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My research is not giving me anything definitive. Are the Apollo asteroids orbits less stable than the Centaurs or any Trojans, or is another class of objects in less stable solar orbits? On what timescale are those orbits unstable?
The least stable orbits are likely the temporarily captured orbiters of Earth and other planets. These are bodies that have been captured by the Earth/Moon gravity well and move from solar orbits to terrestrial orbits. They normally have complex orbits that take them well beyond the orbit of the Moon and most don't last long.
Most such objects are small, typically less than 1 m in diameter. There are probably about 1000 such bodies with sizes of 10cm and up orbiting the Earth. Normally, they are undetected but occasionally a larger object will be discovered. Most recently, 2020 CD3 was captured in the winter of 2015-16 and escaped Earth's gravity in May 2020.
The orbits of such temporary orbiters are not stable, they normally escape the Earth/Moon gravitational well after a few years, and return to heliocentric orbits.
Signs of a hidden Planet Nine in the solar system may not hold up
Planet Nine might be a mirage. What once looked like evidence for a massive planet hiding at the solar system’s edge may be an illusion, a new study suggests.
“We can’t rule it out,” says Kevin Napier, a physicist at the University of Michigan in Ann Arbor. “But there’s not necessarily a reason to rule it in.”
Previous work has suggested that a number of far-out objects in the solar system cluster in the sky as if they are being shepherded by an unseen giant planet, at least 10 times the mass of Earth. Astronomers dubbed the invisible world Planet Nine or Planet X.
Now, a new analysis of 14 of those remote bodies shows no evidence for such clustering, knocking down the primary reason to believe in Planet Nine. Napier and colleagues reported the results February 10 at arXiv.org in a paper to appear in the Planetary Science Journal.
The idea of a distant planet lurking far beyond Neptune received a surge in interest in 2014, when astronomers Chad Trujillo of Northern Arizona University and Scott Sheppard of the Carnegie Institution for Science reported a collection of distant solar system bodies called trans-Neptunian objects with strangely bunched-up orbits (SN: 11/14/14).
In 2016, Caltech planetary scientists Mike Brown and Konstantin Batygin used six trans-Neptunian objects to refine the possible properties of Planet Nine, pinning it to an orbit between 500 and 600 times as far from the sun as Earth’s (SN: 7/5/16).
But those earlier studies all relied on just a handful of objects that may not have represented everything that’s out there, says Gary Bernstein, an astronomer at the University of Pennsylvania. The objects might have seemed to show up in certain parts of the sky only because that’s where astronomers happened to look.
“It’s important to know what you couldn’t see, in addition to what you did see,” he says.
To account for that uncertainty, Napier, Bernstein and colleagues combined observations from three surveys — the Dark Energy Survey, the Outer Solar System Origins Survey and the original survey run by Sheppard and Trujillo — to assess 14 trans-Neptunian objects, more than twice as many as in the 2016 study. These objects all reside between 233 and 1,560 times as far from the sun as Earth.
The team then ran computer simulations of about 10 billion fake trans-Neptunian objects, distributed randomly all around the sky, and checked to see if their positions matched what the surveys should be able to see. They did.
“It really looks like we just find things where we look,” Napier says. It’s sort of like if you lost your keys at night and searched for them under a streetlamp, not because you thought they were there, but because that’s where the light was. The new study basically points out the streetlamps.
“Once you see where the lampposts really are, it becomes more clear that there is some serious selection bias going on with the discovery of these objects,” Napier says. That means the objects are just as likely to be distributed randomly across the sky as they are to be clumped up.
That doesn’t necessarily mean Planet Nine is done for, he says.
“On Twitter, people have been very into saying that this kills Planet Nine,” Napier says. “I want to be very careful to mention that this does not kill Planet Nine. But it’s not good for Planet Nine.”
There are other mysteries of the solar system that Planet Nine would have neatly explained, says astronomer Samantha Lawler of the University of Regina in Canada, who was not involved in the new study. A distant planet could explain why some far-out solar system objects have orbits that are tilted relative to those of the larger planets or where proto-comets called centaurs come from (SN: 8/18/20). That was part of the appeal of the Planet Nine hypothesis.
“But the entire reason for it was the clustering of these orbits,” she says. “If that clustering is not real, then there’s no reason to believe there is a giant planet in the distant solar system that we haven’t discovered yet.”
Batygin, one of the authors of the 2016 paper, isn’t ready to give up. “I’m still quite optimistic about Planet Nine,” he says. He compares Napier’s argument to seeing a group of bears in the forest: If you see a bunch of bears to the east, you might think there was a bear cave there. “But Napier is saying the bears are all around us, because we haven’t checked everywhere,” Batygin says. “That logical jump is not one you can make.”
Evidence for Planet Nine should show up only in the orbits of objects that are stable over billions of years, Batygin adds. But the new study, he says, is “strongly contaminated” by unstable objects — bodies that may have been nudged by Neptune and lost their position in the cluster or could be on their way to leaving the solar system entirely. “If you mix dirt with your ice cream, you’re going to mostly taste dirt,” he says.
Lawler says there’s not a consensus among people who study trans-Neptunian objects about which ones are stable and which ones are not.
Everyone agrees, though, that in order to prove Planet Nine’s existence or nonexistence, astronomers need to discover more trans-Neptunian objects. The Vera Rubin Observatory in Chile should find hundreds more after it begins surveying the sky in 2023 (SN: 1/10/20).
“There always may be some gap in our understanding,” Napier says. “That’s why we keep looking.”
Which solar system objects have the least stable solar orbits? - Astronomy
The Solar System is the astronomical name for the Sun, the Earth, and the collection of planets and other rocky and icy objects moving around, or orbiting, the Sun.
The main component of the Solar System is the Sun, which contains 98.6 percent of the system's mass and whose gravity holds everything else in orbit.
The Earth's orbit around the Sun is nearly a perfect circle, but when mapped it is found that the Earth moves around the Sun in a very slightly oval shaped, or elliptical orbit. The other planets in the Solar System also circle the Sun in slightly elliptical orbits. Mercury has a more elliptical orbit than the others, and some of the smaller objects orbit the Sun in very eccentric orbits.
- (1) Mercury
- (2) Venus
- (3) Earth
- (4) Mars
- (5) Jupiter
- (6) Saturn
- (7) Uranus
- (8) Neptune
The planets are the biggest objects that go around Sun. It took people many years of looking carefully through telescopes to find the farthest away ones. No one expects to find new planets, but more small objects are found every year. Most of the planets have moons that orbit around them. There are at least 173 of these moons in the solar system.
Pluto had been called a planet since it was discovered in 1930, but in 2006 astronomers meeting at the International Astronomical Union decided for the first time on the definition of a planet, and Pluto didn't fit. Instead they defined a new category of dwarf planet, into which Pluto did fit along with some other objects.
Pluto is now one of five dwarf planets, here they are in order of their distance from the Sun.:
Astronomers think they will find more dwarf planets soon.
Before the discovery of Uranus, ancients thought the solar system consisted only of the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
Until the 16th and 17th centuries, western scientists thought the Sun and the planets orbited the Earth. This was what ancient civilizations thought too, except for a few individuals of the Greek, Indian and Muslim civilizations.
- Uranus, discovered in 1781
- Ceres, discovered in 1801, recently (2006) defined as a dwarf planet
- Neptune, discovered in 1846
- Pluto, discovered in 1930
- Eris, discovered in 2005
The Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the retinue of objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly-flat disc called the ecliptic plane. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials.
The Solar System is also home to two regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these regions, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognised to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies in those two regions, various other small body populations, such as comets, centaurs and interplanetary dust, freely travel between regions.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
The Sun is the star at the center of the Solar System. The Sun has a diameter of about 1,392,000 kilometres (865,000 mi) (about 109 Earths), and by itself accounts for about 99.86% of the Solar System's mass the remainder consists of the planets (including Earth), asteroids, meteoroids, comets, and dust in orbit. About three-fourths of the Sun's mass consists of hydrogen, most of the rest is helium. Less than 2% consists of other elements, including iron, oxygen, carbon, neon, and others.
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.
A terrestrial planet, telluric planet or rocky planet is a planet that is primarily composed of silicate rocks. Within the solar system, the terrestrial planets are the inner planets closest to the Sun (Mercury, Venus, Earth, Mars, and one terrestrial dwarf planet, Ceres.). The terms are derived from Latin words for Earth (Terra and Tellus), and an alternative definition would be that these are planets which are, in some notable fashion, "Earth-like".
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt region is also termed the main belt to distinguish it from other concentrations of minor planets within the Solar System, such as the Kuiper belt and scattered disc.
A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars.
A comet is a small solar system body bigger than a meteoroid that, when close enough to the Sun, exhibits a visible coma (fuzzy "atmosphere"), and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few hundred metres to tens of kilometres across.
The Kuiper belt, sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. It is home to at least three dwarf planets – Pluto, Haumea and Makemake.
The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered disc objects have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units. These extreme orbits are believed to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune. While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects "among the most distant and cold objects in the Solar System". The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects known as the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper.
The Oort Cloud is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and scattered disc, the other two known reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.
The vulcanoids are a hypothetical population of asteroids that may orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, whose existence was disproven in 1915. No vulcanoids have yet been discovered, and it is not clear if any exist.
Planets beyond Neptune. Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.
The formation and evolution of the Solar System is estimated to have begun 4.55 to 4.56 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the centre, forming the Sun, while the rest flattened into a protoplanetary disc out of which the planets, moons, asteroids, and other small Solar System bodies formed.
New Research Helps Explain Peculiar Orbits and Clustering of Outer Solar System’s Detached Objects
Our Solar System contains a large population of icy bodies stretching well beyond the orbit of Neptune. These objects are remnants from the early formation of the Solar System that were scattered outward from their birth location by Neptune. New research from the University of Colorado Boulder may help solve the mystery why so many of these trans-Neptunian icy bodies don’t circle the Sun the way they should.
An artist’s concept of the minor planet Sedna, one of the detached trans-Neptunian objects. Image credit: NASA / JPL-Caltech.
The orbits of extreme trans-Neptunian objects, which astronomers call ‘detached objects,’ tilt and buckle out of the plane of the Solar System, among other unusual behaviors.
Some scientists have suggested that something big could be to blame — like an as-of-yet-unseen ninth planet lurking beyond Neptune — that scatters objects in its wake.
But University of Colorado Boulder astronomer Ann-Marie Madigan and colleagues prefer to think smaller.
Drawing on exhaustive computer simulations, the researchers make the case that these detached objects may have disrupted their own orbits — through tiny gravitational nudges that added up over millions of years.
“The findings provide a tantalizing hint to what may be going on in this mysterious region of space,” Dr. Madigan said.
The scientists used supercomputers to recreate the dynamics of the outer Solar System in greater detail than ever before.
“We modeled something that may have once existed in the outer Solar System and also added in the gravitational influence of the giant planets like Jupiter,” said Alexander Zderic, a graduate student at the University of Colorado Boulder.
In the process, the team discovered something unusual: the icy objects in their simulations started off orbiting the Sun like normal. But then, over time, they began to pull and push on each other.
As a result, their orbits grew wonkier until they eventually resembled the real thing.
What was most remarkable was that they did it all on their own — the asteroids and minor planets didn’t need a big planet to throw them for a loop.
Comparison of the largest trans-Neptunian objects: Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, 2002 MS4, Orcus and Salacia. Image credit: Lexicon / CC BY-SA 3.0.
“Individually, all of the gravitational interactions between these small bodies are weak. But if you have enough of them, that becomes important,” Dr. Madigan said.
The authors had seen hints of similar patterns in earlier research, but their latest results provide the most exhaustive evidence yet.
In order to make their theory of ‘collective gravity’ work, the outer Solar System once needed to contain a huge amount of stuff.
“You needed objects that added up to something on the order of 20 Earth masses,” Dr. Madigan said.
“That’s theoretically possible, but it’s definitely going to be bumping up against people’s beliefs.”
“One way or another, scientists should find out soon. A new telescope called the Vera C. Rubin Observatory is scheduled to come online in Chile in 2022 and will begin to shine a new light on this unknown stretch of space.”
“A lot of the recent fascination with the outer Solar System is related to technological advances. You really need the newest generation of telescopes to observe these bodies,” Zderic said.
Alexander Zderic & Ann-Marie Madigan. 2020. Giant-planet Influence on the Collective Gravity of a Primordial Scattered Disk. AJ 160, 50 doi: 10.3847/1538-3881/ab962f
Alexander Zderic et al. 2020. Apsidal Clustering following the Inclination Instability. ApJL 895, L27 doi: 10.3847/2041-8213/ab91a0
What are Lagrange Points?
Lagrange Points are positions in space where the gravitational forces of a two body system like the Sun and the Earth produce enhanced regions of attraction and repulsion. These can be used by spacecraft as "parking spots" in space to remain in a fixed position with minimal fuel consumtion.
There are five special points where a small mass can orbit in a constant pattern with two larger masses. The Lagrange Points are positions where the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. This mathematical problem, known as the "General Three-Body Problem" was considered by Lagrange in his prize winning paper (Essai sur le Problème des Trois Corps, 1772).
Of the five Lagrange points, three are unstable and two are stable. The unstable Lagrange points - labeled L1, L2 and L3 - lie along the line connecting the two large masses. The stable Lagrange points - labeled L4 and L5 - form the apex of two equilateral triangles that have the large masses at their vertices. L4 leads the orbit of earth and L5 follows.
The L1 point of the Earth-Sun system affords an uninterrupted view of the sun and is currently home to the Solar and Heliospheric Observatory Satellite SOHO. The L2 point of the Earth-Sun system was the home to the WMAP spacecraft, current home of Planck, and future home of the James Webb Space Telescope. L2 is ideal for astronomy because a spacecraft is close enough to readily communicate with Earth, can keep Sun, Earth and Moon behind the spacecraft for solar power and (with appropriate shielding) provides a clear view of deep space for our telescopes. The L1 and L2 points are unstable on a time scale of approximately 23 days, which requires satellites orbiting these positions to undergo regular course and attitude corrections.
NASA is unlikely to find any use for the L3 point since it remains hidden behind the Sun at all times. The idea of a hidden "Planet-X" at the L3 point has been a popular topic in science fiction writing. The instability of Planet X's orbit (on a time scale of 150 years) didn't stop Hollywood from turning out classics like The Man from Planet X.
The L4 and L5 points are home to stable orbits so long as the mass ratio between the two large masses exceeds 24.96. This condition is satisfied for both the Earth-Sun and Earth-Moon systems, and for many other pairs of bodies in the solar system. Objects found orbiting at the L4 and L5 points are often called Trojans after the three large asteroids Agamemnon, Achilles and Hector that orbit in the L4 and L5 points of the Jupiter-Sun system. (According to Homer, Hector was the Trojan champion slain by Achilles during King Agamemnon's siege of Troy). There are hundreds of Trojan Asteroids in the solar system. Most orbit with Jupiter, but others orbit with Mars. In addition, several of Saturn's moons have Trojan companions. In 1956 the Polish astronomer Kordylewski discovered large concentrations of dust at the Trojan points of the Earth-Moon system. The DIRBE instrument on the COBE satellite confirmed earlier IRAS observations of a dust ring following the Earth's orbit around the Sun. The existence of this ring is closely related to the Trojan points, but the story is complicated by the effects of radiation pressure on the dust grains. In 2010 NASA's WISE telescope finally confirmed the first Trojan asteroid (2010 TK7) around Earth's leading Lagrange point.
Finding the Lagrange Points
The easiest way to understand Lagrange points is to adopt a frame of reference that rotates with the system. The forces exerted on a body at rest in this frame can be derived from an effective potential in much the same way that wind speeds can be inferred from a weather map. The forces are strongest when the contours of the effective potential are closest together and weakest when the contours are far apart.
Orbits Evolving Under Gravity
The solar system extends well beyond Pluto, encompassing small objects on their own unusual orbits around the Sun. How did they get there? A new study attempts to answer this question with simulations.
Models and Moving Objects
The largest objects in the solar system wield the most influence. Models that account for the Sun and the outer planets — Jupiter, Saturn, Uranus, and Neptune — can produce realistic approximations of the solar system’s overall gravitational influence.
So if you have a model of the major gravitational forces at play, you can drop in orbiting objects and see what they do over time. This sounds simple, but it’s a powerful tool when it comes to understanding the current structure of our solar system.
The evolution of the surface density of the disk with time (starting from the upper-left) as seen face-on (top) and edge-on (bottom). Click to enlarge. Yellow regions have a higher density than blue regions. The timescale P represents 1,000 years. The authors note the “cone” of orbits present prior to t = 4,300 P, as well as the coherent ring of orbits most prominent at t = 9,900 P, which corresponds to an “m = 1 mode”. [Zderic et al. 2020]
A Disk on the Outskirts
The disk being examined by Zderic and collaborators consists of objects orbiting at roughly 100 to 1,000 astronomical units (au) from the Sun. For context, Pluto’s farthest distance from the Sun is just 50 au, so these distances definitely qualify as the outer solar system. The orbits of the disk objects all start off in the same plane (which is also the plane in which the solar system’s planets orbit), but they have a higher than average eccentricity (as conditions in the outer solar system require).
In previous studies with higher mass disks, the disk conditions have been shown to reach a consistent state within 660 million years of simulated time. Zderic and collaborators were interested in this consistent state, which reflects the long-term behavior of the disk. To reach this state more quickly, the authors used an equivalent setup: they started with a less massive disk, and they ran their simulation for just under 10 million years.
Evolution of two orbital parameters for a particular object, specifically eccentricity (y-axis) and the longitude of perihelion (x-axis the sum of two other orbital properties that sets the orientation of the orbit relative to a plane). Between 5,000 P and 9,000 P, the object under consideration is in the m = 1 mode. [Zderic et al. 2020]
Modes in Models
As the disk of objects evolves, the authors show that the collective gravity of the small bodies can induce an instability. As a result, the final state of the disk has a significant feature: orbits appear to cluster in a particular region. This is called being in a “mode”, which is shorthand for a group of orbital parameters having specific values. Zderic and collaborators note that later in the simulation, objects tend to settle into the m = 1 mode, though objects also fall in and out of the mode. Additionally, adding more particles to the simulation shows that objects stay in the mode longer. Extrapolating to the solar system, the mode may be stable for as long as the solar system is around.
Why is this interesting? These simulations show that the collective gravity of small bodies in a disk can naturally reproduce many of the observed behaviors of objects in our outer solar system — including extreme trans-Neptunian objects (TNOs), small bodies beyond the orbit of Neptune that are on very unusual orbits.
Schematic showing the observed alignment of the orbits of detached extreme TNOs and the proposed orbit of a hypothetical super-Earth-mass planet (in green). But is Planet Nine actually necessary to explain the extreme TNO orbits? [Sheppard et al. 2019]
Further work will require the simulation of high mass disks that are more similar to the early solar system. Keep an eye out for future studies exploring the cause of our solar system’s structure!
“Apsidal Clustering following the Inclination Instability,” Alexander Zderic et al 2020 ApJL 895 L27. doi:10.3847/2041-8213/ab91a0
Traditionally, devices like a blink comparator were used in astronomy to detect objects in the Solar System, because these objects would move between two exposures—this involved time-consuming steps like exposing and developing photographic plates or films, and people then using a blink comparator to manually detect prospective objects. During the 1980s, the use of CCD-based cameras in telescopes made it possible to directly produce electronic images that could then be readily digitized and transferred to digital images. Because the CCD captured more light than film (about 90% versus 10% of incoming light) and the blinking could now be done at an adjustable computer screen, the surveys allowed for higher throughput. A flood of new discoveries was the result: over a thousand trans-Neptunian objects were detected between 1992 and 2006. 
The first scattered-disc object (SDO) to be recognised as such was 1996 TL66 ,   originally identified in 1996 by astronomers based at Mauna Kea in Hawaii. Three more were identified by the same survey in 1999: 1999 CV118 , 1999 CY118 , and 1999 CF119 .  The first object presently classified as an SDO to be discovered was 1995 TL8 , found in 1995 by Spacewatch. 
As of 2011, over 200 SDOs have been identified,  including Gǃkúnǁʼhòmdímà (discovered by Schwamb, Brown, and Rabinowitz),  2002 TC302 (NEAT), Eris (Brown, Trujillo, and Rabinowitz),  Sedna (Brown, Trujillo, and Rabinowitz)  and 2004 VN 112 (Deep Ecliptic Survey).  Although the numbers of objects in the Kuiper belt and the scattered disc are hypothesized to be roughly equal, observational bias due to their greater distance means that far fewer SDOs have been observed to date. 
Known trans-Neptunian objects are often divided into two subpopulations: the Kuiper belt and the scattered disc.  A third reservoir of trans-Neptunian objects, the Oort cloud, has been hypothesized, although no confirmed direct observations of the Oort cloud have been made.  Some researchers further suggest a transitional space between the scattered disc and the inner Oort cloud, populated with "detached objects". 
Scattered disc versus Kuiper belt Edit
The Kuiper belt is a relatively thick torus (or "doughnut") of space, extending from about 30 to 50 AU  comprising two main populations of Kuiper belt objects (KBOs): the classical Kuiper-belt objects (or "cubewanos"), which lie in orbits untouched by Neptune, and the resonant Kuiper-belt objects those which Neptune has locked into a precise orbital ratio such as 2:3 (the object goes around twice for every three Neptune orbits) and 1:2 (the object goes around once for every two Neptune orbits). These ratios, called orbital resonances, allow KBOs to persist in regions which Neptune's gravitational influence would otherwise have cleared out over the age of the Solar System, since the objects are never close enough to Neptune to be scattered by its gravity. Those in 2:3 resonances are known as "plutinos", because Pluto is the largest member of their group, whereas those in 1:2 resonances are known as "twotinos".
In contrast to the Kuiper belt, the scattered-disc population can be disturbed by Neptune.  Scattered-disc objects come within gravitational range of Neptune at their closest approaches (
30 AU) but their farthest distances reach many times that.  Ongoing research  suggests that the centaurs, a class of icy planetoids that orbit between Jupiter and Neptune, may simply be SDOs thrown into the inner reaches of the Solar System by Neptune, making them "cis-Neptunian" rather than trans-Neptunian scattered objects.  Some objects, like (29981) 1999 TD10, blur the distinction  and the Minor Planet Center (MPC), which officially catalogues all trans-Neptunian objects, now lists centaurs and SDOs together. 
The MPC, however, makes a clear distinction between the Kuiper belt and the scattered disc, separating those objects in stable orbits (the Kuiper belt) from those in scattered orbits (the scattered disc and the centaurs).  However, the difference between the Kuiper belt and the scattered disc is not clear-cut, and many astronomers see the scattered disc not as a separate population but as an outward region of the Kuiper belt. Another term used is "scattered Kuiper-belt object" (or SKBO) for bodies of the scattered disc. 
Morbidelli and Brown propose that the difference between objects in the Kuiper belt and scattered-disc objects is that the latter bodies "are transported in semi-major axis by close and distant encounters with Neptune,"  but the former experienced no such close encounters. This delineation is inadequate (as they note) over the age of the Solar System, since bodies "trapped in resonances" could "pass from a scattering phase to a non-scattering phase (and vice versa) numerous times."  That is, trans-Neptunian objects could travel back and forth between the Kuiper belt and the scattered disc over time. Therefore, they chose instead to define the regions, rather than the objects, defining the scattered disc as "the region of orbital space that can be visited by bodies that have encountered Neptune" within the radius of a Hill sphere, and the Kuiper belt as its "complement . in the a > 30 AU region" the region of the Solar System populated by objects with semi-major axes greater than 30 AU. 
Detached objects Edit
The Minor Planet Center classifies the trans-Neptunian object 90377 Sedna as a scattered-disc object. Its discoverer Michael E. Brown has suggested instead that it should be considered an inner Oort-cloud object rather than a member of the scattered disc, because, with a perihelion distance of 76 AU, it is too remote to be affected by the gravitational attraction of the outer planets.  Under this definition, an object with a perihelion greater than 40 AU could be classified as outside the scattered disc. 
Sedna is not the only such object: (148209) 2000 CR 105 (discovered before Sedna) and 2004 VN 112 have a perihelion too far away from Neptune to be influenced by it. This led to a discussion among astronomers about a new minor planet set, called the extended scattered disc (E-SDO).  2000 CR 105 may also be an inner Oort-cloud object or (more likely) a transitional object between the scattered disc and the inner Oort cloud. More recently, these objects have been referred to as "detached",  or distant detached objects (DDO). 
There are no clear boundaries between the scattered and detached regions.  Gomes et al. define SDOs as having "highly eccentric orbits, perihelia beyond Neptune, and semi-major axes beyond the 1:2 resonance." By this definition, all distant detached objects are SDOs.  Since detached objects' orbits cannot be produced by Neptune scattering, alternative scattering mechanisms have been put forward, including a passing star   or a distant, planet-sized object.  Alternative, it has been suggested that these objects have been captured from a passing star. 
A scheme introduced by a 2005 report from the Deep Ecliptic Survey by J. L. Elliott et al. distinguishes between two categories: scattered-near (i.e. typical SDOs) and scattered-extended (i.e. detached objects).  Scattered-near objects are those whose orbits are non-resonant, non-planetary-orbit-crossing and have a Tisserand parameter (relative to Neptune) less than 3.  Scattered-extended objects have a Tisserand parameter (relative to Neptune) greater than 3 and have a time-averaged eccentricity greater than 0.2. 
An alternative classification, introduced by B. J. Gladman, B. G. Marsden and C. Van Laerhoven in 2007, uses 10-million-year orbit integration instead of the Tisserand parameter.  An object qualifies as an SDO if its orbit is not resonant, has a semi-major axis no greater than 2000 AU, and, during the integration, its semi-major axis shows an excursion of 1.5 AU or more.  Gladman et al. suggest the term scattering disk object to emphasize this present mobility.  If the object is not an SDO as per the above definition, but the eccentricity of its orbit is greater than 0.240, it is classified as a detached TNO.  (Objects with smaller eccentricity are considered classical.) In this scheme, the disc extends from the orbit of Neptune to 2000 AU, the region referred to as the inner Oort cloud.
The scattered disc is a very dynamic environment.  Because they are still capable of being perturbed by Neptune, SDOs' orbits are always in danger of disruption either of being sent outward to the Oort cloud or inward into the centaur population and ultimately the Jupiter family of comets.  For this reason Gladman et al. prefer to refer to the region as the scattering disc, rather than scattered.  Unlike Kuiper-belt objects (KBOs), the orbits of scattered-disc objects can be inclined as much as 40° from the ecliptic. 
SDOs are typically characterized by orbits with medium and high eccentricities with a semi-major axis greater than 50 AU, but their perihelia bring them within influence of Neptune.  Having a perihelion of roughly 30 AU is one of the defining characteristics of scattered objects, as it allows Neptune to exert its gravitational influence. 
The classical objects (cubewanos) are very different from the scattered objects: more than 30% of all cubewanos are on low-inclination, near-circular orbits whose eccentricities peak at 0.25.  Classical objects possess eccentricities ranging from 0.2 to 0.8. Though the inclinations of scattered objects are similar to the more extreme KBOs, very few scattered objects have orbits as close to the ecliptic as much of the KBO population. 
Although motions in the scattered disc are random, they do tend to follow similar directions, which means that SDOs can become trapped in temporary resonances with Neptune. Examples of possible resonant orbits within the scattered disc include 1:3, 2:7, 3:11, 5:22 and 4:79. 
The scattered disc is still poorly understood: no model of the formation of the Kuiper belt and the scattered disc has yet been proposed that explains all their observed properties. 
According to contemporary models, the scattered disc formed when Kuiper belt objects (KBOs) were "scattered" into eccentric and inclined orbits by gravitational interaction with Neptune and the other outer planets.  The amount of time for this process to occur remains uncertain. One hypothesis estimates a period equal to the entire age of the Solar System  a second posits that the scattering took place relatively quickly, during Neptune's early migration epoch. 
Models for a continuous formation throughout the age of the Solar System illustrate that at weak resonances within the Kuiper belt (such as 5:7 or 8:1), or at the boundaries of stronger resonances, objects can develop weak orbital instabilities over millions of years. The 4:7 resonance in particular has large instability. KBOs can also be shifted into unstable orbits by close passage of massive objects, or through collisions. Over time, the scattered disc would gradually form from these isolated events. 
Computer simulations have also suggested a more rapid and earlier formation for the scattered disc. Modern theories indicate that neither Uranus nor Neptune could have formed in situ beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets, and Saturn, may have formed closer to Jupiter, but were flung outwards during the early evolution of the Solar System, perhaps through exchanges of angular momentum with scattered objects.  Once the orbits of Jupiter and Saturn shifted to a 2:1 resonance (two Jupiter orbits for each orbit of Saturn), their combined gravitational pull disrupted the orbits of Uranus and Neptune, sending Neptune into the temporary "chaos" of the proto-Kuiper belt.  As Neptune traveled outward, it scattered many trans-Neptunian objects into higher and more eccentric orbits.   This model states that 90% or more of the objects in the scattered disc may have been "promoted into these eccentric orbits by Neptune's resonances during the migration epoch. [therefore] the scattered disc might not be so scattered." 
Scattered objects, like other trans-Neptunian objects, have low densities and are composed largely of frozen volatiles such as water and methane.  Spectral analysis of selected Kuiper belt and scattered objects has revealed signatures of similar compounds. Both Pluto and Eris, for instance, show signatures for methane. 
Astronomers originally supposed that the entire trans-Neptunian population would show a similar red surface colour, as they were thought to have originated in the same region and subjected to the same physical processes.  Specifically, SDOs were expected to have large amounts of surface methane, chemically altered into tholins by sunlight from the Sun. This would absorb blue light, creating a reddish hue.  Most classical objects display this colour, but scattered objects do not instead, they present a white or greyish appearance. 
One explanation is the exposure of whiter subsurface layers by impacts another is that the scattered objects' greater distance from the Sun creates a composition gradient, analogous to the composition gradient of the terrestrial and gas giant planets.  Michael E. Brown, discoverer of the scattered object Eris, suggests that its paler colour could be because, at its current distance from the Sun, its atmosphere of methane is frozen over its entire surface, creating an inches-thick layer of bright white ice. Pluto, conversely, being closer to the Sun, would be warm enough that methane would freeze only onto cooler, high-albedo regions, leaving low-albedo tholin-covered regions bare of ice. 
The Kuiper belt was initially thought to be the source of the Solar System's ecliptic comets. However, studies of the region since 1992 have shown that the orbits within the Kuiper belt are relatively stable, and that ecliptic comets originate from the scattered disc, where orbits are generally less stable. 
Comets can loosely be divided into two categories: short-period and long-period—the latter being thought to originate in the Oort cloud. The two major categories of short-period comets are Jupiter-family comets (JFCs) and Halley-type comets.  Halley-type comets, which are named after their prototype, Halley's Comet, are thought to have originated in the Oort cloud but to have been drawn into the inner Solar System by the gravity of the giant planets,  whereas the JFCs are thought to have originated in the scattered disc.  The centaurs are thought to be a dynamically intermediate stage between the scattered disc and the Jupiter family. 
There are many differences between SDOs and JFCs, even though many of the Jupiter-family comets may have originated in the scattered disc. Although the centaurs share a reddish or neutral coloration with many SDOs, their nuclei are bluer, indicating a fundamental chemical or physical difference.  One hypothesis is that comet nuclei are resurfaced as they approach the Sun by subsurface materials which subsequently bury the older material. 
Sedna (provisionally designated 2003 VB12) was discovered by Michael Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) on 14 November 2003. The discovery formed part of a survey begun in 2001 with the Samuel Oschin telescope at Palomar Observatory near San Diego, California, using Yale's 160-megapixel Palomar Quest camera. On that day, an object was observed to move by 4.6 arcseconds over 3.1 hours relative to stars, which indicated that its distance was about 100 AU. Follow-up observations were made in November–December 2003 with the SMARTS telescope at Cerro Tololo Inter-American Observatory in Chile, the Tenagra IV telescope in Nogales, Arizona, and the Keck Observatory on Mauna Kea in Hawaii. Combining those with precovery observations taken at the Samuel Oschin telescope in August 2003, and from the Near-Earth Asteroid Tracking consortium in 2001–2002, allowed accurate determination of its orbit. The calculations showed that the object was moving along a distant highly eccentric orbit, at a distance of 90.3 AU from the Sun.   Precovery images have later been found in images of the Palomar Digitized Sky Survey dating back to 25 September 1990. 
Brown initially nicknamed Sedna "The Flying Dutchman", or "Dutch", after a legendary ghost ship, because its slow movement had initially masked its presence from his team.  For an official name for the object, Brown settled on "Sedna", a name from Inuit mythology, which Brown chose partly because he mistakenly thought the Inuit were the closest polar culture to his home in Pasadena, and partly because the name, unlike Quaoar, would be easily pronounceable.  On his website, he wrote:
Our newly discovered object is the coldest, most distant place known in the Solar System, so we feel it is appropriate to name it in honor of Sedna, the Inuit goddess of the sea, who is thought to live at the bottom of the frigid Arctic Ocean. 
Brown also suggested to the International Astronomical Union's (IAU) Minor Planet Center that any future objects discovered in Sedna's orbital region should also be named after entities in arctic mythologies.  The team made the name "Sedna" public before the object had been officially numbered.  Brian Marsden, the head of the Minor Planet Center, said that such an action was a violation of protocol, and that some members of the IAU might vote against it.  No objection was raised to the name, and no competing names were suggested. The IAU's Committee on Small Body Nomenclature accepted the name in September 2004,  and also considered that, in similar cases of extraordinary interest, it might in the future allow names to be announced before they were officially numbered. 
The customary English spelling "Sedna" was popularized by Franz Boas.  The modern pronunciation in the region (southern Baffin Island) is 'Sanna', with dn perhaps becoming nn over the years. 
Sedna has the second longest orbital period of any known object in the Solar System of comparable size or larger, [c] calculated at around 11,400 years.  [a] Its orbit is extremely eccentric, with an aphelion estimated at 937 AU  and a perihelion at about 76 AU. This perihelion was the largest of that of any known Solar System object until the discovery of 2012 VP 113 .   At its aphelion, Sedna orbits the Sun at a mere 1.3% of Earth's orbital speed. When Sedna was discovered it was 89.6 AU  from the Sun approaching perihelion, and was the most distant object in the Solar System observed. Sedna was later surpassed by Eris, which was detected by the same survey near aphelion at 97 AU. The orbits of some long-period comets extend farther than that of Sedna they are too dim to be discovered except when approaching perihelion in the inner Solar System. Even as Sedna nears its perihelion in mid-2076,  [d] the Sun would appear merely as an extremely bright star-like pinpoint in its sky, 100 times brighter than a full moon on Earth (for comparison, the Sun appears from Earth to be roughly 400,000 times brighter than the full Moon), and too far away to be visible as a disc to the naked eye. 
When first discovered, Sedna was thought to have an unusually long rotational period (20 to 50 days).  It was initially speculated that Sedna's rotation was slowed by the gravitational pull of a large binary companion, similar to Pluto's moon Charon.  A search for such a satellite by the Hubble Space Telescope in March 2004 found nothing,  [e] and subsequent measurements from the MMT telescope suggest a much shorter rotation period of about 10 hours, more typical for a body of its size. 
Sedna has a V-band absolute magnitude (H) of about 1.8, and it is estimated to have an albedo of about 0.32, thus giving it a diameter of approximately 1,000 km.  At the time of its discovery it was the intrinsically brightest object found in the Solar System since Pluto in 1930. In 2004, the discoverers placed an upper limit of 1,800 km on its diameter,  but by 2007 this was revised downward to less than 1,600 km after observation by the Spitzer Space Telescope.  In 2012, measurements from the Herschel Space Observatory suggested that Sedna's diameter was 995 ± 80 km , which would make it smaller than Pluto's moon Charon.  Because Sedna has no known moons, determining its mass is currently impossible without sending a space probe. Sedna is currently the largest trans-Neptunian Sun-orbiting object not known to have a satellite.  Only a single attempt has been made to find a satellite,   and it has been suggested that there is a chance of up to 25% that a satellite could have been missed.  
Observations from the SMARTS telescope show that in visible light Sedna is one of the reddest objects in the Solar System, nearly as red as Mars.  Chad Trujillo and his colleagues suggest that Sedna's dark red colour is caused by a surface coating of hydrocarbon sludge, or tholin, formed from simpler organic compounds after long exposure to ultraviolet radiation.  Its surface is homogeneous in colour and spectrum this may be because Sedna, unlike objects nearer the Sun, is rarely impacted by other bodies, which would expose bright patches of fresh icy material like that on 8405 Asbolus.  Sedna and two other very distant objects – 2006 SQ 372 and (87269) 2000 OO 67 – share their color with outer classical Kuiper belt objects and the centaur 5145 Pholus, suggesting a similar region of origin. 
Trujillo and colleagues have placed upper limits in Sedna's surface composition of 60% for methane ice and 70% for water ice.  The presence of methane further supports the existence of tholins on Sedna's surface, because they are produced by irradiation of methane.  Barucci and colleagues compared Sedna's spectrum with that of Triton and detected weak absorption bands belonging to methane and nitrogen ices. From these observations, they suggested the following model of the surface: 24% Triton-type tholins, 7% amorphous carbon, 10% nitrogen ices, 26% methanol, and 33% methane.  The detection of methane and water ices was confirmed in 2006 by the Spitzer Space Telescope mid-infrared photometry.  The presence of nitrogen on the surface suggests the possibility that, at least for a short time, Sedna may have a tenuous atmosphere. During a 200-year period near perihelion, the maximum temperature on Sedna should exceed 35.6 K (−237.6 °C), the transition temperature between alpha-phase solid N2 and the beta-phase seen on Triton. At 38 K, the N2 vapor pressure would be 14 microbar (1.4 Pa or 0.000014 atm).  Its deep red spectral slope is indicative of high concentrations of organic material on its surface, and its weak methane absorption bands indicate that methane on Sedna's surface is ancient, rather than freshly deposited. This means that Sedna is too cold for methane to evaporate from its surface and then fall back as snow, which happens on Triton and probably on Pluto. 
Models of internal heating via radioactive decay suggest that Sedna might be capable of supporting a subsurface ocean of liquid water. 
In their paper announcing the discovery of Sedna, Mike Brown and his colleagues described it as the first observed body belonging to the Oort cloud, the hypothetical cloud of comets thought to exist nearly a light-year from the Sun. They observed that, unlike scattered disc objects such as Eris, Sedna's perihelion (76 AU) is too distant for it to have been scattered by the gravitational influence of Neptune.  Because it is a great deal closer to the Sun than was expected for an Oort cloud object, and has an inclination roughly in line with the planets and the Kuiper belt, they described the planetoid as being an "inner Oort cloud object", situated in the disc reaching from the Kuiper belt to the spherical part of the cloud.  
If Sedna formed in its current location, the Sun's original protoplanetary disc must have extended as far as 75 AU into space.  Also, Sedna's initial orbit must have been approximately circular, otherwise its formation by the accretion of smaller bodies into a whole would not have been possible, because the large relative velocities between planetesimals would have been too disruptive. Therefore, it must have been tugged into its current eccentric orbit by a gravitational interaction with another body.  In their initial paper, Brown, Rabinowitz and colleagues suggested three possible candidates for the perturbing body: an unseen planet beyond the Kuiper belt, a single passing star, or one of the young stars embedded with the Sun in the stellar cluster in which it formed. 
Mike Brown and his team favored the hypothesis that Sedna was lifted into its current orbit by a star from the Sun's birth cluster, arguing that Sedna's aphelion of about 1,000 AU, which is relatively close compared to those of long-period comets, is not distant enough to be affected by passing stars at their current distances from the Sun. They propose that Sedna's orbit is best explained by the Sun having formed in an open cluster of several stars that gradually disassociated over time.    That hypothesis has also been advanced by both Alessandro Morbidelli and Scott Jay Kenyon.   Computer simulations by Julio A. Fernandez and Adrian Brunini suggest that multiple close passes by young stars in such a cluster would pull many objects into Sedna-like orbits.  A study by Morbidelli and Levison suggested that the most likely explanation for Sedna's orbit was that it had been perturbed by a close (approximately 800 AU) pass by another star in the first 100 million years or so of the Solar System's existence.  
The trans-Neptunian planet hypothesis has been advanced in several forms by a number of astronomers, including Rodney Gomes and Patryk Lykawka. One scenario involves perturbations of Sedna's orbit by a hypothetical planetary-sized body in the Hills cloud. Recent simulations show that Sedna's orbital traits could be explained by perturbations by a Neptune-mass object at 2,000 AU (or less), a Jupiter-mass ( M J) object at 5,000 AU, or even an Earth-mass object at 1,000 AU.   Computer simulations by Patryk Lykawka have suggested that Sedna's orbit may have been caused by a body roughly the size of Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 80 and 170 AU from the Sun.  Mike Brown's various sky surveys have not detected any Earth-sized objects out to a distance of about 100 AU. It is possible that such an object may have been scattered out of the Solar System after the formation of the inner Oort cloud. 
Caltech researchers Konstantin Batygin and Mike Brown have hypothesised the existence of a giant planet in the outer Solar System, nicknamed Planet Nine. The planet would be about 10 times as massive as Earth. It would have a highly eccentric orbit, and its average distance from the Sun would be about 20 times that of Neptune (which orbits at an average distance of 30.1 astronomical units (4.50 × 10 9 km)). Its orbital period would be 10,000 to 20,000 years. The planet's existence was hypothesised using mathematical modeling and computer simulations, but it has not been observed directly. It may explain the orbits of a group of objects that includes Sedna.  
It has been suggested that Sedna's orbit is the result of influence by a large binary companion to the Sun, thousands of AU distant. One such hypothetical companion is Nemesis, a dim companion to the Sun that has been proposed to be responsible for the supposed periodicity of mass extinctions on Earth from cometary impacts, the lunar impact record, and the common orbital elements of a number of long-period comets.   No direct evidence of Nemesis has been found, and many lines of evidence (such as crater counts) have thrown its existence into doubt.   John J. Matese and Daniel P. Whitmire, longtime proponents of the possibility of a wide binary companion to the Sun, have suggested that an object of 5 M J lying at roughly 7,850 AU from the Sun could produce a body in Sedna's orbit. 
Morbidelli and Kenyon have also suggested that Sedna did not originate in the Solar System, but was captured by the Sun from a passing extrasolar planetary system, specifically that of a brown dwarf about 1/20th the mass of the Sun ( M ☉)    or a main-sequence star 80 percent more massive than our Sun, which, owing to its larger mass, may now be a white dwarf. In either case, the stellar encounter had likely occurred early after the Sun's formation, about less than 100 million years after the Sun had formed.    Stellar encounters during this time would have minimal effect on the Oort cloud's final mass and population since the Sun had excess material for replenishing the Oort cloud population. 
Sedna's highly elliptical orbit means that the probability of its detection was roughly 1 in 80, which suggests that, unless its discovery was a fluke, another 40–120 Sedna-sized objects would exist within the same region.   Another object, 2000 CR 105 , has a similar but less extreme orbit: it has a perihelion of 44.3 AU, an aphelion of 394 AU, and an orbital period of 3,240 years. It may have been affected by the same processes as Sedna. 
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 was responsible, all such objects would share roughly the same perihelion (about 80 AU). If Sedna were captured from another planetary system that rotated in the same direction as the Solar System, then all of its population would have orbits on relatively low inclinations and have semi-major axes ranging from 100 to 500 AU. If it rotated in the opposite direction, then 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 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, Rabinowitz and Megan 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 likely dwarf planet Gonggong, it detected no new sednoid.  Subsequent simulations incorporating the new data suggested about 40 Sedna-sized objects probably exist in this region, with the brightest being about Eris's magnitude (−1.0). 
In 2014, Chad Trujillo and Scott Sheppard announced the discovery of 2012 VP 113 ,  an object half the size of Sedna in a 4,200-year orbit similar to Sedna's and a perihelion within Sedna's range of roughly 80 AU,  which led some to speculate that it offered evidence of a trans-Neptunian planet.  Another high-perihelion trans-Neptunian object was announced by Sheppard and colleagues in 2018, provisionally designated 2015 TG387 and now named Leleākūhonua.  With a perihelion of 65 AU and an even more distant orbit of 40,000 years, its longitude of perihelion (the location where it makes its closest approach to the Sun) appears to be aligned in the directions of both Sedna and 2012 VP113 , strengthening the case for an apparent orbital clustering of trans-Neptunian objects suspected to be influenced by a hypothetical distant planet, dubbed Planet Nine. In a study detailing Sedna's population and Leleākūhonua's orbital dynamics, Sheppard concluded that the discovery implies a population of about 2 million inner Oort Cloud objects larger than 40 km, with a total mass in the range of 1 × 10 22 kg (several times the mass of the asteroid belt and 80% the mass of Pluto). 
The Minor Planet Center, which officially catalogs the objects in the Solar System, classifies Sedna as a scattered object.  This grouping is heavily questioned, and many astronomers have suggested that it, together with a few other objects (e.g. 2000 CR 105 ), be placed in a new category of distant objects named extended scattered disc objects (E-SDO),  detached objects,  distant detached objects (DDO),  or scattered-extended in the formal classification by the Deep Ecliptic Survey. 
The discovery of Sedna resurrected the question of which astronomical objects should be considered planets and which should not. On 15 March 2004, articles on Sedna in the popular press reported that a tenth planet had been discovered. This question was answered under the International Astronomical Union definition of a planet, adopted on 24 August 2006, which mandated that a planet must have cleared the neighborhood around its orbit. Sedna has a Stern–Levison parameter estimated to be much less than 1, [f] and therefore cannot be considered to have cleared the neighborhood, even though no other objects have yet been discovered in its vicinity. To be a dwarf planet, Sedna must be in hydrostatic equilibrium. It is bright enough, and therefore large enough, that this is expected to be the case,  and several astronomers have called it one.     
Sedna will come to perihelion around July 2076.  [d] This close approach to the Sun provides an opportunity for study that will not occur again for 12,000 years. Although Sedna is listed on NASA's Solar System exploration website,  NASA is not known to be considering any type of mission at this time.  It was calculated that a flyby mission to Sedna could take 24.48 years using a Jupiter gravity assist, based on launch dates of 6 May 2033 or 23 June 2046. Sedna would be 77.27 or 76.43 AU from the Sun when the spacecraft arrived near the end of 2057 or 2073, respectively. 
In May 2018, astrophysicist Ethan Siegel publicly advocated for a space probe mission to study Sedna as it approaches perihelion. Siegel characterized Sedna as an attractive target due to its status as a possible inner Oort cloud object. Because of Sedna's long orbital period, "we will not get the opportunity to study it this close to the Sun for many millennia again."  Such a mission could be facilitated by Dual-Stage 4-Grid ion thrusters that might cut cruise times considerably if powered, for example, by a fusion reactor. 
Orbits of the Planets
Today, Newton&rsquos work enables us to calculate and predict the orbits of the planets with marvelous precision. We know eight planets, beginning with Mercury closest to the Sun and extending outward to Neptune. The average orbital data for the planets are summarized in Table (PageIndex<1>). (Ceres is the largest of the asteroids, now considered a dwarf planet.)
According to Kepler&rsquos laws, Mercury must have the shortest orbital period (88 Earth-days) thus, it has the highest orbital speed, averaging 48 kilometers per second. At the opposite extreme, Neptune has a period of 165 years and an average orbital speed of just 5 kilometers per second.
|Planet||Semimajor Axis (AU)||Period (y)||Eccentricity|
All the planets have orbits of rather low eccentricity. The most eccentric orbit is that of Mercury (0.21) the rest have eccentricities smaller than 0.1. It is fortunate that among the rest, Mars has an eccentricity greater than that of many of the other planets. Otherwise the pre-telescopic observations of Brahe would not have been sufficient for Kepler to deduce that its orbit had the shape of an ellipse rather than a circle.
The planetary orbits are also confined close to a common plane, which is near the plane of Earth&rsquos orbit (called the ecliptic). The strange orbit of the dwarf planet Pluto is inclined about 17° to the ecliptic, and that of the dwarf planet Eris (orbiting even farther away from the Sun than Pluto) by 44°, but all the major planets lie within 10° of the common plane of the solar system.
You can use an orbital simulator to design your own mini solar system with up to four bodies. Adjust masses, velocities, and positions of the planets, and see what happens to their orbits as a result.
The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 100 billion stars.  The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion&ndashCygnus Arm or Local Spur.  The Sun lies between 25,000 and 28,000 light-years from the Galactic Centre,  and its speed within the Milky Way is about 220 km/s, so that it completes one revolution every 225&ndash250 million years. This revolution is known as the Solar System's galactic year.  The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.  The plane of the ecliptic lies at an angle of about 60° to the galactic plane. [i]
The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms.   Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve.  The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.  Even at the Solar System's current location, some scientists have speculated that recent supernovae may have adversely affected life in the last 35,000 years, by flinging pieces of expelled stellar core towards the Sun, as radioactive dust grains and larger, comet-like bodies. 
The Solar System is in the Local Interstellar Cloud or Local Fluff. It is thought to be near the neighbouring G-Cloud but it is not known if the Solar System is embedded in the Local Interstellar Cloud, or if it is in the region where the Local Interstellar Cloud and G-Cloud are interacting.   The Local Interstellar Cloud is an area of denser cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years (ly) across. The bubble is suffused with high-temperature plasma, that suggests it is the product of several recent supernovae. 
There are relatively few stars within ten light-years of the Sun. The closest is the triple star system Alpha Centauri, which is about 4.4 light-years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the small red dwarf, Proxima Centauri, orbits the pair at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was confirmed to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.  The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5.9 ly), Wolf 359 (7.8 ly), and Lalande 21185 (8.3 ly).
The largest nearby star is Sirius, a bright main-sequence star roughly 8.6 light-years away and roughly twice the Sun's mass and that is orbited by a white dwarf, Sirius B. The nearest brown dwarfs are the binary Luhman 16 system at 6.6 light-years. Other systems within ten light-years are the binary red-dwarf system Luyten 726-8 (8.7 ly) and the solitary red dwarf Ross 154 (9.7 ly).  The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. It has roughly 80% of the Sun's mass but only 60% of its luminosity.  The closest known free-floating planetary-mass object to the Sun is WISE 0855&minus0714,  an object with a mass less than 10 Jupiter masses roughly 7 light-years away.
Comparison with extrasolar systems
Compared to other planetary systems the Solar System stands out in lacking planets interior to the orbit of Mercury.   The known Solar System also lacks super-Earths (Planet Nine could be a super-Earth beyond the known Solar System).  Uncommonly, it has only small rocky planets and large gas giants elsewhere planets of intermediate size are typical&mdashboth rocky and gas&mdashso there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). Also, these super-Earths have closer orbits than Mercury.  This led to hypothesis that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.  
The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller orbital eccentricity.  Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.