The most widely accepted model in cosmogony for explaining the birth and evolution of our solar system is the nebular hypothesis (as well as other planetary systems). According to this theory, the Solar System is made up of gas and dust that revolves around the Sun. Immanuel Kant proposed it in his 1755 work, The Universal Natural History and Theory of the Heavens, and Pierre Laplace further refined it in 1796. There are now theories that we can observe planetary system development throughout the universe. SNDM, often known as the solar nebular model, is the most widely accepted current variation of the nebular theory. The almost round and coplanar orbits of the planets, as well as their velocity in the same direction as the Sun's spin, were all explained by this theory. Modern theories of planet formation retain some aspects of the original nebular idea, although most of them have been discarded.
According to the nebular theory, there are massive and thick clouds of molecular hydrogen where stars are formed (GMC). Matter coalesces into dense clumps within these clouds, which rotate and collapse to become stars. A protoplanetary disk (proplyd) is always formed around a newborn star during star formation. Planets could form in specific situations, but this is not well understood now. Because of this, planetary systems are assumed to be the outcome of star formation. The protoplanetary disk of a Sun-like star takes 10–100 million years to evolve into a planetary system once it forms in the first million years of its life.
Preplanetary disks are star-feeding rings of gas and dust. During the T Tauri star stage, tiny dust grains composed of rocks and ice can develop when the disk cools from its initial state of extreme heat. Eventually, the grains will coalesce into planetesimal-sized bodies. A sufficiently massive disk will trigger runaway accretions, rapidly forming planets the size of the Moon to Mars within 100,000 to 300,000 years. The planetary embryos near the star undergo a stage of severe mergers, resulting in a few terrestrial planets. The last phase takes between 100 million to a billion years on average.
Gigantic planets are more challenging to produce. Ice-based planetary embryos are thought to exist beyond the frost line, where they are predominantly composed. Thus, they are several times more massive than the innermost regions of their disks. It's unclear what happens next once the embryo develops. When they reach a mass of 5–10 Earth masses, some embryos appear to continue to expand, which is required to begin accretion of the disk's hydrogen-helium gas. It takes many million years for the protoplanet to acquire a mass of 30 Earth masses (MEarth). At this point, the gas accumulation by the core accelerates and proceeds in a runaway fashion. The bulk of Jupiter's and Saturn-like planets' mass are predicted to accumulate in just 10,000 years. When the gas runs out, the accretion process comes to an end. Created planets can travel over enormous distances during or after their birth. The cores of Uranus and Neptune are assumed to have formed too late, when the disk was nearly gone, and are hence ice giants.
Emanuel Swedenborg initially presented the nebular idea in 1734. Kant, familiar with Swedenborg's work, extended the theory further in 1755, arguing that gaseous clouds (nebulae) slowly rotate, gradually collapse, and flatten owing to gravity, finally producing stars and planets.
Pierre-Simon Laplace's "Exposition du systeme du monde," published in 1796, offered a similar approach. Initially, he hypothesized that the Sun's heated gaseous atmosphere extended throughout the entire Solar System. Using the protosolar nebula, he proposed a shrinking and cooling protosolar cloud. To him, the planets formed from the material ejected from the planets as they became cooler and more compact as they cooled down. Similar to Kant's, but more detailed and scaled-down, his model was a work in progress. Despite its popularity in the 19th century, the Laplacian nebular model suffered several obstacles. The most pressing issue was the allocation of angular momentum between the Sun and the planets. The nebular model could not explain the planets' 99 percent angular momentum, so the nebular model must be wrong. As a result, astronomers around the dawn of the twentieth century mainly abandoned this notion of planet creation.
James Clerk Maxwell (1831–1879) made a significant rebuttal in the 19th century, arguing that material condensation could not occur because of the ring's inner and outer rotations rotating at different speeds. It was also denied by astronomer Sir David Brewster in 1876 when he opined that "those who believe in the Nebular Theory view it as certain that our Earth derives its solid substance and its atmosphere, from the same mechanism, our Earth's moon was cast off from the solar atmosphere." "The Moon must necessarily have drawn out water and air from Earth's aqueous and airy areas and must have an atmosphere," he contended under such a premise. Newton was reported by Brewster as saying that "the development of new systems out of old ones without the mediation of a Divine agency seemed to him preposterous" about nebular theories.
The Laplacian model's perceived flaws prompted scientists to look for a substitute. For example, Thomas Chamberlin and Forest Moulton proposed a planetesimal theory in 1901, followed by the tidal model proposed by James Jeans in 1917, Otto Schmidt in 1944, and William McCrea in 1960 before Michael Woolfson proposed the capture theory. Laplacian concepts about planet formation were revived in 1978 by Andrew Prentice, who established the contemporary Laplacian hypothesis. Many offered ideas were descriptive, and none of these attempts were effective.
SNDM (solar nebular disk model) was developed by Soviet astronomer Victor Safronov and is now regarded mainly as a theory of planet creation. The evolution of the protoplanetary cloud and its development, published in 1969, had a long-lasting impact on scientists' understanding of the formation of the planets, which was translated into English in 1972. Almost all significant planetary formation issues have been defined, and some have been answered in this book. George Wetherill, who found runaway accretion, further extended Safronov's concepts. Astronomers have identified 5,017 extrasolar planets in our galaxy as of 1 May 2022, thanks partly to the SNDM, which was once assumed to apply only to our solar system.
Accretion disks emerge naturally around newly formed stars as the process of star formation proceeds. Approximately one million years after their formation, all-stars may have disks of disks. Evidence from observations of protostars and T Tauri stars, as well as from theory, point to this conclusion. On a thousand-year scale, these disks reveal that the dust grains inside them expand in size, creating one-centimeter-sized particles. Observations
Today, scientists understand how planetesimals expand from 1 km in size to 1,000 km in size through an accretion process. In disks where the number of planetesimals is sufficiently great, this process takes hold and proceeds in a chaotic fashion. Oligarchic accretion takes over as growth slows down. Finally, planets of various sizes are formed depending on the star's distance from Earth. The merger of embryos in the inner section of the protoplanetary disk results in the development a few Earth-sized planets. It is now widely accepted that the origin of terrestrial planets has been solved.
There are some issues with the physics of accretion disks. The most critical factor is the protostar's ability to lose angular momentum when the matter is accreted. Hannes Alfvén proposed that the solar wind shed angular momentum during its T Tauri star phase. Viscous strains convey the momentum to the disk's periphery. Macroscopic turbulence causes viscosity, but the method by which it does so is still a mystery. Alternatively, the star's magnetic field can be used to transmit the star's spin into the surrounding disk, so reducing its angular momentum. Viscous diffusion and photo-evaporation are the primary causes of gas loss in disks.
The nebular disk model's significant unsolved difficulty is the production of planetesimals. It's a wonder how one cm-sized particles turn into 1 km planetesimals. This technique appears to be the key to the riddle of why some stars have planets while others do not.
Giant planet formation time is also a significant issue. Old theories couldn't explain how their cores could form quickly enough to accumulate large amounts of gas from the rapidly vanishing protoplanetary disk, and they couldn't explain. There was some evidence that the disks' average lifespan, which is less than ten million (107) years, is shorter than that required for the development of the core. Current planet formation models can generate Jupiter (or other massive planets) in around 4 million years or less, which is well within the usual lifetime of gaseous disks.
Another potential issue with the development of giant planets is the migration of their orbits. If this rapid inward migration is not halted, the planet will eventually reach the "core regions still as a sub-Jovian entity," according to specific calculations. According to more current simulations, this difficulty can be alleviated by disk evolution during migration.
Giant clouds of cold molecular hydrogen, around the mass of the Sun and 20 parsecs in diameter, are thought to be where stars are formed. Massive molecular clouds are prone to breaking up and disintegrating over billions of years. The small, compact cores formed by these particles eventually collapse into stars. Protostellar (protosolar) nebulae have core masses ranging from a few tens of solar masses. They have a particle number density of 10,000–100,000 cm3 and a diameter of 0.01–0.1 pc (2,000–20,000 AU).
It takes about 100,000 years for a protostar to begin to disintegrate. There is a certain amount of angular momentum at the beginning of every nebula. As the gas in the nebula's core compresses quickly due to its low angular momentum, a hot hydrostatic core forms, comprising only a tiny percentage of the original nebula's mass. This nucleus is the germ of a future star. Because of angular momentum conservation, a faster rotation of the infalling envelope prevents gas from immediately accreting onto the core as the collapse progresses. A disk forms around the equator, which accretes onto the core, forcing the gas to flow outwards. As its mass increases, the core transforms into a young, hot protostar. The protostar and its disk are hidden by the infalling envelope at this stage and cannot be observed directly. Millimeter-wave radiation has problems getting out of the remaining envelope because of its high opacity. Millimeter and submillimeter waves are these objects' primary wavelengths of radiation. Spectral Class 0 protostars are the most common type of star. Outflows (jets) that originate along the disk's rotating axis are common during a collapse. Herbig–Haro (HH) objects are commonly seen in star-forming regions. A solar-mass protostar can radiate up to 100 solar luminosities in Class 0 protostars. As their cores are still too cool to start nuclear fusion, this energy comes from gravitational collapse.
A young stellar object (YSO) becomes visible in the visible spectrum when the disk's envelope grows thin and transparent due to the inflow of material. Deuterium fusion begins at this point in the protostar's development, and hydrogen fusion occurs if the protostar has a mass of at least 80 Jupiter masses (MJ). A brown dwarf, on the other hand, is formed when an object's mass is too low. About 100,000 years after the collapse began, a new star formed. Class I protostars, also known as young T Tauri stars, developed protostars, or young stellar objects, are objects in this stage of evolution. Even though the growing star has gained most of its mass by now, the disk and surviving envelope only account for 10–20 percent of the YSO's total mass.
The envelope dissolves at this point, having been absorbed by the disk, and the protostar becomes a typical T Tauri star. After nearly a million years, something happens. One to three percent of a traditional T Tauri star's mass can be found in its surrounding disk, which accretes material annually in the tens of millions. The presence of two bipolar jets is also common. Magnetic activity, photometric variability, and jets are explained by accretion in classical T Tauri stars. The accretion process is responsible for all of the strange traits of classical T Tauri stars. A star's magnetic poles are where the accreted gas strikes the star's "surface," forming emission lines. Accumulation produces the jets, which remove extra angular momentum from the system. There are approximately 10 million years in the T Tauri stage of evolution. At some point, the disk will vanish due to accretion onto the central star, planet formation, jet ejection, and photoevaporation from the star's UV radiation and other surrounding lights. This results in the formation of a T Tauri star, which gradually transforms into an ordinary Sun-like star over tens of millions of years.
The disk, which can now be referred to as protoplanetary, can give rise to a planetary system in specific circumstances. It has been discovered that protoplanetary disks orbit a large percentage of the stars in newborn star clusters. When a star first forms, they are invisible to the naked eye because the star's surrounding envelope is opaque. Class 0 protostars are predicted to have large disks that are extremely hot. An accretion disk feeds the center protostar. Temperatures of 400 K and 1,000 K can be reached within 5 AU and 1 AU, respectively. Due to the viscous dissipation of turbulence and the gas influx from the nebula, the disk gets heated. Water, organics, and even some rocks in the inner disk evaporate at such a high temperature that only the most refractory metals like iron remain. Only the disk's outermost rim can support the ice's existence.
Researchers have struggled to understand how accretion disks generate and maintain their high effective viscosity for many years. Thought to play a role in the shifting of mass and momentum, turbulent viscosity is a critical factor in disk formation. This is critical for accretion because the core protostar can only take in gas that has lost most of its angular momentum. If the nebula has a large enough starting angular momentum, this process will expand both the protostar and the disk radius, increasing to 1,000 AU. Many star-forming areas, such as the Orion nebula, are known to have large disks.
The accretion disks have a half-life of around 10 million years on average. The disk thins and cools when the star approaches the classical T-Tauri stage. The crystalline silicates in the 0.1–1 m dust grains begin to condense near the center of the cloud. Organic materials and other volatiles may be incorporated into the newly created dust grains via material movement from the outer disk. These interstellar grains and refractory inclusions in comets can be explained by this mixing, which can explain the oddities in the composition of Solar System planets.
Dust particles in the dense disk environment clump together, forming bigger particles up to several centimeters. The infrared spectra of young disks show the effects of dust processing and coagulation. Planetesimals, the basic building blocks of planets, can be formed via further aggregation and reach a diameter of 1 km or more. Another unsolved problem in disk physics is the production of planetesimals because simple adhering fails as dust particles grow larger.
The gravitational instability could be a factor in the development of this object. Particles at least a few centimeters in diameter fall into a dense layer less than 100 kilometers across in the middle of the disk. These aggregates can then collapse into planetesimals if the gravity of this stratum is too strong. Gravitational instability prevents the gas disk from becoming thin enough to fracture because of turbulence caused by velocity differences between the disk and the solids at the mid-plane. Using gravitational instabilities to generate planetesimals may be limited by the concentrations of solids in the disk.
In addition to the streaming instability, another probable process for generating planetesimals is the feedback effect caused by the drag felt by particles orbiting through the gas. These concentrations push the gas back, resulting in a less-vigorous headwind for the particles. Thus, the concentration of particles can increase and condense at a higher rate and drift less radially than before. As they are overrun or drift inward, isolated particles join these concentrations, leading them to expand in mass. Massive filaments grow from these concentrations, and as they break apart and collapse under the gravitational pull of the Sun, planetesimals the size of larger asteroids arise.
Gravitational instability can also cause the disk to fragment, resulting in planets forming. They may collapse if dense enough, resulting in the rapid production of gas giant planets and even brown dwarf stars in 1,000 years. Tides from the star can cause significant mass loss when these clusters migrate inward, leaving behind a smaller body. However, this is only viable in disks that are at least 0.3 M in diameter. As a comparison, disk masses range from 0.01–0.03 M on the low end. This technique of planet formation is assumed to be uncommon since large, more common disks are also rare. There is a possibility that it could have a significant role in the development of brown dwarfs.
Several diverse mechanisms are responsible for the final dissipation of protoplanetary disks. The inner half of the disk is either accreted by the star or ejected by the bipolar jets. Still, the outside section may evaporate under the star's intense UV radiation during the T Tauri stage or by neighboring stars. Small dust particles are ejected by the central star's radiation pressure, while the core region gas can either be accreted or ejected by the expanding planets. If planetesimals failed to develop, all that's left is a planetary system, a dusty remnant, or nothing.
Some planetesimals survive the formation of a planetary system because they are so numerous and dispersed over the protoplanetary disk. Astronomers believe asteroids and comets are formed from debris left over from the formation of planetesimals. In contrast, comets are formed from planetesimals that originate in the outer regions of a planetary system. There is a wealth of information about the origin of the Solar System to be found in meteorites and fragments of planetesimals that have landed on Earth. When a planetesimal is broken into smaller pieces, it is called a "primitive" meteorite; when a planetesimal is broken into larger pieces, it is known as a "processed" one. The nascent Solar system may have assimilated extrasolar planets and other celestial bodies.
When a protoplanetary disk first forms, rocky planets are thought to be born within its frost line, where temperatures are high enough to prohibit water ice and other volatiles from condensing into grains. As a result, only rocky grains coagulate, and planetesimals are made of rock formations. The inner 3–4 AU of the disk of a Sun-like star is thought to have such circumstances.
Runaway accretion begins after the formation of tiny planetesimals of around 1 km in diameter. R4M4/3, where R and M are the radius and mass, respectively, are used as the growth rate of the growing body. Growth in specific (measured as a percentage of total mass) accelerates as mass accumulates. As a result, larger bodies expand more rapidly than smaller ones. It takes 10,000 and 100,000 years for the runaway accretion to halt when the largest bodies reach a diameter of 1,000 kilometers. Large entities on the remaining planetesimals are causing gravitational perturbations, which slow down the accretion. Smaller bodies, on the other hand, cannot grow because of the effect of larger ones.
Oligarchic accretion is the following step. Oligarchs, some of the largest entities, continue to accrete planetesimals and dominate the system. The oligarchs are the only ones who can grow. Right now, the accretion rate is proportional to R2, calculated from an oligarch's geometric cross-section. R2 = 1. Accrual rate decreases as body mass increases; it is inversely related to M1/3. As a result, lesser oligarchs can catch up to larger ones in terms of power. To maintain this distance, the surviving planetesimals exert their influence on the oligarchs, keeping them about ten times as far apart as they would be otherwise. Both the eccentricity and inclination of their orbits are very tiny. The oligarchs continue to grow until the disk around them is depleted of planetesimals. Occasionally, oligarchs from the same region unite. The isolation mass determines an oligarch's final mass based on the distance from the star and the density of the planetesimals. It can be as large as 0.1 MEarth, or the mass of Mars, for rocky planets. When we reach the oligarchic stage, we'll have a constellation of around 100 planet embryos, each roughly the size of the Moon or Mars, evenly spaced throughout time. They're assumed to be tucked away in gaps in the disk, separated by rings of planetesimals still in the neighborhood. A few hundred thousand years are estimated to have passed since the beginning of this phase of the life cycle.
The merging stage is the final step in developing a rocky planet. Few planetesimals are left, and as they grow in mass, the embryos interact with one another, causing their orbits to become unstable. They begin to expel the remaining planetesimals and collide with each other at this point in the development process. It takes between 10 and 100 million years for a few Earth-sized entities to form throughout this process. According to computer simulations, there are between two and five still-existing planets in our solar system. Earth and Venus may be examples of these planets' development in our solar system. Ten to twenty planetary embryos were combined to form both planets, while an equal number were ejected from the Solar System. According to specific theories, asteroid belt embryos may have transported water to Earth. Mars and Mercury may be viewed as the last of the embryos to emerge from the crucible of that conflict. Rock planets that have managed to coalesce eventually settle in more or less stable orbits, which explains why planetary systems always appear to be on the verge of instability.
This is a significant problem in the field of planetary sciences. In the context of the solar nebula paradigm, there are two possible explanations for their origin. Giant planets are formed in huge protoplanetary disks due to their gravitational fragmentation in the first disk instability model (see above). The second option is the core accretion model, often known as the nucleated instability model. Giant planets can form in disks with masses as low as 0.1 M, which is why it is regarded as the most likely scenario. Accretion of a 10 MEarth core is the first step in this model's significant planet creation, followed by accretion from the disk. Brown dwarfs can be created using either method. As of 2011, searches revealed that core accretion was the most critical mechanism for forming new stars.
The formation of the cores of giant planets is considered to follow the same pattern as terrestrial planets. Initially, planetesimals proliferate before slowing down to the oligarchic stage. Because of the minimal likelihood of planetary embryos colliding in the outer reaches of planetary systems, theories do not foresee a merger stage. Giant planets, on the other hand, originate above the so-called frost line, where the planetesimals are mostly made of ice, with an ice to rock ratio of around 4 to 1. As a result, planetesimals have a fourfold increase in mass. Within 10 million years, the minimum mass nebula required for the formation of terrestrial planets can only produce 1–2 MEarth cores in the vicinity of Jupiter (5 AU). According to this figure, the gaseous disks that orbit Sun-like stars have an average lifespan of 1.2 billion years. Among the potential solutions are a tenfold increase in disk mass, protoplanet migration, and accelerated accretion due to drag in the embryos' gaseous envelopes.
Any one or more of the theories mentioned above could account for how gas giant planets like Jupiter and Saturn got their cores. As far as Uranus and Neptune go, no theory has been able to explain how their cores formed in situ at a distance of 20–30 AU from the star that gave rise to them. It's possible that they were first gathered in the vicinity of Jupiter and Saturn, then dispersed and traveled to where they are now. Alternatively, it is plausible that the cores of the giant planets could expand by pebble accretion. Gas drag slows down pebbles between a centimeter and a meter in diameter so they can spiral into an enormous bulk and be accreted. Pebble accretion maybe 1000 times faster than planetesimal accretion in terms of growth rate.
Once the cores have accumulated enough mass (5–10 MEarth), they begin acquiring gas from the disk around them. The increase in mass is gradual at first, taking a few million years to reach a core mass of 30 MEarth. Accumulation rates skyrocket after that, and the remaining 90% of the mass is amassed within 10,000 years. When the disk's supply of gas is depleted, gas accretion stops. A disk and disk dispersal density gap lead to this progressive process. Uranus and Neptune in this model are ice giants that had failed cores because they started accumulating gas too late when almost all the gas had evaporated. The newly formed giant planets migrate, and gas accretion continues slowly in the post-runaway-gas-accretion stage. The planet in the void interacts with the remaining disk, resulting in migration. There is no further progress if the protoplanetary disk vanishes or the disk's end is reached. These so-called hot Jupiters are thought to have halted their migration when they reached the inner hole in the protoplanetary disk, which fits the second scenario better.
Giant planets can strongly influence the creation of Earth's terrestrial planets. The existence of giants in the terrestrial planet zone increases the eccentricities and inclinations of planetesimals and embryos (see the Kozai mechanism) (inside 4 AU in the Solar System). Inner planet accretion can be slowed or halted if giant planets form prematurely. Because of their ability to affect planetary embryo mergers, they may form after the oligarchic stage, as in our solar system. As a result, there will be fewer and larger terrestrial planets as a result, and terrestrial planets will form closer to the central star, reducing the overall system size. The impact of the Solar System's giant planets, like Jupiter, is assumed to have been restricted because they are far from the terrestrial planets.
The area of a planetary system next to the massive planets will be affected uniquely. Embryos may be ejected from the system if their eccentricities are sufficiently severe and they pass close to an enormous planet. In this location, no planets will develop if all embryos are removed. In addition, many small planetesimals will remain because massive planets are incapable of cleaning them all out without the help of embryos. More than 99 percent of the remaining planetesimals will be minuscule in mass, as their gravitational pull will have removed them before being ejected into space. There will eventually be an asteroid belt like the one in the Solar System located between 2 and 4 AU away from our Sun.
In the last two decades, thousands of new exoplanets have been discovered. Mostly, these planets and solar system bodies don't follow the same orbital paths as the sun-orbiting planets. Hot-Jupiters, warm-Jupiters, super-Earths, and systems of densely packed inner planets have all been detected.
After their formation, the "hot-Jupiters" and "warm-Jupiters" may have migrated to their current orbits. There have been several hypotheses put up as to how this migration might take place. The semimajor axis of Jupiter's orbit could be reduced by Type I or Type II migration, resulting in a warm- or hot-Jupiter. Tidal interactions with the star can circularize a planet's orbit after other planets scatter it onto an eccentric orbit with a perihelion close to the star. In the presence of a massive companion planet or star-inclined orbit, the Kozai mechanism can raise eccentricities and lower perihelions, leading to circularization and a close orbit. Eccentric orbits are typical among Jupiter-sized planets, which may suggest that the planets have collided gravitationally; however, migration during resonance can also stimulate eccentricities. It's also been recommended that hot Jupiters could form inside the solar system by accreting mass from nearby super-Earths. According to this theory, the star's cores might have developed near the star or formed farther away and then migrated inward.
Super-Earths and other planets in close orbit around the Sun are either assumed to have developed in situ or ex-situ or migrated inward from their original positions. Small solids from farther out in the disk would have to be radially displaced to generate super-Earths that orbit in close proximity to one another without an enormous disk. They are more likely to have been Type I migratory super-Earths or embryos that collided to generate them due to their lesser masses. Exoplanet systems that are in resonance with each other suggest that migration has occurred, while systems that aren't in resonance imply that instability has occurred after the gas has been dissipated. The lack of Super-Earths and other planets with close orbits in our Solar System may be because of Jupiter's birth.
A super-Earth that originated in situ might have different gas accumulation rates depending on when the planet's embryos fused owing to massive impacts and the gas disk dissipated. In a transition disk, a super-Earth with a gas envelope containing a few percent of its mass may form if the mergers occur after the gas disk fades, allowing for the formation of terrestrial planets. Runaway gas accretion could lead to the development of a gas giant if mergers occur too early. When the dynamical friction of gas disk collisions is insufficient to prevent them, mergers begin a process that begins sooner in a higher metallicity disk. Alternatively, runaway gas accretion could be delayed until the core reaches a mass of 15 Earth masses since the envelopes are not in hydrostatic equilibrium and gas flows through them, limiting their expansion.
Misnomering the protoplanetary disk as an "accretion disk" causes misunderstandings about how planets form. Because gaseous material may still be falling onto the young T Tauri-like protostar from the disk's inner edge while it is still contracting, the protoplanetary disk is frequently referred to as an accretion disk. There is a net mass flow from bigger radii to smaller radii in an accretion disk.
It's important to remember that the term "accretion" does not refer to the formation of planets. It's important to understand that accretion, in this case, refers to the steady growth of planetesimals and the collisions between large planetesimals in the protoplanetary disk as cooled, solidified dust and ice grains.
In addition, the giant planets likely had their own accretion disks. Every protoplanet's surface was blanketed in hydrogen and helium from the condensing clouds forming in the disk's center. Meanwhile, the solid bodies in the disk accreted to form the planets' regular moon systems.