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Dwarf Planets By Anton Komarov
Planets, Dwarf Planets and Small Solar System Bodies. Questions and Answers
In plain language, what is the new definition of planet?
[A] A planet is an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round (or near-spherical) shape. In addition a planet orbits in a clear path around the Sun. If any object ventures near the orbit of a planet, it will either collide with the planet, and thereby be accreted, or be ejected into another orbit. [*]
Currently there are five objects accepted as dwarf planets. Ceres, Pluto, Eris, Makemake and Haumea. Many more dwarf planets are likely to be discovered soon.

Dwarfs Planets are a class apart in the eclectic Gas, Rock or Ice bestiary which constitutes the populace of our Solar System. Their classification is subject of hot debate and is not the subject of this article. They are smaller than the eight main planetary bodies of the solar system but size is not the main reason for instance that Pluto does not have the status of a planet. Among the three criteria necessary to be classified as a planet are

• International Astronomy Union: A Planet is in orbit around the Sun
• Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
• Has cleared the neighborhood

Pluto not having cleared its neighborhood was demoted to the indignation of its millions of supporters down to the less glamorous title of Dwarf Planet. But Pluto may not have said its last word in this affair and it was not the first to be a victim of reclassification.

When we think about Dwarf Planets, we always figure that they are located in the outer far fringes of the solar system, which is true for the vast majority of them. The terminologies with which they are sometimes associated says it all, TNO for Trans Neptunian Objects, KBO for Kuiper Belt Object or even at times, member of the scattered disk and so on which all denotes their remote location.

However the first member of this fascinating family lies in the Asteroid Belt and was also once upon a time classified as a planet. When Ceres was discovered in 1801, it was thought to be a fully-fledged planet filling the apparent gap in between Mars and Jupiter. However, 49 years later in 1850 many other objects sharing similar orbits were being discovered and Ceres was declassified.

Does Ceres Originates where it is presently located?

Despite its relatively small size, Ceres still sports a diameter of 950 km and differs so much from the other rocky denizens of the Asteroid Belt that in 2006, the same year Pluto was declassified, Ceres climbed up from being an asteroid to become a Dwarf Planet.

Just one year after this celestial promotion, the space probe Dawn was launched (2007) and propelled through interplanetary space by an Ion Engine on a trajectory which allowed her to visit the two colossus of the Asteroid Belt, Vesta and Ceres. Vesta though full of surprises, revealed more of what scientists, illustrators or science fiction writers thought a massive asteroid would turn out to be like.

Vesta is actually an old proto planet which would have been not much different from the other terrestrial planets (Mercury, Mars, Venus, and Earth) if not for the negative gravitational influence of the giant planet Jupiter which impeded the growth of the asteroid. Before the arrival of the probe, we knew relatively very few things about the former asteroid Vesta besides the basics. [*]

The subsurface ocean of Ceres

Finally in 2015, Dawn became the first spacecraft to visit a Dwarf Planet and the mini world was found to be not only active geologically but more hydrated than thought before.[*]

“It is a tantalizing perspective in terms of astrobiology and would make of Ceres a strong candidate for harboring life. ”

Dawn found hints of possible large reservoirs of underground water as well as it became accepted that once Ceres had a global subsurface ocean. Recent studies based on data obtained by Dawn revealed that remnants of this subsurface ocean could still be there. [*]

The scientists studied the relatively young Haulani crater which was the result of an impact which occurred 20 million years ago and found that the impactor would have penetrated the icy crust and hit the underground reservoir. This could have remained in a liquid state due to its extreme saltiness and a type of hydrate which would change the distribution of heat through the crust of the mini world. It would seem that the same mechanism is in action on Ceres’s cousin, the most famous of the dwarf worlds, Pluto. It is a tantalizing perspective in terms of astrobiology and would make of Ceres (in the Roman Mythology she was a goddess of agriculture, grain crops, fertility and motherly relationships) a strong candidate for harboring life. Less than twenty-four hours after this announcement on August 12, 2020, Scientific America was reporting that other researchers were disputing the hypothesis. [*]

Mikhail Zolotov of Arizona State University who was consulted (but not part of), in one of the seven studies published in Nature, even qualified the idea of a sub-surface ocean as wishful thinking. This was a comment which went round on the internet last year (2020) in the world of astrobiology and astronomy. Some are not even convinced totally of the existence of a past ocean. The website Solar System of NASA has on its page on Ceres this info ‘Dawn reinforced that dwarf planets, not just icy moons, could have hosted oceans during a large part of their history—and potentially still do. The main Issue remains to explain where the energy comes from to maintain large bodies of water at liquid state whether salty or not. Contrary to the Icy moons around the Gas and Icy giants whose oceans are maintained by the gravitational tides caused by their parent planet, Ceres does not orbit such a world. One thing is certain, there is something cooking geologically on the Dwarf planet as the team of Dawn also noticed that the surface of this world shows fewer craters than previously, expected a clear indication that some process is rejuvenating the surface.

The solitary giant mountain

It sounds like the beginning of a science fiction story. A lone giant mountain of four kilometers high stood on this dwarf world.

Mountains do not generally come one by one for an entire planet. Even the fictional little Prince of Saint Exupery had three volcanoes on his small asteroid, two which were active and one which was dead.

The International Outsider
N.B: Ahuna is not the only high elevation feature on Ceres but the only one of that kind and of such height.

Many attempts have been made to try to explain this unique formation on the Dwarf Planet and recently a study by (ESA), the German Aerospace Center (DLR), the Jet propulsion laboratory of NASA (JPL) and the Sapienza Università claimed that “an ascending plume made of a mixture of salty water and rock rose from within the dwarf planet. Once that material erupted onto the surface, it formed a mountain 2.5 miles (4 kilometers) high and 10.5 miles (17 kilometers) wide. While this process occurred at very cold temperatures on Ceres, probably below 32 degrees Fahrenheit (0 degrees Celsius), it is similar to plumes of magma forming volcanoes on Earth”.

So where do you come from Ceres?

With all those characteristics, Ceres seems more and more alien to the environment in which it is located and with the absence of ceroids (Vesta, which is the second largest body in the Asteroid Belt with a diameter of 525 kilometers, has near its orbit, a collection of smaller asteroids thought to be the result of the many impacts in its history. Evolving in the same region, it is hard to explain why Ceres is devoid of ceroids as it should have experienced the same periodic impacts. Though many maintain that Ceres is a classic proto planet which did not accrete sufficient mass to become a classical terrestrial planet, there are many others who argue that Ceres could have formed further in the Kuiper Belt and then migrated into the inner solar system. A class of compounds and elements (among which is ammonia which would show that Ceres formed beyond the Ice line in the Solar System) found on its surface, seems to corroborate this hypothesis.[*]

Proposal for a future mission to Ceres: Project Gauss, (Genesis of Asteroids and the Evolution of the Solar System) a sample Return Mission to Ceres. Find PDF url at the end of the article [*]

Executive summary. The goal of Project GAUSS is to return samples from the dwarf planet Ceres. Ceres is the most accessible ocean world candidate and the largest reservoir of water in the inner solar system. It shows active cryovolcanism and hydrothermal activities in recent history that resulted in minerals not found on any other planets to date, except for Earth’s upper crust. The possible occurrence of a recent subsurface ocean on Ceres and the complex geochemistry, suggest possible past habitability and even the potential for ongoing habitability. Aiming to answer a broad spectrum of questions about the origin and evolution of Ceres and its potential habitability, GAUSS will return samples from this possible ocean world for the first time.

During the Obama presidency, there was a plan for a manned mission to an Asteroid; some of those consulted even pushed the logic to make the target more challenging by proposing a manned mission to the dwarf planet [*]

To find our next dwarf planet, we have to go well beyond the Gas and Ice giants of the Solar System to where lies Pluto but on our way, we should have a look at potential dwarf candidates. Mike Brown (Caltech) the discoverer of Sedna on the 14 November 2003. His team also discovered Eris both of which are considered to be dwarf planets. Brown estimated that of the thousands of Trans Neptunian Objects catalogued (628 numbered, 2000 unnumbered (as of 2020 Data Minor Planet Center MPC) 130 could be considered as dwarf planets. [*]

Ten objects with a diameter of over 900 kilometers as of 2020 are nearly certain to belong to this category, it is important to note that with such a diameter, any object would be in hydrostatic equilibrium meaning it would be round even if its composition is majorly rock. There are 17 objects with a diameter of over 600 kilometers who are highly likely to be so too and a further 41 objects with a diameter of more than 500 kilometer if they are mainly composed of rock. Another 62 objects with a diameter of more than 400 kilometers are probably Dwarf Planets if they are primarily constituted of ice. Mike Brown’s assessment contains other candidates and the list seems to keep on growing with the addition of objects with a diameter of over 200 kilometers taking as an example some Icy Satellites which conserve a round shape, provided this is true also concerning KBO (Kuiper Belt Objects). As of 2020, the list was given an additional 611 such objects.

In our chase for dwarf planets, we will stop at Neptune where in orbit around the Icy Giant lies Triton, by far the largest moon in the system. Once upon a time she could well have been a dwarf planet.

Triton orbits in the opposite direction of the Neptune’s rotation that contrary to all the other large moons of the Solar System. This strange behavior and other characteristics, led scientists to postulate that Triton could have been a Kuiper Belt Object captured by Neptune millions of years ago. In an intentionally (provocative) title, Ethan Siegel wrote in Forbes on November 30-2018, that it is not Eris with the highest density nor Pluto with the greatest diameter but Triton who is the true King of the Kuiper Belt as it is the innermost of the Trans-Neptunian Objects. [*]

Triton revealed herself with the flyby of Neptune by Voyager 2 on the 26 August 1986 and what a revelation it was. With a diameter of 2700 kilometers, Triton has a thin atmosphere comparable to the one found on Pluto, It has active geysers shooting nitrogen high into the atmosphere. In a 2012 paper in the journal Icarus, Jodi Gaeman and colleagues outlined a possible scenario for a subsurface ocean on Triton which seems to be more and more a common feature among the icy moons/dwarf planets multiplying by many factors the chances of finding life’s nurseries all around the Solar System.

Pluto

This demoted planet is the new official King of the Kuiper Belt as it is the innermost representative of this class of objects and it is more than probable that future discoveries will uncover worlds with larger diameters. We will pass over the passion caused by this demoting, a passion still burning today and we will come back on the subject in another article. All we can say is that we are glad that Clyde William Tombaugh, the discoverer of Pluto in 1930 was not alive to see the drama unfolding as in 2006; he sadly passed away in 1997. When the space probe New Horizons finally arrived and performed a flyby of the former planet on the 14 July 2015, the passion sky rocketed like cryovolcanism on Triton.

Pluto is geologically active with mountain peaks cumulating at a height of 3.5 kilometers and is formed on a bed rock of water ice. The thin atmosphere extends to 1600 kilometers above the surface and is composed mainly of methane and nitrogen. The surface of Pluto is covered at 98 percent by nitrogen ice with traces of methane and carbon monoxide. Just outside the southern tip of Sputnik Planitia two craters were identified of volcanoes baptized Wright and Picard Mons. They are the biggest ice volcanoes of the outer system at a height of about 4 kilometers and 150 kilometers across, supporting the idea that Pluto had and certainly still has, cryovolcanism on its surface.
Sputnik Planitia

It is impossible to speak about Pluto without speaking about the now famous Heart of Pluto which is a young surface devoid of craters in the shape of a gigantic heart the size of the state of Texas. It is in fact the biggest glacier in the Solar System. Scientists think that the feature was originally created by an impact; it slowly filled up with volatile ices, mainly nitrogen ice. The largest area of Sputnik Planitia which was named after the first artificial satellite is found in the Northern Hemisphere though it extends through the Equator. The impactor would have been a large one, at around 150 or up to 300 kilometers, it is the most likely origin of this curious formation though alternatively, it could have been created by the accumulation of ices in this location which depressed the surface there, leading to the formation of a basin via a positive feedback process without an impact event (Source Wikipedia). Sputnik Planitia is a unique feature in the Solar System; it plays an important part in the complex dynamics of the tenuous atmosphere of Pluto, a little bit like the role played by Greenland on Earth but to a greater extent as Sputnik Planitia occupies an important proportion of the total surface of Pluto.

Then in a virtual conference in June 2020 [*]researchers after analysing the data of New Horizons indicated that Pluto’s sub surface global ocean could still be present. The main proof being the ripples at the surface of the dwarf planet, those dark ripples are located on the far side of Pluto.[*]

These ripples show evidence of expansion and not contraction validating the presence of an ocean. If Pluto had a ‘hot start’ and not a cold one as previously suspected, it would mean that this ocean would have been present since the dwarf planets early days and for this model to hold on, the ocean should be at about at least 150 kilometers deep and contain minerals[*] that form through the interactions between rock and water. Like the hypothetical sub surface ocean of Ceres, if Pluto’s less hypothetical ocean has been present during billions of years, what does it tells us about the habitability of this world and by extension to all the other sizable Kuiper Belt Objects? Will the Kuiper Belt or even the Oort Cloud reveal to be the most abundant reservoir of life in the Solar System? And could it be the most frequently found one in the universe itself? Pluto has many other features which make it a world apart and add ground to those still willing to get her the status of planet back. One has to know the issue is not only an emotional one but a scientific one too.

Charon

Pluto has five companion moons, Charon, Styx, Nix, Kerberos, and Hydra and among them Charon plays a key role making the system unique from all points of view. First of all if Triton orbiting around Neptune can be considered as a once upon a time dwarf planet, then Charon if orbiting the Sun by itself would have been considered as a fully fledged dwarf planet. Charon is enormous in size compared to its parent planet. With a diameter of 1,212 kilometers compared to Pluto’s 2376,6 kilometers, Charon is more than half the size of Pluto, and as if that was not sufficient, Charon orbits very closely to Pluto at a distance of 19,800 kilometers! If our moon was proportionally as big as Charon and orbiting as close, it would have take a substantial proportion of the night sky.

With this size, Charon exceeds the diameter of dwarf planet Ceres by more than 250 kilometers. And it looks like it is currently in hydrostatic equilibrium so keeping a rounded shape. Charon's' surface composition of is mainly madecof the less volatile water ices and the global composition of the moon is of 55 percent rock and 45 percent water ice.

In 2007 [*] The Gemini Observatory detected several patches of water crystal and ammonia hydrates strongly suggesting cryovolcanic activities on the surface of the moon and if it is confirmed it will make Charon geologically active. One can automatically noticed on the pictures of Charon the reddish cap which covers its North Pole, what is this material and where does it comes from? It is hard to speak about KBO’s without speaking about thollins which covers partly or almost totally, the surface of a lot of these worlds,( tholins (after the Greek θολός (tholós) "hazy" or "muddy" from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide (CO2), methane (CH4) or ethane (C2H6), often in combination with nitrogen (N2) or water (H2O).tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups, typically nitriles and hydrocarbons and their degraded forms such as amines and phenyls. Tholins do not form naturally on modern-day Earth, but they are found in great abundance on the surfaces of icy bodies in the outer Solar System, and as reddish aerosols in the atmospheres of outer Solar System planets and moons.). On Pluto, distribution of thollins is done by the wind and atmospheric currents when it is spewed out by the mouth of cryovolcanoes. Some of this material escapes the dwarf planet into space and starts a migration through the 19,800 km distance which separates Pluto and Charon and ends their journey on the South Pole of the moon and which by accumulation, gives the moon its reddish color.

But wait a minute! We said higher up in the article that ‘…if Charon was orbiting the sun by itself it would have been considered as a fully fledged dwarf planet…,’ actually it does and it should have been considered a dwarf planet by itself according to the strict definition of the IAU (International Astronomy Union). But how can this be? The IAU in its definition of a dwarf planet contends that it has to orbit the sun on its own. In fact true moons have their barycenter (In astronomy, the barycenter (or barycentre; from the Ancient Greek heavy center) which is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit) inside their parent planet. This is not the case of Charon and Pluto which orbit around a barycenter outside of both objects. Making it de facto a binary object so Pluto should be classified as a double dwarf planet with the status of Charon changing from moon to fully fledged dwarf planet and a part of this binary system. This complicates the matter of classification rather than simplifying it and when you think about it, even a regular moon revolves around the sun simply by accompanying its main parent body around our star during its orbital period.

Haumea

If the first dwarf planet in the solar system Ceres is situated at a distance of 413 million kilometers, roughly 2.8 astronomical units away from the sun and Pluto is at an average distance of 30.5, Au 5.9 billion kilometers, then we need to go the distance of 6,452,000,000 kilometers to get to Haumea which is 43 astronomical units away from the Sun. We do not know a lot about Haumea as no space probe has ever visited the small dwarf planet discovered by Mike Brown and his team at the Palomar Observatory in 2004. But for the little that we know about it, Haumea is a world of fascinating mysteries. With a diameter of 1,960 km – 1,518 kilometers which contrasts heavily with the polar diameter of 996 kilometers. The surface appears white and young and a process of geological rejuvenation must be in place to explain the apparently young surface. When this kind of situation occurs the first reflex is to think about cryovolcanism or recent impacts to explain what could be the mechanism in place. Those two explanations are invalidated by the fact that the Gemini observatory in December 2004 revealed the composition of the ice and determined that it was mainly constituted of crystalline water ice and that the absence of ammonia was inconsistent with cryovolcanism. As for the impact event, it appears that it would have happened 100 million years ago making of it a non viable explanation as to why the dwarf Planet is white as snow. The weird shape[*]of Haumea is explained by its insane rotation rapidity. If it takes this Kuiper belt object 285 earth years to rotate around the Sun, then it takes the dwarf planet a mere 3.9 hours to compete a full revolution around itself making of it the fastest rotating object with a diameter of more than 100 kilometers in the whole solar system.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. [*]

This exceptional configuration could be explained by an impact event which would have also created the two know satellites Hiʻiaka (300 kilometer diameter) and Namaka (170 kilometer diameter). The collision would have also in the same event have created the collision family of Haumea. It would have also created the curious proportion of rock/ice which is predominantly rocky with a density comparable to our moon. This suggests that the bulk of Haumea is rock covered with a relatively thin layer of ice. It is possible that a thicker ice mantle that is more typical of Kuiper Belt Objects existed in the past, but was blasted off during the impact.

The Ring

Haumea is also the first Trans-Neptunian Object to join the club of the objects with rings in the Solar System which is more numerous than one could guess a priori. People think generally of Saturn when it comes to planetary rings and uncontestably Saturn is the true lord of the rings and without any doubt, the jewel of the solar system. However all the outer solar system giants, Jupiter, Saturn, Uranus and Neptune have at least a ring around them. Despite some reports in 2009 that Rhea, a moon of Saturn had its own ring system, the theory has been rejected after the Cassini probe made detailed imaging of the region. 10199 Chariklo a minor planet in the Centaur class of objects which is in the list of Mike Brown as being a possible dwarf planet has a two ring system, 2060 Chiron, is also suspected to have a pair of rings. [*]

In fact no one has ever observed directly the ring of Haumea, and no spacecraft took a shot at it. The discovery of the ring was deduced after occultation of the star poetically named URAT1 533-182543. A fluke discovery as the experiment was originally designed to measure the size density of the dwarf planet and only incidentally discovered the rings. All we know about it is that it has a radius of about 2,287 kilometers and a width of 70 kilometers. The ring particles don't orbit the dwarf planet with a resonance of 1:3, but do orbit Haumea periodically in a slightly different path that puts them close to that 1:3 resonance.

Makemake

Makemake was discovered on March 31, 2005 by a team led by Michael E. Brown. A rather large object of the classical group her diameter of 715 kilometers makes it smaller in size than the moon Charon of Pluto and it orbits at 5, 61 billion kilometers, 45.8 astronomical units away from the Sun. She completes a round trip of our star in 305.34 Earth years. Despite attempts to detect an atmosphere by occultation of a 18 magnitude star on the 23 April 2011, no presence of an atmosphere was revealed in the results. It may be that it does have a transitional atmosphere when it is at its closest point to the sun, a little bit like Pluto. The surface of Makemake is partly covered by methane which makes it redder in the spectrum like Pluto. In addition to methane, large amounts of ethane and tholins as well as smaller amounts of ethylene, acetylene and high-mass alkanes (like propane) may be present. Makemake has a moon dark like charcoal which has not been named yet and known under the astronomical classification, S/2015 (136472) nicknamed MK 2. MK 2 orbits at 20921 Kilometers from the dwarf planet, and its diameter is estimated to be about 160 kilometers.

Eris

As with Haumea, the Dwarf Planet Eris has not been visited by any space probes. It was discovered by Mike Brown and his team in January 2005. There is not a lot of information on this object and all the sites on the internet speaking about Eris are very repetitive, however there are few notable points about the object but first the basics. Eris is the most massive known Dwarf Planet and is shy only of a few kilometers to match the diameter of Pluto. Eris’s diameter is 2,326 ± 12 kilometers with Pluto being only 2 376,6 kilometers diameter. Eris is about 68 astronomical units away from the Sun with an average distance of 10,125,000,000 kilometers. At a temperature of about -217 degrees Celsius it is certainly one of the lowest temperatures on a massive world of the Solar System. Eris would probably have a rocky surface not much different than Pluto’s. The Dwarf Planet has only one moon known as Dysnomia and likely the second-largest known moon of a Dwarf Planet, after Pluto’s moon Charon. The moon is 500 times less bright in the visible band than Eris. The satellite probable darker surface with quite more redder spectrum shows that it is probably partly covered with Thollins and the moon's diameter is of 700 +- 115 kilometers (25% to 35% of Eris's diameter), it is expected to be of a round shape. Dysnomia's average orbital distance from Eris is approximately 37,300 km [*]

Contrary to its moon Dysnomia, the surface of Eris is extremely bright and this whiteness is not due to water like on Haumea but rather methane which could condense at its surface due to extreme low temperatures and cover potential deposits of red thollins telling us that there is certainly a process of resurfacing in action. Cryovolcanoes or geysers as well who knows, a subsurface ocean could be the mechanism that remodels Eris’s surface but to date, it is only speculation.

Eris has a tenuous atmosphere and the proposed thin white layer that apparently covers the majority of its surface, could be due to the result of the dwarf planet's atmosphere condensing as frost on the surface as it moves away from the sun.

The Dwarf Planet has an highly eccentric, orbit with an orbital period of 559 years. Its maximum possible distance from the Sun (aphelion) is 97.5 AU, and its closest (perihelion) is 38 AU Using numerical integration JPL Horizons shows Eris came to perihelion around 1699 the year The Republic of Venice, Polish–Lithuanian Commonwealth and Holy Roman Empire sign the Treaty of Karlowitz with the Ottoman Empire, marking an end to the major phase of the Ottoman–Habsburg wars. It came to aphelion around 1977, the year that the incredibly successful probe Voyager 2 was launched.which ultimately gave us at the end of its grand tour of the giants of the Solar System in August 1989 the first glimpse of what a Dwarf Planet could look like when flying by the once upon a time Dwarf Planet Triton. Eris’s elongated orbit takes her out of the elliptical disk on which the major planets orbits. Around 2820 AD, Eris will be closer to the Sun than Pluto for some time. [*] [*]]

Apart from being classified as a Dwarf Planet, it is also a TNO. Eris is also a member of the mysterious scattered disk of the solar system, a distant circumstellar disc that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. Although the closest scattered-disc objects approach the Sun at about 30–35 AU, their orbits can extend well beyond 100 AU.

Some Highly likely candidates to the status of Dwarf Planets.
Sedna

Sedna is unique as she belongs yet to another group and 12.8 billion kilometers is the distance that separates Sedna from the Sun. Sedna’s diameter is 995 kilometers, and a survey detected no moon with a probability of missing a potential moon at 25 %. Sedna’s surface is composed of a mixture of methane, nitrogen ice and water with thollins . This combination makes the surface of Sedna look strongly red, maybe even more reddish than the surface of Mars,

At its furthest distance from the sun (aphelion), Sedna would be located at a distance 31 times further than Neptune, the last planet of the system which is a whopping 937 Astronomical Units to the Sun. The heliopause, the most commonly accepted boundary of the Solar System as measured by the interstellar mission Voyager 2, lies at a distance of around 121 Astronomical units. It gives an idea to what extent is the gravitational grip of the sun. Actually at it furthest is at 1.5% of a light-year or approximately 5.5 light-days. At such a distance, Sedna is technically in interstellar space. Sedna is not a Kuiper Belt Object but a denizen of the inner limits of the hill cloud though the Minor Planet Center currently places Sedna in the scattered disc . In astronomy, the Hills cloud (also called the inner Oort cloud[1] and inner cloud[2]) is a vast theoretical circumstellar disc, interior to the Oort cloud, whose outer border would be located at around 20,000 to 30,000 astronomical units (AU) from the Sun, and whose inner border, less well-defined, is hypothetically located at 250–1500 AU.

With an orbital period of 11400 years Sedna is the second furthest known object in the Solar System. The record breaking one being 2012 VP113 a minor planet possibly a dwarf planet discovered on the 5 November 2012. The diameter of 2012 VP113 is unknown but estimations put it between 300 and 1000 kilometers, well in the range to have a round shape due to hydrostatic equilibrium thus a potential fully fledged dwarf planet which at its perihelion is at 80.389 AU and at its furthest 436.143 AU.[*] [*]

For most of its orbit, it is even farther from the Sun than at present, with its aphelion estimated at 937 AU [5] (31 times Neptune's distance, about 1.5% of a light-year or approximately 5.5 light-days).

The classification of a dwarf planet is a minefield or should we say a field of active cryovolcanic geysers? Should a rocky ancient proto planet like Vesta be considered a dwarf planet? what of cases like the couple Pluto and Charon? What happens if we find a Mercury size object far away at the edges of the Hill Cloud which has not cleared its environment? Would this planetary sized body be called a dwarf planet or a giant dwarf planet?

We end our tour of the Dwarf Planets of the solar System. There are many more others to discover and they all have individual characteristics and featureson and more active than previously thought. They have exotic names like )

Quaoar, Chaos or Orcus (also with a large moon, If Orcus is a dwarf planet, Vanth would be the third-largest known dwarf-planet moon, after Charon and Dysnomia. The ratio of masses of Orcus and Vanth is uncertain, possibly anywhere from 1:33 to 1:12.[*])

https://en.wikipedia.org/wiki/90482_Orcus , Gonggong 1,230 km in diameter one moon, Chiron discovered in 1977 and previously classified as an asteroid is one possible candidate to be a Dwarf planet according the list of Mike Brown with a diameter of only 206 kilometers, Hygiea is a major asteroid located in the main asteroid belt. With a diameter of 434 kilometers, Salacia with 850 kilometers in diameter ,one moon 286 km in diameter , Dziewanna 470 kilometers in diameter or even Chaos 600 kilometers in diameter.

References Tour of the Dwarf Planets of The Solar System

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