Hills_cloud - Wikiparency

# Hills cloud  **Microsim — three.js (Wikitube framework):** *The Hills cloud in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=hills&embed=1" data-title="The Hills cloud in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=hills`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: drag to an edge-on view and compare the two clouds: the Hills cloud's points stay within a band around the plane of the planets while the outer Oort cloud's points fill every direction; scroll in until the orbits of the planets shrink to a dot at the centre, far inside the cloud's inner edge; then set scale to true and read the caption, which reports that true scale cannot show the cloud and keeps the logarithmic distances.* The **Hills cloud**, also called the inner [[Oort_cloud|Oort cloud]] or the inner cloud, is a theorised disc-like or toroidal reservoir of comets lying inside the classical Oort cloud and roughly aligned with the plane of the planets.[^duncan1987][^nesvorny2025] Its outer border would lie at about 20,000–30,000 [[Astronomical_unit|AU]] from the [[Sun]]; its inner border is poorly defined, with estimates ranging from a few hundred AU to several thousand, in any case well beyond the planets and the [[Kuiper_belt|Kuiper belt]].[^duncan1987][^levison2007] If it exists, it may hold about five times as many comets as the outer cloud.[^duncan1987] The cloud is named after Jack G. Hills, who proposed it in 1981 to explain why the Oort cloud has not been used up.[^hills1981] Its bodies are too distant and faint to be observed directly, but several distant objects, among them [[Sedna_(dwarf_planet)|Sedna]], have been proposed as members, and some long-period [[Comet|comets]] with aphelia of a few thousand AU may come from it.[^brown2004][^planetary1991][^horizons-comets] The explorer at the top of this page draws the Hills cloud as a static sample of ILLUSTRATIVE points between about 3,000 and 20,000 AU, kept within about 15° of the ecliptic, inside the spherical sample that stands for the outer Oort cloud. ## Overview The case for the Hills cloud rests on the fragility of the outer cloud. Bodies in the outer Oort cloud are so loosely held by the Sun that passing stars and the tide of the [[Milky_Way]] keep altering their orbits. Over the age of the [[PORTAL_Solar_System|Solar System]] a significant fraction should have been stripped away into interstellar space or sent inward, where they are worn away by sunlight, fall into the Sun, strike planets or are thrown out by the giant planets. On that reckoning the outer cloud should be badly depleted by now, yet long-period comets still arrive at a steady rate.[^hills1981][^fernandez1997] A denser inner reservoir resolves the difficulty. Bodies there are bound more tightly and are disturbed less, but over time some are pushed outward into the outer cloud, replenishing it; the Hills cloud then acts as a store from which the classical cloud is topped up.[^fernandez1997][^hills1981] Models make the inner cloud the largest concentration of comets in the Solar System, and some estimates make it many times as massive as the outer cloud.[^duncan1987][^bailey1988] The two regions also differ in shape. Stellar encounters and the galactic tide have randomised the orbits of the outer cloud's bodies, making it a sphere, but the inner cloud is bound tightly enough to keep some memory of the flat disc in which its material formed.[^morbidelli2006] A modelling study in 2025 predicted that the galactic tide should twist part of the inner cloud, at roughly 1,000–10,000 AU, into a spiral structure.[^nesvorny2025] ## History ### Original Oort cloud model Until the 1980s the [[Oort_cloud|Oort cloud]] and, later, the [[Kuiper_belt|Kuiper belt]] were regarded as the Solar System's only reservoirs of comets. In 1932 the Estonian astronomer Ernst Öpik suggested that [[Comet|comets]] come from a cloud orbiting at the edge of the Solar System.[^opik1932] Jan Oort revived the idea independently in 1950 to resolve a contradiction: comets are destroyed after relatively few passages through the inner Solar System, so none that had been doing so since the Solar System formed should still be visible.[^oort1950] Oort studied the original orbits of well-observed long-period comets and found that the distribution of their reciprocal semi-major axes, 1/a, peaked at values corresponding to tens of thousands of AU. He took this to mean that the comets were stored at such distances, near the limit of the Sun's sphere of influence, where passing stars could nudge some of them inward and others out of the Solar System.[^oort1950] Because 1/a measures how tightly an orbit is bound, the peak marks a bound population that is only just being released, not comets arriving from interstellar space. ## New model Jack G. Hills, of the Los Alamos National Laboratory, proposed the inner cloud in 1981. He noted that the new [[Comet|comets]] arriving from the [[Oort_cloud|Oort cloud]] have semi-major axes of more than about 10,000 AU, and argued that this apparent inner edge reflects which comets can be delivered, not where comets are stored. Comets on tighter orbits are shielded from the weak everyday perturbations, and reach the planets only when a star passes unusually close and sets off a "comet shower", an episode that he linked to possible mass extinctions on [[Earth]].[^hills1981] He concluded that an inner cloud could hold tens or hundreds of times as many comet nuclei as the outer halo, and would resupply it.[^hills1981] Later work took up the idea. Sidney van den Bergh in 1982 and Mark Bailey in 1983 discussed the structure of such a cloud, and in 1986 Bailey argued that most of the Solar System's comets lie in an inner cloud with typical semi-major axes of about 5,000 AU. Victor Clube and Bill Napier in 1987 and Richard Stothers in 1988 extended these studies.[^bailey1988] Duncan, Quinn and Tremaine's simulations of cloud formation, also published in 1987, produced a centrally concentrated cloud whose inner part holds most of the comets.[^duncan1987] In the resulting picture the inner cloud begins at roughly 2,000–3,000 AU and merges into the classical cloud at about 20,000 AU.[^levison2007][^duncan1987] At those distances an orbit takes between about 89,000 years (2,000 AU) and 2.8 million years (20,000 AU) to complete, from [[Kepler's_laws_of_planetary_motion|Kepler's third law]] (derived). ## Characteristics ### Structure and composition Estimates of the Hills cloud's extent vary with the model, but it is generally placed at a few thousand to about 20,000 AU, inside the outer cloud's range of about 20,000 to 50,000 AU or more.[^levison2007][^duncan1987] Its mass is unknown. Duncan and colleagues' models suggest it could hold about five times as much material as the outer [[Oort_cloud|Oort cloud]], and Bailey and Stagg estimated its mass at 13.8 Earth masses if most of its bodies lie near 10,000 AU, about 8 × 10²⁵ kg (derived, using Earth's mass of 5.97 × 10²⁴ kg).[^duncan1987][^bailey1988][^nasa-fs] If comets are representative samples, most Hills cloud objects are made of ices of [[Water|water]], methane, ethane, carbon monoxide and hydrogen cyanide.[^gibb2003] The object 1996 PW, an asteroid-like body on an orbit typical of a long-period comet, suggests that the cloud may also contain some rocky bodies.[^weissman1997] Carbon and nitrogen isotope ratios in long-period comets and in Jupiter-family comets differ little, consistent with a common origin in the [[Protoplanetary_disk|protoplanetary disc]]; the 2005 Deep Impact results on comet 9P/Tempel 1 fit the same picture.[^hutsemekers2005][^mumma2005] ### Formation One proposed origin links the inner cloud to Sedna. Morbidelli and Levison found that a star passing about 800 AU from the Sun early in the Solar System's history, probably within the first 100 million years or so, could lift the perihelia of distant bodies and so explain Sedna's orbit, which neither [[Neptune]] nor the galactic tide can account for.[^morbidelli2004] If an encounter of this kind also shaped the inner cloud, the Hills cloud could be younger than the outer cloud, whose bodies were scattered out over a longer period.[^morbidelli2004][^morbidelli2006] A different route into and out of the inner cloud is shown by 2006 SQ372, whose perihelion lies inside Neptune's orbit and which Kaib and colleagues interpret as a long-period comet that originated in the inner cloud.[^kaib2009] ## Possible Hills cloud objects No body has been confirmed as a Hills cloud member, but several [[Trans-Neptunian_object|trans-Neptunian objects]] are candidates, among them [[Sedna_(dwarf_planet)|Sedna]] and other [[Extreme_trans-Neptunian_object|extreme trans-Neptunian objects]]. The table gives barycentric orbits from JPL Horizons at the epoch of 18 September 2026; elements for such long orbits vary slightly with epoch.[^horizons] | Object | Perihelion (AU) | Aphelion (AU) | Discovered | |---|---|---|---| | Sedna | 76.2 | 937 | 2003 | | 2012 VP113 | 80.5 | 444 | 2012 | | 541132 Leleākūhonua | 64.8 | 2,361 | 2015 | | 2000 CR105 | 44.1 | 400 | 2000 | | 2006 SQ372 | 24.2 | 1,569 | 2006 | 2006 SQ372 crosses the orbits of [[Neptune]] and [[Uranus]].[^horizons][^kaib2009] The retrograde trans-Neptunian object 2008 KV42 has also been suggested as a body that might come from the Hills cloud or the outer Oort cloud.[^futura2008] ### Comets [[Comet|Comets]] with aphelia between about 1,000 AU, well beyond the [[Kuiper_belt|Kuiper belt]], and 10,000 AU, still inside the outer cloud, are the natural candidates for Hills cloud comets. Several bright comets qualify. Barycentric elements from JPL Horizons, computed far from perihelion where the planets' influence is small, give an inbound aphelion of about 1,800 AU for comet Lovejoy (C/2007 E2), about 5,000 AU for comet Machholz (C/2004 Q2), an outbound aphelion of about 3,500 AU for comet Hyakutake (C/1996 B2), and about 4,100 AU for comet McNaught (C/2006 P1).[^horizons-comets] A comet's aphelion changes at every passage through the planetary region: McNaught's inbound orbit reached about 67,000 AU, but it will go back out only to about 4,100 AU (derived from the same elements).[^horizons-comets] ### Sedna Sedna was discovered by Michael Brown, Chad Trujillo and David Rabinowitz on 14 November 2003. In announcing it, they called it the first observed Oort cloud body: unlike [[Scattered_disc|scattered disc]] objects such as [[Eris_(dwarf_planet)|Eris]], its perihelion of 76 AU is too far out for Neptune to have shaped its orbit.[^brown2004] Because it is much closer than expected for an Oort cloud body and orbits near the plane of the planets, they described it as an inner Oort cloud object, lying between the Kuiper belt and the spherical cloud.[^brown2004][^lykawka2007] Whether it counts as a Hills cloud object depends on the definition: one definition takes the Hills cloud to be the region of orbits between 1,500 and 10,000 AU, and Sedna's aphelion of about 937 AU falls short of it.[^planetary1991][^horizons] It takes about 11,400 years to complete one orbit and next reaches perihelion in 2076.[^horizons] ### 2012 VP113 2012 VP113, the second body found on a [[Sedna_(dwarf_planet)|Sedna]]-like orbit, was announced on 26 March 2014. Its orbit resembles Sedna's, with a perihelion even farther from Neptune, about 80 AU, and an aphelion of about 444 AU.[^trujillo2014][^planetary2014][^horizons] Trujillo and Sheppard regarded it, with Sedna, as a member of the inner Oort cloud; the two were the first known [[Sednoid|sednoids]].[^trujillo2014] ## Footnotes Derived values are marked "(derived)". Orbital periods follow Kepler's third law, P = a^1.5 (P in years, a in AU). The explorer's Hills cloud and Oort cloud points are ILLUSTRATIVE samples, not catalogued bodies, and the points do not move. ## References [^duncan1987]: Duncan, M.; Quinn, T.; Tremaine, S. (1987). "The formation and extent of the solar system comet cloud". *The Astronomical Journal* 94: 1330. https://doi.org/10.1086/114571 [^nesvorny2025]: Nesvorný, D.; Dones, L.; Vokrouhlický, D.; Levison, H. F.; et al. (2025). "A spiral structure in the inner Oort cloud". *The Astrophysical Journal* 983: 74. https://doi.org/10.3847/1538-4357/adbf9b [^hills1981]: Hills, J. G. (1981). "Comet showers and the steady-state infall of comets from the Oort cloud". *The Astronomical Journal* 86: 1730–1740. https://doi.org/10.1086/113058 [^brown2004]: Brown, M. E.; Trujillo, C.; Rabinowitz, D. (2004). "Discovery of a candidate inner Oort cloud planetoid". *The Astrophysical Journal* 617: 645–649. https://doi.org/10.1086/422095 [^planetary1991]: Donahue, T. M.; Trivers, K. K.; Abramson, D. M. (eds.) (1991). *Planetary Sciences: American and Soviet Research. Proceedings from the U.S.–U.S.S.R. Workshop on Planetary Sciences*. National Academy Press, p. 247. ISBN 978-0-309-04333-5. https://doi.org/10.17226/1790 [^fernandez1997]: Fernández, J. A. (1997). "The formation of the Oort cloud and the primitive galactic environment". *Icarus* 129: 106–119. https://doi.org/10.1006/icar.1997.5754 [^bailey1988]: Bailey, M. E.; Stagg, C. R. (1988). "Cratering constraints on the inner Oort cloud: steady-state models". *Monthly Notices of the Royal Astronomical Society* 235: 1–32. https://doi.org/10.1093/mnras/235.1.1 [^morbidelli2006]: Morbidelli, A. (2006). "Origin and dynamical evolution of comets and their reservoirs of water, ammonia and methane". arXiv:astro-ph/0512256. https://arxiv.org/abs/astro-ph/0512256 [^opik1932]: Öpik, E. J. (1932). "Note on stellar perturbations of nearly parabolic orbits". *Proceedings of the American Academy of Arts and Sciences* 67: 169–182. https://doi.org/10.2307/20022899 [^oort1950]: Oort, J. H. (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. Bibcode 1950BAN....11...91O. [^levison2007]: Levison, H. F.; Donnes, L. (2007). "Comet populations and cometary dynamics". In McFadden, L.-A.; Weissman, P. R.; Johnson, T. V. (eds.), *Encyclopedia of the Solar System* (2nd ed.). Academic Press, pp. 575–588. ISBN 978-0-12-088589-3. [^nasa-fs]: NASA NSSDCA. "Planetary fact sheet" (fetched 2026-09-18). https://nssdc.gsfc.nasa.gov/planetary/factsheet/ [^gibb2003]: Gibb, E. L.; Mumma, M. J.; Dello Russo, N.; DiSanti, M. A.; Magee-Sauer, K. (2003). "Methane in Oort cloud comets". *Icarus* 165: 391–406. https://doi.org/10.1016/S0019-1035(03)00201-X [^weissman1997]: Weissman, P. R.; Levison, H. F. (1997). "Origin and evolution of the unusual object 1996 PW: asteroids from the Oort cloud?". *The Astrophysical Journal* 488: L133–L136. https://doi.org/10.1086/310940 [^hutsemekers2005]: Hutsemékers, D.; Manfroid, J.; Jehin, E.; Arpigny, C.; Cochran, A.; Schulz, R.; et al. (2005). "Isotopic abundances of carbon and nitrogen in Jupiter-family and Oort cloud comets". *Astronomy & Astrophysics* 440: L21–L24. https://doi.org/10.1051/0004-6361:200500160 [^mumma2005]: Mumma, M. J.; DiSanti, M. A.; Magee-Sauer, K.; et al. (2005). "Parent volatiles in comet 9P/Tempel 1: before and after impact". *Science* 310: 270–274. https://doi.org/10.1126/science.1119337 [^morbidelli2004]: Morbidelli, A.; Levison, H. F. (2004). "Scenarios for the origin of the orbits of the trans-Neptunian objects 2000 CR105 and 2003 VB12 (Sedna)". *The Astronomical Journal* 128: 2564–2576. https://doi.org/10.1086/424617 [^kaib2009]: Kaib, N. A.; Becker, A. C.; Jones, R. L.; et al. (2009). "2006 SQ372: a likely long-period comet from the inner Oort cloud". *The Astrophysical Journal* 695: 268–275. https://doi.org/10.1088/0004-637X/695/1/268 [^horizons]: JPL Horizons On-Line Ephemeris System, barycentric osculating orbital elements at epoch 2026-09-18 (retrieved 2026-09-18). NASA Jet Propulsion Laboratory. https://ssd.jpl.nasa.gov/horizons/ [^futura2008]: Etienne, J. (5 September 2008). "2008 KV42, l'astéroïde qui tourne à l'envers". *Futura*. https://www.futura-sciences.com/sciences/actualites/astronomie-2008-kv42-asteroide-tourne-envers-16593/ [^horizons-comets]: JPL Horizons On-Line Ephemeris System, barycentric osculating orbital elements of C/2007 E2, C/2004 Q2, C/1996 B2 and C/2006 P1 at epochs 1900-01-01 (inbound) and 2100-01-01 (outbound) (retrieved 2026-09-18). NASA Jet Propulsion Laboratory. https://ssd.jpl.nasa.gov/horizons/ [^lykawka2007]: Lykawka, P. S.; Mukai, T. (2007). "Dynamical classification of trans-Neptunian objects: probing their origin, evolution, and interrelation". *Icarus* 189: 213–232. https://doi.org/10.1016/j.icarus.2007.01.001 [^trujillo2014]: Trujillo, C. A.; Sheppard, S. S. (2014). "A Sedna-like body with a perihelion of 80 astronomical units". *Nature* 507: 471–474. https://doi.org/10.1038/nature13156 [^planetary2014]: Lakdawalla, E. (26 March 2014). "A second Sedna! What does it mean?". The Planetary Society. https://www.planetary.org/articles/03261345-a-second-sedna-what-does-it-mean ## Further reading - Dones, L.; Brasser, R.; Kaib, N.; Rickman, H. (2015). "Origin and evolution of the cometary reservoirs". *Space Science Reviews* 197: 191–269. https://doi.org/10.1007/s11214-015-0223-2 - Morbidelli, A. (2006). "Origin and dynamical evolution of comets and their reservoirs of water, ammonia and methane". arXiv:astro-ph/0512256. ## External links - NASA Science. "Oort cloud". https://science.nasa.gov/solar-system/oort-cloud/ - JPL Horizons On-Line Ephemeris System. https://ssd.jpl.nasa.gov/horizons/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Hills_cloud) : [Wikitube](https://en.wikitube.io/wiki/Hills_cloud) · pinned revision [1370779979](https://en.wikipedia.org/w/index.php?oldid=1370779979) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-079 · explorer state `?obj=hills`.*