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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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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.

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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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• Published: 22 December 2021

PLANET FORMATION

Archaeology of the Solar System

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A new model for the origin of the Solar System proposes planet building blocks formed fast from material that was transported outwards to cooler regions. It claims to be consistent with the properties of ancient meteorites.

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Formation of Planetesimals: The Building Blocks of Planets

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Planetesimals are traditionally defined as solid objects (rocky or icy or a combination of both) whose internal strength is dominated by self-gravity rather than material strength. This corresponds to bodies of approximately 100 m to 1 km in size (Benz 2000 ). However, this definition does not take into account the role of the body in the planet-building process. Planetesimals of km in size are vulnerable to erosion and fragmentation in collisions with other planetesimals at the speeds relevant in protoplanetary disks (Ida et al. 2008 ). One can instead define the planetesimal formation stage as the growth of bodies to sufficient sizes to be insensitive to disruption in collisions with equal-sized bodies. This stage of planet formation may extend to as large as 1,000 km in size, depending on the strength of the gas turbulence which is responsible for inducing high planetesimal-planetesimal speeds.

Planet formation takes place in protoplanetary disks of gas and dust...

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Johansen, A. (2014). Formation of Planetesimals: The Building Blocks of Planets. In: Amils, R., et al. Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-27833-4_5251-1

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Chamberlin-Moulton planetesimal hypothesis

The Chamberlin-Moulton planetesimal hypothesis is a catastrophic hypothesis , proposed by Thomas Chamberlin and Forest Moulton in 1905, in which the planets of the Solar System are seen to arise from an encounter between the Sun and another star. In this scenario, the gravity of the passing star tears a succession of bolts from the solar surface. Bolts coming from the side nearer the star are thrown out to distances comparable with those of the giant plants, while those from the far side of the Sun are ejected less violently to the distances of the terrestrial planets. From the inner remains of these bolts formed the initial cores of the planets. The outer parts expanded and cooled into a huge swarm of solid particles spread out in a disk rotating about the Sun in a plane determined by the motion of the passing star. The cores gradually grew into planets by gathering in the planetesimals , most of the growth taking place in the outer parts of the Solar System where material was more plentiful.

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Planetesimals

Planetesimals are small bodies of rock and/or ice that form by accretion in the protoplanetary disks of protostellar systems. These small objects continue to accrete and merge until finally a planetary system is formed. In our own Solar System, small asteroids are examples of leftover planetesimals.

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Examples Of Planetesimals

The Characteristics of Comets, Meteors & Asteroids

Terrestrial planets, gas giants, comets, moons, and asteroids number among the numerous types of heavenly bodies that make up our solar system. Planetesimals form when clumps of rock and matter start to congeal together; they are thought to be the building blocks of planet formation. They are located in many parts of the solar system, and some astronomers believe they are key to the history of planets and moons. Planetesimal matter such as rock and dust may have combined with gravity to form a number of the masses orbiting the sun.

Planetesimal Particulars

Russian astronomer Viktor Safronov theorized that, during the formation of the solar system, the attractive force of gravity pulled bits from nebulae – clouds of dust, gasses and plasma – together, creating rocky planetesimals of various sizes. If the planetesimals nearest the sun were composed of matter that had high melting points, they may have formed the four inner planets.

A similar conjecture, the Chamberlin–Moulton planetesimal hypothesis, proposed similar ideas of planetesimal objects forming in the accretion around planets.

The outer planets could have come from planetesimals made from different materials that formed dense cores, attracting light gases such as hydrogen and helium. This may have resulted in the four giant planets known as gas giants.

In the early solar system, planets started off as planetesimals and debris started to accrete and form protoplanets. These were times of great chaos and collision, but eventually, full planetary systems were formed from the wreckage.

There are many types of nebular formations and nebulae. The sun likely formed out of a solar nebula, and the left over gases and materials would continue to coalesce into smaller bodies. Everything from meteors to Uranus would eventually form from these remaining building blocks.

Meteors are only classified as meteorites by space agencies like NASA once they make contact with a planet’s surface.

Pluto's New Category

Pluto was once considered one of the nine planets in earth’s solar system. However, in the latter part of the 20th century, many astronomers believed that Pluto was simply not large enough to be considered a major planet. Some of these scientists began referring to Pluto as a planetesimal. By 2006, most astronomers in the International Astronomical Union generally agreed that Pluto was not a planet, although this was a controversial decision for some scientists and nonscientists. Dropping Pluto from the planetary list was intended as a reclassification rather than a demotion.

Ceres is another famous dwarf planet that resides in the asteroid belt.

In 1943, Irish astronomer Kenneth Edgeworth suggested that undiscovered objects lay near the outer boundary of the solar system. In 1951, Gerard Kuiper offered further evidence to support this idea. In fact, a ring of icy bodies, now commonly known as the Kuiper belt, orbits the sun beyond Neptune. Some of the larger objects in the belt are considered planetesimals or "super comets." Since 1992, many have been identified. Pluto is one of the larger bodies within this grouping. There are countless smaller Kuiper Belt objects, and there may even be larger planets or celestial bodies waiting to be discovered.

Beyond this Kuiper Belt lies an expanse of far-away icy comets, asteroids, and rocky planets called the Oort Cloud. These objects are hard to directly confirm because of their distance and size, but their existence is supported by the appearance of comets and small bodies appearing periodically in the solar system.

Many of the moons orbiting planets are sometimes considered planetesimals. The largest of Neptune’s 13 moons, Triton, falls into this category. One of Saturn’s 53 moons, Phoebe, is a planetesimal, as well as both of Mars’ moons, Phobos and Deimos. In addition, Jupiter has 50 moons, and several of these match the criteria for planetesimals.

There is some debate as to the differentiation of planetesimals and moons. Some of the moons orbiting these planets are thought to be left overs of planetary formation or the protoplanetary disk, but it is hard to determine conclusively.

Venus and Mercury are the only two planets in the solar system without moons.

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• Rochester Institute of Technology: Terrestrial Bodies in the Solar System
• Universe Today: Planetesimals
• NASA: Earth Rocks on the Moon
• Encyclopedia Britannica: Planetesimal
• Universe Today: What Is A Nebula?
• Harvard University: International Comet Quarterly: Is Pluto a giant comet?
• National Geographic: Pluto Not a Planet, Astronomers Rule
• Encyclopedia Britannica: Kuiper Belt
• University of Maryland, Baltimore County: The Kuiper Belt
• NASA: Neptune: Moons
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The history of scientific thought about the Formation and evolution of the Solar System begins with the Copernican Revolution. The first recorded use of the term "Solar System" dates from 1704.

1. Contemporary View

The most widely accepted theory of planetary formation, known as the nebular hypothesis, maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud which was light years across. Several stars, including the Sun, formed within the collapsing cloud. The gas that formed the Solar System was slightly more massive than the Sun itself. Most of the mass collected in the centre, forming the Sun; the rest of the mass flattened into a protoplanetary disc, out of which the planets and other bodies in the Solar System formed.

There are, however, arguments against this hypothesis.

2. Formation Hypothesis

French philosopher and mathematician René Descartes was the first to propose a model for the origin of the Solar System in his Le Monde (ou Traité de lumière) which he wrote in 1632 and 1633 and for which he delayed publication because of the Inquisition and it was published only after his death in 1664. In his view, the Universe was filled with vortices of swirling particles and the Sun and planets had condensed from a particularly large vortex that had somehow contracted, which explained the circular motion of the planets and was on the right track with condensation and contraction. However, this was before Newton's theory of gravity and we now know matter does not behave in this fashion. [ 1 ]

The vortex model of 1944, [ 1 ] formulated by German physicist and philosopher Baron Carl Friedrich von Weizsäcker, which harkens back to the Cartesian model, involved a pattern of turbulence-induced eddies in a Laplacian nebular disc. In it a suitable combination of clockwise rotation of each vortex and anti-clockwise rotation of the whole system can lead to individual elements moving around the central mass in Keplerian orbits so there would be little dissipation of energy due to the overall motion of the system but material would be colliding at high relative velocity in the inter-vortex boundaries and in these regions small roller-bearing eddies would coalesce to give annular condensations. It was much criticized as turbulence is a phenomenon associated with disorder and would not spontaneously produce the highly ordered structure required by the hypothesis. As well, it does not provide a solution to the angular momentum problem and does not explain lunar formation nor other very basic characteristics of the Solar System. [ 2 ]

The Weizsäcker model was modified [ 1 ] in 1948 by Dutch theoretical physicist Dirk Ter Haar, in that regular eddies were discarded and replaced by random turbulence which would lead to a very thick nebula where gravitational instability would not occur. He concluded the planets must have formed by accretion and explained the compositional difference (solid and liquid planets) as due to the temperature difference between the inner and outer regions, the former being hotter and the latter being cooler, so only refractories (non-volatiles) condensed in the inner region. A major difficulty is that in this supposition turbulent dissipation takes place in a time scale of only about a millennium which does not give enough time for planets to form.

The nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg [ 3 ] and later elaborated and expanded upon by Immanuel Kant in 1755. A similar theory was independently formulated by Pierre-Simon Laplace in 1796. [ 4 ]

In 1749, Georges-Louis Leclerc, Comte de Buffon conceived the idea that the planets were formed when a comet collided with the Sun, sending matter out to form the planets. However, Laplace refuted this idea in 1796, showing that any planets formed in such a way would eventually crash into the Sun. Laplace felt that the near-circular orbits of the planets were a necessary consequence of their formation. [ 5 ] Today, comets are known to be far too small to have created the Solar System in this way. [ 5 ]

In 1755, Immanuel Kant speculated that observed nebulae may in fact be regions of star and planet formation. In 1796, Laplace elaborated by arguing that the nebula collapsed into a star, and, as it did so, the remaining material gradually spun outward into a flat disc, which then formed the planets. [ 5 ]

2.1. Alternative Theories

However plausible it may appear at first sight, the nebular hypothesis still faces the obstacle of angular momentum; if the Sun had indeed formed from the collapse of such a cloud, the planets should be rotating far more slowly. The Sun, though it contains almost 99.9 percent of the system's mass, contains just 1 percent of its angular momentum. [ 6 ] This means that the Sun should be spinning much more rapidly.

Tidal theory

Attempts to resolve the angular momentum problem led to the temporary abandonment of the nebular hypothesis in favour of a return to "two-body" theories. [ 5 ] For several decades, many astronomers preferred the tidal or near-collision hypothesis put forward by James Jeans in 1917, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets. [ 5 ] However, in 1929 astronomer Harold Jeffreys countered that such a near-collision was massively unlikely. [ 5 ] Objections to the hypothesis were also raised by the American astronomer Henry Norris Russell, who showed that it ran into problems with angular momentum for the outer planets, with the planets struggling to avoid being reabsorbed by the Sun. [ 7 ]

The Chamberlin-Moulton model

Forest Moulton in 1900 had also shown that the nebular hypothesis was inconsistent with observations because of the angular momentum. Moulton and Chamberlin in 1904 originated the planetesimal hypothesis [ 8 ] (see Chamberlin–Moulton planetesimal hypothesis). Along with many astronomers of the day they came to believe the pictures of "spiral nebulas" from the Lick Observatory were direct evidence of forming solar systems. These turned out to be galaxies instead but the Shapley-Curtis debate about these was still 16 years in the future. One of the most fundamental issues in the history of astronomy was distinguishing between nebulas and galaxies.

Moulton and Chamberlin suggested that a star had passed close to the Sun early in its life to cause tidal bulges and that this, along with the internal process that leads to solar prominences, resulted in the ejection of filaments of matter from both stars. While most of the material would have fallen back, part of it would remain in orbit. The filaments cooled into numerous, tiny, solid fragments, ‘planetesimals’, and a few larger protoplanets. This model received favourable support for about 3 decades but passed out of favour by the late '30s and was discarded in the '40s by the realization it was incompatible with the angular momentum of Jupiter, but a part of it, planetesimal accretion, was retained. [ 1 ]

Lyttleton's scenario [ 1 ]

In 1937 and 1940, Ray Lyttleton postulated that a companion star to the Sun collided with a passing star. Such a scenario was already suggested and rejected by Henry Russell in 1935. Lyttleton showed terrestrial planets were too small to condense on their own so suggested one very large proto-planet broke in two because of rotational instability, forming Jupiter and Saturn, with a connecting filament from which the other planets formed. A later model, from 1940 and 1941, involves a triple star system, a binary plus the Sun, in which the binary merges and later breaks up because of rotational instability and escapes from the system leaving a filament that formed between them to be captured by the Sun. Objections of Lyman Spitzer apply to this model also. [ clarification needed ]

Band-structure model

In 1954, 1975, and 1978 [ 9 ] Swedish astrophysicist Hannes Alfvén included electromagnetic effects in equations of particle motions, and angular momentum distribution and compositional differences were explained. In 1954 he first proposed the band structure in which he distinguished an A-cloud, containing mostly helium, but with some solid- particle impurities ("meteor rain"), a B-cloud, with mostly hydrogen, a C-cloud, having mainly carbon, and a D-cloud, made mainly of silicon and iron. Impurities in the A-cloud form Mars and the Moon (later captured by Earth), in the B-cloud they condense into Mercury, Venus, and Earth, in the C-cloud they condense into the outer planets, and Pluto and Triton may have formed from the D-cloud.

Interstellar cloud theory

In 1943, the Soviet astronomer Otto Schmidt proposed that the Sun, in its present form, passed through a dense interstellar cloud, emerging enveloped in a cloud of dust and gas, from which the planets eventually formed. This solved the angular momentum problem by assuming that the Sun's slow rotation was peculiar to it, and that the planets did not form at the same time as the Sun. [ 5 ] Extensions of the model, together forming the Russian school, include Gurevich and Lebedinsky (in 1950), Safronov (in 1967,1969), Safronov and Vityazeff (in 1985), Safronov and Ruskol (in 1994), and Ruskol (in 1981), among others [ 10 ] However, this hypothesis was severely dented by Victor Safronov who showed that the amount of time required to form the planets from such a diffuse envelope would far exceed the Solar System's determined age. [ 5 ]

Ray Lyttleton modified the theory by showing that a 3rd body was not necessary and proposing that a mechanism of line accretion described by Bondi and Hoyle in 1944 would enable cloud material to be captured by the star (Williams and Cremin, 1968, loc. cit.)

Hoyle's hypothesis

In this model [ 1 ] (from 1944) the companion went nova with ejected material captured by the Sun and planets forming from this material. In a version a year later it was a supernova. In 1955 he proposed a similar system to Laplace, and with more mathematical detail in 1960. It differs from Laplace in that a magnetic torque occurs between the disk and the Sun, which comes into effect immediately or else more and more matter would be ejected resulting in a much too massive planetary system, one comparable to the Sun. The torque causes a magnetic coupling and acts to transfer angular momentum from the Sun to the disk. The magnetic field strength would have to be 1 gauss. The existence of torque depends on magnetic lines of force being frozen into the disk (a consequence of a well-known MHD (magnetohydrodynamic) theorem on frozen-in lines of force). As the solar condensation temperature when the disk was ejected could not be much more than 1000 degrees K., a number of refractories must be solid, probably as fine smoke particles, which would grow with condensation and accretion. These particles would be swept out with the disk only if their diameter at the Earth's orbit was less than 1 meter so as the disk moved outward a subsidiary disk consisting of only refractories remains behind where the terrestrial planets would form. The model is in good agreement with the mass and composition of the planets and angular momentum distribution provided the magnetic coupling is an acceptable idea, but not explained are twinning, the low mass of Mars and Mercury, and the planetoid belts. It was Alfvén who formulated the concept of frozen-in magnetic field lines.

Kuiper's theory

Gerard Kuiper (in 1944) [ 1 ] argued, like Ter Haar, that regular eddies would be impossible and postulated that large gravitational instabilities might occur in the solar nebula, forming condensations. In this, the solar nebula could be either co-genetic with the Sun or captured by it. Density distribution would determine what could form: either a planetary system or a stellar companion. The 2 types of planets were assumed to be due to the Roche limit. No explanation was offered for the Sun's slow rotation which Kuiper saw as a larger G-star problem.

Whipple's theory

In Fred Whipple's 1948 scenario [ 1 ] a smoke cloud about 60,000 AU in diameter and with 1 solar mass ( M ☉ ) contracts and produces the Sun. It has a negligible angular momentum thus accounting for the Sun's similar property. This smoke cloud captures a smaller one with a large angular momentum. The collapse time for the large smoke and gas nebula is about 100 million years and the rate is slow at first, increasing in later stages. The planets would condense from small clouds developed in, or captured by, the 2nd cloud, the orbits would be nearly circular because accretion would reduce eccentricity due to the influence of the resisting medium, orbital orientations would be similar because the small cloud was originally small and the motions would be in a common direction. The protoplanets might have heated up to such high degrees that the more volatile compounds would have been lost and the orbital velocity decreases with increasing distance so that the terrestrial planets would have been more affected. The weaknesses of this scenario are that practically all the final regularities are introduced as a priori assumptions and most of the hypothesizing was not supported by quantitative calculations. For these reasons it did not gain wide acceptance.

• Williams, I.O., Cremin, A.W. 1968. A survey of theories relating to the origin of the solar system. Qtly. Rev. RAS 9: 40–62. ads.abs.harvard.edu/abs
• Woolfson, Michael Mark, The Origin and Evolution of universe and the Solar System, Taylor and Francis, 2000 ; completely considered that collision of the two suns produce the solar system and universe in the entire 100,00 years of the evolution.
• Swedenborg, Emanuel. 1734, (Principia) Latin: Opera Philosophica et Mineralia (English: Philosophical and Mineralogical Works), (Principia, Volume 1)
• See, T. J. J. (1909). "The Past History of the Earth as Inferred from the Mode of Formation of the Solar System". Proceedings of the American Philosophical Society 48 (191): 119–128. 
• Michael Mark (1993). "The Solar System: Its Origin and Evolution". Journal of the Royal Astronomical Society 34: 1–20. Bibcode: 1993QJRAS..34....1W. "Physics Department, University of New York".  http://adsabs.harvard.edu/abs/1993QJRAS..34....1W
• Woolfson, Michael Mark (1984). "Rotation in the Solar System". Philosophical Transactions of the Royal Society of London 313 (1524): 5. doi:10.1098/rsta.1984.0078. Bibcode: 1984RSPTA.313....5W.  https://dx.doi.org/10.1098%2Frsta.1984.0078
• Benjamin Crowell (1998–2006). "5". Conservation Laws. lightandmatter.com. ISBN 0-9704670-2-8. http://www.lightandmatter.com/html_books/2cl/ch05/ch05.html. 
• Sherrill, T.J. 1999. A Career of Controversy: the Anomaly of T.J.J. See. J. Hist. Astrn. ads.abs.harvard.edu/abs/1999JHA.
• Alfvén, H. 1978. Band Structure of the Solar System. In Origin of the Solar System, S.F. Dermot, ed, pp. 41–48. Wiley.
• Williams, I.O., Cremin, A.W. 1968. A survey of theories relating to the origin of the solar system. Qtly. Rev. RAS 9: 40–62. ads.abs.harvard.edu/abs.

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