Editing Oort cloud (section)

== Development of theory ==

By the early 20th century, astronomers had identified two main types of comets: short-period comets (also called ''[[ecliptic]]'' comets) and long-period comets (also called ''nearly [[isotropy|isotropic]]'' comets).<ref>{{cite book | last1 = Duncan | first1 = Martin J. | last2 = Levison | first2 = Harold F. | last3 = Dones | first3 = Luke | chapter = Dynamical Evolution of Ecliptic Comets | editor = Michel Festou, H. U. Keller, and H. A. Weaver | title = Comets II | publisher = University of Arizona Press | year = 2004 | pages = 193–204 | url = https://www.lpi.usra.edu/books/CometsII/7010.pdf }}</ref> Ecliptic comets have relatively small orbits aligned near the [[plane of the ecliptic|ecliptic plane]] and are not found much farther than the [[Kuiper cliff]] around 50 AU from the Sun (the orbit of [[Neptune]] averages about 30 AU and [[177P/Barnard]] has aphelion around 48 AU). Long-period comets, on the other hand, travel in very large orbits thousands of AU from the Sun and are isotropically distributed. This means long-period comets appear from every direction in the sky, both above and below the ecliptic plane.<ref name="book" /> The origin of these comets was not well understood, and many long-period comets were initially assumed to be on parabolic trajectories, making them one-time visitors to the Sun from interstellar space.

In 1907, [[Armin Otto Leuschner]] suggested that many of the comets then thought to have parabolic orbits in fact moved along extremely large elliptical orbits that would return them to the inner Solar System after long intervals during which they were invisible to Earth-based astronomy.<ref name="ley1967204">{{Cite magazine
 |last=Ley
 |first=Willy
 |date=April 1967
 |title=The Orbits of the Comets
 |department=For Your Information
 |url=https://archive.org/stream/Galaxy_v25n04_1967-04#page/n55/mode/2up
 |magazine=Galaxy Science Fiction|volume=25|issue=4
 |pages=55–63
}}</ref> In 1932, the [[Estonia]]n astronomer [[Ernst Öpik]] proposed a reservoir of long-period comets in the form of an orbiting cloud at the outermost edge of the [[Solar System]].<ref name="Öpik-1932">{{cite journal
 |author= Ernst Julius Öpik
 |date= 1932
 |title= Note on Stellar Perturbations of Nearby Parabolic Orbits
 |journal= [[Proceedings of the American Academy of Arts and Sciences]]
 |volume= 67|issue=6|pages=169–182
 |doi= 10.2307/20022899
 |jstor= 20022899
|bibcode= 1932PAAAS..67..169O
 }}</ref> [[Dutch people|Dutch]] astronomer [[Jan Oort]] revived this basic idea in 1950 to resolve a paradox about the origin of comets. The following facts are not easily reconcilable with the highly elliptical orbits in which long-period comets are always found:
* Over millions and billions of years the orbits of Oort cloud comets are unstable. [[Dynamics (physics)|Celestial dynamics]] will eventually dictate that a comet must be pulled away by a passing star, collide with the Sun or a planet, or be ejected from the Solar System through planetary [[Perturbation (astronomy)|perturbations]].
* Moreover, the volatile composition of comets means that as they repeatedly approach the Sun [[Electromagnetic radiation|radiation]] gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further [[outgassing]].<ref name="Oort" />

Oort reasoned that comets with orbits that closely approach the Sun cannot have been doing so since the condensation of the protoplanetary disc, more than 4.5 billion years ago. Hence long-period comets could not have formed in the current orbits in which they are always discovered and must have been held in an outer reservoir for nearly all of their existence.<ref name=Oort /><ref name=dave /><ref name=book />

Oort also studied tables of [[Ephemeris|ephemerides]] for long-period comets and discovered that there is a curious concentration of long-period comets whose farthest retreat from the Sun (their [[aphelion|aphelia]]) cluster around 20,000&nbsp;AU. This suggested a reservoir at that distance with a spherical, [[Isotropy|isotropic]] distribution. He also proposed that the relatively rare comets with orbits of about 10,000&nbsp;AU probably went through one or more orbits into the inner Solar System and there had their orbits drawn inward by the [[gravity]] of the planets.<ref name="book" />