Radiation astronomy/Scattered disks

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The diagram shows scattered disc objects out to 100 AU. Credit: Eurocommuter.

Scattered Disk Objects (up to 100 AU): Kuiper Belt objects are shown in grey, resonant objects within the Scattered Disk are shown in green.

The position of an object represents

  • its orbit’s semi-major axis a in AU and the orbital period in years (horizontal axis)
  • its orbit’s inclination i in degrees (vertical axis).

The size of the circle illustrates the object’s size relative to others. For a few large objects, the diameter drawn represents the best current estimates. For all others, the circles represent the absolute magnitude of the object.

The eccentricity of the orbit is shown indirectly by a segment extending from the left (perihelion) to the aphelion to the right. In other words, the segment illustrates the variations of the object's distance from the Sun. Objects with nearly circular orbits will show short segments while highly elliptical orbits will be represented by long segments.

Main resonances with Neptune are marked with vertical bars; 1:1 marks the position of Neptune’s orbit (and its Trojan asteroids), 2:3 marks the orbit of Pluto (and plutinos) etc.

The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5 x 109 km; 2.8 x 109 mi.). While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System".[1]

The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects traditionally called the Kuiper belt,[2] but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the Kuiper belt proper. The literature is inconsistent in the use of the phrases "scattered disc" and "Kuiper belt"; for some, they are distinct populations; for others, the scattered disc is part of the Kuiper belt and authors may even switch between these two uses in a single publication.[3]

Periodic comets[edit | edit source]

This image shows Comet 67P/Churyumov-Gerasimenko. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/ UPM/DASP/IDA.
This image shows Comet 67P/Churyumov-Gerasimenko rotated around a vertical axis from the right. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/ UPM/DASP/IDA.
This is an image of the nucleus of Comet 67P/Churyumov-Gerasimenko by Rosetta. Credit: ESA Rosetta Mission.{{free media}}
Single frame Rosetta spacecrast NAVCAM image of Comet 67P/C-G was taken on 6 March from a distance of 82.9 km to the comet. Credit: ESA/Rosetta/NAVCAM.{{free media}}
Images taken by the Rosetta navigation camera (NAVCAM) on 19 September 2014 at 28.6 km (17.8 mi) from the centre of comet 67P/Churyumov–Gerasimenko. Credit: ESA/Rosetta/NAVCAM.{{free media}}
Four-image montage comprises images taken by Rosetta's navigation camera from a distance of 9.8 km from the centre of comet 67P/C-G – about 7.8 km from the surface. Credit: ESA/Rosetta/NAVCAM.{{free media}}
Image is taken by Rosetta's navigation camera from a distance of 9.8 km from the centre of comet 67P/C-G Credit: ESA/Rosetta/NAVCAM.{{free media}}

The image at the right is an optical astronomy image of the comet 67P/Churyumov-Gerasimenko. Rosetta's OSIRIS narrow-angle camera made the image on 3 August 2014 from a distance of 285 km. The image resolution is 5.3 metres/pixel.

The left image is rotated 90° from the right. The location of the right image is the front view of the left side just out of view in the left image. The object rotates by the right hand rule from the left image to the right.

Note that due to the evaporation of volatiles, the surface of the rocky object appears pitted or cratered.

Def. a "comet which orbits the Sun and which returns to the innermost point of its orbit at known, regular intervals"[4] is called a periodic comet. "The short period comets have orbital periods <20 years and low inclination. Their orbits are controlled by Jupiter and thus they are also called Jupiter Family comets. [...] Because the orbit crosses that of Jupiter, the comet will have gravitational interactions with this massive planet. The objects orbit will gradually change from these interactions and eventually the object will either be thrown out of the Solar System or collide with a planet or the Sun."[5]

Perihleion distance in AU = 1.243, eccentricity = 0.641, inclination = 7.0, and orbital period in years = 2.745.[6]

Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets in the Solar System, with the centaurs, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object's migration from the disc to the inner Solar System.[7]

Centaurs[edit | edit source]

Positions of known outer Solar System objects.
The centaurs lie generally inwards of the Kuiper belt and outside the Jupiter trojans.
  Jupiter trojans (6,178)
  Scattered disc (>300)   Neptune trojans (9)
  Giant planets: J ··· N
  Centaurs (44,000)
  Kuiper belt (>100,000)
Credit: WilyD.
Colour distribution of centaurs is shown. Credit: Eurocommuter~commonswiki.

Def. an "icy planetoid that orbits the Sun between Jupiter and Neptune"[8] is called a Centaur.

"The recent investigation of the orbital distribution of Centaurs (Emel’yanenko et al., 2005) showed that there are two dynamically distinct classes of Centaurs, a dominant group with semimajor axes a > 60 AU and a minority group with a < 60 AU."[9] "[T]he intrinsic number of such objects is roughly an order of magnitude greater than that for a<60 AU".[9]

Centaurs are small Solar System bodies with a semi-major axis between those of the outer planets, generally have unstable orbits because they cross or have crossed the orbits of one or more of the giant planets; almost all their orbits have dynamic lifetimes of only a few million years.[7] There is one centaur, 514107 Kaʻepaokaʻawela, which may be in a stable (though retrograde) orbit.[10] Centaurs typically behave with characteristics of both asteroids and comets and are named after the mythological centaurs that were a mixture of horse and human. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 kilometer.[7]

No centaur has been photographed up close, although there is evidence that Saturn's moon Phoebe, imaged by the Cassini–Huygens (Cassini) probe in 2004, may be a captured centaur that originated in the Kuiper belt.[11]

Even centaurs such as 2000 GM137 and 2001 XZ255}, which do not currently cross the orbit of any planet, are in gradually changing orbits that will be perturbed until they start to cross the orbit of one or more of the giant planets.[7]

The Minor Planet Center (MPC) defines centaurs as having a perihelion beyond the orbit of Jupiter (q > 5.2 AU) and a semi-major axis less than that of Neptune (a < 30.1 AU).[12]

The Jet Propulsion Laboratory (JPL) similarly defines centaurs as having a semi-major axis, a, between those of Jupiter (5.5 AU < a) and Neptune (a < 30.1 AU).[13]

The Deep Ecliptic Survey (DES) defines centaurs using a dynamical classification scheme. These classifications are based on the simulated change in behavior of the present orbit when extended over 10 million years. The DES defines centaurs as non-resonant objects whose instantaneous (osculating) perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation. This definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit.[14]

The collection The Solar System Beyond Neptune (2008) defines objects with a semi-major axis between those of Jupiter and Neptune and a Jupiter – Tisserand's parameter above 3.05 – as centaurs, classifying the objects with a Jupiter Tisserand's parameter below this and, to exclude Kuiper belt objects, an arbitrary perihelion cut-off half-way to Saturn (q < 7.35 AU) as Jupiter-family comets (This would make 60558 Echeclus (q = 5.8 AU, TJ = 3.03) and 52872 Okyrhoe (q = 5.8 AU; TJ = 2.95), which have traditionally been classified as centaurs, and 944 Hidalgo (q = 1.95 AU; TJ = 2.07), which has traditionally been considered an asteroid and is classified as a centaur by JPL, Jupiter-family comets, not centaurs.) and classifying those objects on unstable orbits with a semi-major axis larger than Neptune's as members of the scattered disc.[15]

Centaurs are objects that are non-resonant with a perihelion inside the orbit of Neptune that can be shown to likely cross the Hill sphere of a gas giant within the next 10 million years,[16] so that centaurs can be thought of as objects scattered inwards and that interact more strongly and scatter more quickly than typical scattered-disc objects.

The JPL Small-Body Database lists 452 centaurs.[17] There are an additional 116 trans-Neptunian objects (objects with a semi-major axis further than Neptune's, i.e. a > 30.1 AU) with a perihelion closer than the orbit of Uranus (q < 19.2 AU).[18]

The Committee on Small Body Nomenclature of the International Astronomical Union has adopted the following naming convention for such objects: Befitting their centaur-like transitional orbits between TNOs and comets, "objects on unstable, non-resonant, giant-planet-crossing orbits with semimajor axes greater than Neptune's" are to be named for other hybrid and shape-shifting mythical creatures. Thus far, only the binary objects 65489 Ceto and Phorcys and 42355 Typhon and Echidna have been named according to the new policy.[19]

Centaurs with measured diameters listed as possible dwarf planets include 10199 Chariklo, (523727) 2014 NW65, 2060 Chiron, and 54598 Bienor.[20]

The colours of centaurs are very diverse, which challenges any simple model of surface composition.[21] In the side-diagram, the colour indices are measures of apparent magnitude of an object through blue (B), visible (V) (i.e. green-yellow) and red (R) filters. The diagram illustrates these differences (in exaggerated colours) for all centaurs with known colour indices. For reference, two moons: Triton and Phoebe, and planet Mars are plotted (yellow labels, size not to scale).

Centaurs appear to be grouped into two classes:

  • very red – for example 5145 Pholus
  • blue (or blue-grey, according to some authors) – for example 2060 Chiron
Name Year Discoverer Half-life[7]
55576 Amycus 2002 Near Earth Asteroid Tracking (NEAT) at Palomar Observatory 11.1 Ma UK
54598 Bienor 2000 Marc W. Buie et al. ? U
10370 Hylonome 1995 Mauna Kea Observatory 6.3 Ma UN
10199 Chariklo 1997 Spacewatch 10.3 Ma U
8405 Asbolus 1995 Spacewatch (James V. Scotti) 0.86 Ma SN
7066 Nessus 1993 Spacewatch (David L. Rabinowitz) 4.9 Ma SK
5145 Pholus 1992 Spacewatch (David L. Rabinowitz) 1.28 Ma SN
2060 Chiron 1977 Charles T. Kowal 1.03 Ma SU

Trans-Neptunian objects[edit | edit source]

Distribution of trans-Neptunian objects, with semi-major axis on the horizontal, and inclination on the vertical axis. Scattered disc objects are shown in grey, objects that are in resonance with Neptune in red. Classical Kuiper belt objects (cubewanos) and sednoids are blue and yellow, respectively.
Comparison is of the largest TNOs: Pluto, Eris, Haumea, Makemake, 2007 OR10, Quaoar, Sedna, 2002 MS4, Orcus and Salacia. All except two of these TNOs (Sedna and 2002 MS4) are known to have moon(s). The top 4 are IAU-accepted dwarf planets while the bottom 6 are dwarf-planet candidates that are accepted as dwarf planets by several astronomers. Credit: Lexicon.{{free media}}

Over a thousand trans-Neptunian objects were detected between 1992 and 2006.[22]

As of October 2018, the catalog of minor planets contains 528 numbered and more than 2,000 unnumbered TNOs.[23] [24] [25][26][27]

Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects (ETNOs).[28]

Discrepancies in the early 1900s between the observed and expected orbits of Uranus and Neptune suggested that there were one or more additional planets beyond Neptune, but revised estimates of Neptune's mass from the Voyager 2 flyby in 1989 showed that the problem was spurious.[29]

While the relatively dimmer bodies, as well as the population as the whole, are reddish (V−I = 0.3–0.6), the bigger objects are often more neutral in colour (infrared index V−I < 0.2). This distinction leads to suggestion that the surface of the largest bodies is covered with ices, hiding the redder, darker areas underneath.[30]

Mean-color indices of dynamical groups in the outer Solar System [31]:35
Color Plutinos Cubewanos Centaurs Scattered disc object (SDOs) Comets Jupiter trojans
B–V 0.895±0.190 0.973±0.174 0.886±0.213 0.875±0.159 0.795±0.035 0.777±0.091
V–R 0.568±0.106 0.622±0.126 0.573±0.127 0.553±0.132 0.441±0.122 0.445±0.048
V–I 1.095±0.201 1.181±0.237 1.104±0.245 1.070±0.220 0.935±0.141 0.861±0.090
R–I 0.536±0.135 0.586±0.148 0.548±0.150 0.517±0.102 0.451±0.059 0.416±0.057

Pluto[edit | edit source]

Pluto fills the frame in an image from NASA's New Horizons spacecraft taken July 13, when the spacecraft was 476,000 miles from the dwarf planet. Credit: NASA EPA.
A composite image of Pluto from 11 July shows high-resolution black-and-white LORRI images colorized with Ralph data. Credit: NASA-JHUAPL-SWRI.
Pluto and its satellites, Charon, Hydra and Nix are imaged with the Hubble Space Telescope. Credit: H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team.
NASA's Hubble Space Telescope has obtained the clearest pictures ever of our solar system's most distant and enigmatic object: the planet Pluto. Credit: NASA on The Commons.{{free media}}
The first close-up image of Pluto released by NASA from the New Horizons mission. Credit: NASA-JHUAPL-SWRI.
Another mountain range is revealed in this new image taken by New Horizons on July 14. Credit: NASA-JHUAPL-SWRI.
The frozen plains of Sputnik Planum on Pluto are photographed by the LORRI instrument aboard New Horizons on 14 July 2015. Credit: NASA/JHUAPL/SwRI.{{free media}}
The International Astronomical Union (IAU) approved names of 14 surface features on Pluto in August 2017. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.{{free media}}

Pluto is the second largest dwarf planet known (after Eris].

In the image on the right are shown from left to right: Pluto, Charon, Nix and Hydra.

The observations on the left by Hubble were made with the European Space Agency's Faint Object Camera.

Pluto is a very peculiar object. Its orbit is tilted and is more elliptical than the orbits of any of the other planets in the solar system. Pluto also rotates upside down with its North Pole below the plane of the solar system in the opposite sense of the Earth and most of the other planets. Pluto is smaller than our own Moon and also denser than any of its neighbors in the outer solar system.

Pluto is currently near its closest approach to the Earth in its 249 year journey around the Sun, and is approximately four and a half billion kilometers away. The bright object at the center of the frame is Pluto while Charon is the fainter object in the lower left. Charon is fainter than Pluto because it is smaller and, probably, because its surface is covered by water ice whereas Pluto is thought to be covered mainly by the more reflective methane frost or snow. As indicated in the diagram at the bottom of the photograph, Charon's orbit around Pluto is a circle seen nearly edge on from Earth, with a radius of almost twenty thousand kilometers - a distance equal to approximately one and a half times the diameter of the Earth. At the time of observation, Charon was near its maximum apparent distance from Pluto, so that its angular separation was about nine tenths of an arcsecond. Because of the peculiar orientation of the Pluto-Charon orbit with respect to our line of sight, Charon approaches to within less than one tenth of an arcsecond of Pluto every three days.

"Over the weekend, scientists discovered that it is a bit bigger than they expected. Previous estimates had put it somewhere between 715 and 746 miles, but new information suggests Pluto's radius is about 736 miles across, putting it solidly at the upper end of the estimate."[32]

"At heights of about 11,000 feet, the Norgay Montes [on the right] most closely approximates the height of the Rockies."[33]

"NASA scientists identified this newfound range of water-ice mountains [on the left] based on high-res images obtained by the New Horizons spacecraft."[34]

"The mountain range is located in the dwarf planet's heart-shaped feature, called Tombaugh Regio after Pluto's discoverer, Clyde Tombaugh. The mountains rise to about the height of the Appalachian Mountains on Earth, which reach a maximum height of about 6,000 feet."[34]

In the latest data from NASA's New Horizons spacecraft, a new close-up image of Pluto reveals a vast, craterless plain that appears to be no more than 100 million years old, and is possibly still being shaped by geologic processes. This frozen region is north of Pluto's icy mountains, in the center-left of the heart feature, informally named "Tombaugh Regio" (Tombaugh Region).

This fascinating icy plains region -- resembling frozen mud cracks on Earth -- has been informally named "Sputnik Planum" (Sputnik Plain) after the Earth's first artificial satellite. It has a broken surface of irregularly-shaped segments, roughly 12 miles (20 kilometers) across, bordered by what appear to be shallow troughs. Some of these troughs have darker material within them, while others are traced by clumps of hills that appear to rise above the surrounding terrain. Elsewhere, the surface appears to be etched by fields of small pits that may have formed by a process called sublimation, in which ice turns directly from solid to gas, just as dry ice does on Earth.

The irregular shapes may be the result of the contraction of surface materials, similar to what happens when mud dries. Alternatively, they may be a product of convection, similar to wax rising in a lava lamp. On Pluto, convection would occur within a surface layer of frozen carbon monoxide, methane and nitrogen, driven by the scant warmth of Pluto's interior.

Pluto's icy plains also display dark streaks that are a few miles long. These streaks appear to be aligned in the same direction and may have been produced by winds blowing across the frozen surface.

The names, listed below, pay homage to the underworld mythology, pioneering space missions, historic pioneers who crossed new horizons in exploration, and scientists and engineers associated with Pluto and the Kuiper Belt.

  • Tombaugh Regio honors Clyde Tombaugh (1906–1997), the U.S. astronomer who discovered Pluto in 1930 from Lowell Observatory in Arizona.
  • Burney crater honors Venetia Burney (1918–2009), who as an 11-year-old schoolgirl suggested the name "Pluto" for Clyde Tombaugh's newly discovered planet. Later in life she taught mathematics and economics.
  • Sputnik Planitia is a large plain named for Sputnik 1, the first space satellite, launched by the Soviet Union in 1957.
  • Tenzing Montes and Hillary Montes are mountain ranges honoring Tenzing Norgay (1914–1986) and Sir Edmund Hillary (1919–2008), the Indian/Nepali Sherpa and New Zealand mountaineer were the first to reach the summit of Mount Everest and return safely.
  • Al-Idrisi Montes honors Ash-Sharif al-Idrisi (1100–1165/66), a noted Arab mapmaker and geographer whose landmark work of medieval geography is sometimes translated as "The Pleasure of Him Who Longs to Cross the Horizons."
  • Djanggawul Fossae defines a network of long, narrow depressions named for the Djanggawuls, three ancestral beings in indigenous Australian mythology who traveled between the island of the dead and Australia, creating the landscape and filling it with vegetation.
  • Sleipnir Fossa is named for the powerful, eight-legged horse of Norse mythology that carried the god Odin into the underworld.
  • Virgil Fossae honors Virgil, one of the greatest Roman poets and Dante's fictional guide through hell and purgatory in the Divine Comedy.
  • Adlivun Cavus is a deep depression named for Adlivun, the underworld in Inuit mythology.
  • Hayabusa Terra is a large land mass saluting the Japanese spacecraft and mission (2003–2010) that performed the first asteroid sample return.
  • Voyager Terra honors the pair of NASA spacecraft, launched in 1977, that performed the first "grand tour" of all four giant planets. The Voyager spacecraft are now probing the boundary between the Sun and interstellar space.
  • Tartarus Dorsa is a ridge named for Tartarus, the deepest, darkest pit of the underworld in Greek mythology.
  • Elliot crater recognizes James Elliot (1943–2011), an MIT researcher who pioneered the use of stellar occultations to study the solar system – leading to discoveries such as the rings of Uranus and the first detection of Pluto's thin atmosphere.

Charon[edit | edit source]

A number of the images of Pluto appeared elongated. Credit: U.S. Naval Observatory.{{free media}}

On 22 June 1978, an astronomer at the U.S. Naval Observatory in Washington, D.C. was making routine measurements of photographic plates taken with the 1.55-meter (61-inch) Kaj Strand Astrometric Reflector at the USNO Flagstaff Station in Arizona. The purpose of these images was to refine the orbit of the far-flung planet Pluto to help compute a better ephemeris for this distant object.

Astronomer James W. Christy had noticed that a number of the images of Pluto appeared elongated, but images of background stars on the same plate did not. Other plates showed the planet as a tiny, round dot. Christy examined a number of Pluto images from the USNO archives, and he noticed the elongations again. Furthermore, the elongations appeared to change position with respect to the stars over time. After eliminating the possibility that the elongations were produced by plate defects and background stars, the only plausible explanation was that they were caused by a previously unknown moon orbiting Pluto at a distance of about 19,600 kilometers (12,100 miles) with a period of just over six days.

On 7 July 1978, the discovery was formally announced to the astronomical community and the world by the IAU Central Bureau for Astronomical Telegrams via IAU Circular 3241. The discovery received the provisional designation "1978 P 1"; Christy proposed the name "Charon", after the mythological ferryman who carried souls across the river Acheron, one of the five mythical rivers that surrounded Pluto's underworld.

Over the course of the next several years, another USNO astronomer, the late Robert S. Harrington, calculated that Pluto and its newly-found moon would undergo a series of mutual eclipses and occultations, beginning in early 1985. On 17 February 1985 the first successful observation of one of these transits was made at with the 0.9-meter (36-inch) reflector at the University of Texas McDonald Observatory, within 40 minutes of Harrington's predicted time. The IAU Circular announcing these confirming observations was issued on 22 February 1985. With this confirmation, the new moon was officially named Charon.

Pluto was discovered at Lowell Observatory in 1930 by the late Clyde W. Tombaugh, an amateur astronomer from Kansas who was hired by the Observatory specifically to photograph the sky with a special camera and search for the planet predicted by the Observatory's founder, Percival Lowell.

Lowell had deduced the existence of a "Planet X" by studying small anomalies in the orbits of Uranus and Neptune. As it turned out, Pluto's discovery was almost entirely serendipitous; Pluto's tiny mass was far too small to account for the anomalies, which were resolved when Voyager 2 determined more precise masses for Uranus and Neptune.

The discovery of Charon has led to a much better understanding of just how tiny Pluto is. Its diameter is about 2274 km (1413 miles), and its mass is 0.25% of the mass of the Earth. Charon has a diameter of about 1172 kilometers (728 miles) and a mass of about 22% that of Pluto. The two worlds circle their common center of mass with a period of 6.387 days and are locked in a "super-synchronous" rotation: observers on Pluto's surface would always see Charon in the same part of the sky relative to their local horizon.

Normally Pluto is considered the most distant world in the solar system, but during the period from January 1979 until February 1999 it was actually closer to the Sun than Neptune. It has the most eccentric and inclinced orbit of any of the major planets. This orbit won't bring Pluto back to its discovery position until the year 2178!

Plasma objects[edit | edit source]

The New Horizons Particles and Plasma team has discovered a region of cold, dense ionized gas tens of thousands of miles beyond Pluto -- the planet's atmosphere being stripped away by the solar wind and lost to space.

2015 TG387[edit | edit source]

2015 TG
(nicknamed The Goblin for the letters TG and because its discovery was near Halloween),[35][36] is a trans-Neptunian object (TNO) and sednoid in the outermost part of the Solar System.[37] It was first observed on October 13, 2015, with the Subaru Telescope at Mauna Kea Observatories, and publicly announced on October 1, 2018.[38][39]

2015 TG
is the third sednoid to be discovered, following 90377 Sedna and 2012 VP
.[40][41] It is estimated to be 300 km (190 mi) in diameter.[40]

Along with the similar orbits of other distant TNOs, the orbit of 2015 TG
suggests, but does not prove, the existence of a hypothetical Planet Nine in the outer Solar System.[40][39]

As of 2018, the object is 80 AU from the Sun; about two-and-a-half times farther out than Pluto’s orbit.[36] As with Sedna, it would not have been found had it not been on the inner leg of its long orbit, which suggests that there may be many similar objects, most too distant to be detected by contemporary technological methods, and implies a population of about 2 million Hills cloud, or inner Oort cloud, objects larger than 40 km (25 mi), with a combined total mass of 1022 kg, which is several times the mass of the asteroid belt.[40]

Cubewanos[edit | edit source]

The classical objects (cubewanos) are very different from the scattered objects: more than 30% of all cubewanos are on low-inclination, near-circular orbits whose eccentricities peak at 0.25.[42] Classical objects possess eccentricities ranging from 0.2 to 0.8. Though the inclinations of scattered objects are similar to the more extreme KBOs, very few scattered objects have orbits as close to the ecliptic as much of the KBO population.[43]

Although motions in the scattered disc are random, they do tend to follow similar directions, which means that SDOs can become trapped in temporary resonances with Neptune. Examples of possible resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79.[44]

Plutinos[edit | edit source]

The Kuiper belt is a relatively thick torus (or "doughnut") of space, extending from about 30 to 50 AU[45] comprising two main populations of Kuiper belt objects (KBOs): the classical Kuiper-belt objects (or "cubewanos"), which lie in orbits untouched by Neptune, and the resonant Kuiper-belt objects; those which Neptune has locked into a precise orbital ratio such as 2:3 (the object goes around twice for every three Neptune orbits) and 1:2 (the object goes around once for every two Neptune orbits). These ratios, called orbital resonances, allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the age of the Solar System, since the objects are never close enough to Neptune to be scattered by its gravity. Those in 2:3 resonances are known as "plutinos", because Pluto is the largest member of their group, whereas those in 1:2 resonances are known as "resonant trans-Neptunian object#1:2 resonance ("twotinos", period ~330 years) twotinos".

Kuiper belts[edit | edit source]

The eccentricity and inclination of the scattered-disc population compared to the classical and 5:2 resonant Kuiper-belt objects. Credit: Eurocommuter~commonswiki.
Known objects in the Kuiper belt, are derived from data from the Minor Planet Center. Credit: WilyD.

The Kuiper belt is a region of the solar system extending from the orbit of Neptune (at 30 AU to approximately 60 AU from the Sun.[46] It consists mainly of small bodies.

"[B]roadband optical photometry of Centaurs and Kuiper Belt objects from the Keck 10 m, the University of Hawaii 2.2 m, and the Cerro Tololo InterAmerican (CTIO) 1.5 m telescopes [shows] a wide dispersion in the optical colors of the objects, indicating nonuniform surface properties. The color dispersion [may] be understood in the context of the expected steady reddening due to bombardment by the ubiquitous flux of cosmic rays."[47]

In the image at right, objects in the main part of the Kuiper belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few known trojans are yellow, while Jupiter's are pink. The scattered objects between Jupiter's orbit and the Kuiper belt are known as centaurs. The scale is in astronomical units. The pronounced gap at the bottom is due to difficulties in detection against the background of the plane of the Milky Way.


Axes list distances in AU, projected onto the ecliptic, with ecliptic longitude zero being to the right, along the "x" axis).

Positions are accurate for January 1st, 2000 (J2000 epoch) with some caveats:

For planets, positions should be exact.

For minor bodies, positions are extrapolated from other epochs assuming purely Keplerian motion. As all data is from an epoch between 1993 and 2007, this should be a reasonable approximation.

Data from the Minor Planet Center[48] or Murray and Dermott[49] as needed.

Radial "spokes" of higher density in this image, or gaps in particular directions are due to observational bias (i.e. where objects were searched for), rather than any real physical structure. The pronounced gap at the bottom is due to obscuration by the band of the Milky Way.

1995 TL 95[edit | edit source]

The first object presently classified as an SDO to be discovered was 1995 TL8, found in 1995 by Spacewatch.[50]

1996 TL 96[edit | edit source]

The first scattered-disc object (SDO) to be recognised as such was 1996 TL66,[51][52] originally identified in 1996 by astronomers based at Mauna Kea in Hawaii.

1999 CV 118[edit | edit source]

Three more were identified by the same survey in 1999: 1999 CV118, 1999 CY118, and 1999 CF119.[53]

2011 SDOs[edit | edit source]

As of 2011, over 200 SDOs have been identified,[54] including 229762 Gǃkúnǁʼhòmdímà|Gǃkúnǁʼhòmdímà (discovered by Schwamb, Brown, and Rabinowitz),[55] 2002 TC302 (Near Earth Asteroid Tracking NEAT), Eris (Brown, Trujillo, and Rabinowitz),[56] 90377 Sedna Sedna (Brown, Trujillo, and Rabinowitz)[57] and 2004 VN112 (Deep Ecliptic Survey).[58] Although the numbers of objects in the Kuiper belt and the scattered disc are hypothesized to be roughly equal, observational bias due to their greater distance means that far fewer SDOs have been observed to date.[43]

Oort clouds[edit | edit source]

This graphic shows the distance from the Oort cloud to the rest of the Solar System and two of the nearest stars measured in astronomical units (AU). The scale is logarithmic, with each specified distance ten times further out than the previous one.
An artist's rendering is of the Oort cloud and the Kuiper belt (inset). Sizes of individual objects have been exaggerated for visibility.

The Oort cloud or the Öpik–Oort cloud[59] is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.[60] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.[61]

Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc.[62] A third reservoir of trans-Neptunian objects, the Oort cloud, has been hypothesized, although no confirmed direct observations of the Oort cloud have been made.[2] Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "detached objects".[44]

Neptune objects[edit | edit source]

In contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune.[63] Scattered-disc objects come within gravitational range of Neptune at their closest approaches (~30 AU) but their farthest distances reach many times that.[44] Ongoing research[64] suggests that the centaurs, a class of icy planetoids that orbit between Jupiter and Neptune, may simply be SDOs thrown into the inner reaches of the Solar System by Neptune, making them "cis-Neptunian" rather than trans-Neptunian scattered objects.[65] Some objects, like (29981) 1999 TD10, blur the distinction[66] and the Minor Planet Center (MPC), which officially catalogues all trans-Neptunian objects, now lists centaurs and SDOs together.[54]

The MPC, however, makes a clear distinction between the Kuiper belt and the scattered disc, separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs).[54] However, the difference between the Kuiper belt and the scattered disc is not clear-cut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Another term used is "scattered Kuiper-belt object" (or SKBO) for bodies of the scattered disc.[67]

The difference between objects in the Kuiper belt and scattered-disc objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune,"[62] but the former experienced no such close encounters. This delineation is inadequate (as they note) over the age of the Solar System, since bodies "trapped in resonances" could "pass from a scattering phase to a non-scattering phase (and vice versa) numerous times."[62] That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the scattered disc over time. Therefore, they chose instead to define the regions, rather than the objects, defining the scattered disc as "the region of orbital space that can be visited by bodies that have encountered Neptune" within the radius of a Hill sphere, and the Kuiper belt as its "complement ... in the a > 30 AU region"; the region of the Solar System populated by objects with semi-major axes greater than 30 AU.[62]

The scattered disc is a very dynamic environment.[43] Because they are still capable of being perturbed by Neptune, SDOs' orbits are always in danger of disruption; either of being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets.[43] For this reason Gladman et al. prefer to refer to the region as the scattering disc, rather than scattered.[15] Unlike Kuiper-belt objects (KBOs), the orbits of scattered-disc objects can be inclined as much as 40° from the ecliptic.[68]

Sedna[edit | edit source]

Here, the presumed distance of the Oort cloud is compared to the rest of the Solar System using the orbit of Sedna. Credit: NASA / JPL-Caltech / R. Hurt.
Sedna, a possible inner Oort cloud object, is a discovery in 2003. Credit: NASA/Caltech.

Sedna was discovered from an image dated 2003-11-14 at coordinates 03 15 10.09 +05 38 16.5. The 3 overexposed stars are apparent magnitude 13. The "bright star" near Sedna is apmag 14.9 and about the same magnitude as Pluto. (Wikisky image of this region) The picture shows an area of the sky equal to the area covered by a pinhead held at arm's length. Sedna is too faint to be seen by all but the most powerful amateur telescopes.

The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object. Sedna should be considered an inner Oort-cloud object rather than a member of the scattered disc, because, with a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets. [69] Under this definition, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc. [70]

2000 CR 105[edit | edit source]

(148209) 2000 CR105 (discovered before Sedna) and 2004 VN112 have a perihelion too far away from Neptune to be influenced by it. This led to a discussion among astronomers about a new minor planet set, called the extended scattered disc (E-SDO).[71] 2000 CR105 may also be an inner Oort-cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud. More recently, these objects have been referred to as "detached",[72] or distant detached objects (DDO).[73]

There are no clear boundaries between the scattered and detached regions.[70] Gomes et al. define SDOs as having "highly eccentric orbits, perihelia beyond Neptune, and semi-major axes beyond the 1:2 resonance." By this definition, all distant detached objects are SDOs.[44] Since detached objects' orbits cannot be produced by Neptune scattering, alternative scattering mechanisms have been put forward, including a passing star[74] or a distant, planet-sized object.[73]

Scattered near objects[edit | edit source]

Scattered-near (i.e. typical SDOs) and scattered-extended (i.e. detached objects),[75] where scattered-near objects are those whose orbits are non-resonant, non-planetary-orbit-crossing and have a Tisserand parameter (relative to Neptune) less than 3,[75] and scattered-extended objects have a Tisserand parameter (relative to Neptune) greater than 3 and have a time-averaged eccentricity greater than 0.2.[75]

10-million-year orbits[edit | edit source]

A 10-million-year orbit integration is used instead of the Tisserand parameter.[15] An object qualifies as an SDO if its orbit is not resonant, has a semi-major axis no greater than 2000 AU, and, during the integration, its semi-major axis shows an excursion of 1.5 AU or more.[15] Gladman et al. suggest the term scattering disk object to emphasize this present mobility.[15] If the object is not an SDO as per the above definition, but the eccentricity of its orbit is greater than 0.240, it is classified as a detached TNO.[15] (Objects with smaller eccentricity are considered classical.) In this scheme, the disc extends from the orbit of Neptune to 2000 AU, the region referred to as the inner Oort cloud.

Medium and high eccentricities[edit | edit source]

SDOs are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them within influence of Neptune.[76] Having a perihelion of roughly 30 AU is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence.[53]

Eris[edit | edit source]

The dwarf planet Eris is the largest known scattered-disc object (center), with its moon Dysnomia (left of object). Credit: NASA, ESA, and Michael E. Brown.

136199 Eris was measured to be 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi) in diameter.[77] Eris's mass is about 0.27% of the Earth mass,[78][79] about 27% more than dwarf planet Pluto, although Pluto is slightly larger by volume.[80]

Eris is a trans-Neptunian object (TNO), a member of a high-orbital eccentricity population known as the scattered disk, with one known natural satellite, Dysnomia, at a distance from the Sun of 96.3 astronomical units (1.441×1010 km; 8.95×109 mi),[81] roughly three times that of Pluto. With the exception of some long-period comets, until 2018 VG18 was discovered on December 17, 2018, Eris and Dysnomia were the most distant known natural objects in the Solar System.[81]

Observations of a stellar occultation by Eris in 2010 showed that its diameter was 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi), very slightly less than Pluto,[82][83] which was measured by New Horizons as 2,376.6 ± 3.6 kilometers (1,476.8 ± 2.2 mi) in July 2015.[84][85]

Eris is a trans-Neptunian dwarf planet (plutoid).[86] Its orbital characteristics more specifically categorize it as a scattered-disk object (SDO), or a TNO that has been "scattered" from the Kuiper belt into more-distant and unusual orbits following gravitational interactions with Neptune as the Solar System was forming. Although its high orbital inclination is unusual among the known SDOs, theoretical models suggest that objects that were originally near the inner edge of the Kuiper belt were scattered into orbits with higher inclinations than objects from the outer belt.[87]

Haumea[edit | edit source]

Hubble Space Telescope image of Haumea (center) and its two moons; Hiʻiaka is above Haumea and Namaka is below. Credit: Renerpho.{{free media}}
Haumea's orbit outside of Neptune is similar to Makemake's. The positions are as of 1 January 2018. Credit: Tomruen.{{free media}}

Haumea (minor-planet designation 136108 Haumea, initially, (136108) 2003 EL61) is a dwarf planet located beyond Neptune's orbit.[88] It was discovered on December 28, 2004, just after Christmas,[89] at the Palomar Observatory.[90] Precovery images of Haumea have been identified back to March 22, 1955.[91]

Haumea is a plutoid, a dwarf planet located beyond Neptune's orbit.[92] The nominal trajectory suggests that Haumea is in a weak 7:12 orbital resonance with Neptune, which would make it a resonant trans-Neptunian object instead.[93] There are precovery images of Haumea dating back to March 22, 1955 from the Palomar Mountain Digitized Sky Survey.[94]

Haumea has an orbital period of 284 Earth years, a perihelion of 35 AU, and an orbital inclination of 28°.[91] It passed aphelion in early 1992,[95] and is currently more than 50 AU from the Sun.[96]

Haumea's orbit has a slightly greater orbital eccentricity than that of the other members of the Haumea family, its collisional family. This is thought to be due to Haumea's weak 7:12 orbital resonance with Neptune gradually modifying its initial orbit over the course of a billion years,[97][98] through the Kozai mechanism, or Kozai effect, which allows the exchange of an orbit's inclination for increased eccentricity.[97][99][100]

With a visual magnitude of 17.3,[96] Haumea is the third-brightest object in the Kuiper belt after Pluto and Makemake, and easily observable with a large amateur telescope.[101] However, because the planets and most small Solar System bodies share a invariable plane, or common orbital alignment, from their formation in the protoplanetary, primordial disk, of the Solar System, most early surveys for distant objects focused on the projection on the sky of this common plane, called the ecliptic.[102] As the region of sky close to the ecliptic became well explored, later sky surveys began looking for objects that had been dynamically excited into orbits with higher inclinations, as well as more distant objects, with slower mean motions across the sky.[103][104]

Haumea displays large fluctuations in brightness over a period of 3.9 hours, which can only be explained by a rotational period of this length.[105] This is faster than any other known equilibrium body in the Solar System, and indeed faster than any other known body larger than 100 km in diameter.[101] While most rotating bodies in equilibrium are flattened into oblate spheroids, Haumea rotates so quickly that it is distorted into a triaxial ellipsoid. If Haumea were to rotate much more rapidly, it would distort itself into a dumbbell shape and split in two.[88] This rapid rotation is thought to have been caused by the impact that created its satellites and collisional family.[97] Because Haumea has moons, the mass of the system can be calculated from their orbits using Kepler's third law. The result is 4.2×1021 kg, 28% the mass of the Plutonian system and 6% that of the Moon. Nearly all of this mass is in Haumea.[106][107]

For most distant objects, the albedo is unknown, but Haumea is large and bright enough for its infrared, thermal emission to be measured, which has given an approximate value for its albedo and thus its size.[108]

The rigid body dynamics, specifically, rotational physics of deformable bodies predicts that over as little as a hundred days,[101] a body rotating as rapidly as Haumea will have been distorted into the hydrostatic equilibrium form of a triaxial ellipsoid. It is thought that most of the fluctuation in Haumea's brightness is caused not by local differences in albedo but by the alternation of the side view and end view as seen from Earth.[101]

If Haumea were in hydrostatic equilibrium and had a low density like Pluto, with a thick mantle of volatiles, such as ice, over a small silicate, rocky core, its rapid rotation would have elongated it to a greater extent than the fluctuations in its brightness allow. Such considerations constrained its density to a range of 2.6–3.3 g/cm3.[109][101]

In 2005, the Gemini Observatory and Keck Observatory telescopes obtained spectra of Haumea which showed strong crystalline water ice features similar to the surface of Pluto's moon Charon.[110] This is peculiar, because crystalline ice forms at temperatures above 110 K, whereas Haumea's surface temperature is below 50 K, a temperature at which amorphous ice is formed.[110] In addition, the structure of crystalline ice is unstable under the constant rain of cosmic rays and energetic particles from the Sun that strike trans-Neptunian objects.[110] The timescale for the crystalline ice to revert to amorphous ice under this bombardment is on the order of ten million years,[111] yet trans-Neptunian objects have been in their present cold-temperature locations for timescales of billions of years.[98] Radiation damage should also redden and darken the surface of trans-Neptunian objects where the common surface materials of organic molecular ices and tholin-like compounds are present, as is the case with Pluto. Therefore, the spectra and colour suggest Haumea and its family members have undergone recent resurfacing that produced fresh ice. However, no plausible resurfacing mechanism has been suggested.[112]

Haumea is as bright as snow, with an albedo in the range of 0.6–0.8, consistent with crystalline ice.[101] Other large TNOs such as Eris appear to have albedos as high or higher.[113] Best-fit modeling of the surface spectra suggested that 66% to 80% of the Haumean surface appears to be pure crystalline water ice, with one contributor to the high albedo possibly hydrogen cyanide or phyllosilicate clays.[110] Inorganic cyanide salts such as copper potassium cyanide may also be present.[110]

Visible and near infrared spectra suggest a homogeneous surface covered by an intimate 1:1 mixture of amorphous and crystalline ice, together with no more than 8% organics. The absence of ammonia hydrate excludes cryovolcanism and the observations confirm that the collisional event must have happened more than 100 million years ago, in agreement with the dynamic studies.[114] The absence of measurable methane in the spectra of Haumea is consistent with a warm collisional history that would have removed such volatiles,[110] in contrast to Makemake.[115]

Interstellar comets[edit | edit source]

This shows the hyperbolic path of extrasolar object ʻOumuamua, the first confirmed interstellar object, discovered in 2017. Credit: Tomruen.{{free media}}
Comet Machholz 1 (96P/Machholz) is viewed by STEREO-A (April 2007). Credit: NASA.
Comet Hyakutake (C/1996 B2) might be an interstellar object captured by the Solar System. Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria.{{free media}}

An interstellar object is an astronomical object that is located in interstellar space including objects that are on an interstellar trajectory but are temporarily passing close to a star, such as certain asteroids and comets (including exocomets[116][117])

The image on the right shows `Oumuamua's hyperbolic trajectory across the full solar system, with annual markers, and planet positions on 1/1/2018.

"A newly discovered comet is screaming away from Earth, and based on its weird orbital trajectory might be the first comet ever observed to come from interstellar space. A sky-surveying telescope in Hawaii spotted the fast-moving object, now called C/2017 U1, on 18 October, after its closest approach to the sun. The following week, astronomers made 34 separate observations of the object and found it has a strange trajectory that doesn't appear to circle the sun."[118]

ʻOumuamua showed no signs of a cometary coma despite its close approach to the Sun, but underwent non-gravitational acceleration which is seen in many icy comets,[119][120] although other reasons have been suggested.[121][122][123]

The object could be a remnant of a disintegrated interstellar comet (or exocomet).[124][125]

It is possible for objects orbiting a star to be ejected due to interaction with a third massive body, such a process was initiated in early 1980s when C/1980 E1, initially gravitationally bound to the Sun, passed near Jupiter and was accelerated sufficiently to reach escape velocity from the Solar System, changing its orbit from elliptical to hyperbolic and making it the most eccentric known object at the time, with an eccentricity of 1.057.[126] It is headed for interstellar space.

Asteroid (514107) 2015 BZ509 may be a former interstellar object, captured some 4.5 billion years ago, as evidenced by its co-orbital motion with Jupiter and its retrograde orbit around the Sun.[127]

An interstellar comet can probably, on rare occasions, be captured into a heliocentric orbit while passing through the Solar System. Computer simulations show that Jupiter is the only planet massive enough to capture one, and that this can be expected to occur once every sixty million years.[128] Comets Machholz 1 and Comet Hyakutake C/1996 B2 are possible examples of such comets, as they have atypical chemical makeups for comets in the Solar System.[129][130]

Current models of Oort cloud formation predict that more comets are ejected into interstellar space than are retained in the Oort cloud, with estimates varying from 3 to 100 times as many.[117] Other simulations suggest that 90–99% of comets are ejected.[131] There is no reason to believe comets formed in other star systems would not be similarly scattered.[116]

A more recent estimate, following the detection of 'Oumuamua, predicts that "The steady-state population of similar, ~100 m scale interstellar objects inside the orbit of Neptune is ~1×104, each with a residence time of ~10 years."[132]

There should be hundreds of 'Oumuamua-size interstellar objects in the Solar System, based on calculated orbital characteristics, with known examples: 2011 SP25, 2017 RR2, 2017 SV13, and 2018 TL6.[133] These are all orbiting the sun, but with unusual orbits, and are assumed to have been trapped at some occasion.

See also[edit | edit source]

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