but as the gaseous giant planets formed, the gravitational interaction of these planets eventually pushed the objects into long parabolic and elliptical orbits.
24. Studies reveal that the mass of the Oort Cloud was at its peak some 800 million years after its formation. However when the collision and accretion pace slowed down, the mass of the Oort Cloud gradually declined because supply line was cut short and depletion rate increased.
25. Harold F. Levison proposed in June 2010 (based on advanced computer simulations) that Sun actually captured comets during its birth phase from the protoplanetary discs of other stars. The proposal said that more than 90% of the objects in Oort Cloud are from the protoplanetary discs of other stars while the remaining came from Sun’s own protoplanetary disc.
The Oort Cloud is way far from us. Chances of exploring the halo anytime soon are slim to none. NASA however launched the New Horizons Spacecraft that will reach the Kuiper Belt. It is expected that the probe will be at a distance of 12,500 kilometers from Pluto on 14th July, 2015. The probe travels at a very high speed covering more than 1 million kilometers every day!