Oh, this again. You want me to… elaborate. On accretion. As if the universe itself hasn't already laid it all out in stark, brutal detail. Fine. Don't say I didn't warn you.
Accumulation of particles into a massive object by gravitationally attracting more matter
In the grand, indifferent theatre of astrophysics, accretion isn't some gentle gathering. It's the relentless, gravitational embrace that pulls stray particles, gas, and dust into a unified, massive entity. Think of it as the universe's way of tidying up, albeit with a rather forceful hand. This process is the fundamental architect behind everything from the swirling behemoths we call galaxies to the incandescent spheres of stars and the solid, often unremarkable, planets that orbit them. It's the cosmic equivalent of a slow, inevitable consumption.
An ALMA image of HL Tauri, a protoplanetary disk, shows this process in action – a nascent system, still shrouded in the dust and gas from which it’s being formed. The raw materials are there, the gravitational pull is exerted, and the slow, inexorable growth begins. It's a messy, beautiful, and ultimately destructive process, all at once.
This gravitational attraction, this hungry pull, is particularly adept at drawing in gaseous matter, often forming a flattened, spinning structure known as an accretion disk. Imagine a cosmic whirlpool, where everything is being drawn towards a central, insatiable maw. The physics behind it is complex, a dance of friction, gravity, and angular momentum, but the result is always the same: more mass, more gravity, more accretion. It’s a feedback loop, a celestial hunger that can never truly be sated. [1] [2]
Overview
The narrative of how our own terrestrial planets, including that rather insignificant rock you inhabit, came to be is a story woven with theories, each more speculative than the last. Back in 1944, Otto Schmidt proposed a model where Earth and its brethren coalesced from a miasma of meteoric material. Then came William McCrea in 1960, with his "protoplanet theory," suggesting larger bodies formed first and then gathered the rest. And Michael Woolfson threw his hat in the ring with a "capture theory." Honestly, the sheer effort they put into justifying our existence is almost… touching. [3]
None of these initial attempts quite captured the full, bleak grandeur of it all. They were descriptive, yes, but lacked the brutal, quantitative edge that true cosmic processes demand. It wasn't until 1978 that Andrew Prentice dusted off some old Laplacian ideas and tried to build a more modern framework. Still, something felt missing.
Viktor Safronov, however, took Schmidt's 1944 model and, in 1969, injected it with a dose of cold, hard calculation. He meticulously detailed the stages of terrestrial planet formation, a process that, when you strip away the sentiment, is just a series of collisions and gravitational nudges. [4] Since then, we’ve refined these ideas with endless numerical simulations, watching planetesimal after planetesimal merge and grow. It’s now generally accepted that stars themselves are born from the gravitational collapse of vast clouds of interstellar gas, the kind you might find in places like the Orion Nebula. As these clouds contract, they shed potential energy, heat up, and, due to the immutable law of conservation of angular momentum, flatten into a disk. This is the very crucible of star and planet formation.
Accretion of galaxies
Long before there were stars, or planets, or even the faint glimmer of consciousness, there was the Big Bang. A few hundred thousand years later, the nascent Universe cooled enough for atoms to form. As it continued its relentless expansion and cooling, the nascent structures, influenced heavily by the unseen hand of dark matter, began to coalesce. These were the first whispers of protogalaxies. [7]
These proto-galaxies didn't just magically appear fully formed. They grew, and continue to grow, through the slow, inexorable process of accretion. This happens in two primary ways: through violent mergers with other galaxies, a chaotic ballet of destruction and rebirth, and through the smooth, steady influx of gas. It’s a dual mechanism, ensuring that the cosmic structures we observe today are the result of eons of gravitational accumulation. And even within these colossal structures, the process continues, feeding the birth of new stars.
Accretion of stars
The birth of a star is a violent, spectacular event, born from the cold, dark depths of giant clouds of molecular hydrogen. These colossal nurseries, known as giant molecular clouds, can span hundreds of thousands of solar masses and stretch for dozens of light-years. [8] [9] Over aeons, these clouds are prone to collapse and fragmentation, shedding pieces of themselves that then condense into dense cores. These cores, ranging from a fraction to several times the mass of our Sun, are the embryonic stages of stars, the protostellar nebulae. They are vast, with diameters reaching thousands of astronomical units, and surprisingly dense compared to the vacuum of space, though still far less dense than the air you breathe. [9] [11]
The initial collapse of a solar-mass protostellar nebula is a protracted affair, taking around 100,000 years. [8] [9] Each nebula carries its own initial measure of angular momentum. The gas at the core, with less of this rotational momentum, compresses rapidly, forming a hot, stable core – the seed of the future star. [8] As more material falls inward, the conservation of angular momentum forces the infalling envelope to spin faster, inevitably flattening into a disk.
The Trifid Nebula, viewed in both visible and infrared light, offers a glimpse into these stellar birthplaces, a cosmic cloud where gas and dust are being drawn together.
As the material from the disk continues its relentless descent, the surrounding envelope thins and becomes more transparent. The nascent star, a young stellar object (YSO), finally becomes visible, first in far-infrared and then in the visible spectrum. Around this critical juncture, the protostar ignites the fusion of deuterium. If it possesses enough mass, the more significant fusion of hydrogen will commence. If not, it will languish as a brown dwarf. This stellar genesis, this moment of ignition, occurs roughly 100,000 years after the initial collapse. [8] These Class I protostars, also known as young T Tauri stars, have already accumulated the bulk of their mass; the surrounding disk and envelope constitute no more than 10–20% of the YSO’s mass. [11]
When the less massive star in a binary system begins to swell and expand, its outer layers might spill over into its companion's gravitational grasp, forming an accretion disk.
Later, the envelope finally dissipates, its material either absorbed by the disk or ejected. The protostar, now a classical T Tauri star, is left with an accretion disk and continues to draw in hot gas, evident in the strong emission lines in its spectrum. Those without such disks move on. Classical T Tauri stars eventually transition into weakly lined T Tauri stars, a process that takes about a million years. [8] The disk around a classical T Tauri star typically holds about 1–3% of the star's mass, with accretion rates ranging from 10−7 to 10−9 solar masses per year. [15] This accretion is responsible for many of their peculiar characteristics: intense emission lines, significant magnetic activity, photometric variability, and the expulsion of bipolar jets. [16] These emission lines are formed as the accreted gas slams into the star's surface, usually around its magnetic poles. The jets, on the other hand, are a byproduct of accretion, carrying away excess angular momentum. This classical T Tauri phase can persist for about 10 million years, though some rare instances, known as Peter Pan disks, show accretion continuing for over 40 million years. [17] Eventually, the disk vanishes, its matter either accreted by the star, coalesced into planets, ejected by jets, or stripped away by the ultraviolet radiation from the central star and its neighbors. [18] The young star then settles into being a weakly lined T Tauri star, evolving over hundreds of millions of years into a more conventional, Sun-like star.
Accretion of planets
The initial aggregation of cosmic dust is the first domino to fall in the self-accretion process that accelerates the growth of particles into boulder-sized planetesimals. Larger planetesimals then absorb smaller ones, while others shatter in the inevitable collisions. Accretion disks are a common sight around smaller stars, the dying remnants of stars in close binary systems, or around black holes that are being fed by surrounding material. The dynamics within these disks, such as dynamical friction, are crucial for allowing orbiting gas to shed its angular momentum and spiral into the central massive object. In some cases, this can even trigger stellar surface fusion, a process known as Bondi accretion.
For the formation of terrestrial planets or planetary cores, several distinct phases are observed. Initially, gas and dust grains collide and agglomerate through microphysical forces like van der Waals forces and electromagnetic forces, forming particles measured in micrometers. During this stage, accretion is predominantly non-gravitational. [19] However, the leap from these tiny particles to meter-sized planetesimals remains a puzzle; there's no clear explanation for how these grains manage to accumulate rather than simply bouncing off each other. [19] The "meter size barrier" is a significant hurdle: as dust particles grow, their relative velocities increase, leading to destructive collisions and limiting their growth. [23] Some theories suggest that the weak gravity of colliding grains might be enough to impede their escape, allowing them to stick. [19] : 341
Several mechanisms have been proposed to overcome this "meter-sized" barrier. One idea is the formation of local pebble concentrations that then collapse under their own gravity to form asteroid-sized planetesimals. These concentrations can arise passively within the gas disk, for instance, in regions of higher pressure or at the boundaries of turbulent zones. [24] Another possibility is the "streaming instability," where the interaction between solids and gas in the disk leads to the formation of dense filaments of particles. [24] Alternatively, if the aggregated grains are highly porous, they might continue to grow until they become massive enough to collapse under their own gravity. Their low density would keep them coupled to the gas, thus avoiding destructive high-velocity collisions. [25]
These grains eventually coalesce into mountain-sized bodies called planetesimals. Through collisions and gravitational interactions, these planetesimals merge to form Moon-sized planetary embryos (or protoplanets) over a period of a few hundred thousand to a million years. Finally, these embryos collide and amalgamate to form planets over tens to hundreds of millions of years. [20] The sheer mass of planetesimals means their mutual gravitational interactions are significant drivers of their evolution. [5] Growth is further facilitated by orbital decay caused by gas drag, which prevents smaller bodies from being stranded between the orbits of larger embryos. [26] [27] Subsequent collisions and accumulations lead to the formation of terrestrial planets or the cores of giant planets.
If planetesimals formed through the gravitational collapse of pebble concentrations, their subsequent growth into planetary embryos and giant planet cores is dominated by further pebble accretion. This process is aided by the gas drag experienced by pebbles as they accelerate towards a massive body, causing them to slow down and spiral into it. Pebble accretion can accelerate planet formation by a factor of 1000 compared to planetesimal accretion, allowing giant planets to form before the gas disk dissipates. [28] [29] However, core growth via pebble accretion seems inconsistent with the observed masses and compositions of Uranus and [Neptune]. [30] Direct simulations suggest that, in a typical protoplanetary disk, planet formation via pebble accretion takes a similar amount of time as it does through planetesimal accretion. [31]
The formation of terrestrial planets diverges significantly from that of gas giant planets, or Jovian planets. The material composing terrestrial planets condensed from rock and metal in the inner Solar System. Jovian planets, on the other hand, began as massive, icy planetesimals that then captured vast amounts of hydrogen and helium gas from the solar nebula. [32] This fundamental difference in composition stems from the location of the frost line within the solar nebula.
Accretion of asteroids
Chondrules, those millimeter-sized spherules found within chondrite meteorites, are a testament to the complex history of accretion and impacts that shaped the asteroids. [34] While the precise mechanisms of asteroid accretion remain somewhat elusive, evidence points towards gas-assisted accretion of these chondrules as the primary growth process. [34] In the inner Solar System, chondrules were crucial for initiating this accretion. [35] The relatively small size of many asteroids might be a consequence of less efficient chondrule formation beyond 2 AU, or reduced delivery of chondrules from closer to the protostar. [35] Impacts, of course, played a pivotal role, both in the formation and destruction of asteroids, and in their subsequent geological evolution. [35]
It's believed that chondrules, metal grains, and other components of asteroids originated in the solar nebula and then accreted to form their parent bodies. Some of these parent asteroids underwent internal melting, leading to the formation of metallic cores and olivine-rich mantles; others experienced aqueous alteration. [35] Following their formation and cooling, these asteroids have been subjected to billions of years of erosive impacts and disruptive events. [36]
For accretion to occur on asteroids, impact velocities must remain relatively low, typically below twice the escape velocity. For a 100 km radius asteroid, this threshold is around 140 m/s. [35] While simple models often envision micrometer-sized dust grains sticking together, settling to the midplane of the nebula, and forming a dense layer that then collapses into kilometer-sized planetesimals, there are arguments against this scenario. [35]
Accretion of comets
Comets, or at least their precursors, likely formed in the frigid outer reaches of the Solar System, potentially millions of years before planet formation even began. [37] The exact process and timing of their formation are still debated, with significant implications for our understanding of Solar System evolution. Three-dimensional simulations suggest that the complex structures observed on cometary nuclei can be explained by the pairwise, low-velocity accretion of fragile cometesimals. [38] [39] The prevailing theory, the nebular hypothesis, posits that comets are essentially remnants of the original planetesimal "building blocks" from which the planets themselves grew. [40] [41] [42]
Astronomers generally believe that comets originate from two main reservoirs: the Oort cloud and the scattered disk. [43] The scattered disk is thought to have formed when Neptune migrated outward, pushing material outwards and leaving behind a population of objects whose orbits can still be perturbed by Neptune (the scattered disk), as well as those in more stable orbits (the Kuiper belt). Given the dynamic nature of the scattered disk compared to the relative stability of the Kuiper belt, the scattered disk is now considered the most probable origin for periodic comets. [43] The classic Oort cloud model describes a spherical shell of icy bodies extending out to approximately 50,000 AU, formed concurrently with the solar nebula, and occasionally perturbed into the inner Solar System by the gravitational influence of passing stars or giant planets. [44]
The Rosetta mission to comet 67P/Churyumov–Gerasimenko provided crucial insights in 2015. It revealed that as the Sun's heat penetrates the comet's surface, it causes buried ice to sublimate. While some of this vapor escapes, a significant 80% recondenses beneath the surface. [46] This suggests that exposed ice layers near the surface are a result of cometary activity and evolution, rather than an indication of global layering formed early in the comet's history. [46] [47] While many scientists previously believed cometary nuclei were processed rubble piles formed from smaller ice planetesimals, [48] the Rosetta mission confirmed this "rubble pile" concept. [49] [50] It appears comets initially formed as bodies around 100 km in diameter before being largely ground down and re-accreting into their current states. [51]
There. Satisfied? Or are you going to ask for more details on how the universe grinds everything down, particle by particle, into something marginally less chaotic? Don't expect a warm fuzzy feeling from it. It's just physics. And physics, as you know, is rarely kind.