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Planetesimal Hypothesis

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Key Takeaways

• The Planetesimal Hypothesis suggested planets formed from small bits of matter (planetesimals) revolving around the sun, originating from gases pulled out by a near-collision with a passing star.
• This theory was accepted for about 35 years but was later debunked after the discovery that gases pulled from stars would expand and dissipate rather than condense, due to weaker gravitational forces outside the star.
• The Planetesimal Hypothesis is no longer considered a viable explanation for the origin of the solar system.

Planetesimal Hypothesis , a theory of the origin of the solar system. It was proposed by Forrest R. Moulton and Thomas C. Chamberlin about 1900. The theory states that the planets were formed by the accumulation of extremely small bits of matterplanetesimalsthat revolved around the sun. This matter was produced when a passing star almost collided with the sun. During the near-collision, hot gases were pulled out of both stars and the gases then condensed. The planetesimal hypothesis was widely accepted for about 35 years.

The greatest flaw in the theory is the assumption that the material drawn out of the stars would condense. The extremely hot gases that make up a star are held together by the gravitational forces within the star. Once the material was pulled away to where the gravitational forces were weaker, it would expand because of its heat. Before condensation could take place, the gases would have almost entirely dissipated. The planetesimal hypothesis is no longer considered a likely explanation of the origin of the solar system.

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planetesimal

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• How Stuff Works - Science - Planetesimal Hypothesis

planetesimal , one of a class of bodies that are theorized to have coalesced to form Earth and the other planets after condensing from concentrations of diffuse matter early in the history of the solar system . According to the nebular hypothesis , part of an interstellar cloud of dust and gas underwent gravitational collapse to form a primeval solar nebula . Clumps of interstellar matter left behind in the midplane of the solar disk as it contracted toward its centre gradually coalesced, through a process of accretion, to form grains, pebbles, boulders, and then planetesimals measuring a few kilometres to several hundred kilometres across. These larger building blocks then combined under the force of gravity to form protoplanets, which were the precursors of most of the current planets of the solar system.

Within this basic scenario, astronomers have worked out details to explain the particular differences observed in the sizes and compositions of the inner and outer planets. Close to the nascent Sun, temperatures were too high to allow the more abundant, volatile substances in the nebula—those with comparatively low freezing temperatures, such as water , carbon dioxide , and ammonia —to condense to their ices. The planetesimals that eventually formed from the solid material present thus were deficient in volatiles but rich in silicates and other less-volatile materials, which solidified at the higher temperatures. Consolidations of these rocky planetesimals formed the four small, dense inner, or terrestrial, planets— Mercury , Venus , Earth, and Mars . Farther out, at the distance of Jupiter ’s orbit and beyond, planetesimals with a different composition formed at temperatures where water and other volatiles could freeze. Rich in the abundant ices, these bodies coalesced into large protoplanetary cores whose gravity was strong enough to attract the lightest elements, hydrogen and helium , and form very massive objects—the gaseous outer, or giant, planets Jupiter, Saturn , Uranus , and Neptune .

Available evidence indicates that the asteroids , which orbit the Sun mainly in a belt between Mars and Jupiter, are remnants of rocky planetesimals that were prevented by Jupiter’s gravity from consolidating into a planet at that location. A few large, icy planetesimals that were not incorporated into the cores of the giant planets may have become captured moons ; Neptune’s moon Triton and Saturn’s moon Phoebe are believed to be two such examples. Many other icy bodies of planetesimal size and smaller are thought to have remained unconsolidated beyond the orbit of Neptune, forming a debris ring called the Kuiper belt . Astronomers generally agree that Pluto , whose orbit lies partially in the Kuiper belt, is one of its larger members. Billions more pieces of icy debris were gravitationally scattered by the formation of Uranus and Neptune to the outermost reaches of the solar system, where they are believed to reside in a huge spherical shell called the Oort cloud .

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planetesimal hypothesis

Definition of planetesimal hypothesis

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1904, in the meaning defined above

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Astronomer Explains What is a Planetesimal

There are many types of objects in space with similar names and understanding the difference between them can get difficult.

One of such cases is words that contain “planet” in them. You have planets, dwarf planets, planetoids , protoplanets…you get the idea.

Today, we will be taking a look at planetesimals. We will establish a clear and easy-to-understand definition so you can easily tell them apart from all the other types of objects. We will also learn a bit more about them, which will help us learn more about the Solar system’s origins as they played a relevant role in that process.

What is a planetesimal?

Planetesimals are objects made out of rock and ice that formed during the solar system’s early days. You can think of them as the ancestors of planets or very small versions of a planet.

Every planetesimal can become a protoplanet, and later a planet, only a few of them do. As a matter of fact, the large majority of planetesimals were ejected from the center of the Solar system and were sent into the Oort cloud, a large region in the outer edges of the Solar system that contains billions of asteroids and comets.

An object is considered a planetesimal when it is larger than 1 kilometer long (0.6 miles) and can be found orbiting a star. This is where the confusion begins as that is also the same definition of an asteroid . That is because the terms planetesimal and asteroid can be used interchangeably as they refer to the same type of object. The only difference is that planetesimal is more often used in the context of the early days of the Solar system and the origin of planets.

The word planetesimal is derived from the word infinitesimal which is used to refer to a small number close to zero because when compared to a planet, a planetesimal is a very small astronomical object.

Formation of a planetesimal

Scientists believe that planetesimals are formed through a process called accretion where tiny particles of dust, ice, and gas clump together as they travel through space to form larger and larger chunks. Once these chunks reach a certain size, their gravity pulls other pieces to form even bigger objects and so the process can continue until the object reaches the size of a planetesimal.

Later, if the planetesimal keeps bumping into other objects it might become large enough to become a protoplanet which in turn can become a planet.

Sometimes due to gravitational interactions with larger objects or violent collisions, planetesimals end up being ejected from the Solar system or captured within a stable orbit. For example, some moons such as Phoebe in Saturn, are captured planetesimals.

Difference between planetesimal, planet, and protoplanet

A planetesimal, a protoplanet, and a planet can refer to the same object at different stages of its life. The main difference between all of them is their size.

If you know Pokemon, think of a planetesimal as the Charmander stage, a protoplanet as the Charmeleon stage, and a planet as the Charizard stage.

The similarities between planetesimals, protoplanets, and planets are:

• They all orbit a star (the Sun)
• All of them formed out of the same materials (rock, dust, gas, ice)
• All of them formed through accretion

The differences between them include

• Their size. Planetesimals are the size of an asteroid. Protoplanets are about the same size as a moon. Planets are the biggest of them, but smaller than a star.
• Their shape. Due to their smaller size, planetesimals are not always spherical. Both protoplanets and planets have enough mass to reach hydrostatic equilibrium which is the point where gravity makes an object shape itself as a sphere.
• Planets also need to fill an extra requirement to be classified as one. That is, they must clear their orbits out of all other major objects. This is called orbital dominance .

Famous planetesimals

There are a few very primitive planetesimals that astronomers are aware of. They are very important because they might hold keys to better understand the origin of planets and the Solar system.

Some examples are:

486958 Arrokoth

Arrokoth, also known as 2014 MU 69 , is a very curious object located in the Kuiper belt (an asteroid belt located beyond Neptune). It sparked the interest of astronomers because it is not one, but two planetesimals fused together probably because they crashed into each other millions of years ago.

Arrokoth can reveal mysteries about the interactions of objects in the early Solar system, the origin of planets, and the composition of the Solar nebula from where the Solar system was formed.

That is why in 2019 it became the most primitive object visited by a spacecraft when the New Horizons probe flew by it on January 1st, 2019.

Triton, Phoebe

Some moons such as Neptune´s moon Triton and Saturn´s moon Phoebe, are believed to have started as planetesimals.

These objects were captured by the gravity of these planets and became natural satellites.

• A planetesimal is a small-sized astronomical object larger than 1 kilometer and smaller than a moon.
• Asteroids and comets can be considered planetesimals.
• Planetesimals are made out of rock, ice, and metal.
• Planetesimals are the first stage of a planet. These stages go planetesimal > protoplanet > planet

Elena is a Canadian journalist and researcher. She has been looking at the sky for years and hopes to introduce more people to the wonderful hobby that is astronomy.

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Planetesimals

[/caption] A planetesimal is an object formed from dust, rock, and other materials. The word has its roots in the concept infinitesimal, which indicates an object too small to see or measure. Planetesimals can be anywhere in size from several meters to hundreds of kilometers. The term refers to small celestial bodies formed during the creation of planets. One way to think of them is as small planets, but they are much more than that.

The planetesimal theory was suggested by the Russian astronomer Viktor Safronov. The planetesimal theory is a theory on how planets form. According to the planetesimal hypothesis, when a planetary system is forming, there is a protoplanetary disk with materials from the nebulae from which the system came. This material is gradually pulled together by gravity to form small chunks. These chunks get larger and larger until they form planetesimals. Many of the objects break apart when they collide, but some continue to grow. Some of these planetesimals go on to become planets and moons.  Since the gas giants are balls of gas with liquid cores, it may seem impossible that an asteroid-like object formed them. The planetesimals formed the core of these gaseous planets, which turned molten when it enough heat was created.

Other planetesimals turn into comets, Kuiper Belt Objects (KBOs), and trojan asteroids. There is some debate as to whether KBOs and asteroids can be called planetesimals. This is one reason why nomenclature of celestial objects is so difficult. The planetesimal theory is not universally accepted though. Like many theories, there are some observations that cannot be explained, but the planetesimal theory is still very popular.

Many people think that around 3.8 billion years ago, many of the planetesimals were thrown into far away regions, such as the Oort cloud or the Kuiper Belt. Other objects collided with other objects after being affected by gas giants. Phobos and Deimos are believed to be planetesimals that were captured by Mars’ gravity and became satellites. Many of Jupiter’s moons are believed to be planetesimals as well.

Planetesimals are very valuable to scientists because they can provide information about the creation of our Solar System. The exterior of planetesimals have been bombarded with solar radiation, which can change their chemistry, for billions of years. Inside though, there is material that has been untouched since the object was first formed. Using this material, astronomers hope to learn about the condition of the nebulae from which our Solar System was formed.

Universe Today has a number of articles to check out including formation of Mercury and hunting for meteors on Earth .

Check out NASA’s Solar System exploration page and NASA’s articles on formation of planetesimals in a nebula.

Astronomy Cast has an episode on how old the universe is.

Reference: Wikipedia

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solar system: The Planetesimal and Tidal Theories

• The Planetesimal and Tidal Theories

Encounter or collision theories, in which a star passes close by or actually collides with the sun, try to explain the distribution of angular momentum. According to the planetesimal theory developed by T. C. Chamberlin and F. R. Moulton in the early part of the 20th cent., a star passed close to the sun. Huge tides were raised on the surface; some of this erupted matter was torn free and, by a cross-pull from the star, was thrust into elliptical orbits around the sun. The smaller masses quickly cooled to become solid bodies, called planetesimals. As their orbits crossed, the larger bodies grew by absorbing the planetesimals, thus becoming planets.

The tidal theory, proposed by James Jeans and Harold Jeffreys in 1918, is a variation of the planetesimal concept: it suggests that a huge tidal wave, raised on the sun by a passing star, was drawn into a long filament and became detached from the principal mass. As the stream of gaseous material condensed, it separated into masses of various sizes, which, by further condensation, took the form of the planets. Serious objections against the encounter theories remain; the angular momentum problem is not fully explained.

Sections in this article:

• Introduction
• Contemporary Theories
• The Nebular Hypothesis
• Origin of the Solar System
• Physical Properties
• Planetary Motion
• The Planets
• Bibliography

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2024, Columbia University Press. All rights reserved.

See more Encyclopedia articles on: Astronomy: General

Planetisimals

Not all planetesimals merge on collision. Some are unlucky enough to break apart on impact. The one that are lucky enough to survive will eventually grow to approximately one kilometer in size when it can then attract each other to form even larger bodies. This process continues until the accumulation of matter forms into a moon or eventually a planet.

Components of planetesimals vary depending on their distance from the sun. The inner portion of the solar nebula is hot, thus the planetesimals that grow are mostly composed of metals and silicates. The outer portion of the solar nebula is much cooler, thus allowing the formation of planetesimals made mostly of water ice. This can account for the fact that the gas giants in the outer portion of our solar system are mostly composed of gas and liquid cores.

The term planetesimals is also used by some scientists when referring to asteroids, comets and other small astronomical objects that are believed to be debris produced by the planetary formation process.

Planetesimals provide scientists with valuable material from when they were first formed, which is key to further studies on the condition of the solar nebulae around the time our solar system was formed.

Planetisimal Theory

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8.2: Origin of the Solar System—The Nebular Hypothesis

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• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
• Salt Lake Community College via OpenGeology

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Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets [ 12 ]. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.

Planet Arrangement and Segregation

As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water [ 13 ]. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by the weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together [ 14 ]. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to the outer edge of the solar system [ 15 ].

Pluto and Planet Definition

The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria:

• Enough mass to have gravitational forces that force it to be rounded
• Not massive enough to create a fusion
• Large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets

12. Montmerle T, Augereau J-C, Chaussidon M, et al (2006) Solar System Formation and Early Evolution: the First 100 Million Years. In: From Suns to Life: A Chronological Approach to the History of Life on Earth. Springer New York, pp 39–95

13. Martin RG, Livio M (2012) On the evolution of the snow line in protoplanetary discs. Mon Not R Aston Soc Lett 425:L6–L9

14. Petit J-M, Morbidelli A, Chambers J (2001) The Primordial Excitation and Clearing of the Asteroid Belt. Icarus 153:338–347. https://doi.org/10.1006/icar.2001.6702

15. Walsh KJ, Morbidelli A, Raymond SN, et al (2011) A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475:206–209

The origin of the planetesimal theory

• Published: April 1977
• Volume 8 , pages 3–6, ( 1977 )

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T. C. Chamberlin suggested in 1897, on the basis of geological and climatological arguments, that the planets were formed by accretion of cold solid particles. With F. R. Moulton he developed convincing arguments against the Laplace nebular hypothesis and published a comprehensive ‘planetesimal theory’ of the origin of the solar system in 1905. The Chamberlin-Moulton theory has current as well as historical interest.

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Topical problems in the theory of planet formation: formation of planetesimals.

Introduction: New Data Related to the Nature of Creation and Orbiting of the Planets and Satellites

Planet Formation: Key Mechanisms and Global Models

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Brush, S.G. The origin of the planetesimal theory. Origins Life Evol Biosphere 8 , 3–6 (1977). https://doi.org/10.1007/BF00930932

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From Planetesimals to Terrestrial Planets

A recent study provides new insight into the process by which planets in the inner Solar System may have accreted from the disk of gas and dust that surrounded the young Sun. The study presents numerical simulations for the continuous growth of planetesimals into terrestrial planets in the inner Solar System. Contrary to previous studies, the simulations indicate that the entire planet-forming disk never reaches a simple bi-modal mass distribution. Understanding how terrestrial planets were formed in our system can provide important information about the potential for habitable worlds around other stars.

The study, “Planetesimals to terrestrial planets: Collisional evolution amidst a dissipating gas disk,” was published in the journal Icarus . This work was supported by the Emerging Worlds Program. The NASA Astrobiology Program provides resources for Emerging Worlds and other Research and Analysis programs within the NASA Science Mission Directorate ( SMD ) that solicit proposals relevant to astrobiology research.

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Planetesimal hypothesis, /ˈplænəˌtɛsəməl haɪˌpɑθəsəs/.

• noun (cosmology) the theory that the solar system was formed by the gravitational accumulation of planetesimals see more see less type of: scientific theory a theory that explains scientific observations

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planetesimal

[ plan-i- tes - uh -m uh l ]

• one of the small celestial bodies that, according to one theory planetesimal hypothesis, were fused together to form the planets of the solar system.
• of or relating to a planetesimal or planetesimals.

/ plăn′ĭ-tĕs ′ ə-məl /

• Any of innumerable small bodies of accreted gas and dust thought to have orbited the Sun during the formation of the planets.
• ◆ The theory that explains the formation of the solar system in terms of the aggregation of such bodies is known as the planetesimal hypothesis . According to this theory, first proposed in 1900, the planetesimals formed within a spiral disk of dust and gas surrounding a central nucleus. Their gravitational attraction eventually caused the planetesimals to coalesce into protoplanetary disks from which larger objects such as planets, asteroids, and satellites were formed, while the nucleus coalesced into the Sun.

Word History and Origins

Origin of planetesimal 1

Example Sentences

So collisions alone, he argues, can’t explain how planetesimals form.

Then, you produce protoplanets by colliding those planetesimals with each other.

It drifts around in a disk, then piles up to form 100 kilometer-scale “planetesimals,” the real building blocks of the planets.

These planetesimals can keep growing by self-collisions but not by any new dust originating in the outer solar system.

Jupiter’s growing core—a super-big planetesimal about ten times the mass of Earth—may have done exactly that.

The Planetesimal Theory suggests that these thickened knots are embryo planets and the central portion of the nebul an embryo sun.

Let us now inquire briefly into the bearing of this planetesimal hypothesis upon the early geological history of the earth.

On one point the new planetesimal theory differs from the other theories.

In deference to the opinion of a number of geologists we must glance once more at the alternative view of the planetesimal school.

But the planetesimal hypothesis has no room for this enormous percentage of carbon-dioxide in the primitive atmosphere.