Planetesimal hypothesis of Chamberlin (1905) – Geography Optional Notes

The redistribution of angular momentum within our solar system has been a topic of controversy. This redistribution deviates from the original angular momentum of Laplace’s nebula and challenges the mono-parental or monistic theories regarding the origin of the earth and other planets. As a result, the concept of a biparental or dualistic origin of the earth gained acceptance among scientists during the first quarter of the 20th century. Theory that supports this idea is the planetesimal hypothesis, which was jointly proposed by Chamberlin and Moulton.

Introduction to the planetesimal hypothesis

The planetesimal hypothesis illuminates not only the origin of the Earth but also sheds light on its structure, the emergence of its atmosphere, and the creation of its continents and ocean basins.

In contrast to monistic concepts like Immanuel Kant’s gaseous hypothesis and Laplace’s nebular hypothesis, the planetesimal hypothesis proposes that the formation of the solar system (and Earth) involved the interaction of two celestial bodies.

According to Chamberlin initially there were two heavenly bodies (stars) in the universe –

• Proto-Sun
• and its Companion Star or Intruding star

Overview of the solar system formation problem

The proto-sun’s behavior and properties were distinct from other stars, as it consisted of small, cold, and solid particles. Unlike Laplace’s nebula, the proto-sun was not hot or gaseous; it was cold and circular in shape. Additionally, there was another star referred to as the “intruding star” or “companion star” that was on a trajectory to pass very close to the proto-sun.

When the intruding star approached the proto-sun closely, an enormous gravitational force caused an infinite number of small particles to detach from the proto-sun’s outer surface. This matter included dust, gases, and rock fragments, which eventually accreted to form planets and other celestial bodies that revolved around the proto-sun.

The planetesimal concept and its key features

The detached particles or planetesimals were initially similar to dust particles, but they varied in size. Some planetesimals near the proto-sun were much larger and became nuclei for the potential formation of planets. Gradually, these larger planetesimals attracted smaller ones.

Consequently, numerous small planetesimals accreted onto the nuclei of the larger planetesimals, causing these large planetesimals to grow into planets through continuous accretion of countless planetesimals. Over time, the proto-sun transformed into the present-day sun. The satellites of the planets were created using the same processes and mechanisms that led to the formation of the planets.

Evidence supporting the planetesimal hypothesis

Meteorites: Meteorites are remnants of the early Solar System and provide important clues about its formation. The composition of chondritic meteorites suggests that they originated from the breakup of planetesimals.

Kuiper Belt Objects: The Kuiper Belt is a region of the Solar System beyond Neptune that contains a large number of small, icy objects. The existence of these objects is consistent with the idea that planetesimals formed beyond the frost line, where temperatures were low enough for water and other volatile substances to freeze.

Orbital Characteristics: The orbits of the planets in the Solar System are consistent with the planetesimal hypothesis. The four inner planets (Mercury, Venus, Earth, and Mars) are relatively small and have rocky compositions, whereas the four outer planets (Jupiter, Saturn, Uranus, and Neptune) are much larger and have gaseous compositions. This pattern is consistent with the idea that the inner planets formed from smaller planetesimals that were primarily composed of rock and metal, while the outer planets formed from larger planetesimals that also contained significant amounts of ice and other volatiles.

Solar Nebula: Observations of other protoplanetary disks around other stars suggest that the planetesimal hypothesis is a plausible mechanism for planet formation. These disks contain evidence of planetesimals and show features similar to those observed in the early Solar System.

Criticisms and challenges to the planetesimal hypothesis

• The theory was unable to account for the presence of a massive passing star that would need to remain in close proximity to the Sun.
• There seems to be an error in the alignment of one star with the Sun, as the vast distances between stars in space make it impossible to travel from one to the other.
• The hypothesis did not provide an explanation for why only nine planets formed and why they differ in size.

Impact and legacy of Chamberlin’s work

Thomas Chrowder Chamberlin (1843-1928) was a prominent American geologist and meteorologist who made significant contributions to the fields of geology, meteorology, and planetary science. His work had a significant impact on our understanding of the Earth’s history and the processes that shape our planet.

Chamberlin’s most significant contributions was his theory of planetesimal formation, which suggested that planets formed from small, solid objects that collided and stuck together. This theory, which was later refined and modified by other scientists, has become one of the most widely accepted theories of planetary formation. Chamberlin’s work on the origin of the solar system also helped establish the field of astrophysics.

Chamberlin’s work on the Earth’s geology was equally important. He studied glacial deposits and proposed that glaciers could erode large valleys and reshape landscapes. He also made important contributions to the study of earthquakes and volcanoes, and was one of the first scientists to propose that the Earth’s internal heat was responsible for the planet’s geological activity.

In addition to his scientific contributions, Chamberlin was an influential educator and mentor. He founded the Journal of Geology and helped establish the Department of Geology at the University of Chicago, where he mentored many students who went on to become leading scientists in their own right.

Chamberlin’s legacy continues to be felt in the fields of geology, planetary science, and astrophysics. His contributions to our understanding of the Earth’s history and the processes that shape our planet continue to inspire and inform research in these fields.

Current status and relevance of the planetesimal hypothesis

The planetesimal hypothesis is still considered a relevant and important theory in planetary science, and it has received support from a variety of observations and experiments. For example, the discovery of numerous asteroids and comets in our solar system suggests that planetesimals were present during the early stages of our solar system’s formation. The planetesimal hypothesis remains a relevant and important theory in planetary science, but it is still an area of active research and ongoing debate.

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