Editing Solar System (section)

===Formation of the Planets===
[[File:NASA-ExocometsAroundBetaPictoris-ArtistView.jpg|thumb|Formation of plantets in the early solar system]]
[[File:Artist's concept of collision at HD 172555.jpg|thumb|Artist's conception of the [[Giant impact hypothesis|giant impact]] thought to have formed the [[Moon]]]]

The process of forming [[planet]]s began inside the spinning disk of [[gas]] and [[dust]] around the young [[Sun]]. It all started with tiny dust grains made of things like [[rock]], [[metal]], and [[ice]]. These grains were spread all throughout the disk. At first, the dust grains were tiny. But as they floated in the gas, they bumped into each other and started to stick together. This happened because of [[electrostatic force]]s, the same kind of [[static electricity]] that makes a person's [[hair]] stand up. Over time, the grains grew bigger and turned into [[pebble]]s, then clumps, and finally into objects that were about a kilometer wide. These larger bodies are called [[planetesimal]]s, and they were the first real building blocks of planets. As the planetesimals (small space rocks) kept growing, something important started to happen, gravity became a big deal. The larger planetesimals had stronger [[gravity]], which let them pull in more nearby dust and rocks. This made them grow even faster. This fast-growing phase is called [[Accretion (astrophysics)|runaway accretion]]. During runaway accretion, the biggest planetesimals kept getting bigger and bigger, leaving the smaller ones behind. These big ones started to take over certain parts of the disk, becoming the main objects in their areas.<ref>{{Citation|last=Raymond|first=Sean|title=Runaway Growth|date=2011|url=https://link.springer.com/rwe/10.1007/978-3-642-11274-4_1397|encyclopedia=Encyclopedia of Astrobiology|pages=1485–1486|publisher=Springer, Berlin, Heidelberg|language=en|doi=10.1007/978-3-642-11274-4_1397|isbn=978-3-642-11274-4|access-date=2025-07-04}}</ref> After most of the smaller pieces were gone, the growth slowed down. This slower stage is called [[Formation and evolution of the Solar System|oligarchic growth]]. In this phase, several large bodies, called planetary embryos, formed. These were about the size of the [[Moon]] or even [[Mars]]. Each embryo controlled a small area of the disk. Over millions of years, the large space rocks called planetary embryos began to crash into each other, join together, or pull on each other with [[gravity]]. These actions helped shape the final layout of the planets in the Solar System.<ref>{{Cite journal|last1=Dencs|first1=Z.|last2=Regály|first2=Zs|date=2021-01-01|title=Oligarchic growth in a fully interacting system|url=https://www.aanda.org/articles/aa/abs/2021/01/aa39567-20/aa39567-20.html|journal=Astronomy & Astrophysics|language=en|volume=645|pages=A65|doi=10.1051/0004-6361/202039567|arxiv=2012.07683 |issn=0004-6361}}</ref> In the inner part of the Solar System, where there were mostly rocky materials, these giant collisions eventually formed the rocky planets like [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. In the outer part, beyond the snow line where it was much colder, some embryos grew so big that their gravity could pull in thick layers of gas, mainly hydrogen and helium. These became the huge gas and ice giants like [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]].<ref>{{Cite web|date=2020-09-01|title=How Do Planets Form? - NASA Science|url=https://science.nasa.gov/exoplanets/how-do-planets-form/|access-date=2025-07-04|language=en-US}}</ref><ref>{{Citation|last=Williams|first=I. P.|title=Planetary Formation|date=1974|url=https://doi.org/10.1007/978-94-010-2206-4_1|work=Exploration of the Planetary System|pages=3–12|editor-last=Woszczyk|editor-first=A.|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-010-2206-4_1|isbn=978-94-010-2206-4|access-date=2025-07-04|editor2-last=Iwaniszewska|editor2-first=C.}}</ref>

One of the most important events in the story of how planets formed is the creation of Earth’s [[Moon]]. Scientists believe the Moon was formed by a giant impact. This idea is called the [[giant impact hypothesis]]. According to this theory, about 4.5 billion years ago, a space object about the size of Mars, which we call [[Theia (planet)|Theia]], crashed into the young Earth. The crash was so powerful that it melted parts of both worlds and sent a huge amount of debris into space around Earth. This debris, made mostly of rock from Earth’s outer layers, slowly came together because of [[gravity]]. Over time, the pieces joined to form the Moon. Scientists believe this theory is true because the Moon is made of materials very similar to Earth’s outer layers, especially the [[Mantle (geology)|mantle]]. Also, the Moon does not have many gases or a large [[Planetary core|iron core]], which supports the idea that it formed from Earth’s surface material, not from a separate object that was captured.<ref>{{Cite web|title=How did the Moon form? {{!}} Natural History Museum|url=https://www.nhm.ac.uk/discover/how-did-the-moon-form.html|access-date=2025-07-04|website=www.nhm.ac.uk|language=en}}</ref><ref>{{Citation|last=Genda|first=Hidenori|title=Giant Impact Hypothesis|date=2018|url=https://link.springer.com/rwe/10.1007/978-3-319-39312-4_338|encyclopedia=Encyclopedia of Geochemistry|series=Encyclopedia of Earth Sciences Series |pages=617–620|publisher=Springer, Cham|language=en|doi=10.1007/978-3-319-39312-4_338|isbn=978-3-319-39312-4|access-date=2025-07-04}}</ref>

As young planets were forming, something important called [[planetary differentiation]] began to happen. This is the process where the inside of a [[planet]] becomes layered, with heavier materials sinking and lighter ones rising. As the planets grew bigger, they got hotter inside. This heat came from [[radioactive decay]], big impacts, and the pressure of [[gravity]] squeezing the planet. The heat was so strong that parts of the inside of the planet began to melt. When a planet is melted, the materials inside can separate by weight. Heavy materials like [[iron]] and [[nickel]] sank to the center to form the core. Lighter materials, such as [[Silicate mineral|silicate rock]]s, floated upward to form the mantle and crust.<ref>{{Citation|last1=Sohl|first1=Frank|title=Differentiation, Planetary|date=2014|url=https://link.springer.com/rwe/10.1007/978-3-642-27833-4_430-2|encyclopedia=Encyclopedia of Astrobiology|pages=1–5|publisher=Springer, Berlin, Heidelberg|language=en|doi=10.1007/978-3-642-27833-4_430-2|isbn=978-3-642-27833-4|access-date=2025-07-04|last2=Breuer|first2=Doris}}</ref> Differentiation helped planets develop many important features. One major result was the formation of metallic cores inside planets like Earth. These cores moved and flowed, creating [[magnetic field]]s that protect the planet from harmful [[Outer space|space]] [[radiation]].<ref>{{Cite web|date=2013-12-16|title=How does the Earth's core generate a magnetic field? {{!}} U.S. Geological Survey|url=https://www.usgs.gov/faqs/how-does-earths-core-generate-a-magnetic-field|access-date=2025-07-04|website=www.usgs.gov|language=en}}</ref> At the same time, [[volcano]]es released gases from inside the planet in a process called [[outgassing]]. These gases, along with materials delivered by space rocks, helped form the planet’s [[atmosphere]]. On [[Earth]], this made it possible for life to eventually develop.<ref>{{Cite web|date=2010-04-14|title=The Origin of the Atmosphere|url=https://volcano.oregonstate.edu/origin-atmosphere|access-date=2025-07-04|website=Volcano World|language=en}}</ref> The leftover heat from the planet’s formation, along with moving materials inside the planet, caused ongoing geological activity. This includes things like [[volcano]]es, [[earthquake]]s, and [[Tectonic plate|tectonic plate movement]]. These processes helped shape the planet’s surface and kept the planet active.<ref>{{Cite web|title=How volcanoes form|url=https://www.bgs.ac.uk/discovering-geology/earth-hazards/volcanoes/how-volcanoes-form-2/|access-date=2025-07-04|website=British Geological Survey|language=en-GB}}</ref>

The [[gas giant]]s, like [[Jupiter]] and [[Saturn]], formed in a different way than the smaller rocky planets like Earth. This is because they were born in the colder, outer parts of the Solar System, beyond a boundary called the [[Frost line (astrophysics)|snow line]], where ices could form and last. Scientists have two main ideas about how these big planets formed. The most popular one is called the [[Core accretion (planet formation)|core accretion model]]. According to this idea, the process began with icy space rocks, called [[planetesimal]]s, slowly sticking together to form a large solid core. This core had to be about 10 times the size of Earth to be strong enough. Once the core got that big, its gravity became powerful enough to pull in large amounts of gas, especially [[hydrogen]] and [[helium]], from the surrounding disk. This made the planets grow very quickly and gain their thick, gassy outer layers. But this had to happen fast, within just a few million years, before all the gas in the [[protoplanetary disk]] disappeared. If the gas was gone, the planet could not grow into a gas giant.<ref>{{Cite web|title=Core accretion|url=https://faculty.ucr.edu/~krice/coreacc.html|access-date=2025-07-04|website=faculty.ucr.edu}}</ref><ref>{{Citation|last=Hubickyj|first=Olenka|title=Core accretion—gas capture model for gas giant planet formation|date=2008|url=https://www.cambridge.org/core/books/decade-of-extrasolar-planets-around-normal-stars/core-accretiongas-capture-model-for-gas-giant-planet-formation/2C8FD98F276C0DA52A451F34178EC475|work=A Decade of Extrasolar Planets around Normal Stars: Proceedings of the Space Telescope Science Institute Symposium, held in Baltimore, Maryland May 2–5, 2005|pages=138–152|editor-last=Valenti|editor-first=Jeff|series=Space Telescope Science Institute Symposium Series|place=Cambridge|publisher=Cambridge University Press|isbn=978-0-521-89784-6|access-date=2025-07-04|editor2-last=Sahu|editor2-first=Kailash|editor3-last=Livio|editor3-first=Mario}}</ref> Another idea about how gas giants like Jupiter and Saturn formed is called the [[Disk instability (planet formation)|disk instability model]]. Instead of starting with a solid core, this theory suggests that parts of the gas disk around the Sun became unstable and collapsed under their own gravity. These collapsing clumps of gas could have turned directly into gas giants, without needing a rocky center first. This model could explain how gas giants formed very quickly, which is something the core accretion model struggles with. However, scientists do not have much direct evidence that this happened in our Solar System, so the core accretion model is still the more popular idea.<ref>{{Cite journal|last=Jang-Condell|first=H.|date=2007|title=Disk Instability vs. Core Accretion: Observable Discriminants|url=https://ui.adsabs.harvard.edu/abs/2007lyot.confE..18J/abstract|journal=In the Spirit of Bernard Lyot: The Direct Detection of Planets and Circumstellar Disks in the 21st Century|language=en|pages=18 |bibcode=2007lyot.confE..18J }}</ref>

The [[ice giant]]s, [[Uranus]] and [[Neptune]], formed differently from both the rocky planets and the gas giants. They were born farther from the Sun, in a place where there was less material to build planets. According to the core accretion model, these planets started out by forming solid cores, just like Jupiter and Saturn. But because there was not as much gas around, or because the gas did not last very long, they could not pull in as much [[hydrogen]] and [[helium]]. So, they ended up with only thin layers of gas.<ref>{{Cite journal|last1=Boss |first1=Alan P. |title=Formation of gas and ice giants|url=https://adsabs.harvard.edu/full/2003ESASP.539..143B|access-date=2025-07-04|journal=Earths: Darwin/TPF and the Search for Extrasolar Terrestrial Planets |date=2003 |volume=539 |page=143 |bibcode=2003ESASP.539..143B }}</ref> Instead of having thick, gassy atmospheres, ice giants are made mostly of [[volatiles]], substances like [[water]], [[ammonia]], and [[methane]]. Even though we call them “ice” giants, much of this material is actually in a hot, thick, fluid form deep inside the planets because of the high [[pressure]]. This is why Uranus and Neptune are smaller than Jupiter and Saturn and have a different makeup.<ref>{{Cite web|title=The Realm of the Ice Giants|url=https://www.planetary.org/articles/simon-the-realm-of-the-ice-giants|access-date=2025-07-04|website=The Planetary Society|language=en}}</ref>  At the same time as the planets were forming, the outer edges of the Solar System were creating [[dwarf planet]]s and other icy objects. Far beyond Neptune, there’s a region called the [[Kuiper Belt]].<ref name=":17">{{Cite web|date=2017-11-14|title=Kuiper Belt: Facts - NASA Science|url=https://science.nasa.gov/solar-system/kuiper-belt/facts/|access-date=2025-07-04|language=en-US}}</ref> In this area, leftover pieces from the early Solar System came together to form objects like [[Pluto]], [[Eris (dwarf planet)|Eris]], and [[Haumea (dwarf planet)|Haumea]]. These icy bodies are like frozen [[time capsule]]s. They have not changed much since they first formed, so they give scientists clues about what the outer Solar System was like long ago. Their icy make-up and stable orbits tell us about the cold, quiet conditions far from the Sun. Some of these objects might not have started out in the Kuiper Belt. They may have formed closer to the Sun and were later pushed outward by the strong [[gravity]] of the moving gas giants, like Jupiter. This idea supports theories like the [[Nice model]], which explains how the planets and other objects ended up where they are today.<ref name=":18">{{Cite web|title=Kuiper Belt Objects (KBOs) {{!}} EBSCO Research Starters|url=https://www.ebsco.com/research-starters/astronomy-and-astrophysics/kuiper-belt-objects-kbos|access-date=2025-07-04|website=www.ebsco.com|language=en}}</ref><ref>{{Cite web|date=2018-12-14|title=10 Things to Know About the Kuiper Belt - NASA Science|url=https://science.nasa.gov/solar-system/kuiper-belt/10-things-to-know-about-the-kuiper-belt/|access-date=2025-07-04|language=en-US}}</ref>