Editing Solar System (section)

==General Characteristics of the Solar System==
===Overall Structure===
[[File:The Solar System (without scale representation and extra facts).svg|thumb|Overall structure of the Solar System]]
The Solar System is held together by [[gravity]], with the [[Sun]] at the center. The Sun is so huge that it makes up more than 99.8% of all the [[mass]] in the entire Solar System. Because of its strong gravity, all the major objects, like [[planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, and [[comet]]s, orbit around the Sun. This setup is called the [[Heliocentrism|heliocentric system]], which means “Sun-centered.” The Solar System is divided into different regions, and each region has its own kinds of space objects and special features.<ref name=":15" />

Closest to the Sun are the [[terrestrial planet]]s: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. These planets are small, rocky, and heavy, made mostly of [[rock]] and [[metal]]. This part of the Solar System is called the inner Solar System, and all the planets here are packed close together.<ref name=":1" /> Just past Mars is the [[asteroid belt]]. This is a doughnut-shaped area filled with many rocky objects. These space rocks did not come together to form a planet, mostly because Jupiter’s strong gravity kept pulling things apart. The asteroid belt is like a border zone between the inner rocky planets and the outer giant planets.<ref name=":19" />

Farther from the Sun are the [[giant planet]]s. First come [[Jupiter]] and [[Saturn]], known as [[gas giant]]s because they are made mostly of [[hydrogen]] and [[helium]].<ref name=":2" /> Then come [[Uranus]] and [[Neptune]], called [[ice giant]]s because they have more icy materials like [[water]], [[ammonia]], and [[methane]].<ref name=":3" /> These outer planets are very big, have many moons, and also have [[ring system]]s, though not all are as big as Saturn’s rings.<ref>{{Cite web|date=2024-08-13|title=A guide to rings around planets and across the Solar System|url=https://www.skyatnightmagazine.com/space-science/rings-of-the-solar-system|access-date=2025-07-07|website=BBC Sky at Night Magazine|language=en}}</ref> Beyond Neptune is the [[Kuiper Belt]], a cold, flat region full of icy objects, including [[dwarf planet]]s like [[Pluto]].<ref name=":17" /><ref name=":20">{{Cite web|date=2023-07-05|title=Pluto & Dwarf Planets|url=https://science.nasa.gov/dwarf-planets/|access-date=2025-07-07|language=en-US}}</ref> There are also many tiny frozen bodies out there.<ref name=":18" /> Even farther away is something called the [[Oort cloud|Oort Cloud]]. It is a giant, round cloud of icy objects that surrounds the Solar System. Scientists think this is where long-period comets come from. It is also where the Sun’s gravity starts to fade.<ref name=":6" />

The objects in the Solar System are grouped into different categories based on how they move and what they are made of. First, there are the [[planet]]s, like [[Earth]] and [[Jupiter]]. These are big enough to control their [[orbit]]s by pulling in or pushing away other objects nearby.<ref>{{Cite web|date=2023-07-10|title=About the Planets|url=https://science.nasa.gov/solar-system/planets/|access-date=2025-07-07|language=en-US}}</ref> Next are [[dwarf planet]]s, like [[Pluto]], [[Eris (dwarf planet)|Eris]], and [[Haumea (dwarf planet)|Haumea]]. These are round like planets, but they do not clear their orbits, which means other objects still share their paths around the Sun.<ref name=":20" /> Then we have [[natural satellite]]s, which are moons that orbit planets.<ref name=":16" /> Finally, there are [[Small solar system body|small solar system bodies]] like [[asteroid]]s, [[comet]]s, and [[meteoroid]]s. These are smaller chunks of rock, metal, or ice that travel through space in all kinds of orbits.<ref>{{Cite web|title=Small body {{!}} Asteroids, Comets & Meteors {{!}} Britannica|url=https://www.britannica.com/science/small-body|access-date=2025-07-07|website=www.britannica.com|language=en}}</ref>

=== Distances and scales ===
[[File:Solar System distance to scale.svg|center|thumb|upright=2.5|Comparison of the distances between planets, with the white bar showing orbital variations. The size of the planets is not to scale.]]
[[File:Orbital distances in the solar system linear scale.png|thumb|Relative orbital distances in the Solar System visualized as a condensed rectangle]]

The Solar System is huge, way bigger than what humans can easily imagine. The [[distance]]s and [[size]]s of the planets and other space objects are so big that it is hard to picture them using everyday ideas.<ref>{{Cite web|title=The Solar System to Scale: Dynamic 2D Model|url=https://solartoscale.com/|access-date=2025-07-08|website=solartoscale.com}}</ref> The [[Sun]] is the biggest thing in the Solar System. It is so massive that it makes up almost all the [[mass]] in the entire Solar System. The Sun’s [[radius]] is about 700,000 kilometers (or 400,000 miles). Even though [[Earth]] feels big to us, about 1.3 million Earth's would fit inside the Sun.<ref name=":10" /> [[Jupiter]] is the largest planet. It is 5.2 times farther from the Sun than Earth and is so big that over 1,300 Earths could fit inside it. Its [[diameter]] is more than 11 times that of Earth.<ref>{{Cite web|date=2017-10-05|title=Jupiter Facts|url=https://science.nasa.gov/jupiter/jupiter-facts/|access-date=2025-07-08|language=en-US}}</ref> On the other hand, [[Mercury (planet)|Mercury]], the smallest planet, is just a little bigger than [[Moon|Earth’s Moon]].<ref>{{Cite web|title=Mercury|url=https://observatory.astro.utah.edu/Mercury.html|access-date=2025-07-08|website=observatory.astro.utah.edu}}</ref> And even smaller are [[dwarf planet]]s like [[Pluto]] and tiny space rocks like [[comet]]s and [[asteroid]]s, which can be just a few kilometers wide.<ref>{{Cite web|date=2025-04-22|title=Planet Sizes and Locations in Our Solar System|url=https://science.nasa.gov/solar-system/planets/planet-sizes-and-locations-in-our-solar-system/|access-date=2025-07-08|language=en-US}}</ref>

Because the Solar System is so big, scientists use a special unit called the [[Astronomical unit|Astronomical Unit (AU)]] to measure distances in space. One AU is the average distance between the Earth and the Sun, which is about 149.6 million kilometers (or 93 million miles). Using AU makes it easier to talk about how far planets are from the Sun.<ref>{{Cite web|date=2025-05-29|title=Astronomical unit (AU, or au) {{!}} Definition, Conversion, & Facts {{!}} Britannica|url=https://www.britannica.com/science/astronomical-unit|access-date=2025-07-08|website=www.britannica.com|language=en}}</ref> For example, [[Venus]] is about 0.72 AU from the Sun, [[Mars]] is about 1.52 AU, and [[Neptune]], the farthest main planet, is about 30 AU away. The [[Kuiper Belt]], which contains icy objects like Pluto, stretches from around 30 AU to 50 AU.<ref>{{Cite web|title=Scale of the Solar System|url=https://web.pa.msu.edu/people/frenchj/solarsystemscale/|access-date=2025-07-08|website=web.pa.msu.edu}}</ref> Even farther out is the [[Oort cloud|Oort Cloud]], a mysterious area that might reach 100,000 AU from the Sun.<ref name=":6" /> Usually, the farther a planet is from the Sun, the bigger the gap between it and the next planet. For instance, Venus is only about 0.33 AU farther from the Sun than Mercury, but Saturn is 4.3 AU farther out than Jupiter, and Neptune is 10.5 AU beyond Uranus.<ref>{{Cite web|title=Comparing the Distance Between Planets|url=https://www.msnucleus.org/membership/html/k-6/uc/solar_system/2/ucss2_2a.html|access-date=2025-07-08|website=www.msnucleus.org}}</ref> In the past, scientists tried to find patterns in these distances, like the [[Titius–Bode law]] or [[Mysterium Cosmographicum|Kepler's ideas]], but as more planets and objects were discovered, those ideas did not hold up.<ref>{{Cite journal|last=Jaki|first=Stanley L.|date=1972-07-01|title=The Early History of the Titius-Bode Law|url=https://pubs.aip.org/ajp/article/40/7/1014/1045433/The-Early-History-of-the-Titius-Bode-Law|journal=American Journal of Physics|language=en|volume=40|issue=7|pages=1014–1023|doi=10.1119/1.1986734|issn=0002-9505}}</ref><ref>{{Cite journal|last=Phillips|first=J. P.|date=1965|title=Kepler's Echinus|url=https://www.journals.uchicago.edu/doi/10.1086/349957|journal=Isis|language=en|volume=56|issue=2|pages=196–200|doi=10.1086/349957|issn=0021-1753}}</ref><ref>{{Cite web|last=Staff|first=Astronomy|date=2006-10-01|title=Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries? {{!}} Astronomy.com|url=https://www.astronomy.com/science/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries/|access-date=2025-07-08|website=Astronomy Magazine|language=en-US}}</ref>

There is a lot of empty space between the planets in our Solar System. Even though the planets follow paths around the Sun, the distances between them are huge compared to how big the planets are.<ref>{{Cite web|title=Distances Between the Planets of the Solar System • The Planets|url=https://theplanets.org/distances-between-planets/|access-date=2025-07-09|website=The Planets|language=en-US}}</ref> For example, Earth is about 12,700 kilometers wide, but the closest it ever gets to Mars is around 96 million kilometers away.<ref>{{Cite web|date=2023-05-30|title=Facts About Earth|url=https://science.nasa.gov/earth/facts/|access-date=2025-07-09|language=en-US}}</ref><ref>{{Cite web|last=|first=|date=2025-01-12|title=Mars closest to Earth on January 12, 2025|url=https://earthsky.org/tonight/mars-closest-to-earth/|access-date=2025-07-09|website=earthsky.org|language=en-US}}</ref> Because of these huge distances, even fast [[spacecraft]] take a long [[time]], months or years, to travel between planets. The farther you go, the longer it takes.<ref>{{Cite web|date=2025-02-19|title=How Long Does it Take to Get to the Moon... Mars... Jupiter? We Asked a NASA Expert: Episode 51 - NASA|url=https://www.nasa.gov/directorates/smd/how-long-does-it-take-to-get-to-the-moon-mars-jupiter-we-asked-a-nasa-expert-episode-51/|access-date=2025-07-09|language=en-US}}</ref> For instance, [[light]] from the Sun takes only 8 minutes to reach Earth, but it takes over 4 hours to reach Neptune.<ref>{{Cite web|date=2024-04-12|title=How long does it take for light from the Sun to reach Earth?|url=https://www.skyatnightmagazine.com/space-science/how-take-light-from-sun-reach-earth|access-date=2025-07-09|website=BBC Sky at Night Magazine|language=en}}</ref><ref>{{Cite web|date=2017-11-10|title=Neptune: Facts - NASA Science|url=https://science.nasa.gov/neptune/neptune-facts/|access-date=2025-07-09|language=en-US}}</ref> All this [[Outer space|space]] makes it harder to explore and communicate with the outer planets. Sending missions to places like Neptune or beyond is a big challenge and a major achievement for [[science]] and [[engineering]].<ref>{{Cite web|title=Challenges of space travel - Space exploration - National 5 Physics Revision|url=https://www.bbc.co.uk/bitesize/guides/zqm4fcw/revision/3|access-date=2025-07-09|website=BBC Bitesize|language=en-GB}}</ref>

Some [[Scientific model|model]]s of the Solar System are made to help people understand just how big and spread out everything really is. Since the real Solar System is way too large to picture easily, scientists and educators sometimes build scale models to shrink it down while keeping the sizes and distances in the right proportion. Some of these models are small and mechanical, like [[Orrery|orreries]], which are devices that show planets moving around the Sun.<ref>{{Cite web|title=Orrery|url=https://www.hsm.ox.ac.uk/orrery|access-date=2025-07-09|website=www.hsm.ox.ac.uk|language=en}}</ref> Others are huge and can stretch across cities or even entire regions. One of the biggest models is the [[Sweden Solar System]].<ref>{{Cite web|title=Sweden Solar System|url=https://www.swedensolarsystem.se/en/|access-date=2025-07-09|website=www.swedensolarsystem.se}}</ref> In it, the [[Avicii Arena]] in [[Stockholm]] represents the Sun, and it is about 110 meters wide (361 feet). In this model, Jupiter is a 7.3-meter ball (about 24 feet wide) located at an [[airport]] 40 kilometers (25 miles) away. [[Sedna]], an object far out in the Solar System, is shown as a 10-centimeter ball (about 4 inches) placed 810 kilometers (500 miles) away.<ref>{{Cite web|title=Luleå är Sedna - Lokalt den 17 november 2005 - Arkiv - Norrbottens-Kuriren|url=http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx|access-date=2025-07-09|website=www.kuriren.nu}}</ref><ref>{{Cite web|title=Tours of Model Solar Systems|url=http://www.psych.illinois.edu/~wbrewer/solarmodel.html|access-date=2025-07-09|website=www.psych.illinois.edu|archive-date=2010-07-24|archive-url=https://web.archive.org/web/20100724064517/http://www.psych.illinois.edu/~wbrewer/solarmodel.html|url-status=dead}}</ref> If we shrank the distance from the Sun to Neptune down to just 100 meters (about the length of a soccer field), then the Sun would only be about 3 centimeters wide (a little bigger than a [[marble]]), the [[Giant planet|giant planets]] would be all smaller than about 3 mm (0.12 in), and Earth and the other small planets would be tiny dots, smaller than a [[flea]]. These kinds of models help us understand that even though planets can be big, they are still tiny compared to the space between them.<ref>{{Cite web|title=Solar System Scale|url=https://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html|access-date=2025-07-09|website=www.nasa.gov|archive-date=2023-03-28|archive-url=https://web.archive.org/web/20230328021134/https://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html|url-status=dead}}</ref>

===Mass and Composition Distribution===
[[File:Solar_system_mass_distribution_ppm_chart.svg|thumb|Solar system mass distribution]]
The Solar System’s mass and the way its parts are spread out tell us a lot about how it formed and changed over time. Most of the Solar System’s [[mass]], about 99.86%, is in the [[Sun]], which is a huge ball made mostly of [[hydrogen]] and [[helium]]. The Sun’s gravity controls the movement of all the planets, asteroids, comets, and space dust. It’s incredibly massive, weighing about 1.989 × 10³⁰ kilograms. This huge mass creates a strong gravitational pull that keeps everything else in the Solar System in [[orbit]], and it also helps determine how things spin and move.<ref>{{Cite web|date=2023-05-31|title=Sun - NASA Science|url=https://science.nasa.gov/sun/|access-date=2025-07-15|language=en-US}}</ref><ref>{{Cite web|title=Sun Fact Sheet|url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html|access-date=2025-07-15|website=nssdc.gsfc.nasa.gov|archive-date=1998-02-22|archive-url=https://web.archive.org/web/19980222081658/https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html|url-status=dead}}</ref> Only about 0.14% of the Solar System’s total mass is in everything else, like the eight [[planet]]s, [[dwarf planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, objects in the [[Kuiper Belt]], and the [[Oort cloud|Oort Cloud]].<ref>{{Cite web|date=2023-07-20|title=Chapter 1: The Solar System - NASA Science|url=https://science.nasa.gov/learn/basics-of-space-flight/chapter1-1/|access-date=2025-07-15|language=en-US}}</ref> Out of this small portion, more than 70% is found in [[Jupiter]], the largest planet. Jupiter weighs about 1.9 × 10²⁷ kilograms, which is 318 times heavier than [[Earth]].<ref>{{Cite web|title=Jupiter |url=https://www.dlr.de/en/research-and-transfer/projects-and-missions/juice/jupiter-gas-giant-and-ringed-planet/jupiter|access-date=2025-07-15|website=www.dlr.de|language=en}}</ref> Because of its size, Jupiter has a very strong gravitational pull. It affects the movement of asteroids, helps keep the orbits of other planets stable, and played a big role in clearing out leftover space rocks when the Solar System was still forming.<ref>{{Cite web|date=2023-05-31|title=Jupiter|url=https://science.nasa.gov/jupiter/|access-date=2025-07-15|language=en-US}}</ref>

The [[giant planet]]s, [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]], make up about 99% of the total mass of all the planets in the Solar System. This shows that the Solar System has a bimodal structure, meaning most of the mass is in these large [[Gas giant|gas]] and [[ice giant]]s, not the smaller rocky planets closer to the Sun.<ref>{{Cite book|url=https://nap.nationalacademies.org/read/13117/chapter/10|title=Read "Vision and Voyages for Planetary Science in the Decade 2013-2022" at NAP.edu|language=en}}</ref> Saturn is the second largest planet, with a mass about 95 times that of Earth. Like Jupiter, it is mostly made of hydrogen and helium, but Saturn has more ices and heavier elements deep inside.<ref>{{Cite web|date=2023-05-31|title=Saturn|url=https://science.nasa.gov/saturn/|access-date=2025-07-15|language=en-US}}</ref> Uranus and Neptune are called ice giants. They also have atmospheres made of hydrogen and helium, but inside they have a lot of [[water]], [[ammonia]], and [[Methane|methane ices]]. Each of them has a mass between 14 and 17 times the mass of Earth.<ref>{{Cite web|title=Uranus/Neptune|url=https://pages.uoregon.edu/jschombe/ast121/lectures/lec20.html#:~:text=Neptune%20is%2017%20times%20the,could%20contain%20nearly%2060%20Earths.|access-date=2025-07-15|website=pages.uoregon.edu}}</ref>

On the other hand, the [[Terrestrial planet|terrestrial (rocky) planet]]s, [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]], have only a tiny amount of the Solar System’s mass. Earth makes up more than half of the total mass of these four rocky planets because it’s the biggest and densest. These inner planets are made mostly of [[Silicate|silicate rock]] and [[metal]], with common elements like [[iron]], [[oxygen]], [[silicon]], and [[magnesium]].<ref name=":1" /> They do not have much hydrogen or helium because, when the Solar System was forming, the area close to the Sun was too hot for light gases to stick around. The Sun’s heat pushed those gases away, so only heavier materials like rock and metal were left behind to form the inner planets.<ref>{{Cite web|title=Planets and How They Formed|url=https://lco.global/spacebook/solar-system/planets-and-how-they-formed/|access-date=2025-07-15|website=lco.global|language=en}}</ref>

Besides the major planets, the Solar System has many smaller objects. Even though these objects have only a small amount of mass compared to the planets, they are very important for helping scientists understand how the planets formed. One group of these small objects is the [[asteroid belt]], which is found between Mars and Jupiter. The asteroid belt has less than 4% of the Moon’s mass, but it contains materials that were part of the early building blocks of the rocky planets. The biggest object in the belt is [[Ceres (dwarf planet)|Ceres]], which is about 940 kilometers wide. Ceres makes up about one-third of the asteroid belt’s total mass and is made mostly of [[rock]] and [[Ice|water ice]].<ref>{{Cite journal|last=Krasinsky|first=G|date=2002|title=Hidden Mass in the Asteroid Belt|url=https://linkinghub.elsevier.com/retrieve/pii/S0019103502968375|journal=Icarus|language=en|volume=158|issue=1|pages=98–105|doi=10.1006/icar.2002.6837}}</ref><ref>{{Cite journal|last1=Pitjeva|first1=E. V.|last2=Pitjev|first2=N. P.|date=2018|title=Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft|url=http://link.springer.com/10.1134/S1063773718090050|journal=Astronomy Letters|language=en|volume=44|issue=8–9|pages=554–566|doi=10.1134/S1063773718090050|arxiv=1811.05191 |issn=1063-7737}}</ref> Farther away from the Sun is the Kuiper Belt, which holds thousands of icy objects. Some of the best-known are [[Pluto (dwarf planet)|Pluto]], [[Eris (dwarf planet)|Eris]], [[Haumea (dwarf planet)|Haumea]], and [[Makemake (dwarf planet)|Makemake]]. These objects are much smaller than the giant planets, but their mix of rock and ices like [[methane]], [[nitrogen]], and [[carbon monoxide]] helps scientists learn about the chemistry of the early outer Solar System. The total mass of the Kuiper Belt is only a small fraction of Earth’s mass, between 0.01 and 0.1 times the mass of Earth, because over billions of years, much of its mass was lost in space due to collisions and gravitational forces.<ref>{{Cite journal|last1=Romanishin|first1=W.|last2=Tegler|first2=S. C.|last3=Rettig|first3=T. W.|last4=Consolmagno|first4=G.|last5=Botthof|first5=B.|date=2001-10-09|title=1998 SM165: A large Kuiper belt object with an irregular shape|journal=Proceedings of the National Academy of Sciences|volume=98|issue=21|pages=11863–11866|doi=10.1073/pnas.211147998 |pmid=11572937 |pmc=59815 |doi-access=free }}</ref><ref>{{Cite journal|last1=Pitjeva|first1=E. V.|last2=Pitjev|first2=N. P.|date=2018|title=Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft|url=http://link.springer.com/10.1134/S1063773718090050|journal=Astronomy Letters|language=en|volume=44|issue=8–9|pages=554–566|doi=10.1134/S1063773718090050|arxiv=1811.05191 |issn=1063-7737}}</ref><ref name=":17" />

Even farther out is the [[Oort cloud|Oort Cloud]], which scientists think is a huge, round shell of icy objects surrounding the Solar System. It is believed to contain trillions of comets made mostly of water ice, [[carbon dioxide]] ice, and other frozen gases. The total mass of the Oort Cloud is not known for sure, but it might be several times the mass of Earth. It stretches as far as 100,000 [[Astronomical unit|astronomical units (AU)]] from the Sun (1 AU is the distance from Earth to the Sun). The Oort Cloud is thought to be the source of [[Comet|long-period comet]]s, comets that take hundreds or thousands of years to orbit the Sun and sometimes come close enough to be seen from Earth.<ref name=":6" />

[[Natural satellite|Moon]]s and [[Planetary ring|ring]]s around planets add a little bit of mass to the Solar System, but not very much compared to the planets. For example, [[Moon|Earth’s Moon]] is the fifth largest moon in the Solar System. It is pretty big compared to Earth, with a mass that is about 1.2% of Earth’s mass. The Moon is mostly made of [[Silicate|silicate rock]], which is similar to the material found in [[Earth's crust|Earth’s crust]] and [[Earth's mantle|mantle]].<ref>{{Cite web|date=2017-11-15|title=Moon Facts - NASA Science|url=https://science.nasa.gov/moon/facts/|access-date=2025-07-16|language=en-US}}</ref> The outer planets have some very large moons too, like [[Ganymede (moon)|Ganymede]], [[Titan (moon)|Titan]], [[Callisto (moon)|Callisto]], and [[Io (moon)|Io]]. These moons are made of a mix of rock and ice. Some of them, like [[Europa (moon)|Europa]] and [[Enceladus (moon)|Enceladus]], even have [[ocean]]s beneath their icy surfaces, which shows that there are many different kinds of icy, possibly habitable places beyond Earth.<ref name=":16" />

The types of [[Chemical element|element]]s found in different parts of the Solar System also tell us a lot about how it formed. These elements were created in earlier generations of [[star]]s and then spread out into [[Outer space|space]]. In the outer Solar System, there are more volatile elements (which are lighter and can turn into gas easily), while the inner Solar System has more refractory materials (which are heavier and can stay solid at high [[temperature]]s). This difference shows how temperature, [[sunlight]], and the movement of planets affected where different materials could collect and form planets.<ref>{{Cite book|title=Encyclopedia of the solar system|url=https://archive.org/details/encyclopediaofso0000unse_m9q5|date=2014|publisher=Elsevier|isbn=978-0-12-415845-0|editor-last=Spohn|editor-first=Tilman|edition=3rd|location=Amsterdam ; Boston|editor-last2=Breuer|editor-first2=Doris|editor-last3=Johnson|editor-first3=T. V.}}</ref>