Event 5

4.6 billion years ago

Moon colliding with a planet
Origin of the moon. When the early Earth was struck by a Mars-sized object, the resulting debris from the collision circled the Earth and finally coalesced into our moon. This image above is an artist’s conception of a similar collision between a celestial body about the size of the moon and a planet the size of Mercury.
Credit: Image is used courtesy of NASA/JPL-Caltech

Around 4.6 billion years ago, our solar dust cloud (nebula) began to be compressed, probably from a supernova explosion from another star system. As this nebula collapsed, this dust contracted to form a large spinning flattened disk of very hot gas, with a bulge at the center that would ultimately become our sun. The outer layer of gas (the “accretion disk”) cooled off sufficiently that solid matter starts to condense into very small particles of metal and then rock. These dust particles accumulated by gravity into larger and larger chunks of matter, called planetoids. At a certain size (perhaps tens of miles in diameter), heat from countless impacts and the additional heat of radioactive decay of matter inside these bodies totally melted them. Strong gravitational forces quickly concentrated most of the matter at the core of this spinning disk of gas and particles, about 99 percent, into our local star, the sun. The sun became a giant nuclear fusion reactor and ignited, generating enormous amounts of heat and light. The residual matter of the solar system, formed from the particles in the outer accretion disk, eventually became the planets, the moons, the asteroids, the meteors, and the comets. In the early solar system, these bodies were like giant billiard balls, often colliding, sometimes merging, sometimes breaking apart. Our moon was almost certainly the product of one of these massive collisions.

Protoplanetary Disk
Planet formation. This is a brown dwarf surrounded by a swirling disk of celestial dust that will be the elements of planet building.
Credit: Image is used courtesy of NASA/JPL-Caltech/T. Pyle (SSC)

The formation of the planets around the developing sun is a fascinating process. Random collisions among different particles in the swirling cloud of matter around the forming sun gradually merged smaller particles into larger and larger clumps of matter, eventually forming boulders and smaller asteroids. As some of these particles got larger and larger, they exerted gravitational pull that gave them an ‘edge’ over smaller particles and enabled them to pull in more and more matter in their surroundings into their mass. These growing globs of mass, the protoplanets of our solar system, were meanwhile rotating or developing an orbit around the massive forming sun at the center.

The growing bodies of matter closer to the sun eventually formed the smaller, rocky, inner planets, Mercury, Venus, Earth, and Mars. Strong solar winds swept the nebular gases far away from the sun, which eventually condensed into large dense bodies of gas in the outer, gaseous planets or ‘gas giants,’ Jupiter, Saturn, Uranus, and Neptune. The planets furthest from the sun, Uranus and Neptune, are far enough from the Sun that they are ‘icy’ – containing frozen water, methane, and ammonia, but also contain rock at their core. Between the inner, rocky planets and the outer gaseous planets is the main asteroid belt (mostly made of rock, metal and ice), and beyond the gaseous planets is the Kuiper belt of icy bodies, probably a remnant of the planetesimals from the protoplanetary disk that formed around our protosun almost 4.6 billion years ago. The formation of the planets is believed to have occurred within about 10 to 20 million years of the collapse of our solar nebula.

The rotation of the spinning disk of gas in the collapsed solar nebula set the course for the basic structure and movement of our mature solar system as it is today, 4.6 billion years later. All the planets revolve around the sun (which began as the bulge in the center of the flattening disk of matter) and in the same direction (counterclockwise, the same direction as the Sun’s rotation), their orbits lie in virtually the same plane (the ‘ecliptic plane’), and their patterns of rotation are also remarkably similar (most in the same direction and with their axis of rotation just about perpendicular to their orbit).

Since the basic solar system was established over four billion years ago, elements of it have continued to evolve and develop. Even after the eight main planets were formed, many of them have had satellite bodies or moons form, some of them growing within the discs of dust that had escaped planet formation and others consisting of objects such as asteroids captured by the gravitational pull of the planet. The Earth’s own Moon was formed when a Mars-sized object (sometimes called Theia, an early ‘protoplanet’) was pulled out of its orbit and collided with the Earth about 4.5 billion years ago, knocking debris from the outer mantle of this object and the Earth off to coalesce nearby into our Moon.

Collisions have continued to occur between different bodies in the solar system, though not nearly as frequently as during the early period of planet formation and the brutal asteroid impacts that ensued for hundreds of millions of years afterwards. Our sun is middle-aged, almost halfway through its expected life right now. It is projected that in a little over 5 billion years it will burn through its supply of hydrogen causing its core to collapse, begin a short life of intensely hot helium fusion, expand into a gigantic red dwarf, and then shrink to a cool, dim white dwarf. Long before this, life on Earth would have disappeared as the sun’s temperature increases and Earth gets so hot that its water and atmosphere would be driven off.


Evidence of the early solar system can be seen in meteorites (meteors that occasionally strike the earth). One type, called carbonaceous chondrites, consists of meteorites very similar in chemical composition to the sun (excluding the volatile gases such as hydrogen and helium, which would have been blown away by solar winds). These meteorites, rich in carbon, locked-up water, and organic molecules such as amino acids, are probably examples of the types of matter that provided the building blocks for life. They are the leftovers of the formation of the sun and the planets, and give us our best clues into the early evolution of the solar system. They can be dated by several methods by measuring their elemental isotope compositions; known radioactive decay rates allow us to calculate their age of formation. The earliest are about 4.6 billion years old, roughly the age that the sun and planets are believed to have formed.


Without the sun, our local star, our world would be a bleak, frigid, dark place. Our earth appears to be located at a perfect distance from the sun to promote life (sometimes called the “Goldilocks principle”, not too hot, not too cold, but just right…). Our sun provides most of the energy that runs the life systems of the earth and allows plants to make energy through photosynthesis. The matter that made up our solar dust cloud, the product of other exploded star systems, was the primeval raw material for the sun, the other planets, the earth, and ourselves. Our sun is one of perhaps 100 billion stars in our galaxy (the Milky Way), and our galaxy is one of perhaps 100 billion galaxies in the universe. In recent years, other planets have been identified in other star systems. The likelihood of life being common in the systems with sun-like stars and earth-like planets seems likely, but has not been proven thus far.


National Geographic interactive site with slides about the formation of our solar system and universe:

A clip from the video accompanying the previous NatGeo link:

This is a website geared toward younger audiences.  It provides an easy to follow geologic time scale beginning at 4.6 billion years ago:


Bevan, Alex and John De Laeter. Meteorites: A Journey through Space and Time. Washington: Smithsonian Institution Press. 2002.

Eales, Stephen. Origins: How the Planets, Stars, Galaxies, and the Universe Began. London: Springer-Verlag London Limited (Springer). 2007.

Jones, Tom and Ellen Stofan. Planetology: Unlocking the Secrets of the Solar System. Washinton, D.C.: National Geographic. 2008.

Krauss, Lawrence M. A Universe From Nothing: Why There Is Something Rather Than Nothing. New York: Free Press (A Division of Simon & Schuster, Inc.) 2012.

Luhr, James F. (Editor). Earth: The Definitive Visual Guide. New York: DK Publishing. 2007.

May, Brian, Patrick Moore, and Chris Lintott. BANG! The Complete History of the Universe. Baltimore: Johns Hopkins University Press. 2008.

Murdin, Paul. Secrets of the Universe: How We Discovered the Cosmos. Chicago: University of Chicago Press. 2009.

Norton, O. Richard. The Cambridge Encyclopedia of Meteorites. Cambridge: Cambridge University Press. 2002.

Ronan, Colin A. Universe: The Cosmos Explained. London: Quantum Publishing. 2007.

Silk, Joseph. The Big Bang: Third Edition. New York: Henry Holt and Company, LLC. 2001.


Cosmic Voyage. Narrated by Morgan Freeman. IMAX and Warner Brothers. 2002.

Origins: Fourteen Billion Years of Cosmic Evolution. NOVA, WGBH, Boston Video. 2004. Hosted by Neil deGrasse Tyson.

The Universe. The History Channel. (A multi-year television series).

Understanding the Universe: Introduction to Astronomy. The Teaching Company. 2007. (96 presentations by Alex Filippenko).

Universe: A Journey from Earth to the Edge of the Cosmos. Quercus. 2008.

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