An enormous molecular cloud collapsed 4.6 billion years ago, giving rise to the Solar System. All but a small portion of the collapsing mass was concentrated in a central region, forming the Sun, while the rest stretched out into a protoplanetary disk.
Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace first proposed the nebular hypothesis in the early 1800s. Astronomy, geology, chemistry, and planetary science have been woven into subsequent development. New observations have challenged and refined the model since its inception in the 1950s and 1990s discoveries of extrasolar planets.
Throughout its history, the Solar System has undergone significant change. Many moons are thought to have formed around their parent planets in a circling disk of gas and dust, while others are thought to have started on their own and were later captured by their parent planets. Others may result from massive collisions, such as the Moon, orbits the Earth. Collisions between planets and other celestial bodies have been a constant feature of the Solar System's history, and gravitational interactions may have caused the planets to move. Much of the solar system's early evolution is now attributed to this planetary migration.
About 5 billion years from today, the Sun will cool and expand to many times its current diameter (becoming a red giant) before shedding its outer layers and forming what is known as the planetary nebula. A stellar remnant called a white dwarf in its place. The gravity of passing stars or star systems will gradually reduce the number of planets orbiting the Sun in the future. Some planets will be annihilated, while others will be expelled into interstellar space. All original bodies that once orbited the Sun are expected to be extinct within a few billion years.
For nearly all that time, no attempt was made to link such theories to the existence of a "Solar System," simply because the Solar System, as we now understand it, was not generally believed to exist. However, theories about the origin and future of the world date back to the earliest known writings. Initially, heliocentrism was widely accepted, which stated that the Sun was the center of our solar system and that the Earth revolved around it. At the end of the 17th century, this idea had been around millennia (Aristarchus of Samos suggested as far back as 250 BC). The term "Solar System" was first used in a written document in 1704.
Since the nebular hypothesis was first proposed by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace in the 18th century, it has fluctuated in popularity. The hypothesis was heavily criticized for failing to explain why the Sun has so little angular momentum compared to the other planets. This hypothesis has recently been re-accepted because studies of young stars have revealed that they are encased in cool dust and gas discs, just as the nebular hypothesis predicts.
Understanding the Sun's power source is essential to predicting how it will evolve in the future. With the confirmation of Albert Einstein's theory of relativity by Arthur Stanley Eddington, he understood that the Sun's energy comes from the fusion of hydrogen into helium in its core during nuclear fusion reactions. In 1935, Eddington proposed that stars could produce more elements besides hydrogen and helium. Based on this premise, Fred Hoyle posited that the cores of evolved stars known as red giants produced numerous elements heavier than hydrogen and helium. A red giant's outer layers would then be recycled to form new star systems when the red giant finally sheds its outermost layers.
According to the nebular hypothesis, the Solar System was formed by the gravitational collapse of a fragment of a massive molecular cloud. About 20 parsecs (65 light-years) across, the cloud had a diameter of about 1 parsec (three and a half light-years). Fragments collided further, forming dense cores measuring 0.01–0.1 parsec (2,000–20,000 AU). The pre-solar nebula, which formed the Solar System, was one of these collapsing fragments. The hydrogen, helium, and traces of lithium produced by Big Bang nucleosynthesis made up about 98 percent of this region's mass, slightly higher than the Sun's (M). Nucleosynthesis in earlier generations of stars produced the remaining 2% of the mass, composed of heavier elements. Heavy elements were ejected into the interstellar medium late in the lives of these stars.
Astronomers have captured an image of the Orion Nebula, a "stellar nursery" about the size of the Milky Way from which the Sun likely formed, using the Hubble Space Telescope's Wide Field Camera 3. Meteorite inclusions 4,568.2 million years old are one definition of how long it has taken for solid matter to form in the pre-Solar System nebula. Researchers have found evidence of stable daughter nuclei of short-lived isotopes like iron-60, which can only be created in exploding, short-lived stars. This suggests the presence of a nearby supernova or multiple supernovae.
By creating dense regions within the cloud, a supernova may have led to the formation of the Sun, and these regions then collapsed. There is no other explanation for the Sun's formation than in a massive star-forming region, perhaps similar to the Orion Nebula. A cluster of 1,000 to 10,000 stars with diameters from 6.5 to 19.5 light-years and a combined mass of 3,000 M is the most likely scenario for the formation of the Sun. This cluster began to disintegrate between 135 million and 535 million years after its formation. Many simulations of our Sun's early interactions with nearby stars over the first 100 million years have produced irregular orbits in the outer Solar System, such as those of detached objects.
Angular momentum conservation led to the nebula spinning faster as it collapsed. The atoms in the nebula collided more frequently as the material inside condensed, converting their kinetic energy into heat. The center of the disc became increasingly hotter than the surrounding disc as the mass accumulated there. Gravity, gas pressure, and magnetic fields worked against each other for about 100,000 years to flatten and flatten until they formed an approximately 200-AU-wide protoplanetary disc, which contained a hot, dense protostar at its core.
We believe the Sun was a T Tauri star at this point. A disc of preplanetary matter 0.001–0.1 M has been observed around several T Tauri stars. The Hubble Space Telescope has observed protoplanetary discs of up to 1000 AU in diameter in star-forming regions such as the Orion Nebula. At their hottest, these discs reach a surface temperature of only about 1,000 K (730 °C; 1,340 °F). Solar hydrogen began fusing within 50 million years, creating an internal energy source to counteract gravitational contraction until that hydrogen attained hydrostatic equilibrium. During this period, known as the main sequence, the Sun entered into its prime phase of existence. Hydrogen/helium fusion in the cores of main-sequence stars provides the star's energy. Today, the Sun is still a primary star. Solar System drifted away from its siblings in the stellar nursery as it continued to evolve, eventually continuing its journey around the Milky Way's center on its own.
The solar nebula, a disc-shaped cloud of gas and dust left remaining after the formation of the Sun, is thought to have produced the various planets. Astronomers believe the planets formed through accretion, where dust grains circling a protostar formed into planets. Through direct contact and self-organization, these dust particles formed larger bodies (planetesimals) up to 10 kilometers (6.2 miles) across, colliding to merge and form larger clumps. Over the next few million years, these grew at a centimeter-per-year rate through further collisions.
Metals like iron and nickel, as well as rocky silicates, could only form planetesimals in the inner Solar System because temperatures were too high for volatile molecules like water and methane to condense. These conditions would form terrestrial planets from these rocky bodies (Mercury, Venus, Earth, and Mars). Only 0.6 percent of the nebula's mass comprises these compounds, meaning terrestrial planets can't grow very large. 0.05 Earth masses (MEarth) were reached by the terrestrial embryos, which stopped accumulating matter around 100,000 years following the formation of the Sun, allowing the terrestrial planets to grow to their current sizes through collisions and mergers (see Terrestrial planets below).
A disk of gas and dust surrounded the planets as they formed. Because the gas's orbit around the Sun was slowed down by pressure, the planets could keep up with it. Angular momentum was transferred due to the drag and, more importantly, gravitational interactions between the planets and their surroundings. Based on mathematical simulations, the disk's density and temperature variations governed this migration rate; however, as the disk dissipated, a net trend was to move the inner planets inward and leave them in their current orbits.
A little further out, past the frost line (the point between Mars's orbit and Jupiter's), the gas giant planets (Jupiter, Saturn, Uranus, and Neptune) formed from the material that was too cold for volatile icy compounds to solidify. While metals and silicates predominate on terrestrial worlds, ices, which formed the Jovian planets, were more abundant, allowing the giant worlds to grow massive enough to capture the lightest and most abundant elements, hydrogen and helium. Within 3 million years, planetesimals beyond the frost line had accumulated up to 4 MEarths. Nearly all of the solar system's mass is concentrated in the four largest planets.
According to some scientists, Jupiter's location near the arctic equator is no coincidence. Frost lines accumulate large amounts of water via evaporation of infalling icy material, creating a lower-pressure region that increases the speed of orbiting dust particles and halts their movement toward the Sun. The frost line effectively served as a barrier at a distance of 5 AU from the Sun, causing the material to rapidly accumulate. An embryo (or core) on the order of 10 MEarth was formed from this excess material, which began accumulating an envelope via the accretion of gas from the surrounding disc at an ever-increasing rate. It took about 105 years for growth to reach about 150 Earth masses and finally peak at 318 MEarth once the envelope mass was roughly equal to the solid core mass. Because Saturn formed a few million years after Jupiter, it may have a lower mass when gas was scarcer.
There are far more T Tauri stars like the young Sun than there are older stars with more stable winds. Jupiter and Saturn are thought to have condensed first, with the solar wind removing a large amount of material from the disc before Uranus and Neptune could form. As a result, only about 1 MEarth of hydrogen and helium was accumulated on those planets. It is common to refer to Uranus and Neptune as "failed cores."
Timescale is a significant issue with planet formation theories for these objects in the solar system. At their current locations, their cores would have taken millions of years to form. This suggests that Uranus and Neptune may have formed closer to the Sun, near or even between Jupiter and Saturn, and then migrated or ejected outward later. (see Planetary migration below). Stardust samples from Comet Wild 2 suggest that materials from early Solar System formation may have migrated from the cooler inner Solar System to the Kuiper belt region during planetesimal era motion.
By the time the young Sun's solar wind had blown away the protoplanetary disc's gas and dust after three to ten million years, the planets' growth would have ended.
Initially, scientists assumed that the planets formed in or near the areas where they are currently orbiting. Throughout the last two decades, this has been questioned. A growing number of planetary scientists believe that the Solar System may have looked very different after its initial formation, with several objects at least as massive as Mercury existing in its inner system, the outer Solar System being much more compact than it is now, and the Kuiper belt being much closer to the Sun.
The theory goes like this:
A total of 50–100 Moon- to Mars-sized planetary embryos lived in the inner Solar System at the end of the planetary formation period. Further expansion was only made possible by the collision and merger of these bodies, which took less than 100 million years to complete. The four terrestrial planets we see today would have formed due to gravitational interactions between these various bodies, pulling on each other's orbits until they collided. One such giant collision is thought to have formed the Earth's Moon (see moons below), while another removed the outer shell of the young Mercury.
Even though colliding proto-terrestrial planets would have required highly eccentric orbits to occur, this model cannot explain how the remarkably stable and nearly circular orbits that exist today were formed. According to one theory, this "eccentricity dumping" can be attributed to the Earth's formation within a disc of gas that the Sun has not yet expelled. The planets' orbits would have been smoothed out by the "gravitational drag" of this residual gas. However, if such a gas existed, it would have kept the orbits of the terrestrial planets from becoming so eccentric.
Alternatively, it is possible that the gravitational drag occurred between the planets and the remaining small bodies rather than the remaining gas. Attraction by the larger planets' gravity caused smaller objects to form a "gravitational wake" in the larger object's path. The increased gravity of the wake caused the larger objects to slow down and settle into more regular orbits as they moved in this direction.
Between 2 and 4 AU from our Sun, there is an area of space referred to as the "asteroid belt." Many planetesimals were formed in the asteroid belt at the beginning of the solar system, and they had enough matter to form 2–3 Earth-like planets. Astronomers have long hypothesized that there were 20–30 Moon-sized planetesimals in this region. Still, the proximity of Jupiter meant that this region had a completely different evolutionary history after this planet formed 3 million years ago. Due to gravitational interactions with larger embryos in the asteroid belt, resonances with Jupiter and Saturn are particularly strong in the asteroid belt. Jupiter's gravity increased the speed of objects within these resonances, causing them to shatter rather than accrete when they collided with other bodies.
Asteroid belt resonances would have swept across the system as Jupiter moved inward following its formation (see Planetary migration below), energizing the population and increasing their relative velocities. As a result of the resonances and embryos, either the planetesimals were expelled from the asteroid belt, or they were exaggerated in their orbital inclinations and eccentricities. In addition to being ejected by Jupiter, some of those massive embryos may have migrated to the inner Solar System and taken part in the final accretion of the terrestrial planets.
When these giant planets and planetary embryos were in their early stages of development, their impact on the asteroid belt reduced it to less than one percent of the Earth's mass, composed mainly of small planetesimals. It's still 10–20 times greater than the current mass of the main belt, which is now about 0.0005 MEarth (megaton Earth equivalent). When Jupiter and Saturn engaged in a temporary 2 to 1 orbital resonance, scientists believe that the asteroid belt went through a second depletion period, reducing its mass to near-present levels (see below).
Significant impacts in the inner Solar System are likely to have contributed to the Earth's current water content (61021 kg) obtained from the earliest asteroid belt. Because water is too volatile to have been present during Earth's formation, it had to have been transported here from further out in the Solar System, where it was colder. Jupiter is thought to have thrown small planetesimals and embryonic planets out of the asteroid belt to deliver the water. The 2006 discovery of a population of main-belt comets has also been suggested as a possible source of water for the Earth. In contrast, only about 6% of Earth's water was delivered by comets from the Kuiper belt or farther regions. Some believe life may have arrived on Earth via this process, a theory known as panspermia.
According to the nebular hypothesis of planetary formation, the outer two planets may be in the "wrong place," according to the nebular hypothesis. The "ice giants" (Uranus and Neptune) are located in a region of the solar nebula where their formation is highly improbable due to the lower density and longer orbital periods. Instead, the two are thought to have formed near Jupiter and Saturn (known as "gas giants"), where more material was available and migrated outward to their current positions over hundreds of years.
There can be no extraterrestrial regions of the solar system without the planets migrating outward. When you get past Neptune, the Solar System continues into the scattered disc and the Oort cloud; these are three clusters of small icy bodies thought to be the origin of most observed comets. For planets to form, accretion had to be too slow at their distance from the Sun, which meant that their initial disc lacked enough mass density to consolidate into a planet.
The Kuiper belt is found at a range of 30 to 55 AU from the Sun, the scattered disc extends to more than 100 AU, and the Oort cloud begins at about 50,000 AU. However, the Kuiper belt was originally much denser and closer to the Sun, with an outer edge approximately 30 AU away from the Sun. During their formation, Uranus and Neptune were much closer to the Sun than they are now (15–20 AU is a reasonable estimate). As a result, the two planets' orbits ended up in opposite places, with Uranus farther from the Sun than Neptune in 50% of simulations.
This model predicts that as many planetesimals as were still present after the Solar System was formed influenced the orbits of all giant planets. When Jupiter and Saturn were about 4 billion years old, they formed a 2 to 1 resonance: Saturn orbited the Sun one time for every two Jupiter orbits. As a result of this gravitational pull, Neptune may have pushed past Uranus and entered the ancient Kuiper belt. Most of the small icy bodies were scattered inwards by the planets as they moved outwards. After that, these planetesimals similarly dispersed to other celestial objects, causing the orbits of those worlds to shift outwards as they moved in.
This continued until the planetesimals came into contact with Jupiter, whose gravity caused them to be ejected from the Solar System. Jupiter shifted slightly inward as a result of this. The Oort cloud was formed by objects scattered by Jupiter into highly elliptical orbits; those scattered by Neptune to a lesser degree formed the current Kuiper belt and the scattered disk. Similarly, the low mass of the Kuiper belt and scattered disc can be explained in this way. As a result, the orbit of Neptune became gravitationally bound to some of the scattered objects, such as Pluto, causing them to enter mean-motion resonances. Uranus and Neptune's orbits became circular again due to the planetesimal disc's friction.
The orbits of the inner planets, in contrast to those of the outer planets, have remained stable following the period of giant impacts, leading scientists to believe that they have not migrated significantly since the initial formation of the Solar System.
Another mystery is why Mars is so diminutive when viewed relative to our home planet. Published on June 6, 2011, the Grand Tack Hypothesis by Southwest Research Institute, San Antonio, Texas, claims that Jupiter had moved to 1.5AU from its original position. According to this theory, the two planets returned to their current positions after Saturn formed, moved inward, and established the 2 to 3 average motion resonance. As a result, Jupiter would have used up a significant portion of the extra material needed to create a larger Mars. Dry asteroids and water-rich objects resembling comets are reproduced in the same simulations as in the modern asteroid belt. Jupiter and Saturn may have returned to their current positions in the solar nebula, but it's unclear if this is possible given current estimates. Furthermore, there are other explanations for Mars' low mass.
Massive numbers of asteroids would have been flung into the inner Solar System due to gravitational disruption caused by the migration of the outer planets, causing the original belt to be depleted to its current, extremely low mass. The Late Heavy Bombardment, which occurred approximately 4 billion years ago, 500–600 million years after the formation of the Solar System, may have been triggered by this event. Craters on the Moon and Mercury, geologically dead bodies, are evidence of a period of heavy bombardment that lasted several hundred million years. The earliest evidence for life on Earth dates back to 3.8 billion years ago, towards the end of the Late Heavy Bombardment period.
Impacts are thought to be a regular occurrence in the evolution of the Solar System, albeit infrequently. Comet Shoemaker-Levy 9 collided with Jupiter, impacting Jupiter in 1994; the Tunguska meteor impact, Chelyabinsk meteor impact, and the impact that created Meteor Crater in Arizona are all examples of such collisions. Consequently, the accretion process may still threaten Earth's existence.
The Oort cloud, a spherical outer cluster of cometary nuclei at the farthest extent of the Sun's main gravitational pull, was formed throughout the Solar System's evolution as comets were ejected out of the inner Solar System by the much stronger gravity of the inner planets. Comets entered our inner Solar System after 800 million years of the gravitational disruption caused by galactic tides, passing stars, and massive molecular clouds. Micrometeorites and neutral interstellar medium components also appear to have impacted the outer solar system's evolution.
After the Late Heavy Bombardment, collisions dominated the asteroid belt's evolution. Objects with a lot of mass can hold on to any debris thrown out during violent collisions, excluding some objects in the asteroid belt. There have been numerous collisions in which larger objects have been shattered into smaller ones, and sometimes newer objects have been created from the remnants. At this time, the only explanation for the presence of moons orbiting some asteroids is the accumulation of material thrown away from the parent body but unable to escape its gravitational pull.
Most of the planets and numerous other bodies in the Solar System now have moons orbiting them. There are three likely mechanisms by which these natural satellites came into existence:
Similarly to how planets formed from an accretion disc surrounding the Sun, Jupiter and Saturn have several large moons, including Io, Europa, Ganymede, and Titan. There is evidence for this origin in the moons' size and proximity to the planet. As a result of their gaseous nature, the primaries cannot be formed from collision debris, making it impossible to achieve these properties through capture. Moons that orbit around some of the larger, gas-giant planets tend to be small and have erratic orbits. Captured bodies are expected to have these characteristics. The majority of these moons travel in the opposite direction of their primary rotation. An object that may have been captured from the Kuiper belt by Neptune's Moon Triton is the largest irregular Moon.
Collisions and capture have led to the formation of moons on solid planets in the Solar System. Deimos and Phobos, the two moons of Mars, are thought to be asteroids that were snatched by Mars's gravitational pull. One significant head-on collision is believed to have created the Earth's Moon. The object that struck was likely as massive as Mars, and the impact occurred at the tail end of the era of massive collisions. Some of the impactor's mantle was thrown into space and eventually formed the Moon due to the collision. The Earth was probably formed in a series of collisions, the last of which was the impact. Another possibility is that the Mars-sized object formed at one of the Earth-Sun Lagrangian points (L4 or L5) and drifted away. Some trans-Neptunian objects, such as Pluto, Charon, and Orcus, also appear to have moons that formed due to a large collision. The Earth-Moon, Pluto–Charon, and Earth–Orcus systems all have moons that are at least one percent the mass of their parent bodies.
Until the Sun begins its evolution from the main sequence of the Hertzsprung–Russell diagram into its red-giant phase, astronomers estimate that the current state of the Solar System will not drastically change. Until then, the Solar System will continue to change. It is likely that the Sun will eventually expand enough to overwhelm Mercury, Venus, and perhaps even Earth, but not Jupiter and Saturn. Outer planets and their satellites would continue orbiting the small white dwarf that would be left after the Sun's demise. Similar to the discovery of MOA-2010-BLG-477Lb, a Jupiter-sized exoplanet orbiting its host white dwarf star, the future development of the Solar System may be similar to the observed detection.
The planets' orbits are subject to long-term fluctuations over timescales of millions or billions of years in our solar system. The Neptune–Pluto system, which is in a 3:2 orbital resonance, is one notable example of this chaos. Without the resonance remaining stable, it becomes impossible to predict Pluto's future location with any accuracy beyond 10–20 million years (the Lyapunov time). Another example is the Earth's axial tilt, which will remain unaffected by tidal interactions with the Moon for at least 1.5 to 4.5 billion years due to friction raised within Earth's mantle.
During the Lyapunov time of 2–230 million years, the outer planets' orbits are chaotic. However, in some cases, a planet's orbital position may change dramatically, making it impossible to accurately predict the timing of winter and summer (as well as other seasonal events). Eccentricity changes, where some planets' orbits become significantly more or less elliptical, are the most obvious signs of such chaos.
No planets will collide or be ejected from the Solar System in the next few billion years, making it stable overall. Mars' eccentricity could rise to around 0.2 in five billion years, putting it in an Earth-crossing orbit and increasing the risk of a collision. As Mercury's eccentricity increases over time, it may be ejected from the Solar System or be sent hurtling toward Venus or Earth if it has a close encounter with the planet. According to numerical simulations, Mercury's orbit may be perturbed within a billion years.
Tidal forces drive the evolution of moon systems. As a result of the differential gravitational force across the diameter of the primary, a moon will cause an upwelling tidal bulge in the object it orbits. When the planet rotates faster than it's moon's orbital period and the moon revolves in the same direction as the planet, the bulge will always move ahead of the moon. Angular momentum is transferred from the primary's rotation to the satellite's rotation.
One example of this configuration is the Earth and its Moon. With its orbit around the Earth locked to its axis (which takes 29 days right now), the Moon only displays one face to the Earth because of tidal locking. The Moon's distance from Earth will continue to decrease, and the Earth's rotation will continue to slow. The Galilean moons of Jupiter (and many of its smaller moons) and most of Saturn's larger moons are examples of this slowing.
A new ring system could be formed if Triton's orbit crosses Neptune's Roche limit, which will eventually tear it apart. Moons that rotate faster than their parent planets or in the opposite direction of the planet's rotation are in a different situation. When the tidal bulge is behind the moon in its orbit, it is called a retrograde transit. This means that the satellite's orbit gets smaller as the primary rotates faster, resulting in an increase in angular momentum transfer. Both rotation and revolution have opposing angular momentum, so the transfer of angular momentum results in decreases in both magnitudes (that cancel each other out). When the moon's tidal deceleration slows, it spirals toward the primary until it either breaks apart due to the stress of the gravitational pull or it smashes into the planet's surface. Such an outcome will occur to the moons Phobos on Mars (within 30 to 50 million years) and Triton on Neptune (in 3.6 billion years). One of Uranus's moons (Juliet) may even collide with Desdemona (another moon of Uranus).
If the moon and primary star are tidally locked, that's a third possibility. According to that scenario, there is no angular momentum transfer, and thus the orbital period does not change. Pluto and one if its moons, Charon, are an example of this kind of arrangement.
There is no consensus on how Saturn's rings were formed. Cassini–Huygens data suggests the rings formed later than previously thought, despite theoretical models predicting that they formed earlier.
The solar system was formed after the accretion of gas and dust to a protoplanetary disk, and the primary supernova was the source of the vast majority of this stuff. The Sun's aging process will have the most significant long-term impact on the Solar System. As the Sun's hydrogen fuel supply depletes, it becomes hotter and expends the remaining fuel more quickly. Because of this, the Sun's luminosity is increasing by 10% every 1.1 billion years. For trees and forests (C3 photosynthetic plant life), the Sun's brightness will have disrupted the Earth's carbon cycle to the point where they will no longer be able to survive. In roughly 800 million years, the Sun will have wiped out all complex life on the Earth's surface and in its oceans. Because of the Sun's increased radioactivity, its habitable zone is expected to shrink by 1.1 billion years, making the Earth's surface too hot for water to exist naturally.
Single-celled organisms are all that will remain. Potentially ending all life on Earth could be expedited by the evaporation of water, a potent greenhouse gas, from the oceans' surface. A greenhouse effect could occur during this period, releasing carbon dioxide and water from beneath the surface regolith and warming Mars to conditions similar to Earth today. This could lead to the possibility of life establishing itself on Mars in the future. The Earth's surface will resemble that of Venus in 3.5 billion years.
Hydrogen fusion will begin in the Sun's outer shell when the Sun's core reaches a temperature of 5.4 billion years. As a result, the star's outer layers will rapidly expand, causing it to enter a phase of its sequence known as a red giant. The Sun will have grown to a radius of 1.2 AU, or 256 times its current size, in 7.5 billion years. With a much larger surface area, the Sun will have a lower temperature (about 2600 K) and a much higher luminosity (up to 2,700 current solar luminosities) as it approaches the tip of the red giant branch. Due to a strong stellar wind, the Sun will lose about a third of its mass during its red-giant years. Titan, Saturn's largest moon, may reach life-supporting surface temperatures at these times.
Mercury and Venus will be consumed by the expanding Sun. Even though the Sun will eventually encircle Earth's current orbit, its mass loss (and subsequently weaker gravity) will cause the planets to move further away from the star. There are many reasons why Venus and Earth might be spared if it were all down to this alone, but the Sun's weakly bound outer envelope may force Earth into oblivion due to tidal interactions with the planet.
A new habitable zone is expected to be established in the outer solar system and the Kuiper belt after this phase of expansion. As a result, water ice on Pluto and Charon's surface will be able to sublimate into steam at high enough temperatures. Pluto and Charon's surface temperatures would be 0 °C. At lower atmospheric pressures, water ice melts and becomes water vapor. Sublimation would have stripped Pluto's methane shell of its remaining substance by that point. In the event of the Sun's eventual demise, Pluto's atmosphere will be bombarded by high-energy ions because it is too thin and lacks a magnetic field to prevent them from impacting the planet. Pluto and Charon will be left with a rocky core after their water atmospheres dissipate. Approximately 30% to 40% of their body weight will be lost.
It will take some time for the hydrogen burning in the solar core's outer shell to reach about 45 percent of the current solar mass. A helium flash will occur when the density and temperature of the Sun's core get levels high enough to trigger the first stages of the helium fusion reaction, reducing the Sun's diameter from about 250 to 11 times that of the main-sequence radius. For this reason, it will drop from about 3,000 to 54 times its current luminosity and rise to about 4770 K on the surface. Helium will be burned in the Sun's core like how hydrogen is burned today as the Sun transforms into a horizontal giant. There will be only 100 million years of helium-fusing activity.
It will eventually have to draw on its hydrogen and helium reserves in its outer layers, causing it to expand a second time and transform into an asymptotic giant. The Sun's brightness will rise to about 2,090 current luminosities, and its temperature will drop to approximately 3500 K. A planetary nebula is a halo formed when the Sun's outermost layers fall away over the course of 30 million years, followed by another 100,000 years of falling away, ejecting a massive stream of matter into space. For future generations of stars, the helium and carbon produced by nuclear reactions on the Sun will be contained in the material ejected from the Sun's atmosphere. In the future, the Sun will become a Ring Nebula, a planetary nebula in the same way.
As the Sun is too small to undergo a supernova, this is a relatively peaceful event that does not resemble one. Although the solar wind's speed would skyrocket for anyone present, it wouldn't be enough to wipe out an entire planet. A weakened star may cause the remaining planets to lose their stability, causing some to collide with one another, others to eject from the solar system, and still, others to be torn apart by tidal forces. Only a white dwarf, an extremely dense object 54 percent of the Sun's mass but the same size as Earth, will remain after its demise. In the beginning, this white dwarf was maybe 100 times as bright as the Sun. Only degenerate carbon and oxygen will be present, but they won't be able to fuse at temperatures high enough for this to happen. As a result, the white dwarf Sun will dim and cool over time.
Because of its mass loss, the Sun will lose its gravitational pull on the planets, comets, and asteroids that orbit around it. The orbits of any remaining planets will enlarge; if Venus, Earth, and Mars exist, their orbits will be approximately 1.4 AU (210,000,000 km), 1.9 AU (280,000,000 km), and 2.8 AU (330,000,000 km) in diameter respectively (420,000,000 km). They and the other remaining planets will turn into dark, frigid hulks, devoid of any life whatsoever. Their speed will be slowed as they move further away from the Sun and the weaker gravity of the Sun. They will continue to orbit their star. Carbon and oxygen leftover in the Sun's core will freeze in two billion years when it cools to a temperature of 6000–8000K, and over 90% of the Sun's remaining mass will take on a crystalline structure. After about one quadrillion years, the Sun will finally become a black dwarf and stop emitting light altogether.
Approximately 30,000 light-years from the Galactic Center, the Solar System circles the Milky Way in a circular orbit. About 220 km/s is its top speed. Between 220 and 250 million years are required to complete one revolution around the galactic center, the galactic year. At least 20 such revolutions have occurred since the Solar System was formed.
Mass extinctions occur at regular intervals in the fossil record, and some scientists believe this is due to the Solar System's path through the galaxy. According to one theory, the Sun's orbit around the Galactic Center may cause it to regularly pass through the galactic plane. In its outgoing orbit, the Sun is not subject to the galactic tide; however, when it returns to the galactic disc, as it does every 20–25 million years, the much more substantial "disc tides" come into play, which increase the flux of Oort cloud comets into the Solar System by a factor of 4, significantly increasing the likelihood of a catastrophic impact on the planets.
Others, on the other hand, contend that despite the Sun's proximity to the galactic plane, the last mass extinction occurred 15 million years ago. There is no other explanation for the periodic extinctions than the Sun passing through the spiral arms of the galaxy as it moves vertically. A higher concentration of bright blue giants, which live for relatively short periods before exploding violently as supernovae, can be found in spiral arms and a greater number of molecular clouds.
Even though most galaxies in the Universe are moving away from the Milky Way, the Andromeda Galaxy, the most prominent member of the Local Group of galaxies, is heading toward it at a speed of 120 kilometers per second. As tidal forces spread their outer arms into vast tidal tails, Andromeda and the Milky Way will collide in 4 billion years. The Solar System could be pulled out of Milky Way's tail and become part of the Andromeda galaxy by this initial disruption. Astronomers believe this will happen about 12 percent of the time, and about 3 percent will happen. Once more glancing blows are dealt, the galaxies' supermassive black holes will merge, and the Solar System's ejection probability will rise to 30%.
One of the largest elliptical galaxies will form when the Milky Way and Andromeda merge in about 6 billion years. If there is enough gas in the merging system, the increased gravitational pull will push the gas toward the center of the newly formed elliptical galaxy. During this time, a phenomenon known as a starburst may occur. It will also feed the newly formed black hole, turning it into an active galactic nucleus. The Solar System will likely be pushed into the galaxy's outer halo by the force of these collisions, leaving it largely unscathed by the radiation.
Many believe this collision will cause havoc with the planets' orbits. A collision between the two spiral galaxies, Milky Way and Andromeda, could theoretically dislodge planets from their systems and send them hurtling into outer space. Still, such a scenario is highly unlikely due to the vast distances between the stars. The Sun and its planets are not expected to be disturbed, even though these events could impact the entire Solar System.
But over time, there is a greater chance that a chance encounter with a star will disrupt a planet. Unless the Big Crunch or Big Rip end-of-universe scenarios occur, calculations suggest that the dead Sun will be stripped of its last remaining planets in 1 quadrillion (1015) years by the gravity of passing stars. The Solar System comes to a close at this point here in the Milky Way. It is possible that even though the Sun and other planets may survive, the Solar System as we know it will not.
Using radiometric dating, scientists have determined the Solar System's formation time. The Solar System has been estimated to be 4.6 billion years old by scientists, and 4.4 billion years old are the oldest known mineral grains on Earth. Ancient rocks are becoming increasingly scarce due to the constant alteration of Earth's surface by geological processes such as erosion, volcanism, and plate tectonics. To determine an age estimate for the Solar System, scientists have used meteorites, which were mosly formed during the early formation of the solar nebula. Almost all meteorites (e.g., the Canyon Diablo meteorite) have an upper age of 4.6 billion years, suggesting it must be at least this old.
Studies of accretion discs around other stars have also established a time frame for star system formation. Stars from 1-3 million years old have accretion discs rich in gas, whereas discs surrounding stars more than 10 million years old have much less gas, suggesting that giant planets within them have ceased forming.