The Birth of Planet Earth

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Aggregation of planetesimals in the protoplanetary disk, the eventual solar system; illustration by Pat Rawlings for NASA-JPL

The Puzzling Problem of Galaxies

A quick glance at our Universe, either in the evening sky or through high-powered telescopes, reveals clumps of stars known as galaxies. Galaxies are discrete collections of stars (numbering 10 million in a dwarf galaxy and 10 trillion in a giant galaxy) and they appear to be the stuff the Universe is made of. Our own galaxy, the Milky Way Galaxy (which comes from the Greek word Galaktos, meaning "milk"), consists of several hundred billion stars, and is shaped like a very thin disk that measures 90,000 light years in diameter. Our Earth (and Sun) is about 30,000 light years from the Center, or about two-thirds of the way from the center to the edge. The Sun carries our Solar System with it at a speed of 230 kilometers per second, and it completes an orbit of the galaxy every 250 million years. Thus, we have circled the galaxy about 18 times since the Earth was formed, approximately 4.6 billion years ago.

Astronomers have found that galaxies don't like to live alone. Thus, they tend to occur in groups. Our own Milky Way and its neighbors are known as the "Local Group". It includes our nearest neighboring galaxy, Andromeda, as mentioned above, and about 15 dwarf galaxies, along with some "clouds" called the Large and Small Magellanic Clouds. In total, our Local Group extends over 10 million light years. Interestingly enough, Andromeda, which is 2.3 million light years away from the Milky Way, is "blue-shifted", meaning it is one of the few galaxies moving towards us. Given its distance, I don't think we have to worry about a collision anytime soon!

While it has been observed that galaxies tend to occur in groups, and while we know that groups of galaxies appear to move in relation to each other according to laws of gravity (fondly known as the Great Cosmic Ballet), we simply don't know how galaxies are formed. Cosmologists have a few ideas (like random quantum fluctuations in the early Universe), but for the most part, the question of galaxy formation remains unanswered. The discovery of fluctuations in the Cosmic Background Radiation has given impetus to the "fluctuation" model of galaxy formation, but nothing is concrete at this point.

The Birth of Planet Earth

Within the last couple years, at least seventeen planets have been discovered outside of our own solar system. While none of these planets appear to be of the type that support life, the discovery of these planets greatly accelerates our knowledge of how planets form and their evolution.

One popular theory for the formation of our solar system is that the Sun and planets evolved from a nebula or the remains of an exploded star. A whirling disc-like cloud of dust and gas was formed, becoming highly turbulent, with dust particles colliding and combining in what has been described as "a demolition derby of cosmic proportions". In the center of this cloud, where the gravitational pull was strongest, the compression of gases caused superheating, leading to thermonuclear reactions that converted hydrogen into helium, producing intense energy. This intense radiant energy became the Sun.

Other whirlpools were formed, colliding and coalescing into larger whirlpools. These whirlpools of dust packed together into larger and larger particles which formed planetesimals, similar in size and shape to the asteroids. As the larger planetesimals grew in size, their gravitational attraction drew more planetesimals towards them, forming the planets. It is believed that if the largest of these planets, Jupiter, had gotten bigger, that we might even have two Suns in our solar system. Binary stars comprise about half the solar systems in our galaxy.

Eventually, a single Sun and at least nine planets and an asteroid belt with planetoids were formed in our solar system. Astronomers speculate that our solar system must have a tenth planet to explain the orbit of Pluto, but it hasn't been discovered yet, to my knowledge. In addition, it is believed that the asteroid belt is a ring of planetesimals that failed to "get it together" to form a planet, or are the result of a collision between two early planets. In the final outcome, Earth stood as the third planet from the Sun, along with the other planets (in order) Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

The formation of the Earth as described above is believed to have occurred about 4.6 billion years ago. At this time, we should note that the rotation of the Earth was considerably faster. Sunrise to sunset took only 2.5 hours, as the Earth rotated every 5 hours.

The formation of our planet was only the beginning of a very significant chain of events for the Earth. Upon its initial formation, the Earth heated over time by three methods:

  1. the accretion of colliding planetesimals
  2. the force of compressing rocks together
  3. radioactive decay of elements within the planet

As planetesimals and meteorites impacted the Earth, the heat of impact managed to keep the Earth in a molten state. This was particularly true in the early stages of development when the Earth had no atmosphere. In addition, as the impacts continued, the Earth grew in size, causing compression of rocks in the interior as the Earth's gravity increased. Finally, the radioactive decay of elements within the rocks also added heat to the Earth. This radioactive decay continues to this day (albeit at a much slower rate) and it is surmised that if you put a gallon-size cube of granite in a perfectly insulated coffee pot and let it stand for 850,000 years, you could make a cup of coffee off the heat of the granite!

One side note on the impacting planetesimals concerns the formation of the moon. Evidence brought back from the moon astronauts suggests that a giant planetesimal, possibly the size of Mars, struck the Earth. The "splash" of the lighter elements of Earth's molten material into space was sufficient to cause the formation of the moon. One still wonders...

Scientists figure that the early Earth had a temperature of 1000 degrees C when first formed as a organized planet. But the rate of heating was faster than the rate of cooling, and the earth heated up. This was very important because after 1 billion years, the temperature became hot enough to melt iron at depths between 400-800 kilometers (250-500 miles).

Iron is abundant in the Earth and because it is heavy, it began to sink as it melted, displacing lighter elements. This "iron sink" caused a tumultuous reorganization of the whole structure of the earth. This process is known as planetary differentiation, and the homogeneous composition of the earth became a stratified one. It has been said that "planetary differentiation of the earth is perhaps the single most important event in the history of the Earth." It led to the formation of the crust and the continents. It also likely led to the escape of gases from the interior of the planet which led to the formation of the atmosphere and the oceans. This "outgassing" has been described as the Big Burp.

After all this overturning and reshuffling of the elements of the earth, things began to settle down and the Earth assumed the layered composition that we know today. In our next lecture, we will examine the formation of the Earth's oceans and look at the role of the seas in the evolution of life on our planet. Of course, we will always be reminded that the physical, chemical, geological, and biological processes on Earth did not evolve independently. This notion of the synergistic evolution of life and the planet will provide more fodder for our notion that the Earth is one giant system.

Question: How is the position of the Earth in our solar system ideally suited for Life? Why is the place Earth occupies sometimes called the Goldilocks position?


Within this grand and faster-than-light-speed survey of our vast Universe, it is appropriate to conclude on a philosophical and poetical note, never forgetting that creativity is the hallmark of great science. What the Big Bang theory has accomplished at the end of this century is truly remarkable. For the first time, we now have a plausible model of the Universe that unites the infinitesimally small with the infinitely large. From elementary particle physics to astronomy in space, the search is on to build a comprehensive pictures of cosmological processes within our Universe.

Still, we shouldn't be worried that everything is going to be figured out in our lifetime, or that everything will ever be figured out to completion. The quantum physics on which we perceive our world is a slippery rascal, and it's not likely to let us out of its grip anytime soon. We must remember that everything we perceive is just that, a perception, and our model of the Universe is just that, a model.

With that in mind, I'll leave you with a bit from the great physicist Erwin Schrodinger (after whom a whole set of well-known quantum physical equations are named), who waxed poetic in his book, My View of the World (Cambridge University Press, 1964). Schrodinger writes:

 All of us living beings belong together in as much as we are all in reality sides or aspects of one single being, which may perhaps in western terminology be called God while in the Upanishads its name is Brahman...For we are all, in every particle of our being, precipitations of consciousness; as are, likewise, the animals and plants, metals cleaving to a magnet and waters tiding to the moon...we are to recognize in this whole Universe a reflection magnified of our own most inward nature; so that we are indeed its ears, its eyes, its thinking, and its speech -- or, on theological terms, God's ears, God's eyes, God's thinking, and God's Word; and, by the same token, participants here and now in an act of creation that is continuous in the whole infinitude of that space of our mind through which the planets fly, and our fellows of earth now among them.

Perhaps the next galaxy isn't so far, far away...

Question: How has your perception of outer space and astronomy changed during your lifetime? What significant "astronomical" events can you recall?

Highly Recommended Reading:

Please review the online tutorial provided by NASA.  It summarizes what I've provided here and offers a more colorful and detailed account of the origins of our solar system and planets.

Online Tutorial for Planetary System Formation, NASA-JPL
Planetary System Formation>>Forming Jupiter and Earth-like planets

Optional Reading:

Deep sea probe tested at Monterey Aquarium

A new aluminum deep sea probe, the prototype of one designed to withstand crushing pressures and extreme temperatures, is set to be lowered to depths of 9 meters (30 feet) in Monterey Bay Aquarium's giant kelp forest July 28 as part of NASA's hunt for clues to life's origins.

Scientists from NASA's Jet Propulsion Laboratory, Pasadena, CA, will sink the new package of underwater cameras, temperature sensors, optics and a spectrometer into the emerald waters of a controlled aquatic environment to test the capabilities of more advanced instruments to explore the interior of volcanic vents. These cracks in the sea floor, occurring at depths of between 500 meters and 4,000 meters (1,650 feet and 13,200 feet), are known to nurture a pageantry of macabre bottom-dwellers such as salps, siphonophores, crustaceans and gelatinous animals only recently discovered at such depths.

(Full Story) 7/26/99

Useful Links:

Project Galielo: Bringing Jupiter to Earth

Project Cassini: Voyage to Saturn

Europa Orbiter Mission: Proposed mission to Jupiter's moon Europa

Pluto-Kuiper Express: Proposed mission to explore the fringes of our solar system

Solar Probe Page: Future mission to study the Sun

Space Infrared Telescope Facility and Infrared Tutorial: A must!

Welcome to the Planets

NASA Planetary Photojournal

Basics of Space Flight Learner's Workshop

Mar's Pathfinder

Discovery of Extrasolar Planets by Dr. Geoff Marcy and Dr. Paul Butler of San Francisco State University

Amazing Space Solar System Trading Cards: Take this Quiz!

Solar System Exploration Site

Solar System Exploration Home Page

The Ulysses Mission Home Page: High-latitude exploration of the sun, launched Oct. 6, 1990

Mars Global Surveyor Home Page