The Oort Cloud is a hypothetical shell of icy objects surrounding our solar system. Also known as the Öpik–Oort cloud, it’s named after Jan Oort and Ernst Öpik, the astronomers who first postulated its existence. The Oort Cloud is a part of our solar system that is still largely theoretical due to its extreme distance and the small size of its constituent objects. It is so far away that no spacecraft have explored it yet.
- The Oort Cloud forms a bubble of icy objects around the solar system.
- The Sun, planets, asteroid belt, and Kuiper Belt are all enclosed within the Oort Cloud.
- Like the asteroid belt and Kuiper Belt, the Oort Cloud contains remnants from the formation of the solar system.
- The cloud contains millions of comets and possibly some dwarf planets.
- Voyager 1 will be the first craft to reach the Oort Cloud, around 300 years from now.
Distance from the Sun and Earth
The Oort Cloud begins at a distance of approximately 2,000 astronomical units (AU) from the Sun and extends outwards to about 100,000-200,000 AU. For comparison, 1 AU is the average distance from the Sun to the Earth, approximately 93 million miles (150 million kilometers). Thus, the Oort Cloud is about 2000 to 100,000 times further away than the distance from the Earth to the Sun.
Discovery and Naming
The region was first postulated by Estonian astronomer Ernst Öpik in 1932. However, takes its name for Dutch astronomer Jan Oort, who independently proposed its existence in 1950. The proposal offered an explanation for the existence of long-period comets. These are comets with orbits that take them far beyond the known boundaries of the solar system.
Shape, Structure, and Composition
Researchers believe the Oort Cloud has a spherical or toroidal shape, extending in all directions from the Sun. This is quite distinct from the flat disk shape of the part of the solar system where the planets reside. Scientists generally agree that the Oort Cloud consists of two connected regions: the outer Oort Cloud and the inner Oort Cloud (sometimes called Hills Cloud or the Oort Hills cloud).
The inner Oort Cloud or Hills Cloud is disc-shaped and lies closer to the rest of the solar system. It starts at a distance of approximately 2,000 to 5,000 astronomical units (AU) from the Sun and extends up to 10,000 to 20,000 AU. The objects in this region feel the influence of the gravity of the gas giant planets.
The outer Oort Cloud is the larger spherical component and extends from the edge of the inner Oort Cloud out to at least 50,000 to 100,000 AU. The comets from this region approach the Sun from any direction, which is why scientists believe the outer cloud is spherical. The objects in the outer cloud are loosely bound to the Sun, and their orbits feel the gravity of nearby stars and the Milky Way galaxy itself.
The cloud likely consists mostly of icy planetesimals. These are small objects made from dust and ice, which are remnants from the formation of the solar system. These planetesimals are so far from the Sun that they react to perturbations from nearby stars or gas clouds.
Origin of the Oort Cloud
The Oort Cloud is probably a remnant of the original protoplanetary disk that formed around the Sun approximately 4.6 billion years ago. It’s believed that the planetesimals within the Oort Cloud started out closer to the Sun but were flung outwards by the gravitational interactions with young planets. As these planetesimals moved further from the Sun, they entered a region of space where the Sun’s gravitational influence is weak.
Comparing the Oort Cloud and the Kuiper Belt
The Oort Cloud and the Kuiper Belt are two separate regions of the solar system, each populated by small, icy bodies.
- Location and Structure: The Kuiper Belt is much closer to the Sun, extending from the orbit of Neptune (at 30 AU) to about 50 AU. Unlike the spherical Oort Cloud, the Kuiper Belt is a disk-shaped region in the plane of the solar system.
- Composition: Both the Oort Cloud and the Kuiper Belt contain small, icy bodies.
- Comets: Both regions are sources of the comets we see in our sky. Short-period comets (with orbits less than 200 years) originate from the Kuiper Belt, while long-period comets (with orbits longer than 200 years) come from the Oort Cloud.
Comets and Other Bodies in the Oort Cloud
The Oort Cloud contains billions or even trillions of comets. Other possible objects in the Oort Cloud might include dwarf planets, similar to Pluto. The existence of such bodies remains purely speculative at this point.
Studying the Oort Cloud
Studying the Oort Cloud provides valuable insight into the early solar system. As a primordial group of objects, the Oort Cloud offers clues to the processes of planet formation and the original properties of the protoplanetary disk.
Furthermore, studying the Oort Cloud helps scientists better understand the dynamics of comets and the risks they pose to Earth. As comets from the Oort Cloud have orbits that potentially intersect with the Earth’s, understanding their behavior is crucial for predicting potential comet impacts.
Reaching the Oort Cloud
Reaching the Oort Cloud with a spacecraft presents a formidable challenge. Even at the incredible speed of the Voyager spacecraft (traveling at more than 35,000 mph or about 56,000 kph), it takes over 300 years to reach the inner edge of the Oort Cloud and up to 30,000 years to pass through it.
References
- Emelyanenko, V. V.; Asher, D. J.; Bailey, M. E. (2007). “The fundamental role of the Oort Cloud in determining the flux of comets through the planetary system”. Monthly Notices of the Royal Astronomical Society. 381 (2): 779–789. doi:10.1111/j.1365-2966.2007.12269.x
- Fernández, Julio A. (1997). “The Formation of the Oort Cloud and the Primitive Galactic Environment”. Icarus. 219 (1): 106–119. doi:10.1006/icar.1997.5754
- Levison, Harold F.; Donnes, Luke (2007). “Comet Populations and Cometary Dynamics”. In Lucy Ann Adams McFadden; Lucy-Ann Adams; Paul Robert Weissman; Torrence V. Johnson (eds.). Encyclopedia of the Solar System (2nd ed.). Amsterdam; Boston: Academic Press. ISBN 978-0-12-088589-3.
- Oort, Jan (1950). “The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin”. Bulletin of the Astronomical Institutes of the Netherlands. 11: 91–110.
- Öpik, Ernst Julius (1932). “Note on Stellar Perturbations of Nearby Parabolic Orbits”. Proceedings of the American Academy of Arts and Sciences. 67 (6): 169–182. doi:10.2307/20022899