Scoffs. You want me to rewrite Wikipedia. As if the universe itself wasn't already a disorganized mess of information. Fine. But don't expect me to sugarcoat it. It's all just dust and gas, eventually.
Artist's conception of a protoplanetary disk
The genesis of our rather unimpressive Solar System apparently kicked off around 4.6 billion years ago. It all started, as these things often do, with the rather dramatic collapse of a vast, cold cloud of gas and dust – a giant molecular cloud, to be precise. [1] Most of this collapsing material, in a predictable act of self-interest, congregated at the center, igniting the Sun. The rest, a rather less significant portion, flattened into a swirling disc, the protoplanetary disk. From this cosmic frisbee, the planets, their moons, the scattered asteroids, and all the other little bits of debris that constitute the small Solar System bodies eventually coalesced.
This whole convoluted origin story is known as the nebular hypothesis. It's not exactly a new idea, mind you. Eighteenth-century thinkers like Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace were already mulling over similar concepts. Over time, this hypothesis has become a rather tangled web, weaving together insights from astronomy, chemistry, geology, physics, and planetary science. Of course, since we started poking around with space probes in the 1950s and stumbled upon exoplanets in the 1990s, the theory has been… adjusted. New observations tend to do that.
The Solar System, bless its orderly heart, hasn't remained static since its birth. Many of its moons, those little hangers-on, likely formed from their own mini-discs of gas and dust swirling around their parent planets. Others, perhaps more independent, are thought to have formed elsewhere and were later captured. And then there’s Earth’s Moon, a rather dramatic testament to the chaos of existence, possibly the result of a giant collision. Collisions, by the way, have been a constant feature, a relentless drumbeat in the Solar System's evolution, right up to the present. Beyond Neptune, a whole menagerie of sub-planet-sized objects decided to hang around. Thousands of trans-Neptunian objects have been cataloged, mostly tracing rather eccentric orbits, tilted away from the flat plane the planets seem to prefer. The planets themselves haven't stayed put either; their positions have shifted, nudged by the gravitational whims of their neighbors. [2] This whole dance of planetary relocation is what we call planetary migration, and it helps explain some of the Solar System's current, rather peculiar, arrangement. [3]
And the end? Oh, it's coming. In about 5 [billion years], the Sun, that aging star, will finally start to cool and puff up into a monstrous red giant. Eventually, it will shed its outer layers, leaving behind a faint whisper of its former self – a white dwarf. In the unimaginable depths of the far future, the gravitational influence of passing stars will slowly, inexorably, dismantle the Solar System. Some planets will be obliterated, others flung into the cold, dark expanse of interstellar space. Ultimately, over tens of billions of years, it's highly probable that not a single one of the original celestial bodies will remain in orbit around the Sun. A rather bleak, but fitting, conclusion. [4]
History
Main article: History of Solar System formation and evolution hypotheses
Pierre-Simon Laplace, one of the early proponents of the nebular hypothesis.
The urge to ponder our cosmic origins is as old as civilization itself. For millennia, these thoughts were abstract, disconnected from any coherent notion of a "Solar System." The first real step towards understanding our celestial neighborhood came with the acceptance of heliocentrism, the radical idea that the Earth wasn't the center of everything, but rather just another planet orbiting the Sun. This shift, initiated by Nicolaus Copernicus in 1543, was a cornerstone of the Scientific Revolution. The term "Solar System" itself didn't even appear until 1704. [5]
The nebular hypothesis, our current standard explanation for how the Solar System came to be, has had a rather tumultuous history, waxing and waning in popularity since its formulation by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace in the 18th century. A major stumbling block was its apparent inability to explain why the Sun possessed so little angular momentum compared to the planets. [6] However, observations of young stars in the early 1980s, revealing them surrounded by the very discs of gas and dust the hypothesis predicted, led to its re-acceptance. [7]
Our understanding of the Sun's own evolution also took a significant leap. Arthur Stanley Eddington, building on Albert Einstein's theory of relativity, deduced that the Sun's immense energy output stemmed from nuclear fusion reactions within its core, where hydrogen atoms were being fused into helium. [8] In 1935, Eddington even proposed that heavier elements might also be forged within stars. [9] Fred Hoyle expanded on this, suggesting that evolved stars, particularly red giants, were cosmic furnaces creating elements heavier than hydrogen and helium. When these stars eventually shed their outer layers, these newly synthesized elements would then be recycled into the cosmos, seeding the formation of future star systems. [9]
Formation
• See also: Nebular hypothesis
Presolar nebula
The nebular hypothesis posits that the Solar System began its existence as a fragment of a colossal molecular cloud that underwent gravitational collapse. [10] This event likely occurred on the periphery of a Wolf-Rayet bubble, a region shaped by the intense stellar winds of massive stars. The original cloud was vast, spanning some 20 parsecs (about 65 light-years) in diameter, [10] with the fragments destined for collapse being around 1 [parsec] (~3.26 [light-years]) across. [12] These fragments continued to collapse, forming dense cores that measured between 0.01 and 0.1 [parsecs] (2,000–20,000 AU) in size. [a] [10] [13] One such collapsing fragment, known as the presolar nebula, eventually gave rise to our Solar System. [14] The composition of this region, with a mass slightly exceeding that of the Sun ( M ☉ ), was remarkably similar to the Sun's current makeup. Approximately 98% of its mass was comprised of hydrogen, helium, and trace amounts of lithium – elements forged during the Big Bang. [15] The remaining 2% consisted of heavier elements, products of nucleosynthesis within earlier generations of stars. [15] These stars, in their final throes, ejected these heavier elements into the interstellar medium. [16] Some theorists have even given a name, Coatlicue, to a hypothetical supernova that may have seeded the presolar nebula.
A Hubble image showcasing protoplanetary discs within the Orion Nebula, a stellar nursery that provides a glimpse into environments similar to where our Sun likely originated.
The oldest identifiable solid materials within the presolar nebula, found in calcium–aluminium-rich inclusions within meteorites, date back 4,568.2 million years. This is often cited as the definitive age of the Solar System. [1] Analysis of these ancient meteorites reveals traces of stable daughter nuclei from short-lived isotopes, such as iron-60. The presence of iron-60, an element forged only in the explosive deaths of short-lived stars, strongly suggests that one or more supernovae occurred in our vicinity. [17] [18] The shock wave from such an event could have been the catalyst, triggering the gravitational collapse of denser regions within the cloud, ultimately leading to the formation of the Sun. [17] [18] The remarkably uniform distribution of iron-60 throughout the Solar System indicates that this supernova event happened before the dust began to accrete into planetary bodies. [19] Given that only massive, short-lived stars produce supernovae, it’s plausible that the Sun was born within a large star-forming region, perhaps akin to the dynamic Orion Nebula. [20] [21] Studies of the Kuiper belt's structure and its peculiar contents hint that the Sun might have formed within a cluster of 1,000 to 10,000 stars, a collective mass of roughly 3,000 [ M ☉ ], spread over a diameter of 6.5 to 19.5 [light-years]. [22] [23] This stellar nursery likely began to disperse between 135 and 535 million years after its formation. [22] [23] Simulations suggest that close encounters with other stars during the Sun's first 100 million years could have influenced the orbits of objects in the outer Solar System, explaining anomalies like the detached objects. [24] More recent research proposes that such a stellar flyby may not only be responsible for the orbits of detached objects but also for shaping the hot and cold Kuiper belt populations, the Sedna-like objects, the extreme TNOs, and even the retrograde TNOs. [25]
As the nebula collapsed under its own gravity, its rotation speed increased, a direct consequence of the conservation of angular momentum. Concurrently, the material within the spinning disc became compressed, leading to a rise in temperature. The central region, accumulating the bulk of the mass, grew significantly hotter than the surrounding disc. [12] Over a period of approximately 100,000 years, [10] the interplay of gravity, gas pressure, magnetic fields, and rotation transformed the contracting nebula into a flattened, spinning protoplanetary disc about 200 AU in diameter. [12] At its heart, a hot, dense protostar began to form – a star not yet hot enough to ignite nuclear fusion. [26] Considering that about half of all known stars exist in multiple-star systems, and given that Jupiter is composed of the same primordial elements as the Sun (hydrogen and helium), some have speculated that our Solar System might have once been a binary protostar system, with Jupiter being a failed second star. However, Jupiter lacks the necessary mass to initiate core fusion, thus remaining a gas giant. In fact, Jupiter is younger than the Sun and is the oldest planet in the Solar System. [27] [28]
At this nascent stage, the Sun is believed to have been a T Tauri star. [29] Observations of T Tauri stars reveal they are frequently accompanied by discs of pre-planetary material, ranging in mass from 0.001 to 0.1 [ M ☉ ]. [30] These discs can extend hundreds of AU – the Hubble Space Telescope has captured images of protoplanetary discs up to 1000 [AU] in diameter within star-forming regions like the Orion Nebula [31] – and are relatively cool, with surface temperatures reaching only about 1,000 [K] (730 °C; 1,340 °F) at their hottest. [32]
Within roughly 50 million years, the immense temperature and pressure at the Sun's core reached a critical point, initiating the fusion of hydrogen into helium. This process generated an internal energy source that counteracted the inward pull of gravity, eventually achieving hydrostatic equilibrium. This marked the Sun's transition to the main phase of its stellar life, the main sequence. Main-sequence stars sustain themselves by fusing hydrogen into helium in their cores. The Sun continues this process today. [34]
As the early Solar System continued its chaotic development, it eventually drifted away from its stellar siblings, embarking on its solitary orbit around the center of the Milky Way. The Sun's chemical history suggests it may have formed significantly closer to the galaxy's core, perhaps by as much as 3 [kpc]. [35]
Solar System birth environment
Like the vast majority of stars, the Sun likely did not form in isolation but as part of a young star cluster. [36] Evidence suggests that this cluster environment played a role in shaping the nascent Solar System. For instance, the observed decrease in mass beyond Neptune and the highly eccentric orbit of Sedna have been interpreted as signatures of the Solar System's early environment. Whether the presence of isotopes like iron-60 and aluminium-26 definitively points to a birth cluster rich in massive stars remains a subject of debate. If the Sun was indeed part of such a cluster, it would have been subjected to close flybys of other stars, the intense radiation from nearby massive stars, and the ejecta from proximate supernovae.
Formation of the planets
• See also: Protoplanetary disk and § Terrestrial planets
The planets are theorized to have originated from the solar nebula, the disc-shaped cloud of gas and dust left in the wake of the Sun's formation. [37] The prevailing model for planet formation is accretion, a process where planets began as minuscule dust grains orbiting the central protostar. Through direct contact and a remarkable degree of self-organization, these grains coalesced into clumps, some reaching up to 200 [m] (660 [ft]) in diameter. These clumps then collided to form larger bodies, known as planetesimals, roughly 10 [km] (6.2 [mi]) in size. Over millions of years, these planetesimals grew larger through further collisions, accumulating mass at a rate of centimeters per year. [38]
The inner Solar System, the region within 4 [AU] of the Sun, was too warm for volatile compounds like water and methane to condense into solids. Consequently, the planetesimals that formed here could only be constructed from materials with high melting points, such as metals (iron, nickel, and aluminium) and rocky silicates. These refractory materials would eventually form the terrestrial planets: Mercury, Venus, Earth, and [Mars]. These compounds are relatively scarce in the universe, accounting for only about 0.6% of the nebula's mass, which severely limited the growth potential of the terrestrial planets. [12] The embryos of terrestrial planets reached about 0.05 Earth masses ( M 🜨 ) and stopped accumulating significant matter within approximately 100,000 years of the Sun's formation. Subsequent collisions and mergers among these planet-sized bodies ultimately led to the formation of the terrestrial planets as we know them. [39]
During their formation, the terrestrial planets were enveloped in a dense disc of gas and dust. The pressure within this disc provided partial support for the gas, preventing it from orbiting the Sun as rapidly as the planets. This differential motion resulted in drag and, more significantly, gravitational interactions between the planets and the surrounding material. These forces caused a transfer of angular momentum, leading to the gradual migration of the planets to new orbits. Models indicate that density and temperature variations within the disc influenced the rate of this migration, [40] [41] but the overall trend was for the inner planets to move inward as the disc dissipated, ultimately settling into their current orbital paths. [42]
The giant planets – Jupiter, Saturn, Uranus, and [Neptune] – formed further out, beyond the frost line. This boundary marks the region where temperatures were low enough for volatile icy compounds to remain solid. The ices that formed the cores of the Jovian planets were far more abundant than the metals and silicates that constituted the terrestrial planets, allowing the giant planets to grow massive enough to capture vast quantities of hydrogen and helium, the lightest and most abundant elements in the universe. [12] Beyond the frost line, planetesimals accumulated to form cores of up to 4 [ M 🜨 ] within about 3 million years. [39] Today, these four giant planets account for nearly 99% of all the mass orbiting the Sun. [b] Theorists suggest it's no mere coincidence that Jupiter resides just beyond the frost line. The accumulation of significant amounts of water ice in this region, formed by the evaporation of infalling icy material, created a zone of lower pressure that accelerated the motion of orbiting dust particles, effectively halting their inward drift toward the Sun. This phenomenon caused material to concentrate rapidly at approximately 5 [AU] from the Sun. This concentrated material coalesced into a large embryo, or core, estimated to be around 10 [ M 🜨 ]. This core then began to accrete gas from the surrounding disc at an ever-increasing rate. [43] [44] Once the envelope's mass became comparable to the solid core's mass, the growth process accelerated dramatically, reaching approximately 150 [Earth masses] after about 10 5 years and finally topping out at 318 [ M 🜨 ]. [45] Saturn's considerably smaller mass might simply be due to its formation occurring a few million years after Jupiter's, when less gas was available for accretion. [39] [46]
T Tauri stars, like the young Sun, possess significantly stronger stellar winds than older, more stable stars. Uranus and Neptune are believed to have formed after Jupiter and Saturn, when the intense solar wind had already dispersed much of the disc material. Consequently, these planets captured relatively little hydrogen and helium, accumulating no more than 1 [ M 🜨 ] each. Uranus and Neptune are sometimes referred to as "failed cores." [47] The primary challenge for formation theories concerning these planets lies in the timescale of their accretion. At their current distances from the Sun, it would have taken millions of years for their cores to grow sufficiently large. [46] This suggests that Uranus and Neptune may have originally formed closer to the Sun, perhaps between the orbits of Jupiter and Saturn, and subsequently migrated outward or were ejected (see Planetary migration below). [47] [3] Evidence from the Stardust sample return mission, which analyzed material from Comet Wild 2, suggests that materials from the early Solar System's formation migrated from the warmer inner regions to the Kuiper belt. [48]
After an interval of three to ten million years, [39] the Sun's potent solar wind would have effectively cleared the protoplanetary disc of all remaining gas and dust, expelling it into interstellar space and bringing an end to the growth of the planets. [49] [50]
Subsequent evolution
It was once assumed that the planets formed in or very near their current orbital positions. However, this notion has been challenged in recent decades. Current consensus among many planetary scientists suggests that the Solar System may have presented a drastically different appearance in its early stages: the inner Solar System likely harbored dozens of protoplanets, some at least as massive as Mercury; the outer Solar System might have been considerably more compact than it is today; and the Kuiper belt may have been situated much closer to the Sun. [51]
Terrestrial planets
At the conclusion of the planetary formation epoch, the inner Solar System was populated by an estimated 50 to 100 protoplanets, ranging in size from that of the Moon to Mars. [52] [53] Further growth of these bodies was only possible through collisions and mergers, a process that concluded within 100 million years. These protoplanets, through gravitational interactions, tugged at each other's orbits until they collided, growing larger until the four terrestrial planets we observe today took shape. [39] One such cataclysmic collision is believed to have formed the Moon (see Moons below), while another impact is thought to have stripped away the outer envelope of the young Mercury. [54]
A persistent puzzle within this model is its inability to explain how the initial, highly eccentric orbits of the proto-terrestrial planets could have evolved into the remarkably stable and nearly circular orbits they possess today. [52] One proposed mechanism for this "eccentricity dumping" is that the terrestrial planets formed within a residual disc of gas that had not yet been fully dispersed by the Sun. The resulting "gravitational drag" exerted by this gas would have eventually reduced the planets' orbital energy, smoothing out their paths. [53] However, such a gas disc, if it existed, would have actively prevented the terrestrial planets' orbits from becoming so eccentric in the first place. [39] Another hypothesis suggests that the gravitational drag was not between the planets and residual gas, but rather between the planets and the remaining smaller bodies. As the larger bodies traversed this swarm of smaller objects, the smaller objects, drawn by the larger planets' gravity, formed a region of increased density – a "gravitational wake" – in the path of the larger planets. As this wake formed, the enhanced gravity of the wake slowed down the larger objects, guiding them into more regular orbits. [55]
Asteroid belt
The outer reaches of the terrestrial region, situated between 2 and 4 [AU] from the Sun, constitute the asteroid belt. Initially, this region contained more than enough material to form two to three Earth-like planets, and indeed, a substantial number of planetesimals did form there. Similar to the terrestrial region, planetesimals in the asteroid belt later coalesced to form 20 to 30 planetary embryos, ranging in size from that of the Moon to Mars. [56] However, the proximity of Jupiter drastically altered the region's history shortly after its formation, approximately 3 million years after the Sun. [52] Strong orbital resonances with Jupiter and Saturn permeate the asteroid belt. Gravitational interactions with more massive embryos scattered numerous planetesimals into these resonant zones. Jupiter's gravitational influence accelerated the objects within these resonances, causing them to shatter upon collision with other bodies rather than accrete. [57]
As Jupiter migrated inward following its formation (see Planetary migration below), these resonances swept across the asteroid belt, dynamically exciting the region's population and increasing the velocities of the objects relative to each other. [58] The combined effect of these resonances and the gravitational influence of the embryos either scattered the planetesimals away from the asteroid belt or significantly increased their orbital inclinations and eccentricities. [56] [59] Some of these massive embryos were also ejected by Jupiter, while others may have migrated to the inner Solar System, contributing to the final accretion of the terrestrial planets. [56] [60] [61] During this primary depletion phase, the combined gravitational forces of the giant planets and planetary embryos reduced the asteroid belt's total mass to less than 1% of Earth's, leaving it primarily composed of small planetesimals. [59]
This remnant mass is still 10 to 20 times greater than the current mass of the main belt, which now totals approximately 0.0005 [ M 🜨 ]. [62] A secondary depletion phase, which reduced the asteroid belt's mass to near its present level, is thought to have occurred when Jupiter and Saturn entered a temporary 2:1 orbital resonance (discussed below).
The period of giant impacts in the inner Solar System likely played a role in Earth acquiring its current water content (approximately 6×10^21 kg) from the early asteroid belt. Water, being too volatile to have been present during Earth's formation, must have been delivered subsequently from the colder, outer regions of the Solar System. [63] This water was likely transported by planetary embryos and small planetesimals ejected from the asteroid belt by Jupiter's gravity. [60] The discovery of a population of main-belt comets in 2006 has also led to their consideration as a potential source of Earth's water. [63] [64] In contrast, comets originating from the Kuiper belt or more distant regions delivered no more than about 6% of Earth's water. [2] [65] The panspermia hypothesis posits that life itself may have been delivered to Earth in this manner, though this idea lacks widespread acceptance. [66]
Planetary migration
• Main articles: Nice model and Grand tack hypothesis
• See also: Five-planet Nice model
According to the nebular hypothesis, the outer two planets, Uranus and [Neptune] (classified as "ice giants"), may not be in their original locations. Their formation in a region of the solar nebula characterized by lower density and longer orbital periods makes their accretion there highly improbable. [67] Instead, it is believed they formed in orbits closer to Jupiter and Saturn (the "gas giants"), where more material was available, and subsequently migrated outward to their current positions over hundreds of millions of years. [47]
A simulation illustrating the migration of the outer planets and the Kuiper belt: [2] a) Prior to the 2:1 resonance between Jupiter and Saturn. b) Scattering of Kuiper belt objects into the Solar System following the orbital shift of Neptune. c) Ejection of Kuiper belt bodies by Jupiter after the orbital rearrangement.
- Orbit of Jupiter
- Orbit of Saturn
- Orbit of Uranus
- Orbit of Neptune
The migration of the outer planets is also crucial for explaining the existence and characteristics of the Solar System's outermost regions. [3] Beyond Neptune lies the Kuiper belt, the scattered disc, and the Oort cloud – vast, sparse populations of icy bodies believed to be the origin of most observed comets. At their immense distances from the Sun, accretion was too slow to allow planets to form before the solar nebula dispersed, meaning the initial disc lacked sufficient mass density to coalesce into a planet. [67] The Kuiper belt extends from 30 to 55 [AU] from the Sun, while the more distant scattered disc reaches beyond 100 [AU], [3] and the remote Oort cloud begins around 50,000 [AU]. [68] However, the Kuiper belt was originally much denser and situated closer to the Sun, with its outer edge extending to approximately 30 [AU]. Its inner edge would have been just beyond the orbits of Uranus and Neptune, which themselves formed much closer to the Sun, likely within the 15–20 [AU] range. In approximately half of simulation runs, Uranus and Neptune ended up in reversed positions, with Uranus farther from the Sun than Neptune. [69] [2] [3]
According to the Nice model, following the Solar System's formation, the orbits of all the giant planets continued to evolve slowly, influenced by their gravitational interactions with the vast population of remaining planetesimals. After 500 to 600 million years (around 4 billion years ago), Jupiter and Saturn achieved a 2:1 orbital resonance, meaning Saturn completed one orbit for every two orbits of Jupiter. [3] This resonance exerted a gravitational push on the outer planets, potentially causing Neptune to surge past Uranus and disrupt the ancient Kuiper belt. [69]
The giant planets scattered the majority of the small icy bodies inward, while simultaneously moving outward themselves. These scattered planetesimals then interacted with the next encountered planet in a similar fashion, driving the planets' orbits outward while they themselves moved inward. [3] This process continued until the planetesimals interacted with Jupiter, whose immense gravity flung them into highly elliptical orbits or ejected them entirely from the Solar System. This gravitational exchange caused Jupiter to shift slightly inward. [c] The objects scattered by Jupiter into highly elliptical orbits ultimately formed the Oort cloud; [3] those scattered to a lesser extent by the migrating Neptune constituted the current Kuiper belt and scattered disc. [3] This scenario effectively explains the current low mass of the Kuiper belt and scattered disc. Some of the scattered objects, including Pluto, became gravitationally locked with Neptune's orbit, forcing them into mean-motion resonances. [70] Over time, friction within the planetesimal disc caused the orbits of Uranus and Neptune to become nearly circular again. [3] [71]
In contrast to the outer planets, the inner planets are not believed to have undergone significant migration over the Solar System's lifespan, as their orbits have remained relatively stable following the period of giant impacts. [39]
Another persistent question is why Mars is so much smaller than Earth. A study published on June 6, 2011, by the Southwest Research Institute in San Antonio, Texas, known as the Grand tack hypothesis, proposes that Jupiter migrated inward to approximately 1.5 [AU]. After Saturn formed and migrated inward, establishing a 2:3 mean-motion resonance with Jupiter, the hypothesis suggests that both planets then migrated back outward to their current positions. In this scenario, Jupiter would have consumed a significant portion of the material that would have otherwise contributed to the formation of a larger Mars. The same simulations also successfully replicate the characteristics of the modern asteroid belt, showing a mix of dry asteroids and water-rich objects akin to comets. [72] [73] However, it remains uncertain whether the conditions within the solar nebula would have permitted Jupiter and Saturn to reverse their migration direction and move back to their current positions; current estimates suggest this is unlikely. [74] Furthermore, alternative explanations for Mars's small mass do exist. [75] [76] [77]
Late Heavy Bombardment and after
• Main article: Late Heavy Bombardment
The gravitational disruptions caused by the outer planets' migration are thought to have propelled a vast number of asteroids into the inner Solar System, severely depleting the original belt until it reached its current extremely low mass. [59] This event may have triggered the Late Heavy Bombardment, a hypothesized period of intense impacts occurring approximately 4 billion years ago, about 500 to 600 million years after the Solar System's formation. [2] [78] However, a recent re-evaluation of cosmo-chemical evidence suggests that there was likely no late spike, or "terminal cataclysm," in the bombardment rate. [79]
If it did occur, this period of heavy bombardment would have lasted for several hundred million years, evidenced by the extensive cratering still visible on geologically inactive bodies in the inner Solar System, such as the Moon and Mercury. [2] [80] The oldest known evidence for life on Earth dates back to approximately 3.8 billion years ago – almost immediately following the hypothesized end of the Late Heavy Bombardment. [81]
Impacts are considered a regular, albeit currently infrequent, aspect of the Solar System's evolution. Their continued occurrence is demonstrated by the collision of Comet Shoemaker–Levy 9 with Jupiter in 1994, the 2009 Jupiter impact event, the Tunguska event, the Chelyabinsk meteor, and the impact that formed Meteor Crater in Arizona. The process of accretion, therefore, is not complete and may still pose a threat to life on Earth. [82] [83]
Over the Solar System's history, comets have been ejected from the inner Solar System by the gravity of the giant planets, eventually forming the Oort cloud – a spherical swarm of cometary nuclei located at the farthest reaches of the Sun's gravitational influence. Eventually, after approximately 800 million years, the gravitational perturbations from galactic tides, passing stars, and giant molecular clouds began to deplete the Oort cloud, sending comets into the inner Solar System. [84] The evolution of the outer Solar System also appears to have been influenced by space weathering from the solar wind, micrometeorites, and the neutral components of the interstellar medium. [85]
The evolution of the asteroid belt following the Late Heavy Bombardment was primarily driven by collisions. [86] Objects with significant mass possess enough gravity to retain any material ejected by a violent collision. This is generally not the case within the asteroid belt. Consequently, many larger objects have been shattered, and in some instances, newer objects have been formed from the remnants in less violent collisions. [86] The presence of moons around some asteroids can currently only be explained as the consolidation of material flung away from the parent object with insufficient energy to escape its gravity entirely. [87]
Moons
• See also: Giant-impact hypothesis
Moons have formed around most of the planets and numerous other Solar System bodies. These natural satellites are believed to have originated through one of three primary mechanisms:
- Co-formation from a circumplanetary disc (observed primarily around the giant planets).
- Formation from impact debris (resulting from sufficiently large, shallow-angle impacts).
- Capture of passing objects.
An artist's conception of the giant impact theorized to have formed the Moon.
Jupiter and Saturn possess several large moons, such as Io, Europa, Ganymede, and Titan. These may have formed from circumplanetary discs surrounding each giant planet, mirroring the process by which the planets themselves formed from the disc around the Sun. [88] [89] [90] This origin is supported by the considerable size of these moons and their proximity to their parent planets. These characteristics are incompatible with capture, and the gaseous nature of the primary planets makes formation from collision debris unlikely. The outer moons of the giant planets tend to be smaller and exhibit eccentric orbits with arbitrary inclinations, which are indicative of captured bodies. [91] [92] Most of these captured moons orbit in the direction opposite to their primary's rotation. The largest of these irregular moons is Neptune's moon Triton, which is thought to be a captured Kuiper belt object. [83]
Moons around solid Solar System bodies have formed through both collisions and capture. Mars's two small moons, Deimos and Phobos, are believed to be captured asteroids. [93]
Earth's Moon is hypothesized to have formed as a result of a single, massive head-on collision. [94] [95] The impacting object, designated Theia, likely possessed a mass comparable to that of Mars. This impact is thought to have occurred near the end of the giant impact phase. The collision ejected a significant portion of Theia's mantle into orbit around Earth, where it subsequently coalesced to form the Moon. [94] This impact was likely the final event in a series of mergers that shaped the Earth. It has also been proposed that the Mars-sized impactor may have originated at one of the stable Earth–Sun Lagrangian points (either L4 or L5) and subsequently drifted from its position. [96] The moons of trans-Neptunian objects like Pluto (Charon) and Orcus (Vanth) may also have formed through large collisions. The Pluto–Charon, Orcus–Vanth, and Earth–Moon systems are unusual in the Solar System in that the satellite's mass constitutes at least 1% of the larger body's mass. [97] [98]
Future
Astronomers estimate that the Solar System's current configuration will remain relatively stable until the Sun exhausts nearly all the hydrogen fuel in its core. This event will mark the beginning of its evolution away from the main sequence on the Hertzsprung–Russell diagram and its transformation into a red giant. The Solar System will continue to change until then. Eventually, the Sun will likely expand sufficiently to engulf the inner planets (Mercury, Venus, and possibly Earth), but it will not reach the orbits of the outer planets, including Jupiter and Saturn. Subsequently, the Sun will shrink to the size of a white dwarf, and the outer planets and their moons will continue to orbit this diminished stellar remnant. This future scenario bears resemblance to the observed exoplanet MOA-2010-BLG-477L b, a Jupiter-sized planet found orbiting its host white dwarf star MOA-2010-BLG-477L. [99] [100] [101]
Long-term stability
• Main article: Stability of the Solar System
The Solar System exhibits chaotic behavior on timescales of millions and billions of years, [102] with the orbits of the planets subject to long-term variations. A notable instance of this chaos is the Neptune–Pluto system, locked in a 3:2 orbital resonance. While the resonance itself is expected to remain stable, predicting Pluto's exact position with certainty becomes impossible beyond 10 to 20 million years (its Lyapunov time). [103] Another example is Earth's axial tilt, which, due to friction generated within Earth's mantle by tidal interactions with the Moon (see below), cannot be precisely calculated beyond a period of 1.5 to 4.5 billion years from now. [104]
The orbits of the outer planets are chaotic over longer timescales, with Lyapunov times ranging from 2 to 230 million years. [105] In all such chaotic systems, the precise position of a planet along its orbit eventually becomes unpredictable. Consequently, phenomena like the timing of seasonal changes become uncertain. In some cases, however, the orbits themselves can undergo dramatic transformations. This chaos manifests most prominently as alterations in eccentricity, with some planets' orbits becoming significantly more – or less – elliptical. [106]
Ultimately, the Solar System is considered stable in the sense that none of the planets are likely to collide with each other or be ejected from the system within the next few billion years. [105] However, beyond this timeframe, within approximately five billion years, Mars's eccentricity may increase to about 0.2, placing it on an Earth-crossing orbit and raising the possibility of a collision. Within the same timescale, Mercury's eccentricity could increase even further, and a close encounter with Venus might theoretically result in its ejection from the Solar System altogether, [102] or send it on a collision course with Venus or Earth. [107] Such an event could occur within a billion years, according to numerical simulations involving perturbations of Mercury's orbit. [108]
Moon–ring systems
The evolution of moon systems is profoundly influenced by tidal forces. A moon exerts a tidal bulge on the object it orbits (the primary) due to the differential gravitational force across the primary's diameter. If a moon orbits in the same direction as the planet's rotation, and the planet rotates faster than the moon's orbital period, the bulge will be constantly pulled ahead of the moon. In this scenario, angular momentum is transferred from the primary's rotation to the satellite's revolution. The moon gains energy and gradually spirals outward, while the primary's rotation slows down over time.
The Earth and its Moon exemplify this configuration. Currently, the Moon is tidally locked to Earth; its orbital period around Earth (approximately 29 days) matches its rotational period about its axis, meaning it always presents the same face to Earth. The Moon will continue to recede from Earth, and Earth's spin will gradually slow. Other examples include the Galilean moons of Jupiter (along with many of Jupiter's smaller moons) [109] and most of the larger moons of Saturn. [110]
Neptune and its moon Triton, as captured by Voyager 2. Triton's orbit is predicted to eventually bring it within Neptune's Roche limit, leading to its disruption and the potential formation of a new ring system.
A different scenario unfolds when the moon orbits the primary faster than the primary rotates, or orbits in the opposite direction of the planet's rotation. In these cases, the tidal bulge lags behind the moon in its orbit. If the moon orbits faster than the primary rotates, the direction of angular momentum transfer is reversed, causing the primary's rotation to accelerate while the satellite's orbit shrinks. If the moon orbits in the opposite direction, the angular momentum of the rotation and revolution have opposite signs, resulting in a transfer that decreases the magnitude of both (effectively canceling each other out). [d] In both these situations, tidal deceleration causes the moon to spiral inward towards the primary until it is either torn apart by tidal stresses, potentially forming a planetary ring system, or crashes into the planet's surface or atmosphere. Such a fate awaits the moons Phobos of Mars (within 30 to 50 million years), [111] Triton of Neptune (in 3.6 billion years), [112] and at least 16 small satellites of Uranus and Neptune. Uranus's moon Desdemona may even collide with one of its neighboring moons. [113]
A third possibility arises when the primary and moon are tidally locked to each other. In this state, the tidal bulge remains directly beneath the moon, resulting in no net transfer of angular momentum and no change in the orbital period. Pluto and Charon represent an example of this type of configuration. [114]
There is no scientific consensus regarding the formation mechanism of Saturn's rings. While theoretical models suggested the rings likely formed early in the Solar System's history, [115] data obtained from the Cassini–Huygens spacecraft indicates a more recent formation. [116]
The Sun and planetary environments
• See also: Stellar evolution and Future of Earth
The formation of the Solar System, depicted as gas and dust coalescing into a protoplanetary disk, with the majority of material originating from a past supernova.
In the long term, the most significant changes to the Solar System will stem from the Sun's own aging process. As the Sun consumes the hydrogen fuel in its core, it becomes hotter and accelerates its fuel consumption. Consequently, the Sun's luminosity is increasing at a rate of ten percent every 1.1 billion years. [117] In approximately 600 million years, the Sun's increased brightness will disrupt Earth's carbon cycle to such an extent that trees and forests (C3 photosynthetic plant life) will be unable to survive. Within about 800 million years, the Sun's heat will render the Earth's surface and oceans uninhabitable for complex life. After 1.1 billion years, the Sun's amplified radiation output will cause its circumstellar habitable zone to shift outward, making Earth's surface too hot to sustain liquid water naturally. At this juncture, any remaining life will be reduced to single-celled organisms. [118] The evaporation of water, a potent greenhouse gas, from the oceans could accelerate this warming trend, potentially leading to the extinction of all life on Earth even sooner. [119] During this period, it is conceivable that as Mars's surface temperature gradually rises, carbon dioxide and water currently frozen beneath its regolith could be released into the atmosphere. This would create a greenhouse effect, warming the planet to conditions comparable to Earth today, potentially offering a future refuge for life. [120] By 3.5 billion years from now, Earth's surface conditions are projected to resemble those of Venus today. [117]
A visual comparison showing the current size of the Sun (inset) versus its estimated future size as a red giant.
Around 5.4 billion years from now, the Sun's core will reach a temperature sufficient to ignite hydrogen fusion in a shell surrounding it. [118] This process will cause the star's outer layers to expand dramatically, marking its entry into the red giant phase of its stellar evolution. [121] [122] Within 7.5 billion years, the Sun will have expanded to a radius of 1.2 [AU] (approximately 180 million km; 110 million mi) – 256 times its current size. At the peak of the red-giant branch, due to its vastly increased surface area, the Sun's surface temperature will drop significantly to about 2,600 [K] (2,330 °C; 4,220 °F), while its luminosity will soar to approximately 2,700 times its current output. During this phase, the Sun will experience a powerful stellar wind, expelling roughly 33% of its mass. [118] [123] [124] It is possible that during this period, Saturn's moon Titan could achieve surface temperatures conducive to life. [125] [126]
As the Sun expands, it will inevitably engulf the planets Mercury and Venus. [127] Earth's fate is less certain; although the Sun will expand beyond Earth's current orbit, the star's mass loss will cause the planets' orbits to move farther outward due to the weakened gravitational pull. [118] If only this effect were considered, Venus and Earth might escape incineration. [123] However, a 2008 study suggests that Earth will likely be swallowed due to tidal interactions with the Sun's loosely bound outer envelope. [118]
Furthermore, the Sun's habitable zone will migrate outward, eventually extending beyond the Kuiper belt by the end of the red-giant phase. This outward shift will cause icy bodies such as Enceladus and Pluto to thaw. During this time, these worlds could potentially support a water-based hydrologic cycle. However, their small size would prevent them from retaining a dense atmosphere like Earth's, leading to extreme temperature variations between day and night. As the Sun transitions from the red-giant branch to the asymptotic giant branch, the habitable zone will abruptly contract to roughly the region between Jupiter and Saturn's current orbits. However, towards the end of the 200-million-year asymptotic giant phase, it will expand outward again to approximately the same distance as before. [128]
Gradually, the hydrogen burning in the shell surrounding the Sun's core will increase the core's mass until it reaches about 45% of the present solar mass. At this point, the density and temperature will become sufficiently high to initiate helium fusion into carbon, triggering a helium flash. The Sun will then contract significantly, from roughly 250 to 11 times its current (main-sequence) radius. Consequently, its luminosity will decrease from approximately 3,000 to 54 times its current level, and its surface temperature will rise to about 4,770 [K] (4,500 °C; 8,130 °F). The Sun will become a horizontal giant, stably fusing helium in its core, much like it currently fuses hydrogen. This helium-fusing stage, powered by the triple-alpha process, will last only about 100 million years. Eventually, it will exhaust its helium reserves and revert to burning hydrogen and helium in its outer layers. It will expand a second time, entering what is known as the asymptotic giant phase. During this phase, the Sun's luminosity will increase again, reaching about 2,090 times its current luminosity, and its surface temperature will cool to approximately 3,500 [K] (3,230 °C; 5,840 °F). [118] This phase will persist for about 30 million years, after which, over a further period of 100,000 years, the Sun's remaining outer layers will be expelled into space, forming a halo known, somewhat misleadingly, as a planetary nebula. This ejected material will contain the helium and carbon produced by the Sun's nuclear reactions, continuing the enrichment of the interstellar medium with heavy elements for future generations of stars and planets. [129]
The Ring Nebula, a planetary nebula similar to what the Sun will eventually become.
This process is relatively gentle, unlike a supernova, which the Sun is too small to undergo during its evolutionary cycle. Any hypothetical observer witnessing this event would see a significant increase in the solar wind's speed, but not enough to completely obliterate a planet. However, the star's mass loss could destabilize the orbits of the surviving planets, leading to collisions, ejections from the Solar System, or disruption by tidal forces. [130] Following this, the Sun will be reduced to a white dwarf – an extraordinarily dense remnant, retaining 54% of its original mass but compressed to the size of Earth. Initially, this white dwarf might be 100 times more luminous than the Sun is today. It will consist entirely of degenerate carbon and [oxygen], but will never reach temperatures sufficient to fuse these elements. Consequently, the white dwarf Sun will gradually cool, becoming progressively dimmer. [131]
As the Sun dies, its gravitational influence on orbiting bodies, including planets, comets, and asteroids, will diminish due to its mass loss. The orbits of all remaining planets will expand. If Venus, Earth, and Mars still exist, their orbits will shift to approximately 1.4 [AU] (210 million km; 130 million mi), 1.9 [AU] (280 million km; 180 million mi), and 2.8 [AU] (420 million km; 260 million mi), respectively. These planets, along with any others that survive, will become desolate, frigid husks, entirely devoid of life. [123] They will continue to orbit their star, their speed reduced due to their increased distance from the Sun and its diminished gravitational pull. Two billion years later, when the Sun's temperature has cooled to the range of 6,000–8,000 [K] (5,730–7,730 °C; 10,340–13,940 °F), the carbon and oxygen in the Sun's core will solidify, and over 90% of its remaining mass will assume a crystalline structure. [132] Eventually, after approximately one quadrillion years, the Sun will cease to emit any light, transforming into a black dwarf. [133]
Galactic interaction
The location of the Solar System within the Milky Way galaxy.
The Solar System travels through the Milky Way in a solitary orbit, approximately 30,000 light years from the Galactic Center. Its orbital speed is about 220 km/s. The time it takes for the Solar System to complete one revolution around the Galactic Center, known as a galactic year, ranges from 220 to 250 million years. Since its formation, the Solar System has completed at least 20 such revolutions. [134]
Various scientists have proposed that the Solar System's trajectory through the galaxy might be linked to the observed periodicity of mass extinctions in Earth's fossil record. One hypothesis suggests that vertical oscillations of the Sun as it orbits the Galactic Center cause it to regularly pass through the galactic plane. When the Sun's orbit takes it outside the galactic disc, the influence of the galactic tide is weaker. However, as it re-enters the galactic disc, an event occurring roughly every 20 to 25 million years, it comes under the influence of the much stronger "disc tides." Mathematical models indicate that these stronger tides can increase the flux of Oort cloud comets into the Solar System by a factor of four, significantly raising the probability of a devastating impact. [135]
Conversely, other researchers argue that the Sun is currently situated near the galactic plane, yet the last major extinction event occurred 15 million years ago. This discrepancy suggests that the Sun's vertical position alone cannot fully explain such periodic extinctions. Instead, extinctions may occur when the Sun traverses the galaxy's spiral arms. These arms are not only regions of higher concentrations of molecular clouds, whose gravity could potentially perturb the Oort cloud, but also host a greater density of bright [blue giants]. These massive stars have short lifespans and end their existence in violent supernovae. [136]
Galactic collision and planetary disruption
• Main article: Andromeda–Milky Way collision
While the vast majority of galaxies in the universe are receding from the Milky Way, the Andromeda Galaxy, the largest member of the Local Group of galaxies, is on a collision course with us, approaching at approximately 120 km/s. [137] In about 4 billion years, Andromeda and the Milky Way will collide, causing both galaxies to deform as tidal forces stretch their outer arms into vast [tidal tails]. If this initial disruption occurs, astronomers calculate a 12% probability that the Solar System will be pulled outward into the Milky Way's tidal tail and a 3% chance that it will become gravitationally bound to Andromeda, becoming part of that galaxy. [137] Following a series of further glancing blows, during which the likelihood of the Solar System's ejection increases to 30%, [138] the galaxies' supermassive black holes will merge. Ultimately, in roughly 6 billion years, the Milky Way and Andromeda will complete their merger, forming a giant elliptical galaxy. During the merger, if sufficient gas is present, the increased gravity will compress the gas towards the center of the forming elliptical galaxy, potentially triggering a brief period of intense star formation known as a starburst. [137] Additionally, the infalling gas will feed the newly formed black hole, transforming it into an active galactic nucleus. The forces involved in these interactions are likely to propel the Solar System into the new galaxy's outer halo, leaving it relatively unscathed by the radiation generated during these cataclysmic events. [137] [138]
It is a common misconception that this galactic collision will disrupt the orbits of the planets within the Solar System. While it is true that the gravity of passing stars can detach planets into interstellar space, the immense distances between stars make the probability of the Milky Way–Andromeda collision causing such disruption to any individual star system negligible. Although the Solar System as a whole might be affected by these events, the Sun and its planets are not expected to be significantly disturbed. [139]
However, over extended periods, the cumulative probability of a chance encounter with a star increases, making the disruption of the planets virtually inevitable. Assuming that the Big Crunch or Big Rip scenarios for the end of the Universe do not occur, calculations suggest that the gravitational influence of passing stars will eventually strip the dead Sun of all its remaining planets within 1 quadrillion (10^15) years. At this point, the Sun and planets may persist, but the Solar System, in any meaningful sense, will cease to exist. [4]
Chronology
The timeframe of the Solar System's formation has been established through radiometric dating. Scientists estimate the Solar System's age to be 4.6 billion years. The oldest known mineral grains found on Earth are approximately 4.4 billion years old. [140] Such ancient rocks are rare due to Earth's dynamic surface, constantly reshaped by erosion, volcanism, and plate tectonics. To accurately date the Solar System, scientists rely on meteorites, which formed during the early condensation of the solar nebula. Nearly all meteorites (including the famous Canyon Diablo meteorite) yield an age of 4.6 billion years, strongly suggesting that the Solar System is at least this old. [141]
Studies of discs surrounding other stars have also provided valuable insights into the timeline of Solar System formation. Stars aged between one and three million years typically exhibit discs rich in gas, whereas discs around stars older than 10 million years show little to no gas, indicating that giant planet formation within them has concluded. [39]
Timeline of Solar System evolution
Note: All dates and times presented in this chronology are approximate and should be considered as indicators of magnitude only.
| Phase | Time since formation of the Sun | Time from present (approximate) | Event