Formation of the solar system: nebular hypothesis, accretion, late heavy bombardment

Intuition Beginner

The solar system did not always exist. About 4.6 billion years ago, the Sun and planets formed together from a vast cloud of gas and dust called the solar nebula. This cloud collapsed under its own gravity. As it shrank, it spun faster — like a spinning ice skater pulling in their arms. The spinning cloud flattened into a disk, with most of the material collecting at the centre to form the Sun.

The leftover material in the disk clumped together to build the planets. Close to the hot young Sun, only rocky and metallic material could survive as solid. These leftovers formed Mercury, Venus, Earth, and Mars — the terrestrial planets. Farther from the Sun, it was cold enough for ice and gas to accumulate. There, Jupiter, Saturn, Uranus, and Neptune grew much larger, capturing thick envelopes of hydrogen and helium.

The young solar system was a violent place. Leftover debris — chunks of rock and ice that never joined a planet — crashed into the newly formed worlds. This period of intense cratering is called the Late Heavy Bombardment. It scarred the Moon and the inner planets with craters between 3.8 and 4.1 billion years ago. Understanding how our solar system formed helps explain why Earth is habitable and whether similar worlds exist around other stars.

Visual Beginner

The table below outlines the major stages of solar system formation, from the initial cloud collapse to the Late Heavy Bombardment.

Stage	Approximate timing	What happened
Cloud collapse	4.567 Ga	Solar nebula contracts under gravity
Protosun and disk	~100,000 yr after collapse	Central mass heats up; rotating disk forms
Planetesimals	~1 million yr	Dust grains stick into kilometre-sized bodies
Protoplanets	~10 million yr	Bodies merge; terrestrial and giant cores grow
Gas dispersal	~5-10 million yr	Solar wind clears remaining gas
Late Heavy Bombardment	~600-800 million yr after start	Debris impacts Moon and inner planets

One astronomical unit (AU) equals about 150 million kilometres, the distance from Earth to the Sun. The frost line — the distance beyond which ice could condense — lay near 2.7 AU, between the present orbits of Mars and Jupiter.

Worked example Beginner

Consider what happened as the solar nebula collapsed. The cloud was rotating slowly at first. As gravity pulled the material inward, the cloud had to spin faster. This is the same effect you see when a spinning figure skater pulls their arms close to their body and speeds up.

The rule behind this is called conservation of angular momentum. The total amount of spin stays the same unless an outside force acts on the system. If the cloud shrinks to half its original radius, the material at the edge must move twice as fast in circles. If the cloud shrinks to one-tenth of its radius, it spins a hundred times faster.

The early solar nebula may have been a light-year across. By the time most of the gas had collected into a disk, material near what is now Earth's orbit was travelling at tens of kilometres per second. This rapid rotation explains a key pattern: all eight planets orbit the Sun in the same direction and in nearly the same flat plane. They inherited their motion from the spinning disk that birthed them.

Check your understanding Beginner

Formal definition Intermediate+

The solar nebula was the rotating cloud of gas and dust from which the solar system formed. Its composition by mass was approximately 73 percent hydrogen, 25 percent helium, and 2 percent heavier elements (silicates, metals, ices, and organic compounds) — the products of nucleosynthesis in earlier generations of stars and supernovae ^{[Ch. 22 Solar system formation]}.

The nebular hypothesis

The nebular hypothesis, proposed independently by Immanuel Kant (1755) and Pierre-Simon Laplace (1796), states that the solar system formed from the gravitational collapse of a rotating interstellar cloud. As the cloud contracted, conservation of angular momentum caused it to spin faster and flatten into a disk. The modern version of this hypothesis is supported by observations of protoplanetary disks around young stars, the common orbital plane and direction of the planets, and the compositional gradient from rocky inner planets to gas-rich outer planets.

Angular momentum and disk formation

For a body of mass $m$ orbiting at radius $r$ with angular velocity $\omega$, the angular momentum is $L=m{r}^2\omega$. During collapse, the total angular momentum $L_{\text{total}}$ of the cloud is conserved (to the extent that external torques are negligible). Material that contracts to a smaller radius must increase its angular velocity: $\omega \propto r^{-2}$. A cloud initially rotating slowly over a scale of light-years produces a disk rotating at kilometres per second at planetary distances.

The frost line

The frost line (or snow line) is the distance from the protostar beyond which temperatures fall below approximately 170 K, allowing volatile compounds — water, ammonia, and methane — to condense into solid ice grains. In the early solar system the frost line was located near 2.7 AU. Inside the frost line, only refractory materials (metals and silicate minerals, accounting for roughly 0.6 percent of the nebula mass) could condense, limiting the growth of terrestrial planets. Outside the frost line, the addition of abundant ices increased the available solid material by a factor of three to four, enabling the growth of much larger cores.

Planetesimals and accretion

A planetesimal is a solid body, roughly 1 to 100 km in diameter, formed by the collisional sticking of dust grains within the protoplanetary disk. Once planetesimals reach kilometre scale, gravitational interactions dominate over electromagnetic sticking, and the process of accretion — the gradual growth of larger bodies by gravitational capture of smaller ones — takes over.

The Safronov parameter $\Theta$ quantifies the role of gravitational focusing during accretion:

$$\Theta =\frac{v_{\text{esc}}^2}{v_{\text{rel}}^2}=\frac{2GM}{R{v}_{\text{rel}}^2}$$

where $v_{\text{esc}}=\sqrt{2GM/R}$ is the escape velocity from the accreting body of mass $M$ and radius $R$, and $v_{\text{rel}}$ is the typical relative velocity of planetesimals. When $\Theta \gg 1$, gravitational focusing is strong and the largest bodies grow fastest — the runaway accretion regime. As the planetesimal population is depleted and relative velocities are pumped up by gravitational scattering, $\Theta$ decreases toward unity and growth enters the oligarchic regime, in which a set of similarly sized bodies grow at comparable rates.

The Late Heavy Bombardment

The Late Heavy Bombardment (LHB) refers to a period of intense impact cratering on the Moon and the terrestrial planets, peaking approximately 3.8 to 4.1 billion years ago. Evidence comes from the radiometric dating of impact melts in lunar samples returned by the Apollo missions and from crater counting on ancient lunar and Mercurian surfaces. The LHB may have been triggered by a dynamical instability in the outer solar system — the Nice model — that scattered planetesimals into the inner solar system.

Key result: planetesimal accretion rates and growth timescales Intermediate+

The Safronov accretion model ^{[Planetesimal accretion model]} gives the rate at which a protoplanet of mass $M$ and physical radius $R$ grows by sweeping up planetesimals from a disk with surface density ${\Sigma }_p$ at orbital distance $a$ from the Sun:

$$\frac{dM}{dt}=\pi R^2{\Sigma }_p\Omega \left(1+\Theta \right)$$

where $\Omega =\sqrt{G{M}_{\odot }/a^3}$ is the Keplerian orbital frequency and the factor $(1+\Theta )$ accounts for gravitational focusing. The term $\pi R^2{\Sigma }_p\Omega$ is the rate at which planetesimals would be swept up by geometric cross-section alone (the "particle-in-a-box" rate), and $(1+\Theta )$ boosts this by the bending of trajectories toward the protoplanet.

Runaway growth

In the runaway regime ($\Theta \gg 1$), the accretion rate simplifies to $dM/dt\propto R^2\Theta \propto M^{4/3}$, since $R\propto M^{1/3}$ and $\Theta \propto M{R}^{-1}\propto M^{2/3}$. Because the growth rate increases faster than linearly in mass, the largest body in a feeding zone grows disproportionately fast, pulling away from its competitors. This regime operates while planetesimal velocities are low (damped by mutual collisions and gas drag), keeping $\Theta$ large.

Oligarchic growth and the isolation mass

As the largest bodies — now protoplanets or oligarchs — grow, they gravitationally stir up the remaining planetesimals, increasing $v_{\text{rel}}$ and reducing $\Theta$. Growth enters the oligarchic regime: each oligarch grows at a rate set by its feeding zone (typically a few Hill radii wide), and neighboring oligarchs grow at comparable rates. Growth halts when a protoplanet has consumed all planetesimals in its feeding zone. The resulting isolation mass is:

$$M_{\text{iso}}\approx {\left(2\pi b{a}^2{\Sigma }_p\right)}^{3/2}{\left(3M_{\odot }\right)}^{-1/2}$$

where $b\approx 2\sqrt{3}$ relates the feeding-zone width to the Hill radius. For nominal disk surface densities (${\Sigma }_p\sim 10$ g/cm${}^2$ at 1 AU), the isolation mass at Earth's orbit is approximately one Earth mass, consistent with the formation of the terrestrial planets over a timescale of $10^7$ to $10^8$ years.

Gas giant formation: core accretion versus disk instability

Beyond the frost line, isolation masses are large enough ($\sim 10M_{\oplus }$) that the protoplanet's gravity can capture hydrogen and helium gas directly from the nebula. In the core accretion model, once the core reaches a critical mass of roughly $10M_{\oplus }$, gas accretion enters a runaway phase and a massive envelope accumulates within $10^5$ years. An alternative — the disk instability model — proposes that a massive, cold disk fragments gravitationally into self-gravitating clumps, forming gas giants directly without a solid core precursor. Core accretion explains the bulk of observed gas giants; disk instability may operate in massive disks at large orbital distances.

The Nice model and dynamical instability

The Nice model ^{[Nature 435 (2005) 459-461; Nice model]} proposes that the four giant planets formed in a compact configuration (between roughly 5 and 17 AU) and subsequently migrated through gravitational interactions with a planetesimal disk extending from 17 to 35 AU. Slow scattering of planetesimals over hundreds of millions of years drove Saturn, Uranus, and Neptune outward and Jupiter slightly inward. When Jupiter and Saturn crossed their mutual 2:1 mean-motion resonance, their eccentricities spiked, triggering a dynamical instability that scattered Uranus and Neptune into the outer disk. This event redistributed planetesimals throughout the solar system and may have delivered the impact flux responsible for the Late Heavy Bombardment at 3.8 to 4.1 Ga.

Exercises Intermediate+

Advanced results Master

Chronological anchors: CAIs, chondrules, and isotopic systems

The oldest dated solids in the solar system are calcium-aluminium-rich inclusions (CAIs) found in carbonaceous chondrite meteorites, with a U-Pb age of 4567.3 $\pm$ 0.16 Ma (Amelin et al. 2002; Connelly et al. 2012). CAIs are centimetre-sized aggregates of refractory oxides and silicates that condensed at temperatures above 1500 K in the inner solar nebula. Chondrules — once-molten silicate spherules 0.1 to 1 mm in diameter — formed 1 to 3 Myr after CAIs, requiring transient heating events whose nature remains debated (shock waves, lightning, impact splashing, or planetesimal bow shocks). The sequence CAIs $\to$ chondrules $\to$ planetesimals $\to$ protoplanets defines the chronological backbone of solar system formation, anchored by multiple isotopic systems: U-Pb for absolute ages, ${}^{26}$Al-${}^{26}$Mg for high-resolution relative chronology (half-life 0.717 Myr), ${}^{182}$Hf-${}^{182}$W for metal-silicate differentiation and core formation (half-life 8.9 Myr), and ${}^{53}$Mn-${}^{53}$Cr for early differentiation.

The short-lived radionuclide ${}^{26}$Al, with its initial ratio ${}^{26}$Al/${}^{27}$Al $\approx 5\times 10^{-5}$ in CAIs, served as the primary heat source for early planetesimal melting. Bodies accreting within the first 1 to 2 Myr — while ${}^{26}$Al was still abundant — underwent sufficient internal heating to differentiate into metallic cores and silicate mantles. Bodies forming later, after ${}^{26}$Al had largely decayed, remained undifferentiated; the survival of porous, undifferentiated asteroids such as Ceres and many C-type bodies reflects late accretion.

Oxygen isotope reservoirs and the structure of the nebula

Oxygen isotope ratios (${}^{16}$O, ${}^{17}$O, ${}^{18}$O) divide solar system materials into two genetically distinct reservoirs. Carbonaceous chondrites and other carbonaceous materials plot along a mixing line distinct from the line defined by non-carbonaceous materials (ordinary chondrites, enstatite chondrites, Earth, Moon, Mars). The carbonaceous reservoir is enriched in ${}^{16}$O relative to the non-carbonaceous reservoir. This bimodality, established by Clayton and Mayeda and sharpened by subsequent high-precision work, implies that the early solar system was physically divided into two regions that exchanged little material. Jupiter's core, forming early and opening a gap in the disk, is the leading explanation for this isotopic barrier — carbonaceous material outside Jupiter's orbit and non-carbonaceous material inside remained separate throughout the accretion epoch.

Pebble accretion and the streaming instability

Classical planetesimal formation by pairwise dust sticking faces a bottleneck at centimetre-to-metre scales: particles of this size experience strong gas drag, spiral into the Sun by radial drift on timescales of $10^2$ to $10^3$ years, and fragment rather than grow upon collision. Two mechanisms address this "metre-size barrier." The streaming instability (Youdin and Goodman 2005; Johansen et al. 2007) is a hydrodynamic effect in which millimetre-to-centimetre particles collectively drag on the gas, producing local over-densities that self-gravitate into planetesimals directly, bypassing the intermediate size range. Numerical simulations produce planetesimals of 100 km diameter in a single gravitational collapse.

Pebble accretion (Lambrechts and Johansen 2012, 2014) describes the rapid capture of centimetre-sized pebbles (large enough to be affected by gas drag, small enough to be captured efficiently) by a protoplanet's Hill sphere. The accretion cross-section can exceed the geometric cross-section by orders of magnitude, allowing cores to reach $10M_{\oplus }$ at 5 AU in under 1 Myr — fast enough to accrete gas before disk dissipation. Pebble accretion halts when the protoplanet reaches the pebble isolation mass, at which it opens a partial gap in the gas disk, cutting off the inward pebble flux and starving further growth. The isolation mass at Jupiter's orbit is approximately $20$ to $50M_{\oplus }$, consistent with the core masses inferred for the giant planets.

Grand Tack, Nice model variants, and the dynamical sculpting of the solar system

The Grand Tack hypothesis (Walsh et al. 2011) resolves two puzzles: the small mass of Mars (approximately $0.1M_{\oplus }$, far below what accretion at 1.5 AU should produce) and the low total mass and compositional mixing of the asteroid belt. Jupiter, forming first among the giants, migrated inward through type-II migration in the gas disk to 1.5 AU, scattering the solid material in its path. Saturn, forming later, caught up and opened its own gap; the two planets then shared a common gap and reversed migration direction (the "tack"), moving outward. This inward-then-outward journey depleted the Mars-forming region and implanted a mixed population of dry inner-belt and hydrated outer-belt asteroids into the 2 to 4 AU zone.

The original Nice model (Tsiganis et al. 2005; Gomes et al. 2005; Morbidelli et al. 2005) explains the current orbital architecture of the giant planets and the timing of the Late Heavy Bombardment through a late dynamical instability. Refined versions — the Nice II model (Levison et al. 2011), the "jumping Jupiter" scenario (Brasser et al. 2009), and five-planet instabilities (Nesvorny and Morbidelli 2012) — impose stronger constraints. The jumping Jupiter mechanism requires close encounters between an ice giant and Jupiter that produce rapid, step-like changes in Jupiter's semi-major axis. This reproduces the observed low eccentricities and inclinations of the terrestrial planets (a slow migration would have destabilised them) and the survival of the asteroid belt in its present form. The ejection of a fifth giant planet — an ice giant perturbed by Jupiter and expelled from the solar system — is required in many simulations to match the present four-planet architecture.

Secular resonance sweeping and Kozai-Lidov cycles

During giant planet migration, secular resonances — commensurabilities in precession frequencies — sweep through the asteroid belt and terrestrial planet region. The ${\nu }_6$ secular resonance (associated with Saturn's nodal precession) and the ${\nu }_{16}$ resonance (associated with the vertical precession of the giant planets' orbital planes) migrate as the giant planets move. As these resonances sweep across the asteroid belt, they excite eccentricities and inclinations, depleting the belt and implanting bodies into terrestrial planet-crossing orbits. This secular resonance sweeping is the primary mechanism by which the Late Heavy Bombardment impactors were delivered to the inner solar system.

Kozai-Lidov cycles — periodic exchange between orbital inclination and eccentricity driven by a perturbing body on an inclined orbit — operate in several early solar system contexts. During the epoch when the giant planets were on eccentric, inclined orbits following the Nice model instability, Kozai-Lidov cycles in the asteroid belt and Kuiper belt would have modulated eccentricities and inclinations, contributing to the sculpting of the small-body populations. In the Kuiper belt, the present-day distribution of inclinations among classical KBOs partially reflects this early dynamical excitation.

Exoplanetary context and the diversity of formation pathways

Comparison with exoplanetary systems reveals that the solar system's architecture is not the only outcome of planet formation. Hot Jupiters — gas giants orbiting within 0.1 AU of their host stars — are absent from our system; their presence in approximately 1 percent of Sun-like stars implies inward migration (disk migration or high-eccentricity tidal migration) that did not operate (or operated differently) in the solar system. Compact systems of super-Earths and mini-Neptunes in short-period orbits, common among Kepler-discovered systems, suggest in-situ formation or inner-disk migration pathways that produced no analog in our solar system. The solar system's combination of widely separated giant planets, a debris-cleared inner region (Jupiter's role as gravitational "vacuum cleaner"), and a single large Moon from a giant impact represents one evolutionary track among many.

Connections Master

• 28.01.01 — The solar system: planets, moons, and small bodies. The formation processes described here produce the observed compositional gradient (terrestrial inside the frost line, gas giants outside), the angular momentum distribution, and the small-body reservoirs (asteroid belt, Kuiper belt, Oort cloud) catalogued in the companion unit.

• 28.02.01 — Stars and stellar evolution. The protosun at the centre of the solar nebula is a protostar passing through the T Tauri phase. The stellar formation process — collapse, disk formation, and the onset of hydrogen fusion — is the stellar counterpart of the planetary formation process described here. The T Tauri wind that dispersed the solar nebula's gas is a direct consequence of protostellar evolution.

• 28.04.01 — Cosmology: the Big Bang, expansion, and fate of the universe. The solar nebula was enriched in heavy elements (the 2 percent beyond hydrogen and helium) by nucleosynthesis in earlier generations of massive stars and supernovae. The cosmic chemical evolution that produced these elements, and the timing of the solar system's formation within the Milky Way's star formation history, provide the cosmological context for the nebular hypothesis.

• 28.05.01 — Exoplanets: detection methods and habitability. The diversity of exoplanetary system architectures (hot Jupiters, compact super-Earth systems, resonant chains) tests planet formation theories developed for the solar system. The core accretion model, disk instability model, and migration scenarios described here are the theoretical framework applied to interpret exoplanet demographics.

• 27.08.01 — Earth history and the geologic time scale. The Hadean Eon — the first 500 Myr of Earth's history, including the Moon-forming impact and the Late Heavy Bombardment — bridges planetary formation and the geological record. Isotopic systems (Hf-W, U-Pb, ${}^{26}$Al-${}^{26}$Mg) used to date accretion and differentiation are shared between planetary science and historical geology.

Historical and philosophical context Master

The nebular hypothesis was proposed by Immanuel Kant in his Universal Natural History and Theory of the Heavens (1755), which argued that the solar system formed from a rotating cloud of gas that collapsed under gravity. Pierre-Simon Laplace independently proposed a similar model in his Exposition du Systeme du Monde (1796), envisioning a contracting, rotating nebula that shed successive rings of material, each ring condensing into a planet. For over a century the nebular hypothesis competed with catastrophic theories, notably the proposal by Georges-Louis Leclerc, Comte de Buffon, and later Thomas Chrowder Chamberlin and Forest Ray Moulton that the planets formed from material torn from the Sun by a passing star. James Jeans and Harold Jeffreys refined this tidal theory in the early twentieth century.

The catastrophic theories lost favour for dynamical reasons: material pulled from a star would disperse or fall back rather than condense into stable planets. The nebular hypothesis, however, faced its own difficulty — the angular momentum problem. If the rotating nebula had collapsed to form the Sun, conservation of angular momentum required the Sun to rotate far faster than observed. The Sun contains 99.86 percent of the solar system's mass but less than 1 percent of its angular momentum; the planets carry the remainder. This discrepancy, which favoured catastrophic theories in the early twentieth century, was resolved by the recognition of magnetic braking in the 1950s: the Sun's magnetic field, threading the ionised solar wind, acts as a lever that transfers angular momentum from the Sun to the outflowing plasma (Schatzman 1962; Weber and Davis 1967).

The modern era of planet formation theory began with Viktor Safronov's monograph Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets (1969), which established the planetesimal accretion framework — dust to planetesimals to protoplanets through gravitational capture and collisional growth — that remains the backbone of the field. George Wetherill's numerical simulations in the 1980s and 1990s demonstrated that terrestrial planet formation by accretion reproduces the masses, orbital spacing, and accretion timescales of the inner planets, including the late giant impacts that shaped their final states.

The discovery of exoplanetary systems beginning in 1995 (Mayor and Queloz) and high-resolution imaging of protoplanetary disks by ALMA and other facilities have transformed the field from a discipline with a single data point — our solar system — to a comparative science. The Nice model, Grand Tack hypothesis, pebble accretion theory, and streaming instability model represent successive refinements motivated by both solar system constraints and exoplanetary discoveries.

Bibliography Master

1. Kant, I. (1755). Universal Natural History and Theory of the Heavens. Early formulation of the nebular hypothesis: the solar system formed from a rotating cloud of gas collapsing under gravity.

2. Laplace, P.-S. (1796). Exposition du Systeme du Monde. Independent statement of the nebular hypothesis, proposing that a contracting nebula shed rings of material that condensed into planets.

3. Safronov, V. S. (1969). Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. Nauka, Moscow (English translation: Israel Program for Scientific Translations, 1972). Foundational monograph establishing planetesimal accretion theory, the Safronov parameter, and the runaway-to-oligarchic growth framework.

4. Wetherill, G. W. (1990). "Formation of the Earth." Annual Review of Earth and Planetary Sciences, 18, 205-256. Numerical synthesis of terrestrial planet formation by accretion, including the role of giant impacts.

5. Tsiganis, K., Gomes, R., Morbidelli, A., and Levison, H. F. (2005). "Origin of the orbital architecture of the giant planets of the Solar System." Nature, 435, 459-461. The original Nice model paper: giant planet migration driven by a 2:1 mean-motion resonance crossing.

6. Gomes, R., Levison, H. F., Tsiganis, K., and Morbidelli, A. (2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets." Nature, 435, 466-469. Links the Nice model dynamical instability to the Late Heavy Bombardment.

7. Walsh, K. J., Morbidelli, A., Raymond, S. N., O'Brien, D. P., and Mandell, A. M. (2011). "A low mass for Mars from Jupiter's early gas-driven migration." Nature, 475, 206-209. The Grand Tack hypothesis: Jupiter migrated inward to 1.5 AU before reversing.

8. Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002). "Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions." Science, 297, 1678-1683. High-precision U-Pb dating establishing 4567.3 Ma as the age of the oldest solar system solids.

9. Youdin, A. N. and Goodman, J. (2005). "Streaming instabilities in protoplanetary disks." Astrophysical Journal, 620, 459-469. Theoretical foundation for the streaming instability as a planetesimal formation mechanism.

10. Lambrechts, M. and Johansen, A. (2012). "Rapid growth of gas giant cores by pebble accretion." Astronomy and Astrophysics, 544, A32. Introduces pebble accretion as a mechanism for rapid gas giant core formation.

11. Morbidelli, A., Lunine, J. I., O'Brien, D. P., Raymond, S. N., and Walsh, K. J. (2012). "Building Terrestrial Planets." Annual Review of Earth and Planetary Sciences, 40, 251-275. Comprehensive review of terrestrial planet formation including Grand Tack and Nice model constraints.

12. Nesvorny, D. and Morbidelli, A. (2012). "Statistical study of the early solar system's instability with four, five, and six giant planets." Astronomical Journal, 144, 117. Demonstrates that a five-planet instability best reproduces the observed solar system architecture.

13. Connelly, J. N., Bizzarro, M., Krot, A. N., Nordlund, A., Wielandt, D., and Ivanova, M. A. (2012). "The absolute chronology and thermal processing of solids in the solar protoplanetary disk." Science, 338, 651-655. Refines the absolute Pb-Pb chronology of CAIs and chondrules.

14. Schatzman, E. (1962). "A theory of the role of magnetic activity during star formation." Annales d'Astrophysique, 25, 18-29. Proposes magnetic braking as the mechanism transferring angular momentum from the young Sun to the solar wind.