Steem vs Tron: The rebellion against a cryptocurrency empire

TheSolar System[d]is thegravitationally boundsystemof theSunand the objects thatorbitit.[11]Itformed about 4.6 billion years agowhen a dense region of amolecular cloudcollapsed, forming the Sun and aprotoplanetary disc. The Sun is a typical star that maintains abalanced equilibriumby thefusionof hydrogen into helium at itscore, releasing this energy from its outerphotosphere. Astronomersclassifyit as aG-type main-sequence star.

There is a strong consensus among astronomers[e]that the Solar System has at least ninedwarf planets:Ceres,Orcus,Pluto,Haumea,Quaoar,Makemake,Gonggong,Eris, andSedna. There are a vast number ofsmall Solar System bodies, such asasteroids,comets,centaurs,meteoroids, andinterplanetary dust clouds. Some of these bodies are in theasteroid belt(between Mars's and Jupiter's orbit) and theKuiper belt(just outside Neptune's orbit).[f]Six planets, seven dwarf planets, and other bodies have orbitingnatural satellites, which are commonly called 'moons'.

The Solar System is constantly flooded by the Sun'scharged particles, thesolar wind, forming theheliosphere. Around 75–90astronomical unitsfrom the Sun,[g]the solar wind is halted, resulting in theheliopause. This is the boundary of the Solar System tointerstellar space. The outermost region of the Solar System is the theorizedOort cloud, the source forlong-period comets, extending to a radius of2,000–200,000 AU. The closest star to the Solar System,Proxima Centauri, is 4.25 light-years (269,000 AU) away. Both stars belong to theMilky Waygalaxy.

The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a largemolecular cloud.[b]This initial cloud was likely several light-years across and probably birthed several stars.[14]As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elementsfusedby previous generations of stars.[15]

As thepre-solar nebula[15]collapsed,conservation of angular momentumcaused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings.[14]As the contracting nebula spun faster, it began to flatten into aprotoplanetary discwith a diameter of roughly200 AU[14][16]and a hot, denseprotostarat the center.[17][18]The planets formed byaccretionfrom this disc,[19]in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftoverminor bodies.[20][21]

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun (within thefrost line). They eventually formed the rocky planets of Mercury, Venus, Earth, and Mars. Because theserefractorymaterials only comprised a small fraction of the solar nebula, the terrestrial planets could not grow very large.[20]

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough forvolatileicy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements.[20]Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.[20]

Following the dissipation of theprotoplanetary disk, theNice modelproposes thatgravitational encountersbetween planetisimals and the gas giants caused each tomigrateinto different orbits. This led to dynamical instability of the entire system, which scattered the planetisimals and ultimately placed the gas giants in their current positions. During this period, thegrand tack hypothesissuggests that a final inward migration of Jupiter dispersed much of the asteroid belt, leading to theLate Heavy Bombardmentof the inner planets.[26][27]

The Solar System remains in a relatively stable, slowly evolving state by following isolated,gravitationally boundorbits around the Sun.[28]Although the Solar System has been fairly stable for billions of years, it is technicallychaotic, and mayeventually be disrupted. There is a small chance that another star will pass through the Solar System in the next few billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today.[29]

The Sun's main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other subsequent phases of the Sun's pre-remnantlife combined.[30]The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become ared giant. Because of its increased surface area, the surface of the Sun will be cooler (2,600 K (4,220 °F) at its coolest) than it is on the main sequence.[30]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth and Mars uninhabitable (possibly destroying Earth as well).[31][32]Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a densewhite dwarf, half the original mass of the Sun but only the size of Earth.[30]The ejected outer layers may form aplanetary nebula, returning some of the material that formed the Sun—but now enriched withheavier elementslike carbon—to theinterstellar medium.[33][34]

Astronomers sometimes divide the Solar System structure into separate regions. Theinner Solar Systemincludes Mercury, Venus, Earth, Mars, and the bodies in theasteroid belt. Theouter Solar Systemincludes Jupiter, Saturn, Uranus, Neptune, and the bodies in theKuiper belt.[35]Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting ofthe objects beyond Neptune.[36]

The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as theecliptic. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.[45][46]Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All of the largest natural satellites are insynchronous rotation, with one face permanently turned toward their parent. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison.[47]

As a result of theformation of the Solar System, planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole.[48]There are exceptions, such asHalley's Comet.[49]Most of the larger moons orbit their planets inprogradedirection, matching the direction of planetary rotation; Neptune's moonTritonis the largest to orbit in the opposite, retrograde manner.[50]Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.[51]

To a good first approximation,Kepler's laws of planetary motiondescribe the orbits of objects around the Sun.[52]: 433–437These laws stipulate that each object travels along anellipsewith the Sun at onefocus, which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called itsperihelion, whereas its most distant point from the Sun is called itsaphelion.[53]: 9-6With the exception of Mercury, the orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these perturbations can be accounted for usingnumerical models,[53]: 9-6but the planetary system can change chaotically over billions of years.[54]

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances, like theTitius–Bode law[60]andJohannes Kepler's modelbased on thePlatonic solids,[61]but ongoing discoveries have invalidated these hypotheses.[62]

SomeSolar System modelsattempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—calledorreries)—whereas others extend across cities or regional areas.[63]The largest such scale model, theSweden Solar System, uses the 110-meter (361-foot)Avicii ArenainStockholmas its substitute Sun, and, following the scale, Jupiter is a 7.5-meter (25-foot) sphere atStockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object,Sedna, is a 10 cm (4 in) sphere inLuleå, 912 km (567 mi) away.[64][65]At that scale, the distance to Proxima Centauri would be roughly 8 times further than the Moon is from Earth.

If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), andEarth's diameteralong with that of the other terrestrial planets would be smaller than aflea(0.3 mm or 0.012 in) at this scale.[66]

Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles calledcosmic rays. The density of cosmic rays in theinterstellar mediumand the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.[67]

Thezone of habitabilityof the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility ofliquid water.[68]Habitability might be possible insubsurface oceansof various outer Solar System moons.[69]

Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.[70][71]The known Solar System lackssuper-Earths, planets between one and ten times as massive as the Earth,[70]although the hypotheticalPlanet Nine, if it does exist, could be a super-Earth orbiting in the edge of the Solar System.[72]

Uncommonly, it has only small terrestrial and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen 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.[70][73]

The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smallerorbital eccentricity.[70]Although there are attempts to explain it partly with a bias in theradial-velocity detection methodand partly with long interactions of a quite high number of planets, the exact causes remain undetermined.[70][74]

The Sun is apopulation I star, having formed in thespiral armsof theMilky Waygalaxy. It has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars in thegalactic bulgeandhalo.[81]Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before theuniversecould be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher metallicity is thought to have been crucial to the Sun's development of aplanetary systembecause the planets formed from the accretion of "metals".[82]

The region of space dominated by the Solarmagnetosphereis theheliosphere, which spans much of the Solar System. Along withlight, the Sun radiates a continuous stream of charged particles (aplasma) called thesolar wind. This stream spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph),[83]filling the vacuum between the bodies of the Solar System. The result is athin, dusty atmosphere, called theinterplanetary medium, which extends to at least100 AU.[84]

Activity on the Sun's surface, such assolar flaresandcoronal mass ejections, disturbs the heliosphere, creatingspace weatherand causinggeomagnetic storms.[85]Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions createauroraeseen near themagnetic poles.[86]The largest stable structure within the heliosphere is theheliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.[87][88]

The inner Solar System is the region comprising theterrestrial planetsand theasteroids.[89]Composed mainly ofsilicatesand metals,[90]the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is within thefrost line, which is a little less than5 AUfrom the Sun.[45]

The four terrestrial or inner planets have dense, rocky compositions, few or nomoons, and noring systems. They are composed largely ofrefractoryminerals such assilicates—which form theircrustsandmantles—and metals such as iron and nickel which form theircores. Three of the four inner planets (Venus, Earth, and Mars) haveatmospheressubstantial enough to generate weather; all have impact craters andtectonicsurface features, such asrift valleysand volcanoes.[91]

Asteroids, except for the largest, Ceres, are classified assmall Solar System bodiesand are composed mainly ofcarbonaceous, refractory rocky and metallic minerals, with some ice.[128][129]They range from a few meters to hundreds of kilometers in size.Many asteroids are divided intoasteroid groupsandfamiliesbased on their orbital characteristics. Someasteroids have natural satellites that orbit them, that is, asteroids that orbit larger asteroids.[130]

Theasteroid beltoccupies a torus-shaped region between 2.3 and3.3 AUfrom the Sun, which lies between the orbits of Mars and Jupiter. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.[142]The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometer in diameter.[143]Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.[40]The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[144]

Below are the descriptions of the three largest bodies in the asteroid belt. They are all considered to be relatively intactprotoplanets, a precursor stage before becoming a fully-formed planet (seeList of exceptional asteroids):[145][146][147]

Hilda asteroidsare in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jovian orbits.[161]They lie in three linked clusters between Jupiter and the main asteroid belt.

Trojansare bodies located within another body's gravitationally stableLagrange points:L4, 60° ahead in its orbit, orL5, 60° behind in its orbit.[162]Every planet except Mercury and Saturn is known to possess at least 1 trojan.[163][164][165]TheJupiter trojanpopulation is roughly equal to that of the asteroid belt.[166]After Jupiter, Neptune possesses the most confirmed trojans, at 28.[167]

The outer region of the Solar System is home to thegiant planetsand their large moons. Thecentaursand manyshort-period cometsorbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia, and methane, than planets of the inner Solar System because their lower temperatures allow these compounds to remain solid, without significantsublimation.[20]

The centaurs are icy, comet-like bodies whosesemi-major axesare longer than Jupiter's and shorter than Neptune's (between 5.5 and 30 AU). These are former Kuiper belt andscattered disc objects(SDOs) that were gravitationallyperturbedcloser to the Sun by the outer planets, and are expected to become comets or be ejected out of the Solar System.[39]While most centaurs are inactive and asteroid-like, some exhibit cometary activity, such as the first centaur discovered,2060 Chiron, which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.[193]The largest known centaur,10199 Chariklo, has a diameter of about 250 km (160 mi) and is one of the few minor planets possessing a ring system.[194][195]

Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which istilted toward the planeof the Solar System and reaches much further out than the Kuiper belt. The entire region is stilllargely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System.[196]

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.[197]It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets.[198]There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.[39]Many Kuiper belt objects have satellites,[199]and most have orbits that are substantially inclined (~10°) to the plane of the ecliptic.[200]

The Kuiper belt can be roughly divided into the "classical" belt and theresonant trans-Neptunian objects.[197]The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU.[201]Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated1992QB1, (and has since been named Albion); they are still in near primordial, low-eccentricity orbits.[202]

There is strong consensus among astronomers that five members of the Kuiper belt aredwarf planets.[198][203]Many dwarf planet candidates are being considered, pending further data for verification.[204]

The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence ofNeptune's early outward migration. Most scattered disc objects have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits can be inclined up to 46.8° from the ecliptic plane.[218]Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as "scattered Kuiper belt objects".[219]Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.[220]

Currently, there is strong consensus among astronomers that two of the bodies in the scattered disc aredwarf planets:

Some objects in the Solar System have a very large orbit, and therefore are much less affected by the known giant planets than other minor planet populations. These bodies are called extreme trans-Neptunian objects, or ETNOs for short.[224]Generally, ETNOs'semi-major axesare at least 150–250 AU wide.[224][225]For example,541132 Leleākūhonuaorbits the Sun once every ~32,000 years, with a distance of 65–2000 AU from the Sun.[D 11]

This population is divided into three subgroups by astronomers. ThescatteredETNOs haveperiheliaaround 38–45 AU and an exceptionally higheccentricityof more than 0.85. As with the regular scattered disc objects, they were likely formed as result ofgravitational scatteringby Neptune and still interact with the giant planets. ThedetachedETNOs, with perihelia approximately between 40–45 and 50–60 AU, are less affected by Neptune than the scattered ETNOs, but are still relatively close to Neptune. Thesednoidsorinner Oort cloudobjects, with perihelia beyond 50–60 AU, are too far from Neptune to be strongly influenced by it.[224]

Currently, there is one ETNO that is classified as a dwarf planet:

The Sun'sstellar-wind bubble, theheliosphere, a region of space dominated by the Sun, has its boundary at thetermination shock. Based on the Sun'speculiar motionrelative to thelocal standard of rest, this boundary is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.[227]Here the solar wind collides with the interstellar medium[228]and dramatically slows, condenses and becomes more turbulent, forming a great oval structure known as theheliosheath.[227]

The heliosheath has been theorized to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind to possibly several thousands of AU.[229][230]Evidence from theCassiniandInterstellar Boundary Explorerspacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field,[231][232]but the actual shape remains unknown.[233]

The shape and form of the outer edge of the heliosphere is likely affected by thefluid dynamicsof interactions with the interstellar medium as well assolar magnetic fieldsprevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere.[227]The heliopause is considered the beginning of the interstellar medium.[84]Beyond the heliopause, at around 230 AU, lies thebow shock: a plasma "wake" left by the Sun as it travels through the Milky Way.[234]Large objects outside the heliopause remain gravitationally bound to the Sun, but the flow of matter in the interstellar medium homogenizes the distribution of micro-scale objects.[84]

Comets aresmall Solar System bodies, typically only a few kilometers across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface tosublimateandionise, creating acoma: a long tail of gas and dust often visible to the naked eye.[235]

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such asHale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as theKreutz sungrazers, formed from the breakup of a single parent.[236]Some comets withhyperbolicorbits may originate outside the Solar System, but determining their precise orbits is difficult.[237]Old comets whose volatiles have mostly been driven out by solar warming are often categorized as asteroids.[238]

Solid objects smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), with the exact division between the two categories being debated over the years.[239]By 2017, the IAU designated any solid object having a diameter between ~30micrometersand 1 meter as meteoroids, and depreciated the micrometeoroid categorization, instead terms smaller particles simply as 'dust particles'.[240]

Some meteoroids formed via disintegration of comets and asteroids, while a few formed via impact debris ejected from planetary bodies. Most meteoroids are made of silicates and heavier metals likenickelandiron.[241]When passing through the Solar System, comets produce a trail of meteoroids; it is hypothesized that this is caused either by vaporization of the comet's material or by simple breakup of dormant comets. When crossing an atmosphere, these meteoroids will produce bright streaks in the sky due toatmospheric entry, calledmeteors. If a stream of meteoroids enter the atmosphere on parallel trajectories, the meteors will seemingly 'radiate' from a point in the sky, hence the phenomenon's name:meteor shower.[242]

The inner Solar System is home to thezodiacal dust cloud, which is visible as the hazyzodiacal lightin dark, unpolluted skies. It may be generated by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is materials from planet Mars.[243]The outer Solar System hosts a cosmic dust cloud. It extends from about10 AUto about40 AU, and was probably created by collisions within the Kuiper belt.[244][245]

Much of the outer Solar System is still unknown. The region beyond 100 AU away is virtually unexplored and learning about this region of space is difficult. Study of this region depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets.[246]Many objects are yet to be discovered in the Solar System's outer region.[247]

TheOort cloudis a theorized spherical shell of up to a trillion icy objects that is thought to be the source for all long-period comets.[248][249]No direct observation of the Oort cloud is possible with present imaging technology.[250]It is theorized to surround the Solar System at roughly 50,000 AU (~0.9ly) from the Sun and possibly to as far as 100,000 AU (~1.8 ly). The Oort cloud is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or thegalactic tide, thetidal forceexerted by the Milky Way.[248][249]

As of the 2020s, a few astronomers have hypothesized thatPlanet Nine(a planet beyond Neptune) might exist, based on statistical variance in the orbit ofextreme trans-Neptunian objects.[251]Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years.[252][253][254]However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence.[255]An alternative hypothesis has a close flyby of another star disrupting the outer Solar System.[256]

The Sun's gravitational field is estimated todominate the gravitational forces of surrounding starsout to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than50,000 AU.[257]Most of the mass is orbiting in the region between 3,000 and100,000 AU.[258]The furthest known objects, such asComet West, have aphelia around70,000 AUfrom the Sun.[259]The Sun'sHill spherewith respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.[260]It was calculated byG. A. Chebotarevto be 230,000 AU.[7]

Within 10 light-years of the Sun there are relatively few stars, the closest being the triple star systemAlpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble'sG-Cloud.[262]Alpha Centauri A and B are a closely tied pair ofSun-like stars, whereas the closest star to the Sun, the smallred dwarfProxima Centauri, orbits the pair at a distance of 0.2 light-years. In 2016, a potentially habitableexoplanetwas found to be orbiting Proxima Centauri, calledProxima Centauri b, the closest confirmed exoplanet to the Sun.[263]

The Solar System is surrounded by theLocal Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge.[264]Multiple otherinterstellar cloudsexist in the region within 300 light-years of the Sun, known as theLocal Bubble.[264]The latter feature is an hourglass-shaped cavity orsuperbubblein the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.[265]

The Local Bubble is a small superbubble compared to the neighboring widerRadcliffe WaveandSplitlinear structures (formerlyGould Belt), each of which are some thousands of light-years in length.[266]All these structures are part of theOrion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye.[267]

Groups of stars form together instar clusters, before dissolving into co-moving associations. A prominent grouping that is visible to the naked eye is theUrsa Major moving group, which is around 80 light-years away within the Local Bubble. The nearest star cluster isHyades, which lies at the edge of the Local Bubble. The closest star-forming regions are theCorona Australis Molecular Cloud, theRho Ophiuchi cloud complexand theTaurus molecular cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.[268]

The Solar System is located in theMilky Way, abarred spiral galaxywith a diameter of about 100,000light-yearscontaining more than 100 billion stars.[271]The Sun is part of one of the Milky Way's outer spiral arms, known as theOrion–Cygnus Armor Local Spur.[272][273]It is a member of thethin diskpopulation of stars orbiting close to the galactic plane.[274]

Its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years.[271]This revolution is known as the Solar System'sgalactic year.[275]Thesolar apex, the direction of the Sun's path through interstellar space, is near the constellationHerculesin the direction of the current location of the bright starVega.[276]The plane of the ecliptic lies at an angle of about 60° to thegalactic plane.[c]

The Sun follows a nearly circular orbit around theGalactic Center(where thesupermassive black holeSagittarius A*resides) at a distance of 26,660 light-years,[278]orbiting at roughly the same speed as that of the spiral arms.[279]If it orbited close to the center, gravitational tugs from nearby stars could perturb bodies in theOort cloudand send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. In this scenario, the intense radiation of the Galactic Center could interfere with the development of complex life.[279]

The Solar System's location in the Milky Way is a factor in theevolutionary history of lifeon Earth. Spiral arms are home to a far larger concentration ofsupernovae, gravitational instabilities, and radiation that could disrupt the Solar System, but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms, this has given Earth long periods of stability for life to evolve.[279]However, according to the controversialShiva hypothesis, the changing position of the Solar System relative to other parts of the Milky Way could explain periodicextinction eventson Earth.[280][281]

Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to theLate Middle Ages–Renaissance, astronomers from Europe to India believed Earth tobe stationary at the centerof the universe[282]and categorically different from the divine or ethereal objects that moved through the sky. Although theGreekphilosopherAristarchus of Samoshad speculated on aheliocentricreordering of the cosmos,Nicolaus Copernicuswas the first person known to have developeda mathematically predictive heliocentric system.[283][284]

Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notablyJohannes Kepler. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical, and the precise observational data ofTycho Brahe, Kepler produced theRudolphine Tables, which enabled accurate computations of the positions of the then-known planets.Pierre Gassendiused them to predict atransit of Mercuryin 1631, andJeremiah Horrocksdid the same for atransit of Venusin 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.[285][286]

In the 17th century,Galileopublicized the use of the telescope in astronomy; he andSimon Mariusindependently discovered that Jupiter had four satellites in orbit around it.[287]Christiaan Huygensfollowed on from these observations by discovering Saturn's moonTitanand the shape of therings of Saturn.[288]In 1677,Edmond Halleyobserved a transit of Mercury across the Sun, leading him to realize that observations of thesolar parallaxof a planet (more ideally using the transit of Venus) could be used totrigonometricallydetermine the distances between Earth,Venus, and the Sun.[289]Halley's friendIsaac Newton, in his magisterialPrincipia Mathematicaof 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the samelaws of motionand ofgravityapply on Earth and in the skies.[52]: 142

Uranus, having occasionally been observed since 1690 and possibly from antiquity, was recognized to be a planet orbiting beyond Saturn by 1783.[294]In 1838,Friedrich Besselsuccessfully measured astellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism.[295]Neptunewas identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus.[296]Mercury's orbital anomalyobservations led to searches forVulcan, a planet interior of Mercury, but these attempts were quashed withAlbert Einstein's theory ofgeneral relativityin 1915.[297]

In the 20th century, humans began their space exploration around the Solar System, starting with placingtelescopes in spacesince the 1960s.[298]By 1989, all eight planets have been visited by space probes.[299]Probes have returned samples from comets[300]and asteroids,[301]as well as flown through theSun's corona[302]and visited two dwarf planets (PlutoandCeres).[303][304]To save on fuel, some space missions make use ofgravity assist maneuvers, such as the twoVoyagerprobesaccelerating when flying by planets in the outer Solar System[305]and theParker Solar Probedecelerating closer towards the Sun after its flyby of Venus.[306]

Humans have landed on the Moon during theApollo programin the 1960s and 1970s[307]and will return to the Moon in the 2020s with theArtemis program.[308]Discoveries in the 20th and 21st century has prompted theredefinition of the termplanetin 2006, hence the demotion of Pluto to a dwarf planet,[309]and further interest intrans-Neptunian objects.[310]

Solar System→Local Interstellar Cloud→Local Bubble→Gould Belt→Orion Arm→Milky Way→Milky Way subgroup→Local Group→Local Sheet→Virgo Supercluster→Laniakea Supercluster→Local Hole→Observable universe→UniverseEach arrow (→) may be read as "within" or "part of".

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Original source: https://en.wikipedia.org/wiki/Solar_System
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