Editing Solar System (section)

== Formation and Evolution of the Solar System ==
{{main|Formation and evolution of the Solar System}}
===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>

===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>

===Evolution of the Solar System===
[[File:Lunar cataclysm.jpg|thumb|upright=1.1|Artist's impression of the [[Moon]] during the Late Heavy Bombardment (above) and today (below)]]
One of the most important steps in the Solar System's history was the clearing of the [[protoplanetary disk]]. This was the spinning cloud of gas and dust around the young Sun where planets were forming. As the [[Sun]] got older and became a full [[main-sequence star]], it started sending out strong [[solar wind]]s, streams of tiny, charged particles. It also gave off intense [[Ultraviolet|ultraviolet radiation]]. Together, these forces began to blow away the leftover gas and dust in the disk. This process is called [[photoevaporation]]. Photoevaporation was especially strong in the inner part of the Solar System, where the Sun's [[heat]] and [[light]] were most powerful.<ref>{{Cite journal|last1=Owen|first1=James E.|last2=Ercolano|first2=Barbara|last3=Clarke|first3=Cathie J.|date=2011-03-21|title=Protoplanetary disc evolution and dispersal: the implications of X-ray photoevaporation: Disc evolution and dispersal|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=412|issue=1|pages=13–25|doi=10.1111/j.1365-2966.2010.17818.x |doi-access=free }}</ref> Once the gas was gone, planets could not grow much more, especially [[gas giant]]s, because the materials they needed had disappeared. Scientists believe this clearing happened within about 10 million years after the Solar System began. We know this by looking at other young stars and their disks in space today. The clearing of the disk marked the end of major planet formation and shaped the Solar System as we know it.<ref>{{Cite journal|last=C. Lin|first=Douglas N.|date=2008|title=The Genesis of Planets|url=https://www.scientificamerican.com/article/the-genesis-of-planets-2008-05|journal=Scientific American|volume=298|issue=5|pages=50–59|doi=10.1038/scientificamerican0508-50|doi-broken-date=12 July 2025|pmid=18444325|issn=0036-8733}}{{Dead link|date=August 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite journal | author=B. G. Elmegreen| title=On the disruption of a protoplanetary disc nebula by a T Tauri like solar wind | journal=Astronomy & Astrophysics |year=1979 | volume=80 | issue=1 | pages=77 | bibcode=1979A&A....80...77E }}</ref><ref>{{cite web |date=24 November 2004 |author=Heng Hao |publisher=Harvard University |title=Disc-Protoplanet interactions |url=http://cfa-www.harvard.edu/~kstanek/astro200/disk-protoplanet.pdf |access-date=2006-11-19 |url-status=dead |archive-url=https://web.archive.org/web/20060907170907/http://cfa-www.harvard.edu/~kstanek/astro200/disk-protoplanet.pdf |archive-date=7 September 2006 }}</ref>

After the planets finished forming and started to settle into their [[orbit]]s, the Solar System went through a chaotic time called the [[Late Heavy Bombardment|Late Heavy Bombardment (LHB)]]. This happened about 4.1 to 3.8 billion years ago and was a period when the inner planets, like [[Earth]], [[Moon|the Moon]], [[Mars]], and [[Mercury (planet)|Mercury]], were hit by tons of [[asteroid]]s and [[comet]]s.<ref>{{Cite web|date=2024-12-19|title=What is the Late Heavy Bombardment? - NASA Science|url=https://science.nasa.gov/moon/lunar-craters/what-is-the-late-heavy-bombardment/|access-date=2025-07-05|language=en-US}}</ref> Scientists know this happened because of rock samples from the Moon that were brought back by the [[Apollo mission]]s. Scientists studied the [[Impact crater|crater]]s on the Moon and used special techniques to figure out how old the rocks were. They found that many of the Moon’s biggest craters, like the [[Mare Imbrium|Imbrium]] and [[Mare Orientale|Orientale basin]]s, were all made around the same time. This shows that there was a sudden increase in [[Impact event|space impact]]s, not just a slow decline over time. The same thing probably happened on Earth and other nearby planets. [[Meteorite]]s found on Earth and information from Mars also support the idea that this was a Solar System-wide event with a huge number of collisions happening in a short period.<ref>{{Citation|last1=Claeys|first1=Philippe|title=Late Heavy Bombardment|date=2015|url=https://link.springer.com/rwe/10.1007/978-3-662-44185-5_869|encyclopedia=Encyclopedia of Astrobiology|pages=1365–1369|publisher=Springer, Berlin, Heidelberg|language=en|doi=10.1007/978-3-662-44185-5_869|isbn=978-3-662-44185-5|access-date=2025-07-05|last2=Morbidelli|first2=Alessandro}}</ref><ref name=":0">{{Cite journal|last1=Gomes|first1=R.|last2=Levison|first2=H. F.|last3=Tsiganis|first3=K.|last4=Morbidelli|first4=A.|date=2005|title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets|url=https://www.nature.com/articles/nature03676|journal=Nature|language=en|volume=435|issue=7041|pages=466–469|doi=10.1038/nature03676|pmid=15917802 |issn=1476-4687}}</ref>

Scientists are still trying to figure out exactly what caused the Late Heavy Bombardment (LHB), but one popular idea is called the [[Nice model]] (named after a city in France). This model suggests that the [[giant planet]]s, like [[Jupiter]] and [[Saturn]], did not always stay in the same [[orbit]]s. They might have moved. At some point in the early Solar System, Jupiter and Saturn may have crossed a special point in their orbits called a [[Orbital resonance|resonance]]. This happens when their orbits line up in a way that their gravitational pulls reinforce each other. In this case, it created a powerful gravitational effect that caused destabilized the orbits of many icy bodies in the [[Kuiper Belt]] and rocky objects in the asteroid belt. These objects were either flung out of the Solar System or sent crashing inward toward the inner planets, like [[Earth]], [[Mars]], and [[Venus]]. This sudden movement of so many objects led to the Late Heavy Bombardment, when the inner planets were hit by lots of [[asteroid]]s and [[comet]]s.<ref name=":0" /> These impacts helped shape the surfaces of the planets and may have even brought [[water]] and [[Organic compound|organic material]]s to Earth, ingredients that later made [[life]] possible.<ref>{{Cite web|last=bgray|date=2016-12-24|title=Late Heavy Bombardment may not have happened|url=https://geologyglasgow.org.uk/headlines/late-heavy-bombardment-may-not-have-happened/|access-date=2025-07-05|website=Geological Society of Glasgow|language=en-GB}}</ref>

One idea in [[planetary science]] is the [[Grand Tack hypothesis]]. This theory tries to explain some strange things about our Solar System by suggesting that Jupiter did not always stay where it is now. Instead, it says that Jupiter once moved inward, getting much closer to the [[Sun]], about 1.5 times the distance from Earth to the Sun, before turning around and moving back outward to where it is today. This big change in direction, or "tack" (like a [[sailboat]] changing course), may have happened because of [[Saturn]]. After Jupiter formed, Saturn also started forming, and the gravity between the two may have caused Jupiter to change direction.<ref>{{Cite web|title=Jupiter's Youthful Travels Redefined Solar System|url=https://www.nasa.gov/topics/solarsystem/features/young-jupiter.html|access-date=2025-07-05|website=www.nasa.gov|archive-date=2017-03-01|archive-url=https://web.archive.org/web/20170301170530/https://www.nasa.gov/topics/solarsystem/features/young-jupiter.html|url-status=dead}}</ref> As Jupiter moved inward, it would have pushed away or scattered a lot of [[dust]], [[rock]]s, and [[planetesimal]]s. This would have cleared out space in the Solar System and changed how the rocky planets, like Earth and Mars, formed. When Jupiter moved back outward, it may have pulled in a new mix of material, adding both rocky and icy objects into the [[asteroid belt]]. This helps explain why the asteroid belt today has a mix of different types of [[asteroid]]s, some rocky, some icy.<ref>{{Cite web|author1=Charles Q. Choi|date=2015-03-23|title=Jupiter's 'Smashing' Migration May Explain Our Oddball Solar System|url=https://www.space.com/28901-wandering-jupiter-oddball-solar-system.html|access-date=2025-07-05|website=Space|language=en}}</ref><ref>{{Cite web|title=How Did Jupiter Shape Our Solar System?|url=https://www.universetoday.com/articles/how-did-jupiter-shape-our-solar-system|access-date=2025-07-05|website=Universe Today|language=en}}</ref>

One big area scientists study is how the [[orbit]]s of [[planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, and [[comet]]s change over time. These changes happen slowly and are often caused by [[gravity]] pulling on objects in special ways. One important idea is called a [[Orbital resonance|resonance]]. This happens when two objects go around the Sun in a regular pattern, like one going around twice for every three times the other one goes around. These simple ratios (like 2:1 or 3:2) can keep orbits stable or sometimes make them unstable.<ref>{{Cite web|title=Orbital resonance|url=https://www.aanda.org/glossary/175-orbital-resonance|access-date=2025-07-05|website=www.aanda.org}}</ref><ref>{{Cite journal|last=Christopher|first=Lee|title=The effect of orbital resonance on the stability of a planetary system|url=https://www.the-iyrc.org/uploads/1/2/9/7/129787256/iyrc2021_14_final.pdf|journal=The International Young Researchers' Conference}}</ref> Resonances help explain why the asteroid belt looks the way it does, why certain moons are in just the right spots, and why icy objects beyond Neptune tend to cluster in certain areas.<ref>{{Cite web|title=Planetary orbits: Couplings and resonances {{!}} EBSCO Research Starters|url=https://www.ebsco.com/research-starters/history/planetary-orbits-couplings-and-resonances|access-date=2025-07-05|website=www.ebsco.com|language=en}}</ref><ref>{{Cite journal|last=Malhotra|first=Renu|date=2019-11-09|title=Resonant Kuiper belt objects: a review|journal=Geoscience Letters|volume=6|issue=1|article-number=12|doi=10.1186/s40562-019-0142-2|doi-access=free |issn=2196-4092|pmc=7067275|pmid=32215241}}</ref> Another idea is called [[secular variation]], which means slow, long-term changes in things like a planet’s orbit shape, tilt, or direction. These changes happen because planets gently pull on each other over millions of years.<ref>{{Cite web|title=Secular evolution of planetary orbits|url=https://farside.ph.utexas.edu/teaching/celestial/Celestial/node91.html#:~:text=78)-,for%20.,to%20the%20ecliptic%20plane,%20is|access-date=2025-07-05|website=farside.ph.utexas.edu}}</ref><ref>{{Citation|last=Rimrott|first=Fred P. J.|title=Secular Variations of the Orbital Elements|date=1989|url=https://doi.org/10.1007/978-3-322-90338-9_7|work=Introductory Orbit Dynamics|pages=101–106|editor-last=Rimrott|editor-first=Fred P. J.|place=Wiesbaden|publisher=Vieweg+Teubner Verlag|language=en|doi=10.1007/978-3-322-90338-9_7|isbn=978-3-322-90338-9|access-date=2025-07-05}}</ref> For example, Earth’s orbit and tilt slowly shift because of the gravity from Jupiter and Saturn. These changes affect Earth’s climate over a long time, in something called [[Milankovitch cycles]].<ref>{{Cite web|date=2020-02-27|title=Milankovitch (Orbital) Cycles and Their Role in Earth's Climate - NASA Science|url=https://science.nasa.gov/science-research/earth-science/milankovitch-orbital-cycles-and-their-role-in-earths-climate/|access-date=2025-07-05|language=en-US}}</ref> Sometimes, gravity can even throw objects off course or capture them. This is called [[gravitational scattering]].<ref>{{Cite web|title=Gravitational Dynamics {{!}} Center for Astrophysics {{!}} Harvard & Smithsonian|url=https://www.cfa.harvard.edu/research/topic/gravitational-dynamics|access-date=2025-07-05|website=www.cfa.harvard.edu}}</ref> For example, Neptune’s moon [[Triton (moon)|Triton]] orbits backward, which suggests it did not form around Neptune but was a Kuiper Belt object that got caught by Neptune’s gravity.<ref>{{Cite journal|last1=Farinella|first1=P.|last2=Milani|first2=A.|last3=Nobili|first3=A.M.|last4=Valsecchi|first4=G.B.|date=1980|title=Some remarks on the capture of Triton and the origin of Pluto|url=https://linkinghub.elsevier.com/retrieve/pii/0019103580901487|journal=Icarus|language=en|volume=44|issue=3|pages=810–812|doi=10.1016/0019-1035(80)90148-7}}</ref><ref>{{Cite web|date=2017-11-21|title=Triton - NASA Science|url=https://science.nasa.gov/neptune/moons/triton/|access-date=2025-07-05|language=en-US}}</ref> Mars’s tiny moons, [[Phobos]] and [[Deimos]], may also be captured asteroids because of their strange shapes and materials.<ref>{{Cite web|date=2017-11-11|title=Mars Moons - NASA Science|url=https://science.nasa.gov/mars/moons/|access-date=2025-07-05|language=en-US}}</ref>

Another important force that helps shape the Solar System is tidal interaction, especially between planets and their moons. [[Tidal force]]s happen because of gravity. Planets pull on their moons, and moons pull back on their planets. This tugging creates bulges, which causes [[energy]] to be lost and leads to slow changes in [[orbit]]s and [[rotation]].<ref>{{Cite web|last=Plait|first=Phil|title=Tides Move Heaven and Earth More Than You Know|url=https://www.scientificamerican.com/article/how-do-tides-shape-earth-and-the-solar-system/|access-date=2025-07-05|website=Scientific American|language=en}}</ref><ref>{{Cite web|date=2016-10-18|title=13.7: Tidal Forces|url=https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/13%3A_Gravitation/13.07%3A_Tidal_Forces|access-date=2025-07-05|website=Physics LibreTexts|language=en}}</ref> One famous example is the [[Earth]] and [[Moon]]. Because of tidal forces, the Moon always shows the same side to Earth. This is called being [[tidally locked]].<ref>{{Cite web|date=2023-06-06|title=Tidal Locking - NASA Science|url=https://science.nasa.gov/moon/tidal-locking/|access-date=2025-07-05|language=en-US}}</ref> Also, Earth’s rotation is slowly slowing down, and the Moon is very slowly moving farther away from us.<ref>{{Cite book|title=Time: from earth rotation to atomic physics|last1=McCarthy|first1=Dennis D.|last2=Seidelmann|first2=P. Kenneth|date=2009|publisher=Wiley-VCH|isbn=978-3-527-62795-0|location=Weinheim}}</ref><ref>{{Cite web|date=2017-11-15|title=Moon Facts - NASA Science|url=https://science.nasa.gov/moon/facts/|access-date=2025-07-05|language=en-US}}</ref> Tidal forces can also create heat inside moons, especially those that orbit big planets like Jupiter and Saturn. For example, [[Europa (moon)|Europa]] (a moon of Jupiter) and [[Enceladus (moon)|Enceladus]] (a moon of Saturn) are squeezed and stretched as they move around their planets.<ref>{{Cite web|title=Europa Tidal Heating|url=http://astronomy.nmsu.edu/candaceg/Europa/Tidal_Heating.html|access-date=2025-07-05|website=astronomy.nmsu.edu}}</ref><ref>{{Cite journal|last1=Meyer|first1=Jennifer|last2=Wisdom|first2=Jack|date=2007|title=Tidal heating in Enceladus|url=https://linkinghub.elsevier.com/retrieve/pii/S0019103507001054|journal=Icarus|language=en|volume=188|issue=2|pages=535–539|doi=10.1016/j.icarus.2007.03.001}}</ref> This squeezing, called [[Tidal heating|tidal flexing]], creates heat inside them.<ref>{{Citation|last=Lunine|first=Jonathan I.|title=Tidal heating|date=1997|url=https://link.springer.com/rwe/10.1007/1-4020-4520-4_408|encyclopedia=Encyclopedia of Planetary Science|series=Encyclopedia of Earth Science |pages=828|publisher=Springer, Dordrecht|language=en|doi=10.1007/1-4020-4520-4_408|isbn=978-1-4020-4520-2|access-date=2025-07-05}}</ref><ref>{{Cite web|title=tidal heating|url=https://evolution.berkeley.edu/glossary/tidal-heating/|access-date=2025-07-05|website=evolution.berkeley.edu}}</ref> That heat might keep liquid [[ocean]]s under their icy surfaces.<ref>{{Cite web|last=Owen|first=Rebecca|date=2024-01-10|title=Scientists Investigate How Heat Rises Through Europa's Ocean|url=https://eos.org/research-spotlights/scientists-investigate-how-heat-rises-through-europas-ocean|access-date=2025-07-05|website=Eos|language=en-US}}</ref> These hidden oceans could have [[geyser]]s or even [[cryovolcanoe]]s (volcanoes that erupt [[ice]] instead of [[lava]]).<ref>{{Cite web|date=2020-08-04|title=Cryovolcanism in the Solar System|url=https://www.skyatnightmagazine.com/space-science/cryovolcanism-in-the-solar-system|access-date=2025-07-05|website=BBC Sky at Night Magazine|language=en}}</ref> Because of this, Europa and Enceladus are interesting places to look for life beyond Earth.<ref>{{Cite web|last=Velasco|first=Rebecca Boyle, Juan|date=2023-05-01|title=The Six Moons Most Likely to Host Life in Our Solar System|url=https://www.scientificamerican.com/article/the-six-moons-most-likely-to-host-life-in-our-solar-system/|access-date=2025-07-05|website=Scientific American|language=en}}</ref>

===The Present-day Conditions of the Solar System===
[[File:Solar System graphic by NASA (wide).png|thumb|The Solar System]]
At the very center of the Solar System is the [[Sun]], which is a middle-aged [[star]] called a [[G-type main-sequence star]]. The Sun is very massive, it holds more than 99.8% of all the [[mass]] in the Solar System. The Sun gives off [[light]] and [[heat]], which make [[life]] on [[Earth]] possible. It also creates the [[gravity]] that keeps all the [[planet]]s, [[Natural satellite|moon]]s, [[asteroid]]s, and [[comet]]s moving in their [[orbit]]s.<ref name=":10" /> Even though the Sun looks calm in the sky, it is actually very active. It gives off [[radiation]] and sends out a stream of charged particles called the [[solar wind]].<ref name=":11" /> This wind stretches far out into space and helps create the edge of a giant bubble called the [[heliosphere]], which surrounds the entire Solar System.<ref name=":7" />

Surrounding the Sun are the eight major planets, which are split into two main groups. The first group is the terrestrial planets, [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]]. These are small, [[Terrestrial planet|rocky planet]]s made mostly of [[rock]] and [[metal]].<ref name=":1" /> The second group includes the [[Gas giant|gas]] and [[ice giant]]s, [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]. These planets are much bigger and made mostly of gases like [[hydrogen]] and [[helium]], and ices such as [[water]], [[ammonia]], and [[methane]].<ref name=":2" /><ref name=":3" /> All the planets move in oval-shaped orbits (called [[elliptical orbit]]s) around the Sun, and they mostly stay in the same flat area called the [[Ecliptic|ecliptic plane]].<ref name=":4" /><ref name=":5" /> Most of them spin in the same direction as the Sun, but there are a couple of exceptions. Venus spins backward, and Uranus spins on its side.<ref>{{Cite web|last=Franzen|first=Harald|title=Why Venus Spins the Wrong Way|url=https://www.scientificamerican.com/article/why-venus-spins-the-wrong/|access-date=2025-07-05|website=Scientific American|language=en}}</ref><ref>{{Cite web|date=2017-11-10|title=Uranus: Facts - NASA Science|url=https://science.nasa.gov/uranus/facts/|access-date=2025-07-05|language=en-US}}</ref> Besides the planets, the Solar System also has lots of smaller objects. Between Mars and Jupiter is the [[asteroid belt]], which contains millions of space rocks. Some as small as dust, and others as big as the [[dwarf planet]] [[Ceres]].<ref name=":19">{{Cite web|date=2025-05-14|title=Asteroid Belt: what is it and how did it form?|url=https://www.skyatnightmagazine.com/space-science/asteroid-belt-facts-formation|access-date=2025-07-05|website=BBC Sky at Night Magazine|language=en}}</ref> Even farther out is the Kuiper Belt, which is full of icy objects like [[Pluto]], [[Haumea (dwarf planet)|Haumea]], and [[Makemake (dwarf planet)|Makemake]]. These are called [[Trans-Neptunian object|trans-Neptunian objects (TNOs)]], and they are leftovers from the Solar System's early days. They never became [[planet]]s because the gravity from the big planets, like Jupiter, kept pulling things around and disturbing them.<ref>{{Cite web|title=Trans-Neptunian object {{!}} EBSCO Research Starters|url=https://www.ebsco.com/research-starters/astronomy-and-astrophysics/trans-neptunian-object|access-date=2025-07-05|website=www.ebsco.com|language=en}}</ref> Way beyond that, scientists think there is something called the [[Oort cloud|Oort Cloud]]. It is a giant, round shell of icy objects that surrounds the Solar System. Scientists have not seen it directly, but they think it is there because it is likely the home of long-period comets. The ones that take hundreds or thousands of years to orbit the Sun. The Oort Cloud is believed to be the very edge of the Sun’s gravitational reach.<ref name=":6" />

The Solar System is always changing. It is not just made of planets and moons, there are lots of smaller objects moving around too, like [[comet]]s, [[meteoroid]]s, and [[Centaur (planetoid)|centaur]]s. Centaurs are unusual because they have features of both asteroids and comets.<ref name=":12">{{Cite web|last=Metcalfe|first=Tom|date=2021-10-07|title=Our solar system's centaurs are half-asteroid, half-comet|url=https://www.astronomy.com/science/our-solar-systems-centaurs-are-half-asteroid-half-comet/|access-date=2025-07-05|website=Astronomy Magazine|language=en-US}}</ref> Sometimes, comets from faraway places like the Kuiper Belt or the Oort Cloud travel into the inner part of the Solar System. As they get closer to the Sun, the heat causes their ice to melt, creating bright tails of gas and dust. These tails can be really beautiful and are often visible from [[Earth]].<ref name=":8" /> [[Meteoroid]]s are small rocks that sometimes enter Earth’s atmosphere. When they burn up in the sky, we see them as [[meteor]]s or "shooting stars." If a bigger piece makes it all the way to the ground, it is called a [[meteorite]].<ref name=":9" /> While big impacts are rare, scientists still keep an eye on [[Near-Earth object|near-Earth objects (NEOs)]] just in case.<ref>{{Cite web|title=Near-Earth Object Program - NASA|url=https://www.nasa.gov/stem-content/near-earth-object-program/|access-date=2025-07-05|language=en-US}}</ref> Centaurs are icy objects with unstable orbits between Jupiter and Neptune.<ref name=":12" /> They might one day become short-period comets that visit the inner Solar System more often.<ref>{{Citation|last1=Peixinho|first1=Nuno|title=Chapter 14 - From Centaurs to comets: 40 Years|date=2020-01-01|url=https://www.sciencedirect.com/science/article/pii/B978012816490700014X|work=The Trans-Neptunian Solar System|pages=307–329|editor-last=Prialnik|editor-first=Dina|publisher=Elsevier|doi=10.1016/b978-0-12-816490-7.00014-x|isbn=978-0-12-816490-7|access-date=2025-07-05|last2=Thirouin|first2=Audrey|last3=Tegler|first3=Stephen C.|last4=Di Sisto|first4=Romina P.|last5=Delsanti|first5=Audrey|last6=Guilbert-Lepoutre|first6=Aurélie|last7=Bauer|first7=James G.|arxiv=1905.08892 |editor2-last=Barucci|editor2-first=M. Antonietta|editor3-last=Young|editor3-first=Leslie A.}}</ref> In recent years, astronomers even discovered objects from outside our Solar System, like ‘[[Oumuamua]] and Comet [[2I/Borisov]]. These interstellar visitors came from far beyond the Sun’s reach, giving us clues about how other star systems might form.<ref>{{Cite web|date=2017-11-29|title='Oumuamua - NASA Science|url=https://science.nasa.gov/solar-system/comets/oumuamua/|access-date=2025-07-05|language=en-US}}</ref><ref>{{Cite web|date=2020-04-17|title=Comet 2I/Borisov - NASA Science|url=https://science.nasa.gov/solar-system/comets/2i-borisov/|access-date=2025-07-05|language=en-US}}</ref>

There are over 800 known [[Natural satellite|moon]]s in our Solar System, and they come in all shapes and sizes.<ref>{{Cite web|date=2023-05-31|title=Moons|url=https://science.nasa.gov/solar-system/moons/|access-date=2025-07-05|language=en-US}}</ref> Some moons are fiery and active, like [[Io (moon)|Io]], which has [[volcano]]es constantly erupting.<ref>{{Cite web|date=2017-11-17|title=Io|url=https://science.nasa.gov/jupiter/jupiter-moons/io/|access-date=2025-07-05|language=en-US}}</ref> Others, like [[Europa (moon)|Europa]] and [[Enceladus (moon)|Enceladus]], are covered in ice but may have liquid [[ocean]]s hidden underneath.<ref>{{Cite web|last=US Department of Commerce|first=National Oceanic and Atmospheric Administration|title=Are there oceans on other planets?|url=https://oceanservice.noaa.gov/facts/et-oceans.html|access-date=2025-07-05|website=oceanservice.noaa.gov|language=EN-US}}</ref> These icy moons are interesting because they might even have the right conditions for [[life]].<ref>{{Cite web|last=Barbuzano|first=Javier|date=2023-08-14|title=Subsurface Oceans Could Boost Exoplanet Habitability|url=https://eos.org/articles/subsurface-oceans-could-boost-exoplanet-habitability|access-date=2025-07-05|website=Eos|language=en-US}}</ref> Some planets, like Saturn and Neptune, have lots of moons, creating huge moon systems that scientists are still exploring and learning about.<ref>{{Cite web|date=2025-03-17|title=Which planet has the most moons?|url=https://www.skyatnightmagazine.com/space-science/which-planet-has-most-moons|access-date=2025-07-05|website=BBC Sky at Night Magazine|language=en}}</ref> Besides natural moons, the Solar System also has many [[spacecraft]] and [[Satellite (artificial)|satellite]]s that humans have sent into space to learn more about our Solar System. Some of these orbit Earth, while others go much farther.<ref>{{Cite web|date=2025-06-18|title=Space exploration - Solar System, Probes, Missions {{!}} Britannica|url=https://www.britannica.com/science/space-exploration/Solar-system-exploration|access-date=2025-07-05|website=www.britannica.com|language=en}}</ref> For example, [[Voyager 1]] has traveled so far that it has entered interstellar space, beyond the edge of the Solar System.<ref>{{Cite web|date=2024-03-10|title=Where Are Voyager 1 and 2 Now? - NASA Science|url=https://science.nasa.gov/mission/voyager/where-are-voyager-1-and-voyager-2-now/|access-date=2025-07-05|language=en-US}}</ref> Other missions like [[Juno (spacecraft)|Juno]] (orbiting Jupiter), [[Perseverance (rover)|Perseverance]] (exploring Mars), the [[James Webb Space Telescope]], and [[New Horizons]] (which flew past Pluto) are helping us study planets, moons, and stars in detail.<ref>{{Cite web|date=2023-06-15|title=Juno - NASA Science|url=https://science.nasa.gov/mission/juno/|access-date=2025-07-05|language=en-US}}</ref><ref>{{Cite web|date=2017-12-05|title=Mars 2020: Perseverance Rover - NASA Science|url=https://science.nasa.gov/mission/mars-2020-perseverance/|access-date=2025-07-05|language=en-US}}</ref><ref>{{Cite web|title=Mission Timeline|url=https://webbtelescope.org/home/news/milestones/mission-timeline|access-date=2025-07-05|website=Webb|language=en}}</ref><ref>{{Cite web|date=2017-12-05|title=New Horizons|url=https://science.nasa.gov/mission/new-horizons/|access-date=2025-07-05|language=en-US}}</ref>

Over billions of years, the planets in our Solar System have settled into stable orbits. This means that even though their paths can slowly change, they mostly stay the same over time. These changes happen very slowly and are caused by [[gravity]] pulling on the planets from other planets and the [[Sun]]. The way the planets are arranged now is called the [[Epoch (astronomy)|current epoch]], and it is expected to stay stable for billions more years. The [[orbit]]s might change a little in shape or tilt, but not in a way that causes big problems. This stability is really important because it means we can predict how [[planet]]s move far into the future. It also helps keep [[Earth]] safe and habitable, making [[life]] possible for a long time.<ref>{{Citation|last=Laskar|first=Jacques|title=Solar system: Stability|date=1997|url=https://link.springer.com/rwe/10.1007/1-4020-4520-4_382|encyclopedia=Encyclopedia of Planetary Science|series=Encyclopedia of Earth Science |pages=762–766|publisher=Springer, Dordrecht|language=en|doi=10.1007/1-4020-4520-4_382|isbn=978-1-4020-4520-2|access-date=2025-07-05}}</ref>

===Future of the Solar System===
[[File:Sun red giant.svg|thumb|The current Sun compared to its peak size in the red-giant phase]]
The future of the Solar System depends a lot on what happens to the [[Sun]], since it is the [[star]] at the center and controls everything with its [[gravity]] and [[energy]]. Right now, the Sun is in the main part of its life, called the main sequence. During this time, it is turning [[hydrogen]] into [[helium]] through a process called [[nuclear fusion]], which creates the [[light]] and [[heat]] [[Earth]] gets. But this would not last forever. In about 5 billion years, the Sun will start to run out of hydrogen fuel. When that happens, the core will shrink, and the outer layers will puff up, making the Sun grow much bigger. This next stage is called the red giant phase. As a [[red giant]], the Sun will become so large that it might swallow up [[Mercury (planet)|Mercury]] and [[Venus]], and it could even reach [[Earth]], depending on how much it expands.<ref name=":13">{{Cite journal|last1=SchrÃ¶der|first1=K.-P.|last2=Connon Smith|first2=Robert|date=2008-05-01|title=Distant future of the Sun and Earth revisited|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=386|issue=1|pages=155–163|doi=10.1111/j.1365-2966.2008.13022.x |doi-access=free }}</ref>

When the Sun becomes a red giant, it will also start to lose a lot of its [[mass]]. This happens because strong [[solar wind]]s will blow material off the Sun and into space. As the Sun gets lighter, its gravity will weaken, and the planets that survive will slowly move into wider [[orbit]]s. Even if Earth is not swallowed by the Sun, it will still be in big trouble. The intense [[heat]] and [[radiation]] will likely destroy [[Atmosphere of Earth|Earth's atmosphere]] and [[ocean]]s, making it impossible for [[life]] to survive. Earth will become dry and lifeless. The inner Solar System will turn into a very dangerous and hostile place. The outer planets, like [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]], will probably survive, but their [[Natural satellite|moon]]s and [[Ring system|ring]]s might change.<ref name=":14">{{Cite web|date=2018-12-11|title=Chapter 6: Aging Into Gianthood - NASA Science|url=https://science.nasa.gov/exoplanets/resources/life-and-death/chapter-6/|access-date=2025-07-07|language=en-US}}</ref> At some point, the core of the Sun will get hot enough to start burning helium. This process is called [[Triple-alpha process|helium fusion]]. However, the Sun will only be able to burn helium for a short time compared to how long it burned hydrogen. The Sun is not big enough or heavy enough to fuse heavier elements like [[carbon]] or [[oxygen]]. So after the helium runs out, the [[nuclear reaction]]s in the core will stop. Without these reactions, the Sun would not be able to hold up its outer layers.<ref name=":14" />

After the Sun finishes its red giant phase, it will shed its outer layers and leave behind a small but very hot core called a white dwarf. A [[white dwarf]] is about the size of Earth, but it still holds about half the mass of the original Sun. Even though it would not make energy anymore, it will still be very hot and dense.<ref name=":13" /> When the Sun's outer layers float away into space, they might create a cloud of glowing gas called a [[planetary nebula]]. This is a bright shell of gas that shines in space for a short time. These outer layers are made of material that once belonged to the Sun, but now they contain heavier elements like [[carbon]] that were made inside the Sun over its lifetime. This gas will go back into the [[interstellar medium]], the space between stars, where it can help form new [[star]]s and [[planet]]s one day.<ref>{{Cite web|title=Planetary Nebulas {{!}} Center for Astrophysics {{!}} Harvard & Smithsonian|url=https://www.cfa.harvard.edu/research/topic/planetary-nebulas|access-date=2025-07-07|website=www.cfa.harvard.edu}}</ref><ref>{{Cite journal|last1=Gesicki|first1=K.|last2=Zijlstra|first2=A. A.|last3=Miller Bertolami|first3=M. M.|date=2018-05-07|title=The mysterious age invariance of the planetary nebula luminosity function bright cut-off|url=https://www.nature.com/articles/s41550-018-0453-9|journal=Nature Astronomy|language=en|volume=2|issue=7|pages=580–584|doi=10.1038/s41550-018-0453-9|hdl=11336/82487 |issn=2397-3366}}</ref>

Over time, the white dwarf will cool down slowly, taking trillions of years to lose its [[heat]]. It would not shine like the Sun does now, but it will still have gravity strong enough to keep the [[outer planet]]s and moons orbiting around it. However, over a very long time, the orbits of those planets might change slightly because there will be less [[gravity]] and other [[force]]s acting on them. Over an even longer period of time, tens or even hundreds of billions of years, the Solar System may start to break apart. Tiny changes, like the pull from passing [[star]]s or forces from the [[galaxy]], can slowly change the orbits of [[planet]]s and other space objects. Some planets or moons might get knocked out of the Solar System and drift off into deep space, becoming free-floating objects. Others might spiral inward and crash into the [[white dwarf]] that used to be the [[Sun]]. Eventually, the Solar System could completely fall apart, with its planets and moons scattered across the galaxy. This slow breakup is a natural part of how gravity works over a long time, and it would be the end of the Solar System.<ref>{{Cite journal|last=Dyson|first=Freeman J.|date=1979-07-01|title=Time without end: Physics and biology in an open universe|url=https://link.aps.org/doi/10.1103/RevModPhys.51.447|journal=Reviews of Modern Physics|language=en|volume=51|issue=3|pages=447–460|doi=10.1103/RevModPhys.51.447|issn=0034-6861}}</ref>