Oort cloud

The distance from the Oort cloud to the interior of the Solar System, and two of the nearest stars, is measured in astronomical units. The scale is logarithmic: each indicated distance is ten times farther out than the previous distance. The red arrow indicates the projected location, in 2025–2027, of the space probe Voyager 1, which may reach the Oort cloud about 300 years later.
An artist's impression of the Oort cloud and the Kuiper belt (inset); the sizes of objects are over-scaled for visibility.

The Oort cloud (pronounced /ɔːrt/ AWT or /ʊərt/ OORT),[1] sometimes called the Öpik–Oort cloud,[2] is theorized to be a cloud of billions of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years).[3][4] The cloud was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was named.[5][6] Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.[7]

The cloud is thought to encompass two regions: a disc-shaped inner Oort cloud aligned with the solar ecliptic (also called its Hills cloud) and a spherical outer Oort cloud enclosing the entire Solar System. Both regions lie well beyond the heliosphere and are in interstellar space.[4][8] The innermost portion of the Oort cloud is more than a thousand times farther from the Sun than the Kuiper belt, the scattered disc and the detached objects—three nearer reservoirs of trans-Neptunian objects.[9]

The outer limit of the Oort cloud defines the cosmographic boundary of the Solar System. This area is defined by the Sun's Hill sphere, and hence lies at the interface between solar and galactic gravitational dominion.[10] The outer Oort cloud is only loosely bound to the Solar System and its constituents are easily affected by the gravitational pulls of passing stars, the Milky Way itself and the cloud's own microgravity.[11] These forces served to moderate and render more circular the highly eccentric orbits of material ejected from the inner Solar System during its early phases of development. The circular orbits of material in the Oort disc are largely thanks to this galactic gravitational torquing.[12][13] By the same token, galactic interference in the motion of Oort bodies occasionally dislodges comets from their orbits within the cloud, sending them into the inner Solar System.[4] Based on their orbits, most but not all of the short-period comets appear to have come from the Oort disc. Other short-period comets may have originated from the far larger spherical cloud.[4][14]

Astronomers hypothesize that the material presently in the Oort cloud formed much closer to the Sun, in the protoplanetary disc, and was then scattered far into space through the gravitational influence of the giant planets.[4] No direct observation of the Oort cloud is possible with present imaging technology.[15] Nevertheless, the cloud is thought to be the source that replenishes most long-period and Halley-type comets, which are eventually consumed by their close approaches to the Sun after entering the inner Solar System. The cloud may also serve the same function for many of the centaurs and Jupiter-family comets.[14]

Types of distant minor planets
  1. ^ "Oort". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  2. ^ Cite error: The named reference Whipple was invoked but never defined (see the help page).
  3. ^ Williams, Matt (August 10, 2015). "What is the Oort Cloud?". Universe Today. Archived from the original on January 23, 2018. Retrieved May 21, 2021.
  4. ^ a b c d e Cite error: The named reference Morbidelli2006 was invoked but never defined (see the help page).
  5. ^ van der Kruit, Pieter C. (2020). Master of Galactic Astronomy: A Biography of Jan Hendrik Oort. Springer. Bibcode:2021mgab.book.....K.
  6. ^ Whipple, Fred L. (1978). "The Oort Cloud". In Tom Gehrels (ed.). Protostars and Planets. University of Arizona Press.
  7. ^ Redd, Nola Taylor (October 4, 2018). "Oort Cloud: The Outer Solar System's Icy Shell". Space.com. Archived from the original on January 26, 2021. Retrieved August 18, 2020.
  8. ^ Cite error: The named reference jpl.PIA17046 was invoked but never defined (see the help page).
  9. ^ Dones, Luke (2004). Hans Rickman and Michael Festou (ed.). The Origin and Evolution of the Oort Cloud. Cambridge University Press. pp. 153–174.
  10. ^ Cite error: The named reference NASA_SSE_oort was invoked but never defined (see the help page).
  11. ^ Correa-Otto, J. A.; Calandra, M. F. (2019). "The stability in the most external region of the Oort Cloud: The evolution of the ejected comets". Monthly Notices of the Royal Astronomical Society. 490 (2): 2495–2504. arXiv:1901.05964. doi:10.1093/mnras/stz2755.
  12. ^ Fouchard, Marc; Froeschlé, Christiane; Valsecchi, Giovanni; Rickman, Hans (27 September 2006). "Long-term effects of the Galactic tide on cometary dynamics". Celestial Mechanics and Dynamical Astronomy. 95 (1–4): 299–326. Bibcode:2006CeMDA..95..299F. doi:10.1007/s10569-006-9027-8.
  13. ^ Raymond, Sean (2023-06-21). "Oort cloud (exo)planets". PLANETPLANET. Archived from the original on 2023-07-01. Retrieved 2023-07-01.
  14. ^ a b Cite error: The named reference emel2007 was invoked but never defined (see the help page).
  15. ^ "Oort Cloud". NASA Solar System Exploration. 20 June 2023. Archived from the original on 2023-06-30. Retrieved 2023-07-01.
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