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Nebular Theory Might Explain How Our Solar System Formed

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Our solar system contains the sun, inner rocky planets, the gas giants , or the outer planets, and other celestial bodies, but how they all formed is something that scientists have debated over time.

The nebular theory , also known as nebular hypothesis , presents one explanation of how the solar system formed. Pierre-Simon, Marquis de Laplace proposed the theory in 1796, stating that solar systems originate from vast clouds of gas and dust, known as solar nebula, within interstellar space.

Learn more about this solar system formation theory and some of the criticism it faced.

What Is the Nebular Theory?

Criticisms of the nebular theory, solar nebular disk model.

Laplace said the material from which the solar system and Earth derived was once a slowly rotating cloud, or nebula, of extremely hot gas. The gas cooled and the nebula began to shrink. As the nebula became smaller, it rotated more rapidly, becoming somewhat flattened at the poles.

A combination of centrifugal force, produced by the nebula's rotation, and gravitational force, from the mass of the nebula, left behind rings of gas as the nebula shrank. These rings condensed into planets and their satellites, while the remaining part of the nebula formed the sun.

The planet formation hypothesis, widely accepted for about a hundred years, has several serious flaws. The most serious concern is the speed of rotation of the sun.

When calculated mathematically on the basis of the known orbital momentum, of the planets, the nebular hypothesis predicts that the sun must rotate about 50 times more rapidly than it actually does. There is also some doubt that the rings pictured by Laplace would ever condense into planets.

In the early 20th century, scientists rejected the nebular hypothesis for the planetesimal hypothesis, which proposes that planets formed from material drawn out of the sun. This theory, too, proved unsatisfactory.

Later theories have revived the concept of a nebular origin for the planets. An educational NASA website states: "You might have heard before that a cloud of gas and dust in space is also called a 'nebula,' so the scientific theory for how stars and planets form from molecular clouds is also sometimes called the Nebular Theory. Nebular Theory tells us that a process known as 'gravitational contraction' occurred, causing parts of the cloud to clump together, which would allow for the Sun and planets to form from it."

Victor Safronov , a Russian astronomer, helped lay the groundwork for the modern understanding of the Solar Nebular Disk Model. His work, particularly in the 1960s and 1970s, was instrumental in shaping our comprehension of how planets form from a protoplanetary disk.

At a time when others did not want to focus on the planetary formation process, Safronov used math to try to explain how the giant planets, inner planets and more came to be. A decade after his research, he published a book presenting his work.

George Wetherill's research also contributed to this area, specifically on the dynamics of planetesimal growth and planetary accretion.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

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• Ask An Astrobiologist
• Resources Graphic Histories Coloring Pages Heroes Posters Life in the extremes Digital Backgrounds SciComm Guild

1. How did matter come together to make planets and life in the first place?

1.2. how did our solar system form.

Grades K-2 or Adult Naive Learner

• NGSS Connections for Teachers
• Concept Boundaries for Scientists

Do you know what a planet is? A planet is a big, round world, floating in space. It can be made mostly of rock or even mostly of gas, just like the air all around us.

You, me, and everyone we know lives on a planet called Earth. Our planet is in space and goes around the Sun. Now, did you know that the Sun is a star? Well, there are also seven other planets going around our star, the Sun. The Sun and the planets are part of what we call the Solar System.

The Solar System is really old. The Sun and all of the planets came from a big cloud of stuff in space. Do you know that raindrops come from clouds in the sky? Well, it turns out that stars and even planets can come from clouds in space. Our Sun came from the middle of a big cloud in space, and the planets of our solar system also formed from that same cloud, moving around the Sun in the same kind of pattern that they follow today.

Disciplinary Core Ideas

ESS1.C: The History of Planet Earth: Some events happen very quickly; others occur very slowly, over a time period much longer than one can observe. (2-ESS1-1)

PS3.B: Conservation of Energy and Energy Transfer: Sunlight warms Earth’s surface. (K-PS3-1, K-PS3-2)

Crosscutting Concepts

Patterns in the natural world can be observed, used to describe phenomena, and used as evidence. (1-ESS1-1, 1-ESS1-2)

Big Ideas: The solar system consists of Earth and seven other planets all spinning around the Sun. Planets are big, round worlds floating in space. The Earth is a planet that goes around a much larger star called the Sun. The Sun and planets formed from a big cloud of gas and dust. The Earth, moon, Sun and planets all move in a pattern called an orbit.

Boundaries: By the end of 2nd grade, seasonal patterns of Sunrise and Sunset can be observed, described and predicted. Temperature (i.e. the Sun warms Earth) is limited to relative measurements such as warmer/cooler. (K-PS3-1)

K-5 The Science of the Sun. In this unit, students focus on the Sun as the center of our solar system and as the source for all energy on Earth. By beginning with what the Sun is and how Earth relates to it in size and distance, students gain a perspective of how powerful the Sun is compared to things we have here on Earth, and the small fraction of its energy we receive. Students also gain an understanding of how Earth relates to the other planets in the solar system. The Sun as a Star (page 17) Students identify the sun as a star. The Scale of Things (page 27). Students explore the scale of the solar system. The Size of Things (page 33) Students describe the relative sizes of the planets in the solar system by making a play-doh model. What is a year (page 37) Students act out the motion of Earth as it travels (revolves) around the Sun. Goddard Space Flight Center/NASA. https://sdo.gsfc.nasa.gov/assets/docs/UnitPlanElementary.pdf

2-12 Toilet Paper Solar System. Even in our own “cosmic neighborhood,” distances in space are so vast they are difficult to imagine. In this activity, participants build a scale model of the distances in the solar system using a roll of toilet paper. https://astrosociety.org/file_download/inline/cfdf9b2c-5947-4c19-9a23-a790ac3c7ae0

Grades 3-5 or Adult Emerging Learner

For us to learn about where we came from, we need to understand how our solar system formed.

The Sun and the planets and all of the asteroids and comets and other stuff in our solar system all formed from a really big cloud of gas and dust in space. There are clouds of gas and dust all around our galaxy. Sometimes these clouds can slowly turn into stars and planets when enough material is available and clumps together forming massive collections of ice and rock.

Do you know what kind of pattern the planets make when they go around the Sun? It kind of looks like a big circle, right? Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same way, with the Sun forming in the middle. That’s why we see the planets moving around the Sun the way that they do today! We call that pattern of how a planet moves around the Sun an “orbit.” Have you heard of anything else that has an “orbit”? Our Moon orbits around our Earth, just like our Earth orbits around our Sun, and our entire solar system is also orbiting around the galaxy. Orbits are really important for us to learn about if we want to know where we came from.

ESS1.C: The History of Planet Earth: Local, regional, and global patterns of rock formations reveal changes over time due to earth forces, such as earthquakes. The presence and location of certain fossil types indicate the order in which rock layers were formed. (4-ESS1-1)

PS1.A: Structure and Properties of Matter: Matter of any type can be subdivided into particles that are too small to see, but even then the matter still exists and can be detected by other means. (5-PS1-1)

PS2.B: Types of Interactions: Objects in contact exert forces on each other. (3-PS2-1) The gravitational force of Earth acting on an object near Earth’s surface pulls that object toward the planet’s center. (5-PS2-1)

Patterns can be used as evidence to support an explanation. (4-ESS1-1, 4-ESS2-2) *Science assumes consistent patterns in natural systems. (4-ESS1-1)

Big Ideas: The Solar system formed through condensation from a big cloud of gas and dust. The solar system consists of Earth and seven other planets all orbiting around the Sun. The Sun, moon, and planets all move in predictable patterns called orbits. Many of these orbits are observable from Earth. The entire solar system orbits around the Milky Way galaxy.

Boundaries: In this grade band, students are learning about the different positions of the Sun, moon, and stars as observable from Earth at different times of the day, month, and year. Students are not yet defining the unseen particles or explaining the atomic-scale mechanism of condensation.

3-5 SpaceMath Problem 543: Timeline for Planet Formation. Students calculate time intervals in millions and billions of years from a timeline of events [Topics: time calculations; integers] https://spacemath.gsfc.nasa.gov/Grade35/10Page6.pdf

3-5 SpaceMath Problem 541: How to Build a Planet. Students study planet growth by using a clay model of planetessimals combining to form a planet by investigating volume addition with spheres. [Topics: graphing; counting] https://spacemath.gsfc.nasa.gov/Grade35/10Page4.pdf

3-5, 6-8, 9-12 Marsbound! In this NGSS aligned activity (three 45-minute sessions), students in grades become NASA project managers and design their own NASA mission to Mars. Mars is significant in astrobiology and more needs to be learned about this planet and its potential for life. Students create a mission that must balance the return of science data with mission limitations such as power, mass and budget. Risk factors play a role and add to the excitement in this interactive mission planning activity. Arizona State University/NASA. http://marsed.asu.edu/lesson_plans/marsbound

3-5 or 6-8 Strange New Planet. This 5E hands-on lesson (2-3 hours) engages students in how scientists gain information from looking at things from different perspectives. Students gain knowledge about simulated planetary surfaces through a variety of missions such as Earth-based telescopes to landed missions. They learn the importance of remote sensing techniques for exploration and observation. NASA /Arizona State University. http://marsed.asu.edu/strange-new-planet

4-8 SpaceMath Problem 300: Does Anybody Really Know What Time It Is? Students use tabulated data for the number of days in a year from 900 million years ago to the present, to estimate the rate at which an Earth day has changed using a linear model. [Topics: graphing; finding slopes; forecasting] https://spacemath.gsfc.nasa.gov/earth/6Page58.pdf

4-12 Meet the Planets. In this activity, kids identify the planets in the solar system, observe and describe their characteristics and features, and build a scale model out of everyday materials. They are also introduced to moons, comets, and asteroids. (Finding life Beyond Earth, page 13) NOVA . https://d43fweuh3sg51.cloudfront.net/media/assets/wgbh/nvfl/nvfl_doc_collection/nvfl_doc_collection.pdf

5-12 Exploring Meteorite Mysteries: The Meteorite Asteroid Connection (4.1). In this lesson, students build an exact-scale model of the inner solar system; the scale allows the model to fit within a normal classroom and also allows the representation of Earth to be visible without magnification. Students chart where most asteroids are, compared to the Earth, and see that a few asteroids come close to the Earth. Students see that the solar system is mostly empty space unlike the way it appears on most charts and maps. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Exploring Meteorite Mysteries: Building Blocks of Planets (10.1). Chondrites are the most primitive type of rock available for study. The chondrules that make up chondrites are considered the building blocks of planets. In this lesson, students experiment with balloons and static electricity to illustrate the theories about how dust particles collected into larger clusters. Students also manipulate magnetic marbles and steel balls to illustrate the accretion of chondritic material into larger bodies like planets and asteroids. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Exploring Meteorite Mysteries: Exploration Proposal (17.1). Exploration of the outer Solar System provides clues to the beginnings of the solar system. This is a group-participation simulation based on the premise that water and other resources from the asteroid belt are required for deep space exploration. Students brainstorm or investigate to identify useful resources, including water, that might be found on an asteroid. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Big Explosions and Strong Gravity. In this one-two day activity, students work in groups to examine the crushing ability of gravity, equilibrium, and a model for the creation of heavy elements through a supernova. This active lesson helps students visualize the variation and life cycle of stars. NASA http://imagine.gsfc.nasa.gov/educators/programs/bigexplosions/activities/supernova_demos.html

Grades 6-8 or Adult Building Learner

Earth is the only world that we know of that has life. All of the plants and animals and microbes and other living things on Earth have evolved here. So, for us to understand where life as we know it came from, we need to understand where our planet came from.

The Sun and the planets and all of the other stuff in our solar system all formed from a really big cloud of gas and dust in space. We call such a cloud a “nebula” and more than one of them we refer to as “nebulae.” There are nebulae all around our galaxy, and it’s from these nebulae that stars and planets form. Nebulae are massive clouds of dust and debris in space and have all the ingredients to form stars and planets. When enough material is available, it begins to stick together forming a large mass. In time, the mass can grow large enough to form a planet or even a new star.

We currently think that our solar system formed from a large nebula, perhaps after the explosion of a nearby star. Some big stars can explode, something called a supernova, and that explosion has enough energy to make the gas and dust in nearby nebulae start swirling and spinning about. As this happened, it caused a lot of the material in the nebula to fall into its center, and that’s where the Sun started forming. Meanwhile, the rest of the gas and dust in the nebula began colliding and sticking together, making little pieces of metal and rock. Those small pieces then collided with each other, forming larger pieces, which then collided with each other to form even larger ones. These were young planets, and eventually, over a long time and through many, many collisions, our eight planets were formed – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

We call the pattern that the planets make when they go around the Sun an “orbit.” Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same direction as the orbits of the planets today, with the Sun forming in the middle and also spinning in the same direction. That’s why we see the planets moving around the Sun the way that they do today!

You might also know that the Moon orbits around Earth. For something to be a moon, it needs to be in orbit around a planet. One thing that makes a planet is that a planet has to be orbiting a star. But star systems also have orbits. They orbit around their entire galaxy. So, orbits are really important for us to learn about if we want to know where we came from.

ESS1.A: The Universe and Its Stars: - Patterns of the apparent motion of the Sun, the Moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1) - Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe. (MS-ESS1-2)

ESS1.B: Earth and the Solar System: - The solar system consists of the Sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the Sun by its gravitational pull on them. (MS-ESS1-2, MS-ESS1-3) - This model of the solar system can explain eclipses of the Sun and the Moon. Earth’s spin axis is fixed in direction over the short-term but tilted relative to its orbit around the Sun. The seasons are a result of that tilt and are caused by the differential intensity of sunlight on different areas of Earth across the year. (MS-ESS1-1) - The solar system appears to have formed from a disk of dust and gas, drawn together by gravity. (MS-ESS1-2)

PS1.A: Structure and Properties of Matter: All substances are made from some 100 different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms. Pure substances are made from a single type of atom or molecule; each pure substance has characteristic physical and chemical properties that can be used to identify it. (MS-PS1-1)

Cause and effect relationships may be used to predict phenomena in natural or designed systems. (MS-PS1-4)

Big Ideas: Condensation causes rain drops to form inside of clouds, and sometimes can cause entire star systems to form inside of clouds. The Solar system formed through condensation from big clouds of gas and dust called nebulae after a supernova, or the explosion of a large star. Planets move around the Sun in an orbit, and the Solar system orbits around the entire galaxy.

Boundaries: Emphasis is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. (MS-ESS1-2) Does not include Kepler’s Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth. (MS-ESS1-2)

6-8 SpaceMath Problem 542: The Late Heavy Bombardment Era. Students estimate the average arrival time of large asteroids that impacted the moon. They work with the formula for the volume of a sphere to estimate how much additional mass was added to the moon and Earth during this era. [Topics: volume of spheres; proportions] https://spacemath.gsfc.nasa.gov/earth/10Page5.pdf

6-8 SpaceMath Problem 60: When is a planet not a planet? In 2003, Dr. Michael Brown and his colleagues at CalTech discovered an object nearly 30% larger than Pluto, which is designated as 2003UB313. Is 2003UB313 really a planet? In this activity, students examine this topic by surveying various internet resources that attempt to define the astronomical term ‘planet’. [Topics: non-mathematical essay; reading to be informed] https://spacemath.gsfc.nasa.gov/astrob/2page17.pdf

6-8 SpaceMath Problem 59: Getting A Round in the Solar System! How big does a body have to be before it becomes round? In this activity, students examine images of asteroids and planetary moons to determine the critical size for an object to become round under the action of its own gravitational field. [Topics: data analysis; decimals; ratios; graphing] https://spacemath.gsfc.nasa.gov/astrob/2page20.pdf

6-8 Explore! Jupiter’s Family Secrets. This one-hour lesson for formal or informal education settings has students connecting their own life story to a cultural creation story and then to the “life” story of Jupiter, including the Big Bang as the beginning of the universe, the creation of elements through stars and the creation of the solar system. JPL /NASA. http://www.lpi.usra.edu/education/explore/solar_system/activities/birthday/

6-9 Rising Stargirls Teaching and Activity Handbook. 1.2. Art & the Cosmic Connection: (page 19). This activity engages students in space and science education by becoming explorers. Using the elements of art: line, color, texture, shape, and value: students learn to analyze the mysterious surfaces of our rocky celestial neighbors; planets, moons, comets and asteroids, as well as the Earth. Name That Planet (page 25) Students communicate their knowledge about the solar system using different modes of communication—visual, verbal, and kinesthetic. Distance Calculation (page 27) Students calculate the distances between planets using a unit of measurement that is personal to them—themselves! Rising Stargirls activities fuse science and the arts to create enlightened future scientists and imaginative thinkers. Rising Stargirls. https://static1.squarespace.com/static/54d01d6be4b07f8719d7f29e/t/5748c58ec2ea517f705c7cc6/1464386959806/Rising_Stargirls_Teaching_Handbook.compressed.pdf

6-12 Science Fiction Stories with Good Astronomy & Physics: A Topical List: Cosmology. 1.2. The Astronomical Society of the Pacific created this list of short stories and novels that use more or less accurate science and can be used for teaching or reinforcing astronomy or physics concepts including the origin of the universe. https://astrosociety.org/file_download/inline/621a63fc-04d5-4794-8d2b-38e7195056e9

6-12 Where are the Small Worlds? Through an immersive digital experience (1-2 hours), students use a simulation/model of the solar system in order to investigate small worlds in order to learn more about the solar system and its origin. The experience can be standalone or has options to track student tasks or modify the simulation as needed by the teacher. Arizona State University. https://infiniscope.org/lesson/where-are-the-small-worlds/

6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include Habitable Zones and Stellar Luminosity (page 57) and Ice or Water? (page 49). NASA . https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf

6-12 Pocket Solar System. This activity involves making a simple model to give students an overview of the distances between the orbits of the planets and other objects in our solar system. It is also a good tool for reviewing fractions. https://astrosociety.org/file_download/inline/5c27818a-e947-46ad-a9dc-f4af157af7d8

6-12 Origins: The Universe. In this web interactive, scientists use a giant eye in the southern sky to unravel how galaxies are born. Video, pictures, and print weave information for the learner as they more deeply understand the scientific pursuit of astrobiology. UW-Madison. https://origins.wisc.edu/

7-9 SpaceMath Problem 8: Making a Model Planet. Students use the formula for a sphere, and the concept of density, to make a mathematical model of a planet based on its mass, radius and the density of several possible materials (ice, silicate rock, iron, basalt). [Topics: volume of sphere; mass = density x volume; decimal math; scientific notation] https://spacemath.gsfc.nasa.gov/astrob/Week14.pdf

Grades 9-12 or Adult Sophisticated Learner

As the physical context for life as we know it, it is important to learn about Earth’s origins so we can understand life’s origins. Although life may exist in situations other than that of a planet orbiting a star, it makes sense to explore the phenomenon of planetary system formation as a context for the emergence and evolution of life.

The story of the formation of our solar system begins in a region of space of called a “giant molecular cloud”. You might have heard before that a cloud of gas and dust in space is also called a “nebula,” so the scientific theory for how stars and planets form from molecular clouds is also sometimes called the Nebular Theory. Nebular Theory tells us that a process known as “gravitational contraction” occurred, causing parts of the cloud to clump together, which would allow for the Sun and planets to form from it.

Before gravitational contraction, the majority of the material within the giant molecular cloud that formed our solar system consisted of hydrogen and helium produced at the time of the big bang, with small amounts of heavier elements such as carbon and oxygen which were made via nucleosynthesis in prior generations of stars (see 1.1 above). The material in this giant cloud was not uniformly distributed – there were regions of higher density (more dust and gas within a specific volume of space) and regions of lower density (less gas and dust within that same volume).

Evidence from meteorites suggests that the energy produced by a nearby exploding star (a supernova) passed through a higher density region in the cloud and caused it to begin to swirl and twist about. This area of the cloud is sometimes called the pre-solar nebula (“pre” = before; “solar” = star or Sun). As molecules in the pre-solar nebula were swirling about, some of them started bumping into each other and sometimes would even stick together. As more and more of these clumps formed, gravity caused them to start sticking together and to fall into the center of the pre-solar nebula, which only caused gravity to pull even more of the material into the center of the cloud, and this is the process that’s referred to as gravitational contraction.

While all of this was happening, the action of molecules bumping into each other over and over slowly caused the pre-solar nebula to flatten into a spinning disk of dust and gas. This is sometimes called a circumstellar disk (“circum” = around; “stellar” = star) or protoplanetary disk (“proto” = first or before). Almost all of the material in the disk collected in the center, giving rise to the young Sun. However, some of the particles in the spinning disk began colliding with each other and sticking together, forming larger and larger fragments. The larger a fragment became, the more mass it had and therefore the more gravitational pull it exerted. Which in turn drew more and more material to it, and the larger it became, and so on. This process is called “accretion,” and resulted in the production of many planetesimals (small objects that build up into planets), and eventually, the planets themselves.

While the young Sun was starting to heat up in the middle of the protoplanetary disk, it warmed up the disk so much that nothing could stay solid really close to the Sun (it all melted). A little further out from the Sun, stuff like metal and rock was able to cool enough to make solid materials for forming the planets. But it was still so hot there that molecules that are often liquids or gases here on Earth (like water, ammonia, carbon dioxide and methane) couldn’t easily stick to the solid planet-forming materials. Those molecules could only really be added to planets that were a lot further from the Sun, where it was cold enough for them to clump together with the other solid stuff. This is why we have gas giant planets like Jupiter and Saturn which are very different from the rocky planets like Earth and Venus.

ESS1.A: The universe and its Stars: Nearly all observable matter in the universe is hydrogen or helium, which formed in the first minutes after the big bang. Elements other than these remnants of the big bang continue to form within the cores of stars. (HS-ESS1-2) *Nuclear fusion within stars produces all atomic nuclei lighter than and including iron, and the process releases the energy seen as starlight. Heavier elements are produced when certain massive stars achieve a supernova stage and explode. (HS-ESS1-2, HS-ESS1-3) *Stars go through a sequence of developmental stages — they are formed; evolve in size, mass, and brightness; and eventually burn out. Material from earlier stars that exploded as supernovas is recycled to form younger stars and their planetary systems.

ESS1.B: Earth and the Solar System: Kepler’s laws describe common features of the motions of orbiting objects, including their elliptical paths around the Sun. (HS-ESS1-4) *The solar system consists of the Sun and a collection of objects of varying sizes and conditions — including planets and their moons — that are held in orbit around the Sun by its gravitational pull on them. This system appears to have formed from a disk of dust and gas, drawn together by gravity.

PS1.C: Nuclear Processes: Nuclear processes, including fusion, fission, and radioactive decays of unstable nuclei, involve release or absorption of energy. The total number of neutrons plus protons does not change in any nuclear process. (HS-PS1-8)

Scientific knowledge is based on the assumption that natural laws operate today as they did in the past and they will continue to doe so in the future (HS-ESS1-2). Science assumes the universe is a vast single system in which basic laws are consistent. (HS-ESS1-2)

Big Ideas: The phenomenon of planetary system formation serves as a context for the emergence and evolution of life. A cloud of gas and dust in space is called a “nebula”. The Nebular Theory is the scientific theory for how stars and planets form from molecular clouds and their own gravity. The majority of the material within the giant molecular cloud that formed our solar system consisted of hydrogen and helium produced at the time of the big bang. Nuclear fusion within stars forms heavier elements under extreme pressure and temperature. The larger the star, the heavier the elements that can be produced through fusion and Supernova. Heavier elements were also made via nucleosynthesis. The circumstellar disk gave rise to the young Sun.

Boundaries: Emphasis is on the way nucleosynthesis, and therefore the different elements created, varies as a function of the mass of a star and the stage of its lifetime.(HS-ESS1-3) Does not include details of the atomic and subatomic processes involved with the Sun’s nuclear fusion. (HS-ESS1-1)

9-10 Voyages through Time: Cosmic Evolution. This comprehensive integrated curriculum includes the universe, the totality of all things that exist, origins (beginning with an explosion of space and time and the expansion of a hot, dense mass of elementary particles and photons), and how it has evolved over billions of years into the stars and galaxies we observe today. Sample lesson on the website and the curriculum is available for purchase. SETI . http://www.voyagesthroughtime.org/cosmic/index.html

9-11 SpaceMath Problem 302: How to Build a Planet from the Inside Out. Students model a planet using a spherical core and shell with different densities. The goal is to create a planet of the right size, and with the correct mass using common planet building materials. [Topics: geometry; volume; scientific notation; mass=density x volume] https://spacemath.gsfc.nasa.gov/astrob/6Page72.pdf

9-12 Genesis Science Modules: Cosmic Chemistry: Planetary Diversity. The goal of this module is to acquaint students with the planets of the solar system and some current models for their origin and evolution. The lessons in the Genesis Science Modules challenge students to look for patterns in data, to generate observations, and critically analyze where the data does not fit with the current nebular model. This mini-unit reveals the essence of scientific research and argument within the context of the formation of solar systems. JPL /NASA http://genesismission.jpl.nasa.gov/educate/scimodule/PlanetaryDiversity/index.html

9-12 A101 Slide Set: From Supernovae to Planets. This slide set explains the discoveries of the SOFIA mission and the implications of the new data explaining how supernovae and dust push planet formation and how this is the physical context for life. SOFIA /NASA https://slideplayer.com/slide/8679314/ Teacher’s Guide:

https://www.astrosociety.org/edu/higher-ed/files/A101ss.SOFIA_SupernovaePlanets.v3.pdf

11-12 SpaceMath Problem 305: From Asteroids to Planets. Students explore how long it takes to form a small planet from a collection of asteroids in a planet-forming disk of matter orbiting a star based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page82.pdf

11-12 SpaceMath Problem 304: From Dust Balls to Asteroids. Students calculate how long it takes to form an asteroid-sized body using a simple differential equation based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page81.pdf

11-12 SpaceMath Problem 303: From Dust Grains to Dust Balls. Students create a model of how dust grains grow to centimeter-sized dust balls as part of forming a planet based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page80.pdf

Storyline Extensions

The planets are named after stories from long ago:.

Our planets are named Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Seven of the planets are named after gods from Roman mythology. These are Mercury, Venus, Mars, Jupiter, Saturn, and Neptune. However, Uranus is a name from Greek mythology (Uranus was the god of the sky). Also, the name for our planet, Earth, comes from Old English, and appears to have come from people who lived in Northern Europe long ago.

Our location in the galaxy:

Our Milky Way galaxy is really big! If we could travel outside of the galaxy and look back at it, it would look like a big disk of dust and gas and stars, with a big bulging sphere of stars near the middle. The disk of the galaxy is about 100,000 lightyears in diameter. That means that it takes light about 100,000 years to travel from one side to the other. Our little solar system (little in comparison to the galaxy, that is) lies about 30,000 lightyears from the center of the galaxy. Just as moons orbit around planets, and planets orbit around stars, star systems also orbit around the center of the galaxy. Our own solar system is traveling through the galaxy at over 500,000 miles per hour! And our very long orbit around the galaxy takes almost 250 million years! But we’re not alone out here. There are lots of other stars and other worlds in the galaxy. Our best estimates right now are that there are about 100-400 billion stars in the Milky Way. And, even though we’ve only just begun finding exoplanets, some astronomers believe there is evidence for more planets than stars in the milky way and other galaxies. That’s an awful lot of worlds!

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8 The Rise and Fall of the Nebular Hypothesis

• Published: August 2023
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The first to explain the origin of the planets and moons was Pierre-Simon Laplace in his 1796 book, Exposition for the System of the World . His theory would dominate science throughout the next century and come to be accepted as a given. He held that the solar system had begun as a hot, rotating gas cloud. As it spun, centrifugal force threw off blobs of gas that coagulated into planets. The planets then repeated the process to create their moons. By the last few decades of the eighteenth century, enough evidence had come to light to call the nebular hypothesis into question, if not to falsify it. This opened the way for three different theories for the origin of the Moon. The fission theory resembled the nebular hypothesis in holding that the gravity of the Sun had pulled off a bulge in the proto-Earth which became the Moon. The co-accretion theory held that the Moon and the Earth had formed near each other and thus were like sister planets. The capture theory imagined that the Moon had started out in some distant region of the solar system but drew near enough to be captured into orbit by the Earth’s gravity.

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Historical Geology

A free online textbook for Historical Geology courses

Nebular theory and the formation of the solar system

In the beginning….

How and when does the story of Earth begin? A logical place to start is with the formation of the planet, but as you’ll soon see, the formation of the planet is part of a larger story, and that story implies some backstory before the story, too. The purpose of this case study is to present our best scientific understanding of the formation of our solar system from a presolar nebula, and to put that nebula in context too.

Nebular theory

The prevailing scientific explanation for the origin of the Earth does a good job of not only explaining the Earth’s formation, but the Sun and all the other planets too. Really, it’s not “the Earth’s origin story” alone so much as it is the origin story of the whole solar system . Not only that, but our Sun is but one star among a hundred million in our galaxy, and our galaxy is one of perhaps a hundred million in the universe. So the lessons we learn by studying our own solar system can likely be applied more generally to the formation of other solar systems elsewhere, including those long ago, in galaxies far, far away. The vice versa is also true: Our understanding of our own solar system’s origin story is being refined as we learn more about exoplanets, some of which defy what we see in our own system; “ hot Jupiters ” and “ super-Earths ,” for instance, are features we see in other star systems but not our own.

When we use powerful telescopes to stare out into the galaxy, we observe plenty of other stars, but we observe other things too, including fuzzy looking features called nebulae. A nebula is a big cloud of gas and dust in space. It’s not as bright as a star because it’s not undergoing thermonuclear fusion, with the tremendous release of energy that accompanies that process. An example of a nebula that you are likely to be able to see is in the constellation Orion. Orion’s “belt,” three stars in a row, is a readily identifiable feature in the northern hemisphere’s night sky in winter. A smaller trio of light spots “dangle” from the belt; this is Orion’s sword scabbard. A cheap pair of binoculars will let you examine these objects for yourself; you will discover that the middle point of light in this smaller trio is not a star. It is a nebula called Messier 42.

Nebulae like Messier 42 are common features of the galaxy, but not as common as stars. Nebulae appear to be short-lived features, as matter is often attracted to other matter. All that stuff distributed in that tremendous volume of space is not as stable as it would be if it were all to be drawn together into a few big clumps. Particles pull together with their neighboring particles under the influence of various forces, including “static cling” or electrostatic attraction. This is the same force that makes tiny dust motes clump up into dust bunnies under your couch!

Now, electrostatic force is quite strong for pulling together small particles over small distances, but if you want to make big things like planets and stars out from a nebula, you’re going to need gravity to take over at some point. Gravity is a rather weak force. After all: every time you take a step, you’ve overcoming the gravitational pull of the entire Earth. But gravity can work very efficiently over distance, if the masses involved are large enough. So static cling was the initial organizer, until the “space dust bunnies” got large enough, then gravity was able to take over, attracting mass to mass. The net result is that the gajillions of tiny pieces of the nebula were drawn together, swirling into a denser and denser amalgamation. The nebula began to spin, flattening out from top to bottom, and flattening out into a spinning disk, something between a Frisbee and a fried egg in shape:

Once a star forms in the center, astronomers call the ring of debris around it a protoplanetary disk. Two important processes that helped organize the protoplanetary disk further were condensation and accretion.

Condensation is the process where gaseous matter sticks together to make liquid or solid matter. We have evidence of condensation in the form of small spherical objects with internal layering, kind of like “space hailstones.” These are chondrules, and they represent the earliest objects formed in our solar system. (Occasionally, we are lucky enough to find chondrules that have survived until the present day, entombed inside certain meteorites of the variety called chondrites.)

Chondrules glommed onto other chondrules, and stuck themselves together into primordial “rocks,” building up larger and larger objects. Eventually, these objects got to be big enough to pull their mass into an round shape, and we would be justified to dub them “planetesimals.” Planetesimals gobbled up nearby asteroids, and smashed into other planetesimals, merging and growing through time through the process of accretion. The kinetic force of these collisions heated the rocky and metallic material of the planetesimals, and their temperature also went up as radioactive decay heated them from within. Once warm, denser material could sink to their middles, and lighter-weight elements and compounds rose up to their surface. So not only were they maturing into spheroidal shapes, but they were also differentiating internally, separating into layers organized by density.

Meteorites that show metallic compositions represent “core” material from these planetesimals; core material that we would never get to glimpse had not their surrounding rocky material been blasted off. Iron meteorites such as the Canyon Diablo meteorite below (responsible for Arizona’s celebrated Meteor Crater) therefore are evidence of differentiation of planetesimals into layered bodies, followed by disaggregation: a polite way of saying they were later violently ripped apart by energetic collisions.

If you were to somehow weigh the nebula before condensation and accretion, and again 4.6 billion years later, we’d find the mass to be the same. Rather than being dispersed in a diffuse cloud of uncountable atoms, the condensation and accretion of the nebula resulted in exactly the same amount of stuff, but organized into a smaller and smaller number of bigger and bigger objects. The biggest of these was the Sun, comprising about 99.86% of all the mass in the solar system. Four-fifths of the remaining 0.14% makes up the planet Jupiter.  Saturn, Neptune, and Uranus are huge gas giants as well. The inner rocky planets (including Earth) make up a tiny, tiny fraction of the total mass of the whole solar system – but of course, just because they are relatively small, that doesn’t mean they are unimportant!

The process of accretion continues into the present day, though at a slower pace than the earliest days of the solar system. One place you can observe this is in the asteroid belt, where there are certain asteroids that are basically nothing more than a big 3D pile of space rocks, held together under their own gravity. Consider the asteroid called Itokawa 25143, for instance:

Only about half a kilometer long, and only a few hundred meters wide, Itokawa doesn’t even have enough gravity to pull itself into a sphere. If you were to land on the surface of Itokawa and kick a soccer-ball-sized boulder, it would readily fly off into space, as the force of your kick would be much higher than the force of gravity causing it to stay put.

Another example of accretion continuing to this day is meteorite impacts. Every time a chunk of rock in space intersects the Earth, its mass is added to that of the planet. In that instant, the solar system gets a little bit cleaner (fewer leftover bits rattling around) and the planet gets a little more massive. A spectacular example of this occurred in 1994 with Comet Shoemaker-Levy 9, a  comet which had only been discovered the previous year. Jupiter’s immense gravity broke the comet into chunks, and then swallowed them up one after another. Astronomers on Earth watched with fascination as the comet chunks, some more than a kilometer across, slammed into Jupiter’s atmosphere at 60 km/second (~134,000 mph), creating a 23,700°C fireball and enormous impact scars that were as large as the entire Earth. These scars lasted for months.

This incredibly dramatic event perhaps raises the hair on our necks, seeing the violence and power of cosmic collisions. It’s a reminder that Earthlings are not safe from accretionary impacts even today – as the dinosaurs found out. For the purposes of our current discussion, though, bear in mind that the collision was really a merger between the masses of Comet Shoemaker-Levy 9 and the planet Jupiter, and after the dust settled, the solar system had one fewer object left off by itself, and Jupiter gained a bit more mass. This is the overall trend of the accretion of our solar system from the presolar nebula: under gravity’s influence, the available mass becomes more and more concentrated through time.

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A star is born

Because the Sun is so massive, it is able to achieve tremendous pressures in its interior. These pressures are so high, they can actually force two atoms into the same space , overcoming their immense repulsion for one another, and causing their two nuclei to merge. As two atoms combine to make one more massive atom, energy is released. This process is thermonuclear fusion. Once it begins, stars begin to give off light.

The ability of stars to make big atoms from small ones is key to understanding the history of our solar system and our planet. Planet Earth is made of a wide variety of chemical elements, both lightweight and heavy. All of these elements must have been present in the nebula, in order for them to be included in Earth’s “starting mixture.” Elements formed in the Sun today stay in the Sun, fusing low-weight atoms into heavier atoms. So all the elements on Earth today came from a pre-Sun star. We can go outside on a spring day and enjoy the Sun’s warmth, but the carbon that makes up the skin that basks in that warmth was forged in the heart of another star, a star that’s gone now, a star that blew up.

This exploding star was the source of the nebula where we began this case study: it’s the backstory that occurred before the opening scene. Our solar system is like a “haunted house,” where billions of years ago, there was a vibrant, healthy main-sequence star right here, in this part of the galaxy. Perhaps it had planets orbiting it. Perhaps some of those planets harbored life. We’ll never know: the explosion wiped the slate clean, and “reset” the solar system for the iteration in which we live. The ghostly remnants of this time before our own still linger, in the very stuff we’re made from. This long-dead star fused hydrogen to build the carbon in our bodies, the iron in our blood, the oxygen we breathe, and the silicon in the rocks of our planet.

This is an incredible realization to embrace: everything you know, everything you trust, everything you are , is stardust.

Age of the solar system

So just when did all this happen? An estimate for the age of the solar system can be made using isotopes of the element lead (Pb). There are several isotopes of lead, but for the purposes of figuring out the age of the solar system, consider these four: 208 Pb, 207 Pb, 206 Pb, and 204 Pb.

208 Pb, 207 Pb, 206 Pb are all radiogenic: that is to say, they stable “daughter” isotopes that are produced from the radioactive “parent” isotopes. Each is produced from a different parent, at a different rate:

204 Pb is, as far as we know, non-radiogenic. It’s relevant to this discussion because it can serve as a ‘standard’ that can allow us to compare the other lead isotopes to one another. Just as if we wanted to compare the currencies of Namibia, Indonesia, and Chile, we might reference all three to the U.S. dollar. The dollar would serve as a standard of comparison, allowing us to better see the value of the Namibian currency relative to the Indonesian currency and the Chilean currency. That’s what 204 Pb is doing for us here.

This is a plot showing the modeled evolution of our three radiogenic lead isotopes relative to 204 Pb. It is constrained by terrestrial lead samples at the young end, and projected back in time in accordance with our measurements of how quickly these three isotopes of lead are produced by their radioactive parents. Of course, if we go back far enough in time, we run out of samples to evaluate. The Earth’s rock cycle has destroyed all its earliest rocks. They’ve been metamorphosed, or weathered, or melted – perhaps many times over! What would be really nice is to find some rocks from the early end of these curves – some samples that could verify these projections back in time are accurate.

Such samples do exist! But they are not from the Earth so much as “from the Earth’s starting materials.” If the nebular theory is correct, then a few leftover scraps of the planet’s starting materials are found in the solar system’s asteroids. Every now and again, bits of these space rocks fall to earth, and if they survive their passage through the atmosphere, we may be lucky enough to collect them, and analyze them. We call these space rocks “meteors” as they streak through the atmosphere, heating through friction and oxidizing as they fall. Those that make it all the way to Earth’s surface are known as “meteorites.” They can be often be distinguished by their scalloped fusion crust, as with this sample:

Meteorites come in several varieties, including rocky and metallic versions. It is very satisfying that when measurements of these meteorites’ lead isotopes are added to the plot above, they all fall exactly where our understanding of lead isotope production would have them: at the start of each of these model evolution curves. Each lead isotope system tells the same answer for the age of the Earth, acting like three independent witnesses corroborating one another’s testimony. And the answer they all give is 4.6 billion years ago (4.6 Ga). That’s what 208 Pb says. That’s what 207 Pb says. And that’s what 206 Pb says. They all agree, and they agree with the predicted curves based on terrestrial (Earth rock) measurements. This agreement gives us great confidence in this number. The Earth, and meteorites (former asteroids), and the solar system of which they are all a part, began about 4.6 billion years ago…

…But what came before that?

The implications of meteorites

In 1969, a meteorite fell through Earth’s atmosphere and broke up over Mexico. A great many pieces of this meteorite were recovered and made available for scientific analysis. It turned out to be a carbonaceous chondrite, the largest of its kind ever documented. It was named the Allende ( “eye-YEN-day” ) meteorite, for the tiny Chihuahuan village closest to the center of the area over which its fragments were scattered.

One of the materials making up Allende’s chondrules was the calcium feldspar called anorthite. Anorthite is an extraordinarily common mineral in Earth’s crust, but the Allende anorthite was different. For some reason, it has a large amount of magnesium in it. When geochemists determined what kind of magnesium this was, they were surprised to find that it was mostly 26 Mg, an uncommon isotope. The abundances of 25 Mg and 24 Mg were found to be about the same level as Earth rocks, but 26 Mg was elevated by about 1.3%.  And after all, magnesium doesn’t even “belong” in a feldspar. The chemical formula of anorthite is CaAl 2 Si 2 O 8 – there’s no “Mg” spot in there. Why was this odd 26 Mg in this chondritic anorthite?

One way to make 26 Mg is the break-down of radioactive 26 Al. The problem with this idea is that there is no 26 Al around today . It’s an example of an extinct isotope: an atom of aluminum so unstable that it falls apart extremely rapidly. The half-life is only 717,000 years. But because these chondrules condensed in the earliest days of the solar system, there may well have been plenty of 26 Al around at that point for them to incorporate. And Al, of course, is a key part of anorthite’s Ca Al 2 Si 2 O 8 crystal structure.

So the idea is that weird extra 26 Mg in the chondrule’s anorthite could be explained by suggesting it wasn’t always 26 Mg: Instead, it started off as 26 Al ,and it belonged in that crystal’s structure. However, over a short amount of time, it all fell apart, and that left the 26 Mg behind to mark where it had once been. If this interpretation is true, it has shocking implications for the story of our solar system.

To understand why, we first need to ask, what came before the nebula? What was the ‘pre-nebula’ situation? Where did the nebula come from, anyhow?

It turns out that nebulae are generated when old stars of a certain size explode.

These explosions are called supernovae (the plural of supernova). The “nova” part of the name comes from the fact that they are very bright in the night sky – an indication of how energetic the explosion is. They look like “new” stars to the casual observer. Nova is Latin for “new.” Supernovae occur when a star has exhausted its supply of lightweight fuel, and it runs out of small atoms that can be fused together under normal conditions. The outward-directed force ceases, and gravitationally-driven inward-directed forces suddenly dominate, collapsing the star in upon itself. This jacks up the pressures to unbelievably high levels, and is responsible for the nuclear fusion of big atoms. Every atom heavier than iron is made instantaneously in the fires of the supernova.

That suite of freshly-minted atoms included a bunch of unstable isotopes, 26 Al among them.

And here’s the kicker: If the 26 Al was made in a supernova, started decaying immediately, and yet enough was still around that a significant portion of it could be woven into the Allende chondrules’ anorthite, that implies a very short amount of time between the obliteration of our Sun’s predecessor, and the first moments of our own solar system. Specifically, the 717,000 year half-life of 26 Al suggests that this “transition between solar systems” played out in less than 5 million years, conceivably in only 2 million years.

That is very, very quickly.

In summary, the planet Earth is part of a solar system centered on the Sun. This solar system, with its star, its classical planets, its dwarf planets, and its “leftover” comets and asteroids, formed from a nebula full of elements in the form of gas and dust. Over time, these many very small pieces stuck together to make bigger concentrations of mass, eventually culminating in a star and a bunch of planets that orbit it. Asteroids (and asteroids that fall to Earth, called meteorites), are leftovers from this process. The starting nebula itself formed from the destruction of a previous star that had exploded in a supernova. The transition from the pre-Sun star to our solar system took place shockingly rapidly.

Further reading

Marcia Bjornerud’s book Reading the Rocks . Basic Books, 2005: 226 pages.

Jennifer A. Johnson (2019), “ Populating the periodic table: Nucleosynthesis of the elements ,” Science. 01 Feb 2019 : 474-478.

Lee, T., D. A. Papanastassiou, and G. J. Wasserburg (1976), Demonstration of 26 Mg excess in Allende and evidence for 26 Al , Geophysical Research Letters , 3(1), 41-44.

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Chapter Contents

• 1 In the beginning…
• 2 Nebular theory
• 3 A star is born
• 4 Age of the solar system
• 5 The implications of meteorites
• 7 Further reading