How Was the Solar System Formed? – The Nebular Hypothesis

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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.

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
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 Sh 2-106 Nebula (or S106 for short), a compact star forming region in the constellation Cygnus (The Swan). 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.

Problems:

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: phl.upr.edu
A list of potentially habitable exoplanets, courtesy of The Planetary Habitability Laboratory. Credit: phl.upr.edu

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?

What is the Life Cycle Of The Sun?

The Sun has always been the center of our cosmological systems. But with the advent of modern astronomy, humans have become aware of the fact that the Sun is merely one of countless stars in our Universe. In essence, it is a perfectly normal example of a G-type main-sequence star (G2V, aka. “yellow dwarf”). And like all stars, it has a lifespan, characterized by a formation, main sequence, and eventual death.

This lifespan began roughly 4.6 billion years ago, and will continue for about another 4.5 – 5.5 billion years, when it will deplete its supply of hydrogen, helium, and collapse into a white dwarf. But this is just the abridged version of the Sun’s lifespan. As always, God (or the Devil, depending on who you ask) is in the details!

To break it down, the Sun is about half way through the most stable part of its life. Over the course of the past four billion years, during which time planet Earth and the entire Solar System was born, it has remained relatively unchanged. This will stay the case for another four billion years, at which point, it will have exhausted its supply of hydrogen fuel. When that happens, some pretty drastic things will take place!

The Birth of the Sun:

According to Nebular 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.

Protoplanet Hypothesis
Artist’s concept of a star surrounded by a molecular cloud to form a swirling disk called a “protoplanetary disk.” Credit: NASA/JPL-Caltech

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.

The ball at the center would eventually form the Sun, while the disk of material would form the planets. The Sun spent about 100,000 years as a collapsing protostar before temperature and pressures in the interior ignited fusion at its core. The Sun started as a T Tauri star – a wildly active star that blasted out an intense solar wind. And just a few million years later, it settled down into its current form. The life cycle of the Sun had begun.

The Main Sequence:

The Sun, like most stars in the Universe, is on the main sequence stage of its life, during which nuclear fusion reactions in its core fuse hydrogen into helium. Every second, 600 million tons of matter are converted into neutrinos, solar radiation, and roughly 4 x 1027 Watts of energy. For the Sun, this process began 4.57 billion years ago, and it has been generating energy this way every since.

However, this process cannot last forever since there is a finite amount of hydrogen in the core of the Sun. So far, the Sun has converted an estimated 100 times the mass of the Earth into helium and solar energy. As more hydrogen is converted into helium, the core continues to shrink, allowing the outer layers of the Sun to move closer to the center and experience a stronger gravitational force.

The Sun captured by NASA's Solar Dynamics Observatory Spacecraft.
The Sun captured by NASA’s Solar Dynamics Observatory Spacecraft.

This places more pressure on the core, which is resisted by a resulting increase in the rate at which fusion occurs. Basically, this means that as the Sun continues to expend hydrogen in its core, the fusion process speeds up and the output of the Sun increases. At present, this is leading to a 1% increase in luminosity every 100 million years, and a 30% increase over the course of the last 4.5 billion years.

In 1.1 billion years from now, the Sun will be 10% brighter than it is today, and this increase in luminosity will also mean an increase in heat energy, which Earth’s atmosphere will absorb. This will trigger a moist greenhouse effect here on Earth that is similar to the runaway warming that turned Venus into the hellish environment we see there today.

In 3.5 billion years from now, the Sun will be 40% brighter than it is right now. This increase will cause the oceans to boil, the ice caps to permanently melt, and all water vapor in the atmosphere to be lost to space. Under these conditions, life as we know it will be unable to survive anywhere on the surface. In short, planet Earth will come to be another hot, dry Venus.

Core Hydrogen Exhaustion:

All things must end. That is true for us, that is true for the Earth, and that is true for the Sun. It’s not going to happen anytime soon, but one day in the distant future, the Sun will run out of hydrogen fuel and slowly slouch towards death. This will begin in approximate 5.4 billion years, at which point the Sun will exit the main sequence of its lifespan.

With its hydrogen exhausted in the core, the inert helium ash that has built up there will become unstable and collapse under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size and enter the Red Giant phase of its evolution. It is calculated that the expanding Sun will grow large enough to encompass the orbit’s of Mercury, Venus, and maybe even Earth. Even if the Earth survives, the intense heat from the red sun will scorch our planet and make it completely impossible for life to survive.

Final Phase and Death:

Once it reaches the Red-Giant-Branch (RGB) phase,  the Sun will haves approximately 120 million years of active life left. But much will happen in this amount of time. First, the core (full of degenerate helium), will ignite violently in a helium flash – where approximately 6% of the core and 40% of the Sun’s mass will be converted into carbon within a matter of minutes.

The Sun will then shrink to around 10 times its current size and 50 times its luminosity, with a temperature a little lower than today. For the next 100 million years, it will continue to burn helium in its core until it is exhausted. By this point, it will be in its Asymptotic-Giant-Branch (AGB) phase, where it will expand again (much faster this time) and become more luminous.

Over the course of the next 20 million years, the Sun will then become unstable and begin losing mass through a series of thermal pulses. These will occur every 100,000 years or so, becoming larger each time and increasing the Sun’s luminosity to 5,000 times its current brightness and its radius to over 1 AU.

At this point, the Sun’s expansion will either encompass the Earth, or leave it entirely inhospitable to life. Planets in the Outer Solar System are likely to change dramatically, as more energy is absorbed from the Sun, causing their water ices to sublimate – perhaps forming dense atmosphere and surface oceans. After 500,000 years or so, only half of the Sun’s current mass will remain and its outer envelope will begin to form a planetary nebula.

The post-AGB evolution will be even faster, as the ejected mass becomes ionized to form a planetary nebula and the exposed core reaches 30,000 K. The final, naked core temperature will be over 100,000 K, after which the remnant will cool towards a white dwarf. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to black.

Ultimate Fate of our Sun:

When people think of stars dying, what typically comes to mind are massive supernovas and the creation of black holes. However, this will not be the case with our Sun, due to the simple fact that it is not nearly massive enough. While it might seem huge to us, but the Sun is a relatively low mass star compared to some of the enormous high mass stars out there in the Universe.

As such, when our Sun runs out of hydrogen fuel, it will expand to become a red giant, puff off its outer layers, and then settle down as a compact white dwarf star, then slowly cooling down for trillions of years. If, however, the Sun had about 10 times its current mass, the final phase of its lifespan would be significantly more (ahem) explosive.

When this super-massive Sun ran out of hydrogen fuel in its core, it would switch over to converting atoms of helium, and then atoms of carbon (just like our own). This process would continue, with the Sun consuming heavier and heavier fuel in concentric layers. Each layer would take less time than the last, all the way up to nickel – which could take just a day to burn through.

Then, iron would starts to build up in the core of the star. Since iron doesn’t give off any energy when it undergoes nuclear fusion, the star would have no more outward pressure in its core to prevent it from collapsing inward. When about 1.38 times the mass of the Sun is iron collected at the core, it would catastrophically implode, releasing an enormous amount of energy.

Within eight minutes, the amount of time it takes for light to travel from the Sun to Earth, an incomprehensible amount of energy would sweep past the Earth and destroy everything in the Solar System. The energy released from this might be enough to briefly outshine the galaxy, and a new nebula (like the Crab Nebula) would be visible from nearby star systems, expanding outward for thousands of years.

All that would remain of the Sun would be a rapidly spinning neutron star, or maybe even a stellar black hole. But of course, this is not to be our Sun’s fate. Given its mass, it will eventually collapse into a white star until it burns itself out. And of course, this won’t be happening for another 6 billion years or so. By that point, humanity will either be long dead or have moved on. In the meantime, we have plenty of days of sunshine to look forward to!

We have written many interesting articles on the Sun here at Universe Today. Here’s What Color Is The Sun?, What Kind of Star is the Sun?, How Does The Sun Produce Energy?, and Could We Terraform the Sun?

Astronomy Cast also has some interesting episodes on the subject. Check them out- Episode 30: The Sun, Spots and AllEpisode 108: The Life of the Sun, Episode 238: Solar Activity.

For more information, check out NASA’s Solar System Guide.

Who Was Democritus?

As the philosopher Nietzsche famously said “He who would learn to fly one day must first learn to stand and walk and run and climb and dance; one cannot fly into flying.” This is certainly true when it comes to humanity’s understanding of the universe, something which has evolved over many thousands of years and been the subject of ongoing discovery.

And along the way, many names stand out as examples of people who achieved breakthroughs and helped lay the foundations of our modern understanding. One such person is Democritus, an ancient Greek philosopher who is viewed by many as being the “father of modern science”. This is due to his theory of universe that is made up of tiny “atoms”, which bears a striking resemblance to modern atomic theory.

Though he is typically viewed as one of Greece’s many pre-Socratic natural philosopher, many historians have argued that he is more rightly classified as a scientist, at least when compared to his contemporaries. There has also been significant controversy – particularly in Germany during the 19th century – over whether or not Democritus deserves credit for atomic theory.

This argument is based on the relationship Democritus had with contemporary philosopher Leucippus, who is renowned for sharing his theory about atoms with him. However, their theories came down to a different basis, a distinction that allows Democritus to be given credit for a theory that would go on to become a staple of the modern scientific tradition.

Hendrik ter Brugghen - Heraclitus, 1628. Credit: rijksmuseum.nl
Democritus, by Hendrik ter Brugghen – Heraclitus, 1628. Credit: rijksmuseum.nl

Birth and Early Life:

The precise date and location of Democritus birth is the subject the debate. While most sources claim he was born in Abdera, located in the northern Greek province of Thrace, around 460 BCE. However, other sources claim he was born in Miletus, a coastal city of ancient Anatolia and modern-day Turkey, and that he was born in 490 BCE.

It has been said that Democritus’ father was from a noble family and so wealthy that he received the Persian king Xerxes on the latter’s march through Abdera during the Second Persian War (480–479 BC). It is further argued that as a reward for his service, the Persian monarch gave his father and other Abderites gifts, and left several Magi among them. Democritus was apparently instructed by these Magi in astronomy and theology.

After his father had died, Democritus used his inheritance to finance a series of travels to distant countries. Desiring to feed his thirst for knowledge, Democritus traveled extensively across the known world, traveling to Asia, Egypt and (according to some sources) venturing as far as India and Ethiopia. His writings include descriptions of the the cities of Babylon and Meroe (in modern-day Sudan).

Upon returning to his native land, he occupied himself with the study of natural philosophy. He also traveled throughout Greece to acquire a better knowledge of its cultures and learned from many of Greece’s famous philosophers. His wealth allowed him to purchase their writings, and he wrote of them in his own works. In time, he would become one of the most famous of the pre-Socratic philosophers.

The ruins of the ancient Greeof Abdera, with the west gate shown. Credit:
The ruins of the ancient Greek city of Abdera, with the west gate shown. Credit: Wikipedia Commons/Marysas

Leucippus of Miletus had the greatest influence on him, becoming his mentor and sharing his theory of atomism with him. Democritus is also said to have known Anaxagoras, Hippocrates and even Socrates himself (though this remains unproven). During his time in Egypt, he learned from Egyptian mathematicians, and is said to have become acquainted with the Chaldean magi in Assyria.

In the tradition of the atomists, Democritus was a thoroughgoing materialists who viewed the world in terms of natural laws and causes. This differentiated him from other Greek philosophers like Plato and Aristotle, for whom philosophy was more teleological in nature – i.e. more concerned with the purpose of events rather than the causes, as well things like essence, the soul, and final causes.

According to the many descriptions and anecdotes about Democritus, he was known for his modesty, simplicity, and commitment to his studies. One story claims he blinded himself on purpose in order to be less distracted by worldly affairs (which is believed to be apocryphal). He was also known for his sense of humor and is commonly referred to as the “Laughing Philosopher” – for his capacity to laugh at human folly. To his fellow citizens, he was also known as “The Mocker”.

Scientific Contributions:

Democritus is renowned for being a pioneer of mathematics and geometry. He was among the first Greek philosophers to observe that a cone or pyramid has one-third the volume of a cylinder or prism with the same base and height. While none of his works on the subject survived the Middle Ages, his mathematical proofs are derived from other works with contain extensive citations to titles like On Numbers, On Geometrics, On Tangencies, On Mapping, and On Irrationals.

Right circular and oblique circular cones. Credit: Dominique Toussaint
Right circular and oblique circular cones. Credit: Dominique Toussaint

Democritus is also known for having spent much of his life experimenting with and examining plants and minerals. Similar to his work in mathematics and geometry, citations from existing works are used to infer the existence of works on the subject. These include On the Nature of Man, the two-volume collection On Flesh, On Mind, On the Senses, On Flavors, On Colors, Causes concerned with Seeds and Plants and Fruits, and to the three-volume collection Causes concerned with Animals.

From his examination of nature, Democritus developed what could be considered some of the first anthropological theories. According to him, human beings lived short lives in archaic times, forced to forage like animals until fear of wild animals then drove them into communities. He theorized that such humans had no language, and only developed it through the need to articulate thoughts and ideas.

Through a process of trial and error, human beings developed not only verbal language, but also symbols with which to communicate (i.e. written language), clothing, fire, the domestication of animals, and agriculture. Each step in this process led to more discoveries, more complex behaviors, and the many things that came to characterize civilized society.

In terms of astronomy and cosmology, Democritus was a proponent of the spherical Earth hypothesis. He believed that in the original chaos from which the universe sprang, the universe was composed of nothing but tiny atoms that came together to form larger units (a theory which bears a striking resemblance to The Big Bang Theory and Nebular Theory). He also believed in the existence of many worlds, which were either in state of growth or decay.

In a similar vein, Democritus advanced a theory of void which challenged the paradoxes raised by his fellow Greek philosophers, Parmenides and Zeno – the founders of metaphysical logic. According to these men, movement cannot exist because such a thing requires there to be a void – which is nothing, and therefore cannot exist. And a void cannot be termed as such if it is in fact a definable, existing thing.

To this, Democritus and other atomists argued that since movement is an observable phenomena, there must be a void. This idea previewed Newton’s theory of absolute space, in which space exists independently of any observer or anything external to it. Einstein’s theory of relativity also provided a resolution to the paradoxes raised by Parmenides and Zeno, where he asserted that space itself is relative and cannot be separated from time.

Democritus’ thoughts on the nature of truth also previewed the development of the modern scientific method. According to Democritus, truth is difficult, because it can only be perceived through senses-impressions which are subjective. Because of this, Aristotle claimed in his Metaphysics that Democritus was of the opinion that “either there is no truth or to us at least it is not evident.”

However, as Diogenes Laertius quoted in his 3rd century CE tract, Lives and Opinions of Eminent Philosophers: “By convention hot, by convention cold, but in reality atoms and void, and also in reality we know nothing, since the truth is at bottom.”

Diogenes Laërtius: Lives and Opinions of Eminent Philosophers. A biography of the Greek philosophers. Title page from year 1594. Credit: Public Domain
Diogenes Laertius, Lives and Opinions of Eminent Philosophers, makes mention of Democritus and his theories. Credit: Public Domain

Ultimately, Democritus’ opinion on truth came down to a distinction between two kinds of knowledge – “legitimate” (or “genuine”) and bastard (or “secret”). The latter is concerned with perception through the senses, which is subjective by nature. This is due to the fact that our sense-perception are influence by the shape and nature of atoms as they flow out from the object in question and make an impression on our senses.

“Legitimate” knowledge, by contrast, is achieved through the intellect, where sense-data is elaborated through reasoning. In this way, one can get from “bastard” impressions to the point where things like connections, patterns and causality can be determined. This is consistent with the inductive reasoning method later elaborated by Renee Descartes, and is a prime example of why Democritus is considered to be an early scientific thinker.

Atomic Theory:

However, Democritus greatest contribution to modern science was arguably the atomic theory he elucidated. According to Democritus’ atomic theory, the universe and all matter obey the following principles:

  • Everything is composed of “atoms”, which are physically, but not geometrically, indivisible
  • Between atoms, there lies empty space
  • Atoms are indestructible
  • Atoms have always been, and always will be, in motion
  • There are an infinite number of atoms, and kinds of atoms, which differ in shape, and size.

He was not alone in proposing atomic theory, as both his mentor Leucippus and Epicurus are believed to have proposed the earliest views on the shapes and connectivity of atoms. Like Democritus, they believed that the solidity of a material corresponded to the shape of the atoms involved – i.e. iron atoms are hard, water atoms are smooth and slippery, fire atoms are light and sharp, and air atoms are light and whirling.

Democritus' model of an atom was one of an intert solid that ineracted mechanically with other atoms. Credit: .science.edu.sg
Democritus’ model of an atom was one of an inert solid that interacted mechanically with other atoms. Credit: .science.edu.sg

However, Democritus is credited with illustrating and popularizing the concept, and for his descriptions of atoms which survived classical antiquity to influence later philosophers. Using analogies from our sense experiences, Democritus gave a picture or an image of an atom that distinguished them from each other by their shape, size, and the arrangement of their parts.

In essence, this model was one of an inert solid that excluded other bodies from its volume, and which interacted with other atoms mechanically. As such, his model included physical links (i.e. hooks and eyes, balls and sockets) that explained how connections occurred between them. While this bears little resemblance to modern atomic theory (where atoms are not inert and interact electromagnetically), it is more closely aligned with that of modern science than any other theory of antiquity.

While there is no clear explanation as to how scholars of classical antiquity came to theorize the existence of atoms, the concept proved to be influential, being picked up by Roman philosopher Lucretius in the 1st century CE and again during the Scientific Revolution. In addition to being indispensable to modern molecular and atomic theory, it also provided an explanation as to why the concept of a void was necessary in nature.

If all matter was composed of tiny, indivisible atoms, then there must also be a great deal of open space between them. This reasoning has also gone on to inform out notions of cosmology and astronomy, where Einstein’s theory of special relativity was able to do away with the concept of a “luminiferous aether” in explaining the behavior of light.

Early atomic theory stated that different materials had differently shaped atoms. Credit: github.com
Early atomic theory stated that different materials had differently shaped atoms. Credit: github.com

Diogenes Laertius summarized Democritus atomic theory as follows in Lives and Opinions of Eminent Philosophers:

“That atoms and the vacuum were the beginning of the universe; and that everything else existed only in opinion. That the worlds were infinite, created, and perishable. But that nothing was created out of nothing, and that nothing was destroyed so as to become nothing. That the atoms were infinite both in magnitude and number, and were borne about through the universe in endless revolutions. And that thus they produced all the combinations that exist; fire, water, air, and earth; for that all these things are only combinations of certain atoms; which combinations are incapable of being affected by external circumstances, and are unchangeable by reason of their solidity.”

Death and Legacy:

Democritus died at the age of ninety, which would place his death at around 370 BCE; though some writers disagree, with some claiming he lived to 104 or even 109. According to Marcus Aurelius’ book Meditations, Democritus was eaten by lice or vermin, although in the same passage he writes that “other lice killed Socrates”, implying that this was meant metaphorically. Since Socrates died at the hands of the Athenian government who condemned him, it is possible that Aurelius attributed Democritus death to human folly or politics.

While Democritus was highly esteemed amongst his contemporaries, there were also those who resented him. This included Plato who, according to some accounts, disliked him so much that he wished that all his books would be burned. However, Plato’s pupil Aristotle was familiar with the works of Democritus and mentioned him in both Metaphysics and Physics, where he described him as a “physicist” who did not concern himself with the ideals of form or essence.

Democritus meditating on the seat of the soul by Léon-Alexandre Delhomme (1868). Credit: Pubic Domain
Democritus meditating on the seat of the soul, by Léon-Alexandre Delhomme (1868). Credit: Pubic Domain

Ultimately, Democritus is credited as being one of the founders of the modern science because his methods and theories closely resemble those of modern astronomers and physicists. And while his version of the atomic model differs greatly from our modern conceptions, his work was of undoubted value, and was a step in an ongoing process that included such scientists as John Dalton, Neils Bohr and even Albert Einstein.

As always, science is an process of continuing discovery, where new breakthroughs are built upon the foundations of the old and every generations attempts to see a little farther by standing on the shoulders of those who came before.

We have many interesting articles about atomic theory here at Universe Today. Here’s one about John Dalton’s atomic model, Neils Bohr’s atomic model, the “Plum Pudding” atomic model.

For more information, check out The History of the Atom – Democritus.

Astronomy Cast has a wonderful episode on the subject, titled Episode 392: The Standard Model – Intro