Editing Solar System (section)

===Formation of the Sun and Protoplanetary Disk===
[[File:Soot-line1.jpg|thumb|upright=1.35|Diagram of the early Solar System's [[protoplanetary disk]], out of which Earth and other Solar System bodies formed]]
The formation and [[wikt:evolution|evolution]] of the Solar System began 4.6 [[1,000,000,000|billion]] years ago with the [[gravitation]]al collapse of a small part of a giant [[molecular cloud]].<ref>{{citation|first1=Audrey|last1=Bouvier|first2=Meenakshi|last2=Wadhwa|date=2010|title=The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion. |doi=10.1038/ngeo941|journal=Nature Geoscience|volume=3|issue=9|pages=637–641|bibcode=2010NatGe...3..637B}}<br>Date based on oldest inclusions found to date in [[meteorite]]s, thought to be among the first solid material to form in the collapsing solar nebula.</ref> The [[solar nebula]] was the cloud of [[gas]] and [[dust]] that eventually formed the [[Sun]] and all the [[planet]]s. This cloud did not just appear out of nowhere. It came from the [[interstellar medium]], which is the thin, spread-out stuff that fills the space between [[star]]s. The part of space that formed our [[solar system]] was probably in a giant [[molecular cloud]].<ref>{{Cite web|title=Solar Nebula|url=https://www.lpi.usra.edu/education/timeline/gallery/slide_1.html|access-date=2025-07-04|website=www.lpi.usra.edu}}</ref> These are cold, thick areas in space where new stars are often born. That’s why scientists sometimes call them stellar nurseries. These clouds were made mostly of [[hydrogen]] and [[helium]], the two lightest [[Chemical element|element]]s. But they also had tiny amounts of heavier elements like [[carbon]], [[oxygen]], [[silicon]], and [[iron]].<ref>{{Citation|last=Irvine|first=William M.|title=The Molecular Composition of Dense Interstellar Clouds|date=1991|url=https://doi.org/10.1007/978-94-009-0695-2_4|work=Chemistry in Space|pages=89–121|editor-last=Greenberg|editor-first=J. M.|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-009-0695-2_4|isbn=978-94-009-0695-2|access-date=2025-07-04|editor2-last=Pirronello|editor2-first=V.}}</ref> These heavier elements were made by older stars that had exploded in huge blasts called [[supernova]]s. When these stars exploded, they sent these elements out into space, mixing them into the cloud.<ref>{{Cite web|title=Origin of the Chemical Elements - T. Rauscher & A. Patkos|url=https://ned.ipac.caltech.edu/level5/Sept16/Rauscher/Rauscher4.html|access-date=2025-07-04|website=ned.ipac.caltech.edu}}</ref><ref>{{Cite web|date=2023-12-07|title=Extreme conditions in stars produce the universe's heaviest elements|url=https://news.umich.edu/extreme-conditions-in-stars-produce-the-universes-heaviest-elements/|access-date=2025-07-04|website=University of Michigan News|language=en-US}}</ref> Because of these ingredients, our part of the cloud had everything needed to form [[Terrestrial planet|rocky planet]]s like [[Earth]] and even the building blocks of life.<ref>{{Cite web|title=Origin of the Universe and the Earth|url=https://www.columbia.edu/~vjd1/solar_nebula.htm|access-date=2025-07-04|website=www.columbia.edu}}</ref>

The formation of our Solar System began with a triggering event, something that caused part of a giant cloud of gas and dust to start collapsing. Scientists are not completely sure what started it, but there are a couple of good guesses. One idea is that a nearby [[supernova]], a huge [[explosion]] at the end of a [[star]]’s life, sent out a powerful [[Shock wave|shockwave]]. This shockwave may have hit the cloud, pushing and squeezing the gas and dust until it became dense enough for [[gravity]] to take over.<ref>{{Cite journal|last1=Schramm|first1=David N.|last2=Clayton|first2=Robert N.|date=1978|title=Did a Supernova Trigger the Formation of the Solar System?|url=https://www.jstor.org/stable/24955827|journal=Scientific American|volume=239|issue=4|pages=124–139|doi=10.1038/scientificamerican1078-124 |jstor=24955827 |issn=0036-8733}}</ref> Another possibility is that small instabilities inside the cloud caused it to start collapsing on its own. Once gravity became stronger than the pressure trying to push the gas outward, the center of the cloud started to collapse inward. As it collapsed, it began to spin and flatten, forming a thicker, spinning area called the [[Formation and evolution of the Solar System|protosolar nebula]]. This was the early stage of what would eventually become the Sun and planets.<ref>{{Cite web|title=The Origin of the Solar System|url=https://www.pas.rochester.edu/~blackman/ast104/nebular.html|access-date=2025-07-04|website=www.pas.rochester.edu}}</ref>

As the center of the giant cloud of gas and dust collapsed, it started to spin faster. This happened because of a rule in [[physics]] called [[conservation of angular momentum]], kind of like how a figure skater spins faster when they pull their arms in. As the spinning increased, the cloud also flattened into a disk. In the middle of this spinning disk, more and more gas and dust gathered, making the center hotter and denser. As this cloud got smaller, most of the material started gathering in the center, forming a growing ball of gas called a [[protostar]]. This was the very early stage of what would become the Sun.<ref>{{Cite web|title=Sky Tellers - Sun|url=http://www.lpi.usra.edu/education/skytellers/sun/|access-date=2025-07-04|website=Lunar and Planetary Institute (LPI)|language=en}}</ref> As more gas and dust fell into the center, the protostar got hotter and denser. The particles inside began to crash into each other more often, causing the [[pressure]] to rise. When the core reached a temperature of about 10 million Kelvin, a powerful reaction called [[nuclear fusion]] started. During fusion, [[Hydrogen|hydrogen atom]]s joined together to form [[helium]], and this released a huge amount of [[energy]]. At this moment, the Sun officially came to life as a [[main-sequence star]].<ref>{{Citation|title=chapter 47 - BEGINNINGS – SOLAR SYSTEM, SUN, UNIVERSE……LIFE?|date=1966-01-01|url=https://www.sciencedirect.com/science/article/pii/B9780080121536500349|work=Abridged Science for High School Students|pages=47–1|editor-last=Messel|editor-first=H.|publisher=Pergamon|doi=10.1016/b978-0-08-012153-6.50034-9|isbn=978-0-08-012153-6|access-date=2025-07-04}}</ref> Once fusion began, the Sun started giving off strong [[light]], [[heat]], and [[solar wind]], a stream of tiny, charged [[particle]]s. This wind helped blow away the extra gas and dust in the inner Solar System. This clearing-out process is called [[photoevaporation]]. It helped stop more material from falling into the Sun and made room for the planets to start forming.<ref>{{Cite journal|last1=Ercolano|first1=Barbara|last2=Picogna|first2=Giovanni|date=2022-12-19|title=Modelling photoevaporation in planet forming discs|url=https://doi.org/10.1140/epjp/s13360-022-03515-8|journal=The European Physical Journal Plus|language=en|volume=137|issue=12|pages=1357|doi=10.1140/epjp/s13360-022-03515-8|issn=2190-5444}}</ref> Even though the Sun was still young, it now had strong gravity and energy that controlled everything around it. The Sun had become the center of the Solar System.<ref>{{Citation|last=Kaula|first=William M.|title=Formation of the Sun and its Planets|date=1986|url=https://doi.org/10.1007/978-94-009-5255-3_1|work=Physics of the Sun: Volume III: Astrophysics and Solar-Terrestrial Relations|pages=1–32|editor-last=Sturrock|editor-first=Peter A.|place=Dordrecht|publisher=Springer Netherlands|language=en|doi=10.1007/978-94-009-5255-3_1|isbn=978-94-009-5255-3|access-date=2025-07-04|editor2-last=Holzer|editor2-first=Thomas E.|editor3-last=Mihalas|editor3-first=Dimitri M.|editor4-last=Ulrich|editor4-first=Roger K.}}</ref>

Surrounding the young Sun was a large, spinning disk of gas and dust called the [[protoplanetary disk]]. This material was left over from the cloud that formed the Sun. The disk was flat like a pancake and rotated around the Sun. This disk was not the same all over, it had different temperatures in different places. The part closest to the Sun was very hot because of the Sun’s strong [[light]] and [[heat]]. In this hot area, only tough materials like [[metal]]s and [[rock]]s could stay solid. These materials came together to form the [[Terrestrial planet|rocky planet]]s like [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. Farther away from the Sun, the disk was much cooler. In this colder area, things like [[water]], [[ammonia]], and [[methane]] could freeze into ice. These icy and gassy materials helped form the [[Gas giant|gas]] and [[ice giant]]s, planets like [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. So, the different temperatures in the disk helped decide what kind of planets would form in different parts of the Solar System.<ref>{{Cite web|title=The Outer Planets: How Planets Form|url=https://lasp.colorado.edu/outerplanets/solsys_planets.php#:~:text=The%20temperature%20across%20this%20protoplanetary,is%20called%20the%20frost%20line.|access-date=2025-07-04|website=lasp.colorado.edu}}</ref> This model, known as the [[nebula]]r [[hypothesis]], was developed in the 18th (1700s) century by [[Emanuel Swedenborg]], [[Immanuel Kant]], and [[Pierre-Simon Laplace]]. It has been adjusted by scientific disciplines such as [[astronomy]], [[physics]], [[geology]], and [[planetary science]]. As our knowledge of space has grown, the models have been changed to account for the new observations.<ref>{{Cite web|date=2025-06-19|title=Solar nebula {{!}} Formation, Accretion, Protoplanetary Disk {{!}} Britannica|url=https://www.britannica.com/science/solar-nebula|access-date=2025-07-04|website=www.britannica.com|language=en}}</ref>

One important part of the protoplanetary disk was something called the [[Frost line (astrophysics)|snow line]] (also known as the ice line). This was an invisible boundary in the disk that marked the point where it was cold enough for water and other [[gas]]es to freeze into ice. Inside the snow line, closer to the Sun, it was too hot, so only [[rock]] and [[metal]] could stick together and form solid objects. That’s why the inner planets, like Earth and Mars, are made mostly of rock. Beyond the snow line, it was much cooler, and there were lots of ices in addition to rock. These ices made it easier for large planet cores to grow quickly. Once these big cores formed, they had strong enough [[gravity]] to pull in lots of gas from the disk around them. This is how the giant planets like [[Jupiter]] and [[Saturn]] were able to form.<ref>{{Cite web|title=Planets and the Snow Line {{!}} Center for Astrophysics {{!}} Harvard & Smithsonian|url=https://www.cfa.harvard.edu/news/planets-and-snow-line|access-date=2025-07-04|website=www.cfa.harvard.edu}}</ref>