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8.2: Origin of the Solar System—The Nebular Hypothesis

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.

The orange disk has zones that are darker, indicating the planets are growing by using that material in the disk.

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.

It shows a ring of ice around the star

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.

The meteorite is polished showing the Widmanstätten Pattern.

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:

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.

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

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nebular hypothesis

Origin of nebular hypothesis

Words nearby nebular hypothesis, how to use nebular hypothesis in a sentence.

Though researchers have struggled to understand exactly what contributes to this gender difference, Dr. Rohan has one hypothesis.

The leading hypothesis for the ultimate source of the Ebola virus, and where it retreats in between outbreaks, lies in bats.

In 1996, John Paul II called the Big Bang theory “more than a hypothesis.”

To be clear: There have been no double-blind or controlled studies that conclusively confirm this hair-loss hypothesis.

The bacteria-driven-ritual hypothesis ignores the huge diversity of reasons that could push someone to perform a religious ritual.

And remember it is by our hypothesis the best possible form and arrangement of that lesson.

Taken in connection with what we know of the nebulæ, the proof of Laplace's nebular hypothesis may fairly be regarded as complete.

What has become of the letter from M. de St. Mars, said to have been discovered some years ago, confirming this last hypothesis?

To admit that there had really been any communication between the dead man and the living one is also an hypothesis.

"I consider it highly probable," asserted Aunt Maria, forgetting her Scandinavian hypothesis.

British Dictionary definitions for nebular hypothesis

Scientific definitions for nebular hypothesis.

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44 The Nebular Theory

A protostar is an object in which no nuclear fusion has occurred, unlike a star that is undergoing nuclear fusion. A protostar becomes a star when nuclear fusion begins. Most likely the next step was that the nebula flattened into a disk called the  Protoplanetary Disk  ; planets eventually formed from and in this disk.

Three processes occurred with the nebular collapse:

The orderly motions of the solar system today are a direct result of the solar system’s beginnings in a spinning, flattened cloud of gas and dust.

Introduction to Astronomy by Lumen Learning is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Wondrium Daily

The Nebular Hypothesis and Formation of the Solar System


December 3, 2021 Science

By  Robert Hazen, Ph.D. ,  George Mason University

The generally accepted model for the formation of the solar system is called the nebular hypothesis. it takes all the different kinds of information and merges them into one hypothesis that seems to match all the observations. the theory was proposed by french mathematician pierre simon laplace more than 200 years ago..

An illustration of the solar system’s birth

Pierre Simon Laplace: The Mathematical Prodigy

Laplace was a French mathematician with a strong interest in physics and astronomy. He was a brilliant mathematical prodigy; he became a university professor at the age of 19. He described his theory of the nebular hypothesis in an extremely popular work, translated to be The Exposition of the System of the World , published in 1796.

Laplace was a scientist’s scientist; he seemed to speak for all scientists. On his deathbed, he said what is a sentiment that’s rarely expressed, “The knowledge we have of things is small indeed, while that of which we are ignorant is immense.”

This is a transcript from the video series  The Joy of Science .  Watch it now, on Wondrium.

Role of Dust and Gas in the Nebular Hypothesis 

Well, Laplace added greatly to our knowledge by pointing out the nebular hypothesis. According to this hypothesis, gravity attracts dust and gas into an ever denser and more compact cloud. The composition of this nebula is going to be primarily hydrogen and helium because those, of course, are the primary elements of the universe. 

The very first stars had only hydrogen and helium and a little bit of lithium to work with; but after the first supernova, there were more and more heavy elements. An estimate of our solar system is about 90 percent hydrogen, and the rest are other elements. That may represent five or six or seven cycles of supernova, feeding into the material that now has gone into our solar system.

The Nebular Hypothesis and the Solar System’s Formation

We have now a collapsing cloud of dust and gas, and as it collapses, the temperature at the center gets hotter and hotter because of the gravitational potential energy that’s released as the dust comes closer and closer in. Also, the cloud begins to rotate. It has little swirls and eddies that were random at first, but as it gets more and more compact and condensed, the predominant eddy becomes in one direction, and the whole system begins to rotate and spiral. 

Eventually, the solar system begins to look like a flattened disk of matter with a central bulge, not unlike the way a galaxy looks, a spiral galaxy in cross-section, with that central bulge and the spiral arms coming out. Most of the mass gets pulled to the center; that’s where the proto-Sun, the pre-Sun, gets hotter and hotter, and pressures get higher and higher inside this hydrogen sphere.

Learn more about why the Earth is unique .

How the Sun and Planets Were Born

At some critical point, nuclear fusion reactions begin. The pressure and temperature get so hot that hydrogen begins to fuse to helium, and then the star is born. When that happens, energy starts radiating out from the middle of the proto-sun to the surface, and then the radiant energy begins to come from the sun to the other parts of the nebula. 

The solar wind starts blowing outward at a very high, intense rate, and this starts changing and modifying the rest of the dust cloud. The inner parts of the dust cloud are stripped of hydrogen and helium, which are much too light; they’re pushed to the outside. What’s left are more rocky, condensed materials, and this becomes the four rocky or terrestrial planets, planets that have very little hydrogen and helium relative to the other objects in the solar system.

Significance of Computer Modeling in Science

Computer modeling is so important in science today—it keeps track of the evolution of the dust cloud under gravity, under the condensing cloud, under the first nuclear-fission reactions that form the stars. What happens to a dust cloud in those situations? One surprise of these computer models is under most conditions, it appears that two stars should form, not one. 

This would form a binary star, and observations confirm this. Most of the stars we see in the heavens—in fact, two-thirds or more of the stars we see—are binary stars, with two large masses orbiting around each other. In that sense, our solar system is a little bit unusual. Not rare, by any means, but certainly less common than the binaries. 

If Jupiter had been 50 to 100 times larger than it is, we too would be living in a binary system. Jupiter too, would be a star—a brown dwarf star, to be sure—but a star. When only one star dominates in a system, calculations show that the pattern of planets we have is very likely. 

Learn more about the ‘theory of everything’ .

Calculations and the Importance of Very-Late-Stage Collisions

An illustration of Venus

One fascinating aspect of these calculations is the importance of very-late-stage collisions, which fix some of the details of planetary behavior. Imagine this: for example, an object like the Earth. Once a large mass forms, it starts sweeping up the other planetesimals in its vicinity; but it’s very possible to have two competing large masses near the same orbit. 

That situation is absolutely unstable; you cannot have two planets occupying the same orbit. Eventually, those two planets have to collide. When they do, in a late-stage event, it’s epic, it’s catastrophic, it’s violent, and it can change some of the details of planetary orbits.

So, for example, Venus appears to be rotating— not counterclockwise, in the same sense as the orbits—Venus has a very, very slow clockwise rotation against the orbits of all the other planets. This, it’s suggested, may have occurred because of a late-stage collision.

Common Questions about the Nebular Hypothesis and Formation of Solar System

Pierre Simon Laplace was a French mathematician with a keen interest in physics and astronomy. Laplace was a mathematical genius and became a university professor at the age of 19. This scientist proposed a famous theory called the nebular hypothesis .

According to the nebular hypothesis , gravity caused gas and dust to turn into larger and more compact clouds. The composition of these clouds is only helium and hydrogen gases since they were the only raw materials of the universe.

According to the nebular hypothesis , massive clouds of hydrogen and helium collapsed. After the collapse, the center of the cloud became hotter and hotter. Then, as it got hotter and more condensed, the system began to rotate. This was the beginning of the solar system’s formation.

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How Was the Solar System Formed? – The Nebular Hypothesis

Since time immemorial, humans have been searching for the answer of how the Universe came to be. However, it has only been within the past few centuries, with the Scientific Revolution, that the predominant theories have been empirical in nature. It was during this time, from the 16th to 18th centuries, that astronomers and physicists began to formulate evidence-based explanations of how our Sun, the planets, and the Universe began.

When it comes to the formation of our Solar System, the most widely accepted view is known as the Nebular Hypothesis . In essence, this theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. Originally proposed to explain the origin of the Solar System, this theory has gone on to become a widely accepted view of how all star systems came to be.

Nebular Hypothesis:

According to this theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

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From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc .

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury , Venus , Earth , and Mars . Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets ( Jupiter , Saturn , Uranus , and Neptune ) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line ). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt , Kuiper Belt , and Oort Cloud .

Artist's impression of the early Solar System, where collision between particles in an accretion disc led to the formation of planetesimals and eventually planets. Credit: NASA/JPL-Caltech

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

History of the Nebular Hypothesis:

The idea that the Solar System originated from a nebula was first proposed in 1734 by Swedish scientist and theologian Emanual Swedenborg. Immanuel Kant, who was familiar with Swedenborg’s work, developed the theory further and published it in his Universal Natural History and Theory of the Heavens  (1755). In this treatise, he argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity and forming stars and planets.

A similar but smaller and more detailed model was proposed by Pierre-Simon Laplace in his treatise Exposition du system du monde (Exposition of the system of the world), which he released in 1796. Laplace theorized that the Sun originally had an extended hot atmosphere throughout the Solar System, and that this “protostar cloud” cooled and contracted. As the cloud spun more rapidly, it threw off material that eventually condensed to form the planets.

This image from the NASA/ESA Hubble Space Telescope shows Sh 2-106, or S106 for short. This is a compact star forming region in the constellation Cygnus (The Swan). A newly-formed star called S106 IR is shrouded in dust at the centre of the image, and is responsible for the surrounding gas cloud’s hourglass-like shape and the turbulence visible within. Light from glowing hydrogen is coloured blue in this image. Credit: NASA/ESA

The Laplacian nebular model was widely accepted during the 19th century, but it had some rather pronounced difficulties. The main issue was angular momentum distribution between the Sun and planets, which the nebular model could not explain. In addition, Scottish scientist James Clerk Maxwell (1831 – 1879) asserted that different rotational velocities between the inner and outer parts of a ring could not allow for condensation of material.

It was also rejected by astronomer Sir David Brewster (1781 – 1868), who stated that:

“those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process… [Under such a view] the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere.”

By the early 20th century, the Laplacian model had fallen out of favor, prompting scientists to seek out new theories. However, it was not until the 1970s that the modern and most widely accepted variant of the nebular hypothesis – the solar nebular disk model (SNDM) – emerged. Credit for this goes to Soviet astronomer Victor Safronov and his book Evolution of the protoplanetary cloud and formation of the Earth and the planets (1972) . In this book, almost all major problems of the planetary formation process were formulated and many were solved.

For example, the SNDM model has been successful in explaining the appearance of accretion discs around young stellar objects. Various simulations have also demonstrated that the accretion of material in these discs leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.

While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe, and has been used to explain the formation of many of the exoplanets that have been discovered throughout our galaxy.

Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to resolve. For example, there is the problem of tilted axes. According to the nebular theory, all planets around a star should be tilted the same way relative to the ecliptic. But as we have learned, the inner planets and outer planets have radically different axial tilts.

Whereas the inner planets range from almost 0 degree tilt, others (like Earth and Mars) are tilted significantly (23.4° and 25°, respectively), outer planets have tilts that range from Jupiter’s minor tilt of 3.13°, to Saturn and Neptune’s more pronounced tilts (26.73° and 28.32°), to Uranus’ extreme tilt of 97.77°, in which its poles are consistently facing towards the Sun.

The latest list of potentially habitable exoplanets, courtesy of The Planetary Habitability Laboratory. Credit:

Also, the study of extrasolar planets have allowed scientists to notice irregularities that cast doubt on the nebular hypothesis. Some of these irregularities have to do with the existence of “hot Jupiters” that orbit closely to their stars with periods of just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but have yet to address all outlying questions.

Alas, it seems that it questions that have to do with origins that are the toughest to answer. Just when we think we have a satisfactory explanation, there remain those troublesome issues it just can’t account for. However, between our current models of star and planet formation, and the birth of our Universe, we have come a long way. As we learn more about neighboring star systems and explore more of the cosmos, our models are likely to mature further.

We have written many articles about the Solar System here at Universe Today. Here’s The Solar System , Did our Solar System Start with a Little Bang? , and What was Here Before the Solar System?

For more information, be sure to check out the origin of the Solar System and how the Sun and planets formed .

Astronomy Cast also has an episode on the subject – Episode 12: Where do Baby Stars Come From?

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5 Replies to “How Was the Solar System Formed? – The Nebular Hypothesis”

So… the transition from the geocentric view and eternal state the way things are evolved with appreciation of dinosaurs and plate tectonics too… and then refining the nebular idea… the Nice model… the Grand Tack model… alittle more? Now maybe the Grand Tack with the assumption of mantle breaking impacts in the early days – those first 10 millions years were heady times!

And the whole idea of “solar siblings” has been busy the last few years…

Nice overview, and I learned a lot. However, there are some salient points that I think I have picked up earlier:

“something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.”

The study of star forming molecular clouds shows that same early, large stars form that way. In the most elaborate model which makes Earth isotope measurements easiest to predict, by free coupling the processes, the 1st generation of super massive stars would go supernova in 1-10 million years.

That blows a 1st geeration of large bubbles with massive, compressed shells that are seeded with supernova elements, as we see Earth started out with. The shells would lead to a more frequent 2nd generation of massive stars with a lifetime of 10-100 million years or so. These stars have powerful solar winds.

That blows a 2nd generation of large bubbles with massive, compressed shells, The shells would lead to a 3d generation of ~ 500 – 1000 stars of Sun size or less. In the case of the Sun the resulting mass was not enough to lead to a closed star cluster as we can see circling the Milky Way, but an open star cluster where the stars would mix with other stars over the ~ 20 orbits we have done around the MW.

“The ices that formed these planets were more plentiful”.

The astronomy course I attended looked at the core collapse model of large planets. (ASs well as the direct collapse scenario.) The core grew large rapidly and triggered gas collapse onto the planet from the disk, a large factor being the stickiness of ices at the grain stage. The terrestrial planets grow by slower accretion, and the material may have started to be cleared from the disk. by star infall or radiation pressure flow outwards, before they are finished.

An interesting problem for terrestrial planets is the “meter size problem” (IIRC the name). It was considered hard to grow grains above a cm, and when they grow they rapidly brake and fall onto the star.

Now scientists have come up with grain collapse scenarios, where grains start to follow each other for reasons of gravity and viscous properties of the disk, I think. All sorts of bodies up to protoplanets can be grown quickly and, when over the problematic size, will start to clear the disk rather than being braked by it.

“But as we have learned, the inner planets and outer planets have radically different axial tilts.”

Jupiter can be considered a clue, too massive to tilt by outside forces. The general explanation tend to be the accretion process, where the tilt would be randomized. (Venus may be an exception, since some claim it is becoming tidally locked to the Sun – Mercury is instead locked in a 3:2 resonance – and it is in fact now retrograde with a putative near axis lock.) Possible Mercury bit at least Earth and Mars (and Moon) show late great impacts.

A recent paper show that terrestrial planets would suffer impacts on the great impact scale, between 1 to 8 as norm with an average of 3. These would not be able to clear out an Earth mass atmosphere or ocean, so if Earth suffered one such impact after having volatiles delivered by late accretion/early bombardment, the Moon could result.

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What Is the Nebular Theory?

Related articles, what are the four factors that influence weather, does mercury have a thin or thick atmosphere, how long does a planetary nebula last.

The nebular theory is an explanation for the formation of solar systems. The word “nebula” is Latin for “cloud,” and according to the explanation, stars are born from clouds of interstellar gas and dust. The transition from an undifferentiated cloud to a star system complete with planets and moons takes about 100 million years. According to this theory, our own solar system formed about 4.6 billion years ago, and others are forming today in distant nebulae.

What the Theory Explains

As it relates to our own solar system, the nebular theory explains three observable facts. The first is that the planets all rotate in the same direction. The second is that they all orbit within 6 degrees of a common plane. The third is that all the terrestrial planets, which are those within the orbit of the Asteroid Belt, are rocky, while those outside it are gaseous. The theory also explains the existence of the Kuiper Belt -- a region on the fringes of the solar system with a high concentration of comets.

A Star Is Born

According to the nebular theory, a solar system begins when an interstellar cloud, containing approximately 75 percent hydrogen, 25 percent helium and traces of other elements, begins to form areas of higher concentration, or clumps. As the clumps grow, gravitational forces increase and get converted to the kinetic energy of the increasingly fast-moving particles, which collide with one another and generate heat. Eventually one clump dominates, and when its temperature reaches 10 million degrees Kelvin (18 million degrees Fahrenheit), nuclear fission begins. The outward pressure created by the fission reactions prevents further collapse, and the clump of burning hydrogen gas stabilizes and becomes a star.

Seeds of Planets

As a proto-star grows in size, the gases in the nebula from which it is born form a disk and spiral more and more quickly around its center. Eventually, elements on the fringe of the disk begin to form into globules with compositions that depend on their distance from the center. At smaller distances, where temperatures are higher, they are formed of heavy elements, while at greater distances they are formed of ices of water, methane and ammonia. These globules collide with each other and stick together to form larger, spherical bodies in a process called accretion. The larger bodies, with diameters of a few kilometers, are called planetesimals.

Planets and Comets

Once planetesimals form, collisions continue, but they tend to be destructive, and only the largest planetesimals survive. These continue to grow by assimilating surrounding material, including smaller planetesimals, to become planets. The composition of planets closer to the center of the system differs from that of those farther away. The planets within a critical distance, where temperatures are warmer, are rocky, while those beyond the critical distance have solid cores and thick, gaseous atmospheres. At the fringes of the solar system, where gravitational forces are weak, planetesimals never coalesce into planets. These icy bodies sometimes wander in eccentric orbits, and when they get close to the sun, we know them as comets.

Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.

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