A sednoid is a trans-Neptunian object with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU. Only three objects are known from this population, 90377 Sedna, 2012 VP113, 2015 TG387, all of which have perihelia greater than 64 AU, but it is suspected that there are many more; these objects lie outside an nearly empty gap in the Solar System starting at about 50 AU, have no significant interaction with the planets. They are grouped with the detached objects; some astronomers, such as Scott Sheppard, consider the sednoids to be inner Oort cloud objects, though the inner Oort cloud, or Hills cloud, was predicted to lie beyond 2,000 AU, beyond the aphelia of the three known sednoids. This definition applies for 2013 SY99 which has a perihelion at 50.02 AU, far beyond the Kuiper cliff, but it is thought not to belong to the Sednoids, but to the same dynamical class as 2004 VN112, 2014 SR349 and 2010 GB174. With these high eccentricities > 0.8 they can be distinguished from the high-perihelion objects with moderate eccentricities which are in a stable resonance with Neptune, 2015 KQ174, 2015 FJ345, 2004 XR190, 2014 FC72 and 2014 FZ71.
The sednoids' orbits cannot be explained by perturbations from the giant planets, nor by interaction with the galactic tides. If they formed in their current locations, their orbits must have been circular, their present elliptical orbits can be explained by several hypotheses: These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster. Their orbits could have been disrupted by an as-yet-unknown planet-sized body beyond the Kuiper belt such as the hypothesized Planet Nine, they could have been captured from around passing stars, most in the Sun's birth cluster. The three published sednoids, like all of the more extreme detached objects, have a similar orientation of ≈ 0°; this is not due to an observational bias and is unexpected, because interaction with the giant planets should have randomized their arguments of perihelion, with precession periods between 40 Myr and 650 Myr and 1.5 Gyr for Sedna.
This suggests that more undiscovered massive perturbers may exist in the outer Solar System. A super-Earth at 250 AU would cause these objects to librate around ω = 0°±60° for billions of years. There are multiple possible configurations and a low-albedo super-Earth at that distance would have an apparent magnitude below the current all-sky-survey detection limits; this hypothetical super-Earth has been dubbed Planet Nine. Larger, more-distant perturbers would be too faint to be detected; as of 2016, 27 known objects have a semi-major axis greater than 150 AU, a perihelion beyond Neptune, an argument of perihelion of 340°±55°, an observation arc of more than 1 year. 2013 SY99 is not considered a sednoid. On 1 October 2018, 2015 TG387 was announced with perihelion of 65 AU and a semimajor axis of 1094 AU. With an aphelion of 2123 AU, it brings the object further out than Sedna. In late 2015, V774104 was announced at the Division for Planetary Science conference as a further candidate sednoid, but its observation arc was only a short 2 weeks, thus too short to know whether its perihelion was outside Neptune's influence..
The talk about V774104 was meant to refer to 2015 TG387 though V774104 is the internal designation for non-Sednoid 2015 TH367. Sednoids might constitute a proper dynamical class; each of the proposed mechanisms for Sedna's extreme orbit would leave a distinct mark on the structure and dynamics of any wider population. If a trans-Neptunian planet were responsible, all such objects would share the same perihelion. If Sedna were captured from another planetary system that rotated in the same direction as the Solar System all of its population would have orbits on low inclinations and have semi-major axes ranging from 100–500 AU. If it rotated in the opposite direction two populations would form, one with low and one with high inclinations; the perturbations from passing stars would produce a wide variety of perihelia and inclinations, each dependent on the number and angle of such encounters. Acquiring a larger sample of such objects would therefore help in determining which scenario is most likely.
"I call Sedna a fossil record of the earliest Solar System", said Brown in 2006. "Eventually, when other fossil records are found, Sedna will help tell us how the Sun formed and the number of stars that were close to the Sun when it formed." A 2007–2008 survey by Brown and Schwamb attempted to locate another member of Sedna's hypothetical population. Although the survey was sensitive to movement out to 1,000 AU and discovered the dwarf planet 2007 OR10, it detected no new sednoids. Subsequent simulations incorporating the new data suggested about 40 Sedna-sized objects exist in this region, with the brightest being about Eris's magnitude. Following the discovery of 2015 TG387, Sheppard et al. concluded that it implies a population of about 2 million Inner Oort Cloud objects larger than 40 km, with a total mass of 1×1022 kg. New icy
A great comet is a comet that becomes exceptionally bright. There is no official definition. Great comets are rare. Although comets are named after their discoverers, great comets are sometimes referred to by the year in which they appeared great, using the formulation "The Great Comet of...", followed by the year. The vast majority of comets are never bright enough to be seen by the naked eye, pass through the inner Solar System unseen by anyone except astronomers; however a comet may brighten to naked eye visibility, more it may become as bright as or brighter than the brightest stars. The requirements for this to occur are: a large and active nucleus, a close approach to the Sun, a close approach to the Earth. A comet fulfilling all three of these criteria will be spectacular. Sometimes, a comet failing on one criterion will still be impressive. For example, Comet Hale–Bopp had an exceptionally large and active nucleus, but did not approach the Sun closely at all, yet it still became an famous and well observed comet.
Comet Hyakutake was a rather small comet, but appeared bright because it passed close to the Earth. Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across; when they approach the Sun, large amounts of gas and dust are ejected by cometary nuclei, due to solar heating. A crucial factor in how bright a comet becomes is how large. After many returns to the inner Solar System, cometary nuclei become depleted in volatile materials and thus are much less bright than comets which are making their first passage through the Solar System; the sudden brightening of comet 17P/Holmes in 2007 showed the importance of the activity of the nucleus in the comet's brightness. On October 23–24, 2007, the comet suffered a sudden outburst which caused it to brighten by factor of about half a million, it unexpectedly brightened from an apparent magnitude of about 17 to about 2.8 in a period of only 42 hours, making it visible to the naked eye. All these temporarily made comet 17P the largest object in the Solar System although its nucleus is estimated to be only about 3.4 km in diameter.
The brightness of a simple reflective body varies with the inverse square of its distance from the Sun. That is, if an object's distance from the Sun is halved, its brightness is quadrupled. However, comets behave differently, due to their ejection of large amounts of volatile gas which also reflect sunlight and may fluoresce, their brightness varies as the inverse cube of their distance from the Sun, meaning that if a comet's distance from the Sun is halved, it will become eight times as bright. This means that the peak brightness of a comet depends on its distance from the Sun. For most comets, the perihelion of their orbit lies outside the Earth's orbit. Any comet approaching the Sun to within 0.5 AU or less may have a chance of becoming a great comet. For a comet to become spectacular, it needs to pass close to the Earth if it is to be seen. Halley's Comet, for example, is very bright when it passes through the inner Solar System every seventy-six years, but during its 1986 apparition, its closest approach to Earth was the most distant possible.
The comet was unspectacular. On the other hand, the intrinsically small and faint Comet Hyakutake appeared bright and spectacular due to its close approach to Earth at its nearest during March 1996, its passage near the Earth was one of the closest cometary approaches on record. Great comets of the past two millennia include the following: The bright-comet chronicles. John E. Bortle Memorable Comets of the Past Gary W. Kronk. Brightest comets seen since 1935
The Kuiper belt called the Edgeworth–Kuiper belt, is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune to 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists of small bodies or remnants from when the Solar System formed. While many asteroids are composed of rock and metal, most Kuiper belt objects are composed of frozen volatiles, such as methane and water; the Kuiper belt is home to three recognized dwarf planets: Pluto and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, may have originated in the region; the Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not predict its existence. In 1992, Albion was discovered, the first Kuiper belt object since Charon. Since its discovery, the number of known KBOs has increased to over a thousand, more than 100,000 KBOs over 100 km in diameter are thought to exist.
The Kuiper belt was thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. Studies since the mid-1990s have shown that the belt is dynamically stable and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago. The Kuiper belt is distinct from the theoretical Oort cloud, a thousand times more distant and is spherical; the objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects. Pluto is the largest and most massive member of the Kuiper belt, the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Considered a planet, Pluto's status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006, it is compositionally similar to many other objects of the Kuiper belt and its orbital period is characteristic of a class of KBOs, known as "plutinos", that share the same 2:3 resonance with Neptune.
After the discovery of Pluto in 1930, many speculated. The region now called, it was only in 1992. The number and variety of prior speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it; the first astronomer to suggest the existence of a trans-Neptunian population was Frederick C. Leonard. Soon after Pluto's discovery by Clyde Tombaugh in 1930, Leonard pondered whether it was "not that in Pluto there has come to light the first of a series of ultra-Neptunian bodies, the remaining members of which still await discovery but which are destined to be detected"; that same year, astronomer Armin O. Leuschner suggested that Pluto "may be one of many long-period planetary objects yet to be discovered." In 1943, in the Journal of the British Astronomical Association, Kenneth Edgeworth hypothesized that, in the region beyond Neptune, the material within the primordial solar nebula was too spaced to condense into planets, so rather condensed into a myriad of smaller bodies.
From this he concluded that "the outer region of the solar system, beyond the orbits of the planets, is occupied by a large number of comparatively small bodies" and that, from time to time, one of their number "wanders from its own sphere and appears as an occasional visitor to the inner solar system", becoming a comet. In 1951, in a paper in Astrophysics: A Topical Symposium, Gerard Kuiper speculated on a similar disc having formed early in the Solar System's evolution, but he did not think that such a belt still existed today. Kuiper was operating on the assumption, common in his time, that Pluto was the size of Earth and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. Were Kuiper's hypothesis correct, there would not be a Kuiper belt today; the hypothesis took many other forms in the following decades. In 1962, physicist Al G. W. Cameron postulated the existence of "a tremendous mass of small material on the outskirts of the solar system". In 1964, Fred Whipple, who popularised the famous "dirty snowball" hypothesis for cometary structure, thought that a "comet belt" might be massive enough to cause the purported discrepancies in the orbit of Uranus that had sparked the search for Planet X, or, at the least, massive enough to affect the orbits of known comets.
Observation ruled out this hypothesis. In 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus, he used a blink comparator, the same device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before. In 1992, another object, 5145 Pholus, was discovered in a similar orbit. Today, an entire population of comet-like bodies, called the centaurs, is known to exist in the region between Jupiter and Neptune; the centaurs' orbits have dynamical lifetimes of a few million years. From the time of Chiron's discovery in 1977, astronomers have speculated that the centaurs therefore must be replenished by some outer reservoir. Further evidence for the existence of the Kuiper belt emerged from the study of comets; that comets have finite lifespans. As they approach the Sun, its heat causes their volatile surfaces to sublimate into space d
Asteroids are minor planets of the inner Solar System. Larger asteroids have been called planetoids; these terms have been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were found to have volatile-rich surfaces similar to comets; as a result, they were distinguished from objects found in the main asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There exist millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets; the vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, S-type. These were named after and are identified with carbon-rich and silicate compositions, respectively; the sizes of asteroids varies greatly. Asteroids are differentiated from meteoroids. In the case of comets, the difference is one of composition: while asteroids are composed of mineral and rock, comets are composed of dust and ice. Furthermore, asteroids formed closer to the sun; the difference between asteroids and meteoroids is one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. Meteoroids can be composed of either cometary or asteroidal materials. Only one asteroid, 4 Vesta, which has a reflective surface, is visible to the naked eye, this only in dark skies when it is favorably positioned. Small asteroids passing close to Earth may be visible to the naked eye for a short time; as of October 2017, the Minor Planet Center had data on 745,000 objects in the inner and outer Solar System, of which 504,000 had enough information to be given numbered designations.
The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908. In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit, but we're not 100 percent sure when." In 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched; the first asteroid to be discovered, was considered to be a new planet.
This was followed by the discovery of other similar bodies, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής, or asteroeidēs, meaning'star-like, star-shaped', derived from the Ancient Greek ἀστήρ astēr'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" were still used interchangeably. Overview of discovery timeline: 10 by 1849 1 Ceres, 1801 2 Pallas – 1802 3 Juno – 1804 4 Vesta – 1807 5 Astraea – 1845 in 1846, planet Neptune was discovered 6 Hebe – July 1847 7 Iris – August 1847 8 Flora – October 1847 9 Metis – 25 April 1848 10 Hygiea – 12 April 1849 tenth asteroid discovered 100 asteroids by 1868 1,000 by 1921 10,000 by 1989 100,000 by 2005 ~700,000 by 2015 Asteroid discovery methods have improved over the past two centuries.
In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would be spotted; the expected motion of the missing planet was about 30 seconds of arc per hour discernible by observers. The first object, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily, he discovered a new star-like object in Taurus and followed the displacement of this object during several nights. That year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, found to be between the planets Mars and Jupiter.
Piazzi named it after Ceres, the Roman goddess of agriculture. Three other asteroids (2 Pallas, 3 Juno, 4 Ves
Main-belt comets are bodies orbiting within the asteroid belt that have shown comet-like activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with a semi-major axis of more than 2 AU but less than 3.2 AU, a perihelion of no less than 1.6 AU. David Jewitt from UCLA points out that these objects are most not comets with sublimating ice, but asteroids that exhibit dust activity, hence he and others started calling these class of objects active asteroids; the first main-belt comet discovered is 7968 Elst–Pizarro. It was discovered in 1979 and was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Unlike comets, which spend most of their orbit at Jupiter-like or greater distances from the Sun, main-belt comets follow near-circular orbits within the asteroid belt that are undistinguishable from the orbits of many standard asteroids. Although quite a few short-period comets have semimajor axes well within Jupiter's orbit, main-belt comets differ in having small eccentricities and inclinations similar to main-belt asteroids.
The first three identified main-belt comets all orbit within the outer part of the asteroid belt. It is not known how an outer Solar System body like the other comets could have made its way into a low-eccentricity orbit typical of the asteroid belt, only weakly perturbed by the planets. Hence it is assumed that unlike other comets, the main-belt comets are icy asteroids, which formed in an inner Solar System orbit close to their present positions, that many outer asteroids may be icy; some main-belt comets display a cometary dust tail only for a part of their orbit near perihelion. This suggests that volatiles at their surfaces are sublimating, driving off the dust. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia; the activity persists for a month or several out of each 5-6 year orbit, is due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation.
When discovered in January 2010, P/2010 A2 was given a cometary designation and considered a member of this group, but P/2010 A2 is now thought to be the remnant of an asteroid-on-asteroid impact. Observations of Scheila indicated that large amounts of dust were kicked up by the impact of another asteroid of 35 meters in diameter. In October 2013, observations of P/2013 R3, taken with the 10.4 m Gran Telescopio Canarias on the island of La Palma, showed that this comet was breaking apart. Inspection of the stacked CCD images obtained on October 11 and 12 showed that the main-belt comet presented a central bright condensation, accompanied on its movement by three more fragments, A,B,C; the brightest A fragment was detected at the reported position in CCD images obtained at the 1.52 m telescope of the Sierra Nevada Observatory in Granada on October 12. NASA reported on a series of images taken by the Hubble Space Telescope between October 29, 2013 and January 14, 2014 that show the increasing separation of the four main bodies.
The Yarkovsky–O'Keefe–Radzievskii–Paddack effect, caused by sunlight, increased the spin rate until the centrifugal force caused the rubble pile to separate. It has been hypothesized that main-belt comets may have been the source of Earth's water, because the deuterium–hydrogen ratio of Earth's oceans is too low for classical comets to have been the principal source. European scientists have proposed a sample-return mission from a MBC called Caroline to analyse the content of volatiles and collect dust samples; the term'main-belt comet' is a classification based on orbit and the presence of an extended morphology. It does not imply that these objects are comets or that the material surrounding their nuclei was ejected by the sublimation of volatiles, as on comets. Identified members of this morphology class include: Castalia is a proposed mission concept for a robotic spacecraft to explore 133P/Elst–Pizarro and make the first in situ measurements of water in the asteroid belt, thus, help solve the mystery of the origin of Earth's water.
The lead is Colin Snodgrass, from The Open University in the UK. Castalia was proposed in 2015 and 2016 to the European Space Agency within the Cosmic Vision programme missions M4 and M5, but it was not selected; the team continues to mature the mission science objectives. Because of the construction time required and orbital dynamics, a launch date of October 2028 was proposed. Centaur Extinct comet Proper elements of active asteroids at Asteroid Families Portal Henry Hsieh's Main-Belt Comets page has extensive details on Main-belt comets David Jewitt. Main Belt Comets Planetary Society article on MBCs Discussion of possible differences in characteristics of the water in MBCs and other comets YouTube Interview with David Jewitt Impact trigger mechanism diagram by David Jewitt Comet-like appearance of Scheila Project T3: Finding Comets in the Asteroid Population The Active Asteroids Discovery of Main-Belt Comet P/2006 VW139 by Pan-STARRS1 New Comet: P/2012 T1 The location of Asteroidal Belt Comets, in a comets' evolutionary diagram: The Lazarus Comets P/2013 R3: a Main Belt Comet, breaking apart.
J. Licandro New images obtained with the GTC
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process, it is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Three quarters of the Sun's mass consists of hydrogen; the Sun is a G-type main-sequence star based on its spectral class. As such, it is informally and not accurately referred to as a yellow dwarf, it formed 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System; the central mass became so hot and dense that it initiated nuclear fusion in its core. It is thought that all stars form by this process.
The Sun is middle-aged. It fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result; this energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to become a red giant, it is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, no longer produce energy by fusion, but still glow and give off heat from its previous fusion; the enormous effect of the Sun on Earth has been recognized since prehistoric times, the Sun has been regarded by some cultures as a deity.
The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of, the predominant calendar in use today. The English proper name Sun may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn; the Latin name for the Sun, Sol, is not used in everyday English. Sol is used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars; the related word solar is the usual adjectival term used for the Sun, in terms such as solar day, solar eclipse, Solar System. A mean Earth solar day is 24 hours, whereas a mean Martian'sol' is 24 hours, 39 minutes, 35.244 seconds. The English weekday name Sunday stems from Old English and is a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου.
The Sun is a G-type main-sequence star. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is heavy-element-rich, star; the formation of the Sun may have been triggered by shockwaves from more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars; the heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is 1 astronomical unit, though the distance varies as Earth moves from perihelion in January to aphelion in July.
At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports all life on Earth by photosynthesis, drives Earth's climate and weather; the Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres; the tidal effect of the planets is weak and does not affect the shape of the Sun. The Sun rotates faster at its equator than at its poles; this differential rotation is caused by convective motion
Classical Kuiper belt object
A classical Kuiper belt object called a cubewano, is a low-eccentricity Kuiper belt object 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 found after Pluto and Charon, 15760 Albion, which until January 2018 had only had the provisional designation 1992 QB1. Similar objects found were called "QB1-o's", or "cubewanos", after this object, though the term "classical" is much more used in the scientific literature. Objects identified as cubewanos include: 15760 Albion Makemake, the largest known cubewano and a dwarf planet 50000 Quaoar and 20000 Varuna, each considered the largest TNO at the time of discovery 19521 Chaos, 58534 Logos, 53311 Deucalion, 66652 Borasisi, 88611 Teharonhiawako 1997 CU29, 2002 TX300, 2002 AW197, 2002 UX25 2014 MU69 Haumea was provisionally listed as a cubewano by the Minor Planet Center in 2006, but turned out to be resonant.
There are two basic dynamical classes of classical Kuiper-belt bodies: those with unperturbed orbits, those with markedly perturbed orbits. Most cubewanos are found between the 2:3 orbital resonance with the 1:2 resonance. 50000 Quaoar, for example, has a near-circular orbit close to the ecliptic. Plutinos, on the other hand, have more eccentric orbits bringing some of them closer to the Sun than Neptune; the majority of classical objects, the so-called cold population, have low inclinations and near-circular orbits, lying between 42 and 47 AU. A smaller population is characterised by inclined, more eccentric orbits; the terms'hot' and'cold' has nothing to do with surface or internal temperatures. Instead, the terms'hot and'cold' refer to the orbits of the objects, by analogy to particles in a gas, which increase their relative velocity as they become heated up; the Deep Ecliptic Survey reports the distributions of the two populations. The vast majority of KBOs have inclinations of less than 5° and eccentricities of less than 0.1.
Their semi-major axes show a preference for the middle of the main belt. The'hot' and'cold' populations are strikingly different: more than 30% of all cubewanos are in low inclination, near-circular orbits; the parameters of the plutinos’ orbits are more evenly distributed, with a local maximum in moderate eccentricities in 0.15–0.2 range and low inclinations 5–10°. See the comparison with scattered disk objects; when the orbital eccentricities of cubewanos and plutinos are compared, it can be seen that the cubewanos form a clear'belt' outside Neptune's orbit, whereas the plutinos approach, or cross Neptune's orbit. When orbital inclinations are compared,'hot' cubewanos can be distinguished by their higher inclinations, as the plutinos keep orbits below 20°. In addition to the distinct orbital characteristics, the two populations display different physical characteristics; the difference in colour between the red cold population, such as 2014 MU69, more heterogeneous hot population was observed as early as in 2002.
Recent studies, based on a larger data set, indicate the cut-off inclination of 12° between the cold and hot populations and confirm the distinction between the homogenous red cold population and the bluish hot population. Another difference between the low-inclination and high-inclination classical objects is the observed number of binary objects. Binaries are quite common on low-inclination orbits and are similar-brightness systems. Binaries are less common on high-inclination orbits and their components differ in brightness; this correlation, together with the differences in colour, support further the suggestion that the observed classical objects belong to at least two different overlapping populations, with different physical properties and orbital history. There is no official definition of'cubewano' or'classical KBO'. However, the terms are used to refer to objects free from significant perturbation from Neptune, thereby excluding KBOs in orbital resonance with Neptune; the Minor Planet Center and the Deep Ecliptic Survey do not list cubewanos using the same criteria.
Many TNOs classified as cubewanos by the MPC are classified as ScatNear by the DES. Dwarf planet Makemake is such a borderline classical cubewano/scatnear object. 2002 KX14 may be an inner cubewano near the plutinos. Furthermore, 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; the traditional usage of the terms is based on the orbit's semi-major axis, includes objects situated between the 2:3 and 1:2 resonances, between 39.4 and 47.8 AU. These definitions lack precision: in particular the boundary between the classical objects and the scattered disk remains bl