Radiation astronomy/Kuiper belts

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Known objects in the Kuiper belt, are derived from data from the Minor Planet Center. Credit: WilyD. Legend:
  Sun
  Giant Planet (6,178)
  Kuiper belt object (>300)   Scattered disc object (9)
  Trojan of Jupiter: J ··· N
  Neptune trojan (44,000)

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.[1] 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."[2]

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[3] or Murray and Dermott[4] 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.

Colors[edit]

"These authors proposed that the whole-disk surface colors of KBOs could be the result of the competition between the effects of irradiation of surface organics by cosmic-rays and the global resurfacing due to impacts. [...] When these high-energy protons collide with an icy target, they penetrate very [deep] under the surface."[5]

Materials[edit]

"The depth of the absorption bands and the continuum reflectance of [Kuiper Belt Object] 1996 TO66 suggest the presence of a black- to slightly blue-colored, spectrally featureless particulate material as a minority component mixed with the water ice."[6]

Centaurs[edit]

Positions of known outer Solar System objects.
The centaurs lie generally inwards of the Kuiper belt and outside the Jupiter trojans.
  Sun
  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"[7] 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."[8] "[T]he intrinsic number of such objects is roughly an order of magnitude greater than that for a<60 AU".[8]

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.[9] 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.[9]

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..[9]

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[9]
(forward)
Class
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

Haumea family[edit]

The collisional family of Haumea (in green), other cubewano, or classical KBO (blue), Plutinos and other resonant objects (red) and scattered disk object (SDO) (grey). Radius is semi-major axis, angle orbital inclination. Credit: Eurocommuter.
Orbits of Haumea family members, sharing semimajor axes around 43 AU, and inclinations around 27°. Credit: Tomruen.

The first known collisional family in the classical Kuiper belt—a group of objects thought to be remnants from the breakup of a single body—is the Haumea family.[22] It includes Haumea, its moons, 2002 TX300 and seven smaller bodies. The objects not only follow similar orbits but also share similar physical characteristics. Unlike many other KBO their surface contains large amounts of ice (H2O) and no or very little tholins.[23] The surface composition is inferred from their neutral (as opposed to red) colour and deep absorption at 1.5 and 2.0 μm in infrared spectrum.[24] Several other collisional families might reside in the classical Kuiper belt.[25][26]

Calculations indicate that it is probably the only trans-Neptunian collisional family.[27]

Brightest Haumea-family members:
Object Absolute magnitude of Solar System bodies (H) Diameter
Astronomical albedo=0.7
Trans-Neptunian object Colors (V–R)[28]
Haumea 0.2 1,460 km 0.33
(55636) 2002 TX300 3.4 332 km 0.36
(120178) 2003 OP32 3.9 276 km 0.39
(145453) RR43 4.1 252 km 0.41
(386723) 2009 YE7 4.5 200 km
(24835) 1995 SM55 4.6 191 km 0.39
(308193) 2005 CB79 4.7 182 km 0.37
(19308) 1996 TO66 4.8 174 km 0.39

The dwarf planet Haumea is the largest member of the family, and the core of the differentiated progenitor; other identified members are the moons of Haumea and the Kuiper belt objects (55636) 2002 TX300, (24835) 1995 SM55, (19308) 1996 TO66, (120178) 2003 OP32, (145453) 2005 RR43, (86047) 1999 OY3, (416400) 2003 UZ117, (308193) 2005 CB79, 2003 SQ317[28] and (386723) 2009 YE7,[29] all with an ejection velocity from Haumea of less than 150 m/s.[30] The brightest Haumeids have absolute magnitudes (H) bright enough to suggest a size between 400 and 700 km in diameter, and so possible dwarf planets, if they had the albedos of typical TNOs; however, they are likely to be much smaller as it is thought they are water-icy bodies with high albedos. The dispersion of the proper orbital elements of the members is a few percent or less (5% for semi-major axis, 1.4° for the inclination and 0.08 for the eccentricity).[26]

The objects' common physical characteristics include neutral colours and deep infrared absorption features (at 1.5 and 2.0 μm) typical of water ice.[24][31]

Haumea[edit]

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.[32] It was discovered on December 28, 2004, just after Christmas,[33] at the Palomar Observatory.[34] Precovery images of Haumea have been identified back to March 22, 1955.[35]

Haumea is a plutoid, a dwarf planet located beyond Neptune's orbit.[36] 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.[37] There are precovery images of Haumea dating back to March 22, 1955 from the Palomar Mountain Digitized Sky Survey.[38]

Haumea has an orbital period of 284 Earth years, a perihelion of 35 AU, and an orbital inclination of 28°.[35] It passed aphelion in early 1992,[39] and is currently more than 50 AU from the Sun.[40]

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,[22][41] through the Kozai mechanism, or Kozai effect, which allows the exchange of an orbit's inclination for increased eccentricity.[22][42][43]

With a visual magnitude of 17.3,[40] Haumea is the third-brightest object in the Kuiper belt after Pluto and Makemake, and easily observable with a large amateur telescope.[44] 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.[45] 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.[46][47]

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.[48] 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.[44] 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.[32] This rapid rotation is thought to have been caused by the impact that created its satellites and collisional family.[22] 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.[49][50]

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.[51]

The rigid body dynamics, specifically, rotational physics of deformable bodies predicts that over as little as a hundred days,[44] 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.[44]

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.[52][44]

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.[53] 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.[53] 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.[53] The timescale for the crystalline ice to revert to amorphous ice under this bombardment is on the order of ten million years,[54] yet trans-Neptunian objects have been in their present cold-temperature locations for timescales of billions of years.[41] 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.[55]

Haumea is as bright as snow, with an albedo in the range of 0.6–0.8, consistent with crystalline ice.[44] Other large TNOs such as Eris appear to have albedos as high or higher.[56] 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.[53] Inorganic cyanide salts such as copper potassium cyanide may also be present.[53]

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.[23] The absence of measurable methane in the spectra of Haumea is consistent with a warm collisional history that would have removed such volatiles,[53] in contrast to Makemake.[57]

Classical Kuiper belt objects[edit]

The orbits of various cubewanos are compared to the orbit of Neptune (blue) and Pluto (pink). Credit: kheider.

A classical Kuiper belt object, also called a cubewano, a term still used by the Minor Planet Center for their list of Distant Minor Planets is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axes in the 40–50 AU range and, unlike Pluto, do not cross Neptune's orbit. That is, they have low-eccentricity and sometimes low-inclination orbits like the classical planets.

The name "cubewano" derives from the first trans-Neptunian object (TNO) found after Pluto and Charon, 15760 Albion, which until January 2018 had only had the provisional designation (15760) 1992 QB1.[58] Similar objects found later were often called "QB1-o's", or "cubewanos", after this object, though the term "classical" is much more frequently used in the scientific literature.

Objects identified as cubewanos include:

  • 15760 Albion[59] (aka 1992 QB1 and gave rise to term 'Cubewano')
  • Makemake, the largest known cubewano and a dwarf planet[59]
  • 50000 Quaoar and 20000 Varuna, each considered the largest TNO at the time of discovery[59]
  • 19521 Chaos, 58534 Logos, 53311 Deucalion, 66652 Borasisi, 88611 Teharonhiawako
  • (33001) 1997 CU29, (55636) 2002 TX300, (55565) 2002 AW197, (55637) 2002 UX25
  • (486958) 2014 MU69 (nicknamed Ultima Thule)

There is evidence that the Kuiper belt has an 'edge', in that an apparent lack of low-inclination objects beyond 47–49 AU was suspected as early as 1998 and shown with more data in 2001.[60] Consequently, the traditional usage of the terms is based on the orbit's semi-major axis, and includes objects situated between the 2:3 and 1:2 resonances, that is between 39.4 and 47.8 AU (with exclusion of these resonances and the minor ones in-between).[61]

The boundary between the classical objects and the scattered disk remains blurred. As of 2010, there are 377 objects with perihelion (q) > 40 AU and aphelion (Q) < 47 AU.[62]

Haumea was provisionally listed as a cubewano by the Minor Planet Center in 2006,[63] but turned out to be resonant.[59]

Ultima Thule[edit]

Ultima Thule is a contact binary object, composed of two individual objects fused together. Credit: Alan Stern, New Horizons, NASA.{{fairuse}}
Polar view is of 2014 MU69. Credit: Johns Hopkins University Applied Physics Laboratory.{{free media}}

Greyscale view of 2014 MU69 on the right was taken by the Ralph, or Multispectral Visible Imaging Camera (MVIC) aboard New Horizons on 1 January 2019, from a distance of 6,700 kilometres (4,200 mi).[64]

"Obtained with the wide-angle Multicolor Visible Imaging Camera (MVIC) component of New Horizons' Ralph instrument, this image was taken when the KBO was 4,200 miles (6,700 kilometers) from the spacecraft, at 05:26 UT (12:26 a.m. EST) on Jan. 1."[64]

The contact binary object is made up of two lobes named "Ultima" (right) and "Thule" (left).[64]

Its axis of rotation is located near the bright "neck" of the object and spins clockwise from this viewpoint.[65]}}

"This movie shows the propeller-like rotation of Ultima Thule in the seven hours between 20:00 UT (3 p.m. ET) on Dec. 31, 2018, and 05:01 UT (12:01 a.m.) on Jan. 1, 2019."[65]

The image on the left is a composite of two photographs taken respectively by the Long Range Reconnaissance Imager (LORRI) and the Ralph (MVIC) instruments aboard New Horizons on 1 January 2019. The spacecraft was 137,000 kilometres (85,000 mi) away from 2014 MU69 when this image was taken.[66]

This image was "taken at a distance of 85,000 miles (137,000 kilometers) at 4:08 Universal Time on January 1, 2019, [...] is an enhanced color image taken by the Multispectral Visible Imaging Camera (MVIC)) [...]."[66]

Kilometre-sized Kuiper belt objects[edit]

Enlargement (b) of the light curves (a) with error bars representing the detector readout noise and target shot noise overlaid with the best-fit theoretical light curve (black line). Credit: Ko Arimatsu, K. Tsumura, F. Usui, Y. Shinnaka, K. Ichikawa, T. Ootsubo, T. Kotani, T. Wada, K. Nagase and J. Watanabe.{{fairuse}}

"Kuiper belt objects (KBOs) [have a] size distribution of kilometre-sized (radius = 1–10 km). [...] These kilometre-sized KBOs are extremely faint, and it is impossible to detect them directly. Instead, the monitoring of stellar occultation events is one possible way to discover these small KBOs6,7,8,9. [This is] the first detection of a single occultation event candidate by a KBO with a radius of ~1.3 km, which was simultaneously provided by two low-cost small telescopes coupled with commercial complementary metal–oxide–semiconductor cameras. [The] surface number density of KBOs with radii exceeding ~1.2 km is ~6 × 105 deg−2. This surface number density favours a theoretical size distribution model with an excess signature at a radius of 1–2 km (ref. 5). If this is a true KBO detection, this implies that planetesimals before their runaway growth phase grew into kilometre-sized objects in the primordial outer Solar System and remain as a major population in the present-day Kuiper belt."[67]

Regarding the two graphs in the right image: "Light curves of the occultation event candidate obtained with the two OASES observation systems. a, Light curves of an occulted star as a function of the time offset t from the central time of the occultation event candidate obtained with OASES-01 (blue line) and OASES-02 (red line), respectively, normalized to average fluxes. The equatorial coordinates of the occulted star are right ascension = 18 h 29 m 02.7 s and declination = −23° 02′ 34.6′′, while the ecliptic coordinates are λ = 276.7° and β = +0.2°. The Gaia G band magnitude31 of the star is 12.1. The central time of the occultation candidate is estimated to be 12 h 56 m 05.283 s ut on 28 June 2016. The signal-to-noise ratios derived from the light curves of OASES-01 and OASES-02 are 4.9 and 5.4, respectively. [...] b, Enlargement of the light curves with error bars representing the detector readout noise and target shot noise overlaid with the best-fit theoretical light curve (black line). The main noise source is the detector readout noise, and typical error bar sizes are ~0.21 and ~0.17 for OASES-01 and OASES-02, respectively. [These] error sizes are comparable to actual standard deviations of the light curves (0.20 and 0.18 for OASES-01 and OASES-02, respectively). Open blue and red circles correspond to the theoretical light curve integrated over each bin (15.4 Hz interval). Note that the timings of the OASES-01 and OASES-02 exposures are not synchronized. Assuming that the spherical occulting object lies on a circular KBO orbit with an inclination of 0.2°, the best-fit KBO radius, impact parameter and distance yield 1.3-0.10.8 km, 0.6-0.31.4 km and 33-3+17 au, respectively. The best-fit χ2 value from the fit is 7.0, with 12 d.f."[67]

Large Kuiper belt objects[edit]

The size of the circle illustrates the object’s size relative to others. Credit: Eurocommuter~commonswiki.{{free media}}
  classical KBOs
  Plutinos, Neptune trojans and other resonant trans-Neptunian objects

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 absolute magnitude values (H) marked at the bottom of the plot are defined as the optical visual magnitude that an object would have if it were located at a distance of 1 astronomical unit from the Sun and viewed from a distance of 1 astronomical unit at a phase of 0 degrees. It should not be confused with the definition of absolute magnitudes used for stars or the infrared photometry H-band.

Scattered disks[edit]

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.

Oort clouds[edit]

An artist's rendering is of the Oort cloud and the Kuiper belt (inset). Credit: NASA.{{free media}}

The Oort cloud or the Öpik–Oort cloud[68] is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.[69] 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.[70]

Neptune trojans[edit]

Neptune's L4 trojans with plutinos for reference. Credit: Eurocommuter.
  Neptune trojans (selection)
  · 2001 QR322
  · 2005 TN53
  · 2007 VL305
  Plutinos
  · Pluto
  · Orcus
  · Ixion

Neptune trojans are bodies that orbit the Sun near one of the stable Lagrangian points of Neptune, have approximately the same orbital period as Neptune and follow roughly the same orbital path. 22 Neptune trojans are currently known, of which 19 orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune[71] and three orbit near Neptune's L5 region 60° behind Neptune.[71]

The discovery of 2005 TN53 in a high-inclination (>25°) orbit was significant, because it suggested a "thick" cloud of trojans[72] (Jupiter trojans have inclinations up to 40°[73]), which is indicative of freeze-in capture instead of in situ or collisional formation.[72] It is suspected that large (radius ≈ 100 km) Neptune trojans could outnumber Jupiter trojans by an order of magnitude.[74][75]

In 2010, the discovery of the first known L5 Neptune trojan, 2008 LC32218}}, was announced.[76] Neptune's trailing L5 region is currently very difficult to observe because it is along the line-of-sight to the center of the Milky Way, an area of the sky crowded with stars.

It would have been possible for the New Horizons spacecraft to investigate 2011 HM102, the only L5 Neptune trojan discovered by 2014 detectable by New Horizons, when it passed through this region of space en route to Pluto.[75] However, New Horizons may not have had sufficient downlink bandwidth, so it was decided to give precedence to the preparations for the Pluto flyby.[77][78]

In 2001, the first Neptune trojan was discovered, 2001 QR322, near Neptune's L4 region, and with it the fifth (After the asteroid belt, the Jupiter trojans, the trans-Neptunian objects and the Mars trojans.) known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan 2005 TN53 has indicated that the Neptune trojans populate thick clouds, which has constrained their possible origins.

On August 12, 2010, the first L5 trojan, 2008 LC18, was announced.[76] It was discovered by a dedicated survey that scanned regions where the light from the stars near the Galactic Center is obscured by dust clouds.[79] This suggests that large {{L5 trojans are as common as large L4 trojans, to within uncertainty,[79] further constraining models about their origins.

It would have been possible for the New Horizons spacecraft to investigate L5 Neptune trojans discovered by 2014, when it passed through this region of space en route to Pluto.[75] Some of the patches where the light from the Galactic Center is obscured by dust clouds are along New Horizons's flight path, allowing detection of objects that the spacecraft could image.[79] 2011 HM102, the highest-inclination Neptune trojan known, was just bright enough for New Horizons to observe it in end-2013 at a distance of 1.2 AU.[80] However, New Horizons may not have had sufficient downlink bandwidth, so it was eventually decided to give precedence to the preparations for the Pluto flyby.[77][78]

An animation showing the path of six of Neptune's L4 trojans in a rotating frame with a period equal to Neptune's orbital period. Neptune is held stationary. (Click to view.) Credit: frankuitaalst from the Gravity Simulator.

The orbits of Neptune trojans are highly stable; Neptune may have retained up to 50% of the original post-migration trojan population over the age of the Solar System.[72] Neptune's L5 can host stable trojans equally well as its L4.[81] Neptune trojans can librate up to 30° from their associated Lagrangian points with a 10,000-year period.[79] Neptune trojans that escape enter orbits similar to centaurs.[81] Although Neptune cannot currently capture stable trojans,[72] roughly 2.8% of the centaurs within 34 AU are predicted to be Neptune co-orbitals. Of these, 54% would be in horseshoe orbits, 10% would be quasi-satellites, and 36% would be trojans (evenly split between the L4 and L5 groups).[82]

The unexpected high-inclination trojans are the key to understanding the origin and evolution of the population as a whole.[81] The existence of high-inclination Neptune trojans points to a capture during planetary migration instead of in situ or collisional formation.[72][79] The estimated equal number of large L5 and L4 trojans indicates that there was no gas drag during capture and points to a common capture mechanism for both L4 and L5 trojans.[79] The capture of Neptune trojans during a migration of the planets occurs via process similar to the chaotic capture of Jupiter trojans in the Nice model. When Uranus and Neptune are near but not in a mean-motion resonance the locations where Uranus passes Neptune can circulate with a period that is in resonance with the libration periods of Neptune trojans. This results in repeated perturbations that increase the libration of existing trojans causing their orbits to become unstable.[83] This process is reversible allowing new trojans to be captured when the planetary migration continues.[84] For high-inclination trojans to be captured the migration must have been slow,[85] or their inclinations must have been acquired previously.[86]

The first four discovered Neptune trojans have similar colors.[72] They are modestly red, slightly redder than the gray Kuiper belt objects, but not as extremely red as the high-perihelion cold classical Kuiper belt objects.[72] This is similar to the colors of the blue lobe of the centaur color distribution, the Jupiter trojans, the irregular satellites of the gas giants, and possibly the comets, which is consistent with a similar origin of these populations of small Solar System bodies.[72]

The Neptune trojans are too faint to efficiently observe spectroscopically with current technology, which means that a large variety of surface compositions are compatible with the observed colors.[72]

In 2015, the International Astronomical Union (IAU) adopted a new naming scheme for Neptune trojans, which are to be named after Amazons, with no differentiation between objects in L4 and L5.[87] The Amazons were an all-female warrior tribe that fought in the Trojan War on the side of the Trojans against the Greeks. As of 2019, the named Neptune trojans are 385571 Otrera (after Otrera, the first Amazonian queen in Greek mythology) and Clete (an Amazon and the attendant to the Amazons queen Penthesilea, who led the Amazons in the Trojan war).[88][89]

The amount of high-inclination objects in such a small sample, in which relatively fewer high-inclination Neptune trojans are known due to observational biases,[72] implies that high-inclination trojans may significantly outnumber low-inclination trojans.[81] The ratio of high- to low-inclination Neptune trojans is estimated to be about 4:1.[72] Assuming albedos of 0.05, there are an expected 400+250
−200
Neptune trojans with radii above 40 km in Neptune's L4.[72] This would indicate that large Neptune trojans are 5 to 20 times more abundant than Jupiter trojans, depending on their albedos.[72] There may be relatively fewer smaller Neptune trojans, which could be because these fragment more readily.[72] Large L5 trojans are estimated to be as common as large L4 trojans.[79]

2001 QR322 and 2008 LC18 display significant dynamical instability.[81] This means they could have been captured after planetary migration, but may as well be a long-term member that happens not to be perfectly dynamically stable.[81]

As of October 2018, 22 Neptune trojans are known, of which 19 orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune,[71] three orbit near Neptune's L5 region 60° behind Neptune, and one orbits on the opposite side of Neptune (L3) but frequently changes location relative to Neptune to L4 and L5.[71] These are listed in the following table. It is constructed from the list of Neptune trojans maintained by the International Astronomical Union (IAU) Minor Planet Center[71] and with diameters from Sheppard and Trujillo's paper on 2008 LC18,[79] unless otherwise noted.

Astronomical naming conventions (Name) Provisional designation in astronomy (Prov.)
designation
Lagrangian point (Lagrangian)
point
Perihelion (q) (AU) Aphelion (Q) (AU) Inclination (i) (°) Absolute magnitude (Abs. mag) Diameter
km)
Year of
identification
Notes Minor Planet Center (MPC)
2001 QR322 L4 29.404 31.011 1.3 8.2 ~140 2001 First Neptune trojan discovered 2001+QR322
2004 KV18 L5 24.553 35.851 13.6 8.9 56[90] 2011 Temporary Neptune trojan 2004+KV18
385571 Otrera 2004 UP10 L4 29.318 30.942 1.4 8.8 ~100 2004 First Neptune trojan numbered and named 385571
2005 TN53 L4 28.092 32.162 25.0 9.0 ~80 2005 First high-inclination trojan discovered[72] 2005+TN53
385695 Clete 2005 TO74 L4 28.469 31.771 5.3 8.5 ~100 2005 385695
2006 RJ103 L4 29.077 31.014 8.2 7.5 ~180 2006 2006+RJ103
(527604) 2007 VL305 L4 28.130 32.028 28.1 8.0 ~160 2007 2007+VL305
2008 LC18 L5 27.365 32.479 27.6 8.4 ~100 2008 First L5 trojan discovered[79] 2008+LC18
316179 2010 EN65 L3 21.109 40.613 19.2 6.9 ~200 Jumping trojan 316179
2010 TS191 L4 28.608 31.253 6.6 8.1 ~120 2016 Announced on 2016/05/31 2010+TS191
2010 TT191 L4 27.913 32.189 4.3 8.0 ~130 2016 Announced on 2016/05/31 2010+TT191
2011 HM102 L5 27.662 32.455 29.4 8.1 90–180[80] 2012 2011+HM102
(530664) 2011 SO277 L4 29.622 30.503 9.6 7.7 ~140 2016 Announced on 2016/05/31 2011+SO277
(530930) 2011 WG157 L4 29.064 30.878 22.3 7.1 ~170 2016 Announced on 2016/05/31 2011+WG157
2012 UV177 L4 27.806 32.259 20.8 9.2 ~80[91] 2012+UV177
2013 KY18 L5 26.598 33.873 6.7 6.8 ~200 2016 Announced on 2016/05/31, stability uncertain 2013+KY18
2014 QO441 L4 26.961 33.215 18.8 8.2 ~130[91] Most eccentric stable Neptune trojan[92] 2014+QO441
2014 QP441 L4 28.022 32.110 19.4 9.1 ~90[91] 2014+QP441
2015 RW277 L4 27.742 32.236 30.8 10.2 ~50 2018 Announced on 2018/10/01 2015+RW277
2015 VV165 L4 27.513 32.497 16.9 8.8 ~90 2018 Announced on 2018/10/01 2015+VV165
2015 VW165 L4 28.488 31.488 5.0 8.1 ~130 2018 Announced on 2018/10/01 2015+VW165
2015 VX165 L4 27.612 32.327 17.2 8.9 ~90 2018 Announced on 2018/10/01 2015+VX165

2005 TN74[93] and (309239) 2007 RW10[94] were thought to be Neptune trojans at the time of their discovery, but further observations have disconfirmed their membership. 2005 TN74 is currently thought to be in a 3:5 trans-Neptunian resonance with Neptune.[95] (309239) 2007 RW10 is currently following a quasi-satellite loop around Neptune.[96]

50000 Quaoar[edit]

Quaoar is imaged by the Hubble Space Telescope in 2002. Credit: NASA and M. Brown (Caltech).{{free media}}
Hubble photo is used to measure size of Quaoar. Credit: NASA.{{free media}}
Polar and ecliptic view of Quaoar's orbit compared to Pluto and various other cubewanos. Quaoar's orbit is colored yellow in the left image Credit: Eurocommuter and blue in the right image Credit: kheider.

50000 Quaoar, provisional designation 2002 LM60, is a non-resonant trans-Neptunian object (classical Kuiper belt object, or cubewano) and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune measuring approximately 1,100 km (680 mi) in diameter, about half the diameter of Pluto, discovered at the Palomar Observatory on 6 June 2002.[97] Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar.[98] A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects.[99] In February 2007, Weywot, a synchronous minor-planet moon in orbit around Quaoar, was discovered by Brown.[100] Weywot is measured to be 80 km (50 mi) across. Both objects were named after mythological figures from the Native American Tongva people in Southern California. Quaoar is the Tongva creator deity and Weywot is his son.[101]

The earliest precovery, or prediscovery image, of Quaoar was found on a photographic plate imaged on 25 May 1954 from the Palomar Observatory Sky Survey.[102]

Quaoar's albedo or reflectivity could be as low as 0.1, which would still be much higher than the lower estimate of 0.04 for 20000 Varuna. This may indicate that fresh ice has disappeared from Quaoar's surface.[103] The surface is moderately red, meaning that Quaoar is relatively more reflective in the red and near-infrared spectrum than in the blue.[104] The Kuiper belt objects Varuna and Ixion are also moderately red in the spectral class. Larger Kuiper belt objects are often much brighter because they are covered in more fresh ice and have a higher albedo, and thus they present a neutral color.[105] A 2006 model of internal heating via radioactive decay suggested that, unlike 90482 Orcus, Quaoar may not be capable of sustaining an internal ocean of liquid water at the mantle–core boundary.[106]

The presence of methane and other volatiles on Quaoar's surface suggest that it may support a tenuous atmosphere produced from the sublimation of volatiles.[107] With a measured mean temperature of ~ 44 K (−229.2 °C), the upper limit of Quaoar's atmospheric pressure is expected to be in the range of a few microbars.[107] Due to Quaoar's small size and mass, the possibility of Quaoar having an atmosphere of nitrogen and carbon monoxide has been ruled out, since the gases would escape from Quaoar.[107] The possibility of a methane atmosphere still remains, with the upper limit being less than 1 microbar.[108][107] In 2013, Quaoar occulted a 15.8 magnitude star and revealed no sign of a substantial atmosphere, placing an upper limit to at least 20 nanobars, under the assumption that Quaoar's mean temperature is 42 K (−231.2 °C) and that its atmosphere consists of mostly methane.[108][107]

Quaoar is thought to be an oblate spheroid around 1,100 km (680 mi) in diameter, being slightly flattened in shape.[108] The estimates come from observations of Quaoar as it occulted a 15.8 magnitude star in 2013.[108] Given that Quaoar has an estimated oblateness value of 0.0897±0.006 and a measured equatorial diameter of 1,138+48
−34
 km
, Quaoar is believed to be in hydrostatic equilibrium, being described as a Maclaurin spheroid.[108] Quaoar is about as large and massive as (if somewhat smaller than) Pluto's moon Charon.[a] Quaoar is roughly half the size of Pluto.[111]

See also[edit]

References[edit]

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