Models of the Solar System

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Approximate sizes of the planets relative to each other. Outward from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Jupiter's diameter is about 11 times that of the Earth's and the Sun's diameter is about 10 times Jupiter's. The planets are not shown at the appropriate distance from the Sun.

Solar Formation Models

Nebular hypothesis

Main Article: Nebular hypothesis

The nebular hypothesis is a widely accepted model explaining the formation and evolution of the Solar System. It proposes that approximately 4.6 billion years ago, the Solar System formed from a large cloud of gas and dust, known as a nebula. Under the influence of gravity, this nebula collapsed into a rotating disk, with the Sun forming at its center. The surrounding material gradually coalesced into planetesimals, which further accreted to become planets, moons, and other celestial bodies. This hypothesis was first proposed by Emanuel Swedenborg in 1734, later expanded by Immanuel Kant in 1755, and further refined by Pierre-Simon Laplace in 1796. Modern observations of protoplanetary disks around young stars provide strong support for this model, suggesting that planetary system formation is a common process throughout the universe.

Dynamical Evolution Models

Grand tack hypothesis

Main Article: Grand tack hypothesis

The Grand tack hypothesis is a model in planetary astronomy that describes the early migration of Jupiter and its significant impact on the formation of our solar system. According to this hypothesis, Jupiter formed approximately 3.5 astronomical units (AU) from the Sun and then migrated inward to about 1.5 AU. This inward movement was halted and reversed when Saturn, also migrating inward, was captured in a 2:3 mean-motion resonance with Jupiter. Together, they migrated outward, with Jupiter eventually settling near its current position at 5.2 AU. This migration is likened to a sailboat tacking, or changing direction, hence the name "Grand tack."

Fission theory

Main Article: Fission theory

The fission theory posits that the Moon originated from Earth's material, having separated early in the planet's history. This hypothesis suggests that during Earth's formative years, a rapid spin caused a portion of its mass to break away, eventually forming the Moon. While this theory offers an explanation for the compositional similarities between Earth and the Moon, it has been largely superseded by the giant impact hypothesis.

Co-formation theory

Main Article: Co-formation theory

The Co-formation theory posits that Earth and the Moon formed simultaneously from the same protoplanetary disk of gas and dust in the early solar system. According to this hypothesis, both bodies coalesced independently but in close proximity, leading to their current positions. This theory accounts for the compositional similarities between Earth and the Moon, particularly their isotopic ratios. However, it struggles to explain differences in core composition and the Moon's lack of volatiles. Additionally, the Co-formation theory does not adequately account for the current angular momentum of the Earth-Moon system.

Capture theory

Main Article: Capture theory

Capture theory posits that the Moon originated elsewhere in the solar system and was subsequently captured by Earth's gravitational pull. This hypothesis accounts for compositional differences between Earth and the Moon, particularly the Moon's lower iron content. However, the theory faces challenges, notably the improbability of Earth capturing such a large object without a collision, given the high relative velocities involved. Additionally, the similar isotopic compositions of Earth and Moon rocks suggest a common origin, which the capture theory struggles to explain.

Migration of Neptune

Main Article: Migration of Neptune

The Migration of Neptune was the shift in the orbital position of Neptune during the early stages of the Solar System's formation, as predicted by modern models of planetary dynamics. This phenomenon is a key element of the Nice model, which proposes that the giant planets of the Solar System initially formed in a more compact configuration before dispersing to their current orbits due to interactions with a massive primordial disk of planetesimals. Neptune is believed to have originally formed closer to the Sun, between 15 and 20 astronomical units (AU), and subsequently migrated outward to its current orbit at approximately 30 AU. This outward migration likely triggered the scattering of smaller objects, contributing to the formation of the Kuiper belt and the disruption of a hypothetical planetesimal disk, leading to the Late Heavy Bombardment approximately 4 billion years ago. The migration process was influenced by gravitational interactions with the other giant planets, particularly Uranus, and the exchange of angular momentum with the planetesimal disk. Evidence supporting Neptune's migration includes the observed distribution of trans-Neptunian objects, resonant populations such as Plutinos in the 3:2 mean-motion resonance with Neptune, and dynamical simulations aligning with the Solar System's current architecture. The migration also has implications for understanding the origins of Neptune's moons, such as Triton, which may have been captured during this period.

Jumping-Jupiter scenario

Main Article: Jumping-Jupiter scenario

The Jumping-Jupiter scenario is a hypothesis that describes a phase of instability in the early Solar System, characterized by the rapid orbital migration of the gas giant planets. This scenario is part of the broader Nice model, which explores the dynamical evolution of the Solar System after its formation. According to this hypothesis, Jupiter, Saturn, Uranus, and Neptune underwent a period of chaotic interactions that included close gravitational encounters, leading to significant changes in their orbital configurations. Jupiter, in particular, is thought to have experienced a series of rapid orbital shifts, or "jumps," as a result of scattering planetesimals or interacting with an additional-Neptune mass planet that was scattered inwards by Saturn. These jumps helped stabilize the inner Solar System, preventing the disruption of terrestrial planet orbits. The Jumping-Jupiter Scenario also explains features of the Kuiper belt, the eccentric orbits of many distant objects, and the Late Heavy Bombardment—a theorized spike in asteroid and comet impacts on the inner planets approximately 4 billion years ago. This hypothesis is supported by numerical simulations and models of planetary migration, but it remains a topic of active research and debate within the scientific community.

Nice model

Main Article: Nice model

The Nice model is a hypothesis that explains the formation and dynamics of the Solar System, particularly the Late Heavy Bombardment (LHB), a period of intense asteroid and comet impacts on the inner planets around 4 billion years ago. The model proposes that the early Solar System consisted of a relatively stable configuration, with the outer planets, especially Jupiter and Saturn, initially forming in a more compact arrangement. However, due to gravitational interactions and a gradual , particularly Jupiter and Saturn, a significant restructuring of the Solar System occurred. This migration caused a destabilization of the orbits of ice giants Uranus and Neptune, which in turn scattered a large number of small bodies in the Kuiper belt and beyond. The interaction between these bodies and the giant planets led to a period of bombardment on the inner planets, the Moon and Earth, which is now known as the Late Heavy Bombardment. The Nice Model is named after the city of Nice, France, where key developments in its formulation were made. It has been supported by various simulations and observations, including the structure of the Kuiper belt, the distribution of asteroid orbits, and isotopic dating of lunar samples. The model has provided significant insights into the early evolution of the Solar System and the factors contributing to the bombardment event that played a crucial role in shaping the planets and their moons.

Nice 2 model

Main Article: Nice 2 model

The Nice 2 model is an advanced framework for understanding the early evolution of the Solar System, building upon the original Nice model. It posits that the giant planets initially resided in a stable quadruple resonance, with each planet in resonance with its nearest neighbors. Over time, interactions with a surrounding planetesimal disk, stirred by Pluto-sized objects, induced inward migration of these planets while maintaining their resonant relationships. This migration increased the eccentricity of the inner ice giant, leading to secular resonance crossings that eventually destabilized the resonant configuration. The resulting gravitational encounters among planets caused significant orbital rearrangements, aligning with observed planetary positions and providing insights into events like the Late Heavy Bombardment.

Five-planet Nice model

Main Article: Five-planet Nice model

The Five-planet Nice model is a theoretical framework that seeks to explain the current configuration of the Solar System, particularly the dynamical history of the outer planets and the scattering of smaller bodies such as comets and asteroids. Developed in 2011 by researcher David Nesvorný, the model suggests that the Solar System originally had five giant planets: Jupiter, Saturn, Uranus, Neptune, and an additional fifth planet, which was likely a massive ice giant. According to the model, a period of gravitational interactions and migrations occurred, with the outer planets—particularly Jupiter, Saturn, Uranus, and Neptune—undergoing significant shifts in their orbits. The migration of these planets caused the fifth planet to be ejected from the Solar System, while the interactions also resulted in the scattering of numerous small objects, contributing to the formation of the Kuiper Belt and the late heavy bombardment. The model is named after the Nice Observatory in France, where much of the early research was conducted. One of the key features of the Five-Planet Nice Model is the explanation of the current orbital distribution of the outer planets and the observed characteristics of the outer Solar System, such as the chaotic orbits of comets and the positioning of trans-Neptunian objects. The model has been influential in advancing the understanding of planetary migration and the evolution of the Solar System, though it remains a subject of ongoing research and debate within the scientific community.

Planetary Formation Models

Giant-impact hypothesis

Main Article: Giant-impact hypothesis

The Giant-impact hypothesis is the leading explanation for the formation of the Moon, suggesting that the Moon was created as a result of a collision between Earth and a Mars-sized protoplanet, often referred to as Theia, approximately 4.5 billion years ago. This hypothesis is supported by a variety of evidence, including the similarity in isotopic compositions between Earth and Moon rocks, particularly oxygen isotopes, which suggests a shared origin. According to the hypothesis, the impact caused a large portion of the Earth's outer layer to be ejected into orbit, where it coalesced to form the Moon. The giant impact is also believed to have caused the Earth to tilt on its axis, leading to the planet’s current rotational characteristics. Models of the collision indicate that it would have been a highly energetic event, melting much of the impacted region and leading to the formation of a hot, molten Moon that eventually cooled and solidified. This hypothesis also helps explain the Moon's relatively small iron core, which contrasts with Earth’s larger core. Over the years, alternative theories, such as the fission theory and the co-accretion theory, have been proposed, but the Giant-impact hypothesis remains the most widely accepted explanation among scientists due to its consistency with current geological and observational data.

Placement Models

Heliocentric model

Main Article: Heliocentric model

The Heliocentric model, also known as the Copernican model, is an astronomical theory that posits the Sun as the center of the solar system, with Earth and the other planets orbiting around it. Proposed by the Polish mathematician and astronomer Nicolaus Copernicus in the 16th century, the model replaced the geocentric model, which had held that Earth was the center of the universe. Copernicus’ heliocentric theory was first published in his seminal work De revolutionibus orbium coelestium in 1543. The model was revolutionary as it challenged long-held beliefs based on the Ptolemaic system and the works of ancient Greek philosophers such as Aristotle and Claudius Ptolemy. Though initially controversial, the heliocentric model laid the groundwork for later astronomical discoveries, including the laws of planetary motion formulated by Johannes Kepler and the development of the telescope by Galileo Galilei. Kepler's discovery of elliptical orbits and Galileo's telescopic observations provided crucial evidence supporting the heliocentric model. The work of Isaac Newton, particularly his law of universal gravitation, further cemented the Sun-centered system. The heliocentric model fundamentally altered humanity's understanding of the universe, leading to the eventual acceptance of the solar system's true structure and influencing the development of modern science, particularly in the fields of astronomy, physics, and cosmology.

Geocentric model

Main Article: Geocentric model

The Geocentric model, also known as the Earth-centered model, is a historical framework for understanding the universe, where Earth is at the center and all other celestial bodies, including the Sun, Moon, planets, and stars, are believed to revolve around it. This model was widely accepted in ancient civilizations, particularly in Greek, Roman, and medieval European astronomy, with significant contributions from philosophers such as Aristotle and Ptolemy. Ptolemy's Almagest, developed in the 2nd century CE, formalized the model, incorporating complex mechanisms such as epicycles (small circular orbits) to explain observed planetary motions, including retrograde motion. The Geocentric Model remained dominant for over a millennium, underpinned by religious and cultural beliefs that placed humans at the center of the cosmos. However, by the 16th century, the work of astronomers such as Nicolaus Copernicus, Johannes Kepler, and Galileo Galilei challenged this view, leading to the gradual acceptance of the Heliocentric model, which positioned the Sun at the center of the solar system. Despite its eventual displacement by heliocentrism, the Geocentric Model remains an important part of the history of science, providing the foundation for early astronomical observations and theories.

References

Models of the Solar System
Solar Formation		Nebular hypothesis
Dynamical Evolution
		Origin of the Moon
	Early Development	Grand tack hypothesis • Fission theory • Co-formation theory • Capture theory • Migration of Neptune • Jumping-Jupiter scenario
	Later Development	Nice model (Nice 2 model • Five-planet Nice model)
Planetary Formation		Giant-impact hypothesis
Placement		Heliocentric model • Geocentric model