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109-565: 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 . The innermost portion of the Oort cloud is more than a thousand times as distant from the Sun as the Kuiper belt ,

218-421: A binary system allows the formation of circumstellar and circumbinary discs. The formation of such a disc will occur for any binary system in which infalling gas contains some degree of angular momentum. A general progression of disc formation is observed with increasing levels of angular momentum: The indicative timescale that governs the short-term evolution of accretion onto binaries within circumbinary disks

327-616: A circumstellar disc can be used to determine the timescales involved in its evolution. For example, observations of the dissipation process in transition discs (discs with large inner holes) estimate the average age of a circumstellar disc to be approximately 10 Myr. Dissipation process and its duration in each stage is not well understood. Several mechanisms, with different predictions for discs' observed properties, have been proposed to explain dispersion in circumstellar discs. Mechanisms like decreasing dust opacity due to grain growth, photoevaporation of material by X-ray or UV photons from

436-413: A coronagraph, adaptive optics or differential images to take an image of the disk with a telescope. These optical and infrared observations, for example with SPHERE , usually take an image of the star light being scattered on the surface of the disk and trace small micron-sized dust particles. Radio arrays like ALMA on the other hand can map larger millimeter-sized dust grains found in the mid-plane of

545-415: A curious concentration of long-period comets whose farthest retreat from the Sun (their aphelia ) cluster around 20,000 AU. This suggested a reservoir at that distance with a spherical, isotropic distribution. He also proposed that the relatively rare comets with orbits of about 10,000 AU probably went through one or more orbits into the inner Solar System and there had their orbits drawn inward by

654-519: A large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart and it is suggested that many—possibly the majority—of Oort cloud objects did not form in close proximity to the Sun. Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as

763-399: A quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material. A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years. Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models,

872-445: A quiet village in the province of Gelderland , where they sat out the war. In Hulshorst, he began writing a book on stellar dynamics. Before the war was over, he initiated, in collaboration with a Utrecht University student, Hendrik van de Hulst , a project that eventually succeeded, in 1951, in detecting the 21-centimeter radio emission from interstellar hydrogen spectral line at radio frequencies. Oort and his colleagues also made

981-453: A radius of 2,000–20,000 AU (0.03–0.32 ly). The inner Oort cloud is sometimes known as the Hills cloud, named for Jack G. Hills , who proposed its existence in 1981. Models predict the inner cloud to be the much denser of the two, having tens or hundreds of times as many cometary nuclei as the outer cloud. The Hills cloud is thought to be necessary to explain the continued existence of

1090-468: A speech in which he started by saying, 'I won't talk about his dismissal and I shall leave the people who did this, below us, but will concentrate on the greatness of the man dismissed by our aggressors.'" This speech (26 November 1940) made such an impression on all his students that on leaving the auditorium they defiantly sang the anthem of the Netherlands and went on strike. Oort was present for

1199-505: A typical dynamically old comet with an origin in the Oort cloud could be C/2018 F4. Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by the tidal force exerted by the Milky Way . Just as the Moon 's tidal force deforms Earth's oceans, causing the tides to rise and fall, the galactic tide also distorts

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1308-917: A very long timescale. As mentioned, circumstellar discs are not equilibrium objects, but instead are constantly evolving. The evolution of the surface density Σ {\displaystyle \Sigma } of the disc, which is the amount of mass per unit area so after the volume density at a particular location in the disc has been integrated over the vertical structure, is given by: ∂ Σ ∂ t = 3 r ∂ ∂ r [ r 1 / 2 ∂ ∂ r ν Σ r 1 / 2 ] {\displaystyle {\frac {\partial \Sigma }{\partial t}}={\frac {3}{r}}{\frac {\partial }{\partial r}}\left[r^{1/2}{\frac {\partial }{\partial r}}\nu \Sigma r^{1/2}\right]} where r {\displaystyle r}

1417-569: A well known astronomer, Jacobus Cornelius Kapteyn , was teaching there, although Oort was unsure whether he wanted to specialize in physics or astronomy. After studying with Kapteyn, Oort decided on astronomy. "It was the personality of Professor Kapteyn which decided me entirely", he later recalled. "He was quite an inspiring teacher and especially his elementary astronomy lectures were fascinating." Oort began working on research with Kapteyn early in his third year. According to Oort one professor at Groningen who had considerable influence on his education

1526-423: Is a torus , pancake or ring-shaped accretion disk of matter composed of gas , dust , planetesimals , asteroids , or collision fragments in orbit around a star . Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place, and around white dwarfs , they indicate that planetary material survived

1635-436: Is an empirical connection between accretion from a disc onto the star and ejections in an outflow. Mid-disc dissipation , occurs at the mid-disc region (1-5 AU ) and is characterized for the presence of much more cooler material than in the inner part of the disc. Consequently, radiation emitted from this region has greater wavelength , indeed in the mid-infrared region, which makes it very difficult to detect and to predict

1744-413: Is approximately five times the binary orbital period. For eccentric binaries, the period of accretion variability is the same as the binary orbital period due to each binary component scooping in matter from the circumbinary disk each time it reaches the apocenter of its orbit. Eccentric binaries also see accretion variability over secular timescales hundreds of times the binary period. This corresponds to

1853-464: Is credited by colleagues with putting the Netherlands in the forefront of postwar astronomy. Oort also investigated the source of the light from the Crab Nebula, finding that it was polarized , and probably produced by synchrotron radiation , confirming a hypothesis by Iosif Shklovsky . Oort went on to study comets, which he formulated a number of revolutionary hypotheses. He hypothesized that

1962-482: Is hypothesized that 70,000 years ago Scholz's Star passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect). During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710 . This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution. In 1984, physicist Richard A. Muller postulated that

2071-499: Is hypothesized that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System. This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in

2180-464: Is known as the T Tauri star stage. Within this disc, the formation of small dust grains made of rocks and ices can occur, and these can coagulate into planetesimals . If the disc is sufficiently massive, the runaway accretions begin, resulting in the appearance of planetary embryos. The formation of planetary systems is thought to be a natural result of star formation. A sun-like star usually takes around 100 million years to form. The infall of gas onto

2289-610: Is quite far away from the subjects I had so far been studying." He later considered his experience at Yale useful as he became interested in "problems of fundamental astronomy that [he] felt was capitalized on later, and which certainly influenced [his] future lectures in Leiden." Personally, he "felt somewhat lonesome in Yale", but also said that "some of my very best friends were made in these years in New Haven." In 1924, Oort returned to

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2398-606: Is responsible for a number of important insights about comets, including the realization that their orbits "implied there was a lot more solar system than the region occupied by the planets." The Oort cloud , the Oort constants , Oort Limit , an impact crater on Pluto ( Oort ), and the asteroid 1691 Oort were all named after him. Oort was born in Franeker , a small town in the Dutch province of Friesland , on April 28, 1900. He

2507-604: Is the binary's orbital period P b {\displaystyle P_{b}} . Accretion into the inner cavity is not constant, and varies depending on e b {\displaystyle e_{b}} and the behavior of the gas along the innermost region of the cavity. For non-eccentric binaries, accretion variability coincides with the Keplerian orbital period of the inner gas, which develops lumps corresponding to m = 1 {\displaystyle m=1} outer Lindblad resonances. This period

2616-446: Is the inner radius. Protoplanetary disks and debris disks can be imaged with different methods. If the disk is seen edge-on, the disk can sometimes block the light of the star and the disk can be directly observed without a coronagraph or other advanced techniques (e.g. Gomez's Hamburger or Flying Saucer Nebula ). Other edge-on disks (e.g. Beta Pictoris or AU Microscopii ) and face-on disks (e.g. IM Lupi or AB Aurigae ) require

2725-406: Is the radial location in the disc and ν {\displaystyle \nu } is the viscosity at location r {\displaystyle r} . This equation assumes axisymmetric symmetry in the disc, but is compatible with any vertical disc structure. Viscosity in the disc, whether molecular, turbulent or other, transports angular momentum outwards in the disc and most of

2834-883: The "Whipple Mission" was proposed. It would monitor distant stars with a photometer, looking for transits up to 10,000 AU away. The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission. It was also suggested that the Kepler space telescope could have been capable of detecting objects in the Oort cloud. Solar System   → Local Interstellar Cloud   → Local Bubble   → Gould Belt   → Orion Arm   → Milky Way   → Milky Way subgroup   → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster   → Local Hole   → Observable universe   → Universe Each arrow ( → ) may be read as "within" or "part of". Circumstellar disc A circumstellar disc (or circumstellar disk )

2943-575: The Estonian 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 . 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: Oort reasoned that comets with orbits that closely approach

3052-405: 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. The origin of these comets

3161-421: The apsidal precession rate of the inner edge of the cavity, which develops its own eccentricity e d {\displaystyle e_{d}} , along with a significant region of the inner circumbinary disk up to ∼ 10 a b {\displaystyle \sim 10a_{b}} . This eccentricity may in turn affect the inner cavity accretion as well as dynamics further out in

3270-435: The gravity of the planets. The Oort cloud is thought to occupy a vast space somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly) from the Sun to as far out as 50,000 AU (0.79 ly) or even 100,000 to 200,000 AU (1.58 to 3.16 ly). The region can be subdivided into a spherical outer Oort cloud with a radius of some 20,000–50,000 AU (0.32–0.79 ly) and a torus -shaped inner Oort cloud with

3379-435: The scattered disc and the detached objects —three nearer reservoirs of trans-Neptunian objects . 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. The outer Oort cloud is only loosely bound to the Solar System and its constituents are easily affected by

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3488-472: The Milky Way and other galaxies and their distribution until shortly before his death at 92. One of Oort's strengths, according to one source, was his ability to "translate abstruse mathematical papers into physical terms," as exemplified by his translation of the difficult mathematical terms of Lindblad's theory of differential galactic rotation into a physical model. Similarly, he "derived the existence of

3597-423: The Netherlands . Soon after, the occupying regime dismissed all Jewish professors from Leiden University and other universities. "Among the professors who were dismissed", Oort later recalled, "was a very famous … professor of law by the name of Meyers. On the day when he got the letter from the authorities that he could no longer teach his classes, the dean of the faculty of law went into his class … and delivered

3706-437: The Netherlands to work at Leiden University , where he served as a research assistant, becoming Conservator in 1926, Lecturer in 1930, and Professor Extraordinary in 1935. In 1926, he received his doctorate from Groningen with a thesis on the properties of high-velocity stars. The next year, Swedish astronomer Bertil Lindblad proposed that the rate of rotation of stars in the outer part of the galaxy decreased with distance from

3815-415: The Oort cloud after billions of years. Because it lies at the interface between the dominion of Solar and galactic gravitation, the objects comprising the outer Oort cloud are only weakly bound to the Sun. This in turn allows small perturbations from nearby stars or the Milky Way itself to inject long-period (and possibly Halley-type ) comets inside the orbit of Neptune . This process ought to have depleted

3924-493: The Solar System have either already stopped functioning or are predicted to stop functioning before they reach the Oort cloud. In the 1980s, there was a concept for a probe that could reach 1,000 AU in 50 years, called TAU ; among its missions would be to look for the Oort cloud. In the 2014 Announcement of Opportunity for the Discovery program , an observatory to detect the objects in the Oort cloud (and Kuiper belt) called

4033-494: The Solar System is surrounded by a massive cloud consisting of billions of comets, many of them "long-period" comets that originate in a cloud far beyond the orbits of Neptune and Pluto . This cloud is now known as the Oort Cloud. He also realized that these external comets, from beyond Pluto, can "become trapped into tighter orbits by Jupiter , and become periodic comets, like Halley's comet ." According to one source, "Oort

4142-439: The Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU. Very long-period comets, such as C/1999 F1 (Catalina) , whose orbits last for millions of years, are thought to originate directly from

4251-464: The Sun as part of the same process that formed the planets and minor planets . After formation, strong gravitational interactions with young gas giants, such as Jupiter, scattered the objects into extremely wide elliptical or parabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region. Recent research has been cited by NASA hypothesizing that

4360-399: 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. Oort also studied tables of ephemerides for long-period comets and discovered that there is

4469-508: The Sun has an as-yet undetected companion, either a brown dwarf or a red dwarf , in an elliptical orbit within the Oort cloud. This object, known as Nemesis , was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as crater counts ), have thrown its existence into doubt. Recent scientific analysis no longer supports

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4578-510: The Sun). The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide. Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward

4687-417: The Sun, in the protoplanetary disc , and was then scattered far into space through the gravitational influence of the giant planets . No direct observation of the Oort cloud is possible with present imaging technology. 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

4796-448: The Sun. The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 au, and marks the outer boundary of the Oort cloud. Some scholars theorize that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia (smallest distances to the Sun) of planetesimals with large aphelia (largest distances to

4905-509: The absorption in the galactic plane was as bad as it turned out to be." Until Oort began his work, he later recalled, "the Leiden Observatory had been concentrating entirely on positional astronomy, meridian circle work and some proper motion work. But no astrophysics or anything that looked like that. No structure of the galaxy, no dynamics of the galaxy. There was no one else in Leiden who was interested in these problems in which I

5014-412: The acceleration of interstellar clouds by O-type stars . He went on to study high-velocity clouds. Oort served as director of the Leiden Observatory until 1970. After his retirement, he wrote comprehensive articles on the galactic center and on superclusters and published several papers on the quasar absorption lines, supporting Yakov Zel'dovich's pancake model of the universe. He also continued researching

5123-452: The agglomeration of larger objects into planetesimals , and the growth and orbital evolution of planetesimals into the planetary systems, like our Solar System or many other stars. Major stages of evolution of circumstellar discs: Material dissipation is one of the processes responsible for circumstellar discs evolution. Together with information about the mass of the central star, observation of material dissipation at different stages of

5232-499: The area of the Oort cloud. Voyager 1 , the once fastest and farthest of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years and would take about 30,000 years to pass through it. However, around 2025, the radioisotope thermoelectric generators on Voyager 1 will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by Voyager 1. The other four probes currently escaping

5341-501: The binary plane, but it is possible for processes such as the Bardeen-Petterson effect, a misaligned dipole magnetic field and radiation pressure to produce a significant warp or tilt to an initially flat disk. Strong evidence of tilted disks is seen in the systems Her X-1, SMC X-1, and SS 433 (among others), where a periodic line-of-sight blockage of X-ray emissions is seen on the order of 50–200 days; much slower than

5450-457: The binary's parameters, such as the mass ratio q b {\displaystyle q_{b}} and eccentricity e b {\displaystyle e_{b}} , as well as the thermodynamics of the accreting gas. Once a circumstellar disk has formed, spiral density waves are created within the circumstellar material via a differential torque due to the binary's gravity. The majority of these discs form axissymmetric to

5559-519: The central star ( stellar wind ), or the dynamical influence of a giant planet forming within the disc are some of the processes that have been proposed to explain dissipation. Dissipation is a process that occurs continuously in circumstellar discs throughout the lifetime of the central star, and at the same time, for the same stage, is a process that is present in different parts of the disc. Dissipation can be divided in inner disc dissipation, mid-disc dissipation, and outer disc dissipation, depending on

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5668-646: The children, of course, to grow up in." Oort's younger brother, John, became a professor of plant diseases at the University of Wageningen . In addition to John, Oort had two younger sisters and an elder brother who died of diabetes when he was a student. Oort attended primary school in Oegstgeest and secondary school in Leiden, and in 1917 went to Groningen University to study physics. He later said that he had become interested in science and astronomy during his high-school years, and conjectured that his interest

5777-400: The comet cloud on the outskirts of the Solar System from the observations, using the mathematics needed in dynamics, but then deduced the origin of this cloud using general physical arguments and a minimum of mathematics." In 1927, Oort married Johanna Maria (Mieke) Graadt van Roggen (1906–1993). They had met at a university celebration at Utrecht, where Oort's brother was studying biology at

5886-659: The director of Leiden Observatory ; the next year he became General Secretary of the International Astronomical Union (IAU), a post he held until 1948; in 1937 he was elected to the Royal Academy . In 1939, he spent half a year in the U.S., and became interested in the Crab Nebula , concluding in a paper, written with American astronomer Nicholas Mayall , that it was the result of a supernova explosion. In 1940, Nazi Germany invaded

5995-502: The disk, such as circumbinary planet formation and migration. It was originally believed that all binaries located within circumbinary disk would evolve towards orbital decay due to the gravitational torque of the circumbinary disk, primarily from material at the innermost edge of the excised cavity. This decay is no longer guaranteed when accretion from the circumbinary disk onto the binary occurs, and can even lead to increased binary separations. The dynamics of orbital evolution depend on

6104-527: The disk. Jan Oort Jan Hendrik Oort ForMemRS ( / ˈ ɔːr t / or / ˈ ʊər t / ; 28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy . The New York Times called him "one of the century's foremost explorers of the universe"; the European Space Agency website describes him as "one of

6213-404: The disk. Radio arrays like ALMA can also detect narrow emission from the gas of the disk. This can reveal the velocity of the gas within and around the disk. In some cases an edge-on protoplanetary disk (e.g. CK 3 or ASR 41 ) can cast a shadow onto the surrounding dusty material. This cast shadow works like a shadow play , and the projection of the disk is much larger than the true size of

6322-540: The early years of her marriage", he recalled. "But my father was less interested in Church matters." In 1903 Oort's parents moved to Oegstgeest , near Leiden , where his father took charge of the Endegeest Psychiatric Clinic. Oort's father, "was a medical director in a sanitorium for nervous illnesses. We lived in the director's house of the sanitorium, in a small forest which was very nice for

6431-490: The eclipsing binary TY CrA). For disks orbiting a low secondary-to-primary mass ratio binary, a tilted circumbinary disc will undergo rigid precession with a period on the order of years. For discs around a binary with a mass ratio of one, differential torques will be strong enough to tear the interior of the disc apart into two or more separate, precessing discs. A study from 2020 using ALMA data showed that circumbinary disks around short period binaries are often aligned with

6540-536: The end of the war, Oort returned to Leiden, took over as director of the Observatory, and became Full Professor of Astronomy. During this immediate postwar period, he led the Dutch group that built radio telescopes at Radio Kootwijk , Dwingeloo, and Westerbork and used the 21-centimeter line to map the Milky Way, including the large-scale spiral structure, the Galactic Center , and gas cloud motions. Oort

6649-469: The first investigation of the central region of the Galaxy, and discovered that "the 21-centimeter radio emission passed un-absorbed through the gas clouds that had hidden the center from optical observation. They found a huge concentration of mass there, later identified as mainly stars, and also discovered that much of the gas in the region was moving rapidly outward away from the center." In June 1945, after

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6758-570: The first map of the Galaxy was made." For a brief period, before the completion of the Jodrell Bank telescope, the Dwingeloo instrument was the largest of its kind on Earth. It has been written that "Oort was probably the first astronomer to realize the importance" of radio astronomy. "In the days before radio telescopes," one source notes, "Oort was one of the few scientists to realise the potential significance of using radio waves to search

6867-586: The formation of a gaseous protoplanetary disc around the young, rotating star. The former is a rotating circumstellar disc of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, mainly in the form of gas which is itself mainly hydrogen . The main accretion phase lasts a few million years, with accretion rates typically between 10 and 10 solar masses per year (rates for typical systems presented in Hartmann et al. ). The disc gradually cools in what

6976-471: The formation of a non-volatile crust on the surface. Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in the outer-planet region would be several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of Jupiter , which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker–Levy 9 in 1994. An example of

7085-440: The galactic core, and Oort, who later said that he believed it was his colleague Willem de Sitter who had first drawn his attention to Lindblad's work, realized that Lindblad was correct and that the truth of his proposition could be demonstrated observationally. Oort provided two formulae that described galactic rotation; the two constants that figured in these formulae are now known as "Oort's constants". Oort "argued that just as

7194-587: The galactic tide or stellar perturbations alone, and that the most likely cause would be a Jupiter -mass object in a distant orbit. This hypothetical gas giant was nicknamed Tyche . The WISE mission , an all-sky survey using parallax measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis. In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it. Space probes have yet to reach

7303-403: The gravitational pulls of both passing stars and the Milky Way itself. 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. By the same token, galactic interference in

7412-501: The greatest astronomers of the 20th century" and states that he "revolutionised astronomy through his ground-breaking discoveries." In 1955, Oort's name appeared in Life magazine's list of the 100 most famous living people. He has been described as "putting the Netherlands in the forefront of postwar astronomy". Oort determined that the Milky Way rotates and overturned the idea that the Sun

7521-403: The heavens. His theoretical research suggested that vast clouds of hydrogen lingered in the spiral arms of the Galaxy. These molecular clouds, he predicted, were the birthplaces of stars." These predictions were confirmed by measurements made at the new radio observatories at Dwingeloo and Westerbork . Oort later said that "it was Grote Reber 's work which first impressed me and convinced me of

7630-488: The idea that extinctions on Earth happen at regular, repeating intervals. Thus, the Nemesis hypothesis is no longer needed to explain current assumptions. A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the postulated Oort cloud than can be explained by

7739-400: The inner Solar System. Besides the galactic tide , the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun's Oort cloud and the gravitational fields of nearby stars or giant molecular clouds . The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems . For example, it

7848-453: The inner Solar System. The cloud may also serve the same function for many of the centaurs and Jupiter-family comets . By the turn of the 20th century, it was understood that there were two main classes of comet: short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits aligned near the ecliptic plane and are not found much farther than

7957-463: The lecture and was greatly impressed. This occasion formed the beginning of the active resistance in Holland. The speech by Rudolph Cleveringa , the dean of the faculty of Law and former graduate student of professor Meijers, was widely circulated during the rest of the war by the resistance groups. Oort was in a little group of professors in Leiden who came together regularly and discussed the problems

8066-646: The mass inwards, eventually accreting onto the central object. The mass accretion onto the star M ˙ {\displaystyle {\dot {M}}} in terms of the disc viscosity ν {\displaystyle \nu } is expressed: M ˙ = 3 π ν Σ [ 1 − r in r ] − 1 {\displaystyle {\dot {M}}=3\pi \nu \Sigma \left[1-{\sqrt {\frac {r_{\text{in}}}{r}}}\right]^{-1}} where r in {\displaystyle r_{\text{in}}}

8175-423: The motion of Oort bodies occasionally dislodges comets from their orbits within the cloud, sending them into the inner Solar System . 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. Astronomers hypothesize that the material presently in the Oort cloud formed much closer to

8284-475: The nearly spherical shape of the outer Oort cloud. On the other hand, the Hills cloud , which is bound more strongly to the Sun, has not acquired a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud's formation, since

8393-421: The number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations. In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in its birth cluster ." Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from

8502-470: The number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected. The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material. Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains

8611-424: The orbit of the binary. Binaries with a period longer than one month showed typically a misalignment of the disk with the binary orbit. Stages in circumstellar discs refer to the structure and the main composition of the disc at different times during its evolution. Stages include the phases when the disc is composed mainly of submicron-sized particles, the evolution of these particles into grains and larger objects,

8720-421: The orbits of bodies in the outer Solar System . In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun, but in the outer reaches of the system, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational Galactic Center compresses it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to

8829-500: The original protosolar cloud, a conclusion also supported by studies of granular size in Oort-cloud comets and by the recent impact study of Jupiter-family comet Tempel 1 . The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago. The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to

8938-475: The outer Oort cloud. Other comets modeled to have come directly from the outer Oort cloud include C/2006 P1 (McNaught) , C/2010 X1 (Elenin) , Comet ISON , C/2013 A1 (Siding Spring) , C/2017 K2 , and C/2017 T2 (PANSTARRS) . The orbits within the Kuiper belt are relatively stable, and so very few comets are thought to originate there. The scattered disc, however, is dynamically active, and is far more likely to be

9047-482: The outer cloud are separated by a significant fraction of 1 AU, tens of millions of kilometres. The outer cloud's total mass is not known, but assuming that Halley's Comet is a suitable proxy for the nuclei composing the outer Oort cloud, their combined mass would be roughly 3 × 10 kilograms (6.6 × 10 lb), or five Earth masses. Formerly the outer cloud was thought to be more massive by two orders of magnitude, containing up to 380 Earth masses, but improved knowledge of

9156-518: The outer planets appear to us to be overtaken and passed by the less distant ones in the solar system, so too with the stars if the Galaxy really rotated", according to the Oxford Dictionary of Scientists . He "was finally able to calculate, on the basis of the various stellar motions, that the Sun was some 30,000 light-years from the center of the Galaxy and took about 225 million years to complete its orbit. He also showed that stars lying in

9265-503: The outer regions of the galactic disk rotated more slowly than those nearer the center. The Galaxy does not therefore rotate as a uniform whole but exhibits what is known as 'differential rotation'." These early discoveries by Oort about the Milky Way overthrew the Kapteyn system , named after his mentor, which had envisioned a galaxy that was symmetrical around the Sun. As Oort later noted, "Kapteyn and his co-workers had not realized that

9374-438: The pace of accretion and collision slowed and depletion began to overtake supply. Models by Julio Ángel Fernández suggest that the scattered disc , which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward toward the Oort cloud, whereas a quarter are shifted inward to Jupiter's orbit, and

9483-410: The part of the disc considered. Inner disc dissipation occurs at the inner part of the disc (< 0.05 – 0.1 AU ). Since it is closest to the star, this region is also the hottest, thus material present there typically emits radiation in the near-infrared region of the electromagnetic spectrum . Study of the radiation emitted by the very hot dust present in that part of the disc indicates that there

9592-508: The place of origin for comets. Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs . These centaurs are then sent farther inward to become the short-period comets. There are two main varieties of short-period comets: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. Halley-family comets, named for their prototype, Halley's Comet , are unusual in that although they are short-period comets, it

9701-413: The protoplanetary discs of other stars." In July 2020 Amir Siraj and Avi Loeb found that a captured origin for the Oort Cloud in the Sun's birth cluster could address the theoretical tension in explaining the observed ratio of outer Oort cloud to scattered disc objects, and in addition could increase the chances of a captured Planet Nine . Comets are thought to have two separate points of origin in

9810-448: The scattered disc. Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No dynamical process is known to explain the smaller number of observed comets than Oort estimated. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles , rendering some comets invisible, or

9919-424: The size distribution of long-period comets has led to lower estimates. No estimates of the mass of the inner Oort cloud have been published as of 2023. If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water , methane , ethane , carbon monoxide and hydrogen cyanide . However, the discovery of the object 1996 PW , an object whose appearance

10028-596: The sparser, outer cloud and yet long-period comets with orbits well above or below the ecliptic continue to be observed. The Hills cloud is thought to be a secondary reservoir of cometary nuclei and the source of replenishment for the tenuous outer cloud as the latter's numbers are gradually depleted through losses to the inner Solar System. The outer Oort cloud may have trillions of objects larger than 1 km (0.6 mi), and billions with diameters of 20-kilometre (12 mi). This corresponds to an absolute magnitude of more than 11. On this analysis, "neighboring" objects in

10137-573: The stars in the galaxy were "travelling independently through space, with those nearer the center rotating much faster than those further away." This breakthrough made Oort famous in the world of astronomy. In the early 1930s he received job offers from Harvard and Columbia University , but chose to stay at Leiden, although he did spend half of 1932 at the Perkins Observatory , in Delaware , Ohio . In 1934, Oort became assistant to

10246-492: The systems' binary orbit of ~1 day. The periodic blockage is believed to result from precession of a circumprimary or circumbinary disk, which normally occurs retrograde to the binary orbit as a result of the same differential torque which creates spiral density waves in an axissymmetric disk. Evidence of tilted circumbinary disks can be seen through warped geometry within circumstellar disks, precession of protostellar jets, and inclined orbits of circumplanetary objects (as seen in

10355-484: The three people who made a new translation of the Bible into Dutch." The reference is to Henricus Oort (1836–1927), who was the grandson of a famous Rotterdam preacher and, through his mother, Dina Maria Blom, the grandson of theologian Abraham Hermanus Blom, a "pioneer of modern biblical research". Several of Oort's uncles were pastors, as was his maternal grandfather. "My mother kept up her interests in that, at least in

10464-446: The time. Oort and his wife had two sons, Coenraad (Coen) and Abraham , and a daughter, Marijke. Abraham became a professor of climatology at Princeton University . According to the website of Leiden University, Oort was very interested in and knowledgeable about art. "[W]hen visiting another country he would always try to take some time off to visit the local museums and exhibitions…and in the fifties served for some years as chairman of

10573-624: The timescale of this region's dissipation. Studies made to determine the dissipation timescale in this region provide a wide range of values, predicting timescales from less than 10 up to 100 Myr. Outer disc dissipation occurs in regions between 50 – 100 AU , where temperatures are much lower and emitted radiation wavelength increases to the millimeter region of the electromagnetic spectrum . Mean dust masses for this region has been reported to be ~ 10 solar masses. Studies of older debris discs (10 - 10 yr) suggest dust masses as low as 10 solar masses, implying that diffusion in outer discs occurs on

10682-425: The unique importance of radio observations for surveying the galaxy." Just before the war, Reber had published a study of galactic radio emissions. Oort later commented, "The work of Grote Reber made it quite clear [radio astronomy] would be a very important tool for investigating the Galaxy, just because it could investigate the whole disc of the galactic system unimpeded by absorption." Oort's work in radio astronomy

10791-536: The university faced in view of the German occupation. Most of the members of this group were put in hostage camps soon after the speech by Cleveringa. Oort refused to collaborate with the occupiers, "and so we went down to live in the country for the rest of the war." Resigning from the Royal Academy, from his professorial post at Leiden, and from his position at the Observatory, Oort took his family to Hulshorst,

10900-401: The whole of stellar evolution. Such a disc can manifest itself in various ways. According to the widely accepted model of star formation, sometimes referred to as the nebular hypothesis , a young star ( protostar ) is formed by the gravitational collapse of a pocket of matter within a giant molecular cloud . The infalling material possesses some amount of angular momentum , which results in

11009-485: Was at its center. He also postulated the existence of the mysterious invisible dark matter in 1932, which is believed to make up roughly 84.5% of the total mass in the Universe and whose gravitational pull causes "the clustering of stars into galaxies and galaxies into connecting strings of galaxies". He discovered the galactic halo , a group of stars orbiting the Milky Way but outside the main disk. Additionally Oort

11118-452: Was consistent with a D-type asteroid in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids. Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from

11227-514: Was helped in this project by the Dutch telecommunications company, PTT, which, he later explained, "had under their care all the radar equipment that was left behind by the Germans on the coast of Holland. This radar equipment consisted in part of reflecting telescopes of 7 1/2 meter aperture.... Our radio astronomy was really started with the aid of one of these instruments… it was in Kootwijk that

11336-536: 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. In 1932,

11445-487: Was one of the few people to have seen Comet Halley on two separate apparitions. At the age of 10, he was with his father on the shore at Noordwijk, Netherlands, when he first saw the comet. In 1986, 76 years later, he went up in a plane and was able to see the famous comet once more." In 1951 Oort and his wife spent several months in Princeton and Pasadena , an interlude that led to a paper by Oort and Lyman Spitzer on

11554-635: Was physicist Frits Zernike . After taking his final exam in 1921, Oort was appointed assistant at Groningen, but in September 1922, he went to the United States to do graduate work at Yale and to serve as an assistant to Frank Schlesinger of the Yale Observatory . At Yale, Oort was responsible for making observations with the Observatory's zenith telescope . "I worked on the problem of latitude variation", he later recalled, "which

11663-498: Was principally interested, so the first years I worked more or less by myself in these projects. De Sitter was interested, but his main line of research was celestial mechanics; at that time the expanding universe had moved away from his direct interest." As the European Space Agency states, Oort "sh[ook] the scientific world by demonstrating that the Milky Way rotates like a giant 'Catherine Wheel'." He showed that all

11772-483: Was stimulated by reading Jules Verne . His one hesitation about studying pure science was the concern that it "might alienate one a bit from people in general", as a result of which "one might not develop the human factor sufficiently." But he overcame this concern and ended up discovering that his later academic positions, which involved considerable administrative responsibilities, afforded a good deal of opportunity for social contact. Oort chose Groningen partly because

11881-430: Was the second son of Abraham Hermanus Oort, a physician, who died on May 12, 1941, and Ruth Hannah Faber, who was the daughter of Jan Faber and Henrietta Sophia Susanna Schaaii, and who died on November 20, 1957. Both of his parents came from families of clergymen, with his paternal grandfather, a Protestant clergyman with liberal ideas, who "was one of the founders of the more liberal Church in Holland" and who "was one of

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