How did the Earth form - An example of the scientific method

From GeoClasses

Question

How did the Earth and the solar system form?

Observations

Looking into the space with the Hubble space telescope we can see clouds of dust and gas, such as the Orion Nebula, with bright young stars.

The Orion Nebula. A cloud of gas with bright young stars. Image taken by the Hubble space telescope (Credit: NASA,ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team)

Deductive Reasoning

We know that all particles, even gas atoms and molecules, are attracted to one another by gravity (Newton's law of gravity).

Hypothesis

Thus we can suggest the hypothesis that: Solar systems and stars form from clouds of interstella gas and dust.

Testing

There are many ways to test a hypothesis.

• Observations
  • What do we expect to find based on our hypothesis. (eg. Only young stars can be found in gas clouds)
• Experiments
  • Laboratory experiments (to create a solar system from a gas cloud?)
  • Mathematical Experiments (can we model the formation of a solar system based on our hypothesis.)

A mathematical model

Mathematical models are constructed based on the equations that govern the behavior of the system.

To construct a model of a solar system forming from a gas cloud we need to apply Newton’s law of gravity Newton’s law states that every object in the universe is attracted to every other object. The force of that attraction depends on the masses of the bodies, and how far apart they are. This can be written as,

Newton's law of Gravity

• F = force of the attraction
• G = gravitational constant (always the same)
• m_a = mass of object a
• m_b = mass of object b
• r = distance between the two objects.

Using this equation we can calculate what will happen to a cloud of particles by creating a cloud and seeing how the gravitational force causes the particles to move. The best way of doing this is with a computer model.

Computer Model

In a computer we create a dust cloud of particles.

• Particles are of random size,
• Particles are randomly placed in the cloud.
• Particles start off with random motions.
• All particles are attracted to one another by Newton's law of gravity.

The particles move because of the attractive force. Can we form a solar system?

Nebula Model

• nebula model
• solar system model

So we create a computer model of a cloud of particles and make them all attracted to one another by Newton's law of gravity. The result is shown here:

Scene 3 from the nebula model showing the orbits of 3 planets. Note how the orbits are changed by the approach of other planets.

• Dust cloud collapses at start of simulation.
  • Scene 1 of nebula collapse - at the start of the simulation a central star quickly forms.
• Early in the simulation the solar system has lots of junk flying around but they are constantly colliding with one another.
  • Scene 3 - shows trails of 3 particles orbiting the central star.
• Late in the simulation collisions between particles have left only a few planets orbiting the star
  • Scene 10 - late in the simulation.

Running the nebula model yourself

You can run and observe the nebula model yourself. The model is interactive so you can view from multiple perspectives (and even tag a ride along with a planet).

Comparing the nebula model to a model of the solar system

Meteor crator in the southewestern USA.

To see how well the nebula model simulates the formation of the solar system, take a look at the solar system model to compare.

Conclusions

The Earth and solar system probably formed from a nebula of dust and gas. The model, by implementing Newton's Law of gravity, does a reasonable job of simulating;

• the formation of a central star
• the formation of planets orbiting the star in elliptical orbits.
• the eventual accretion of all the dust in the cloud into a few discrete bodies/planets.

The model leaves a number of outstanding questions however, such as;

• why are the planets in the solar system aligned close to the plane of the ecliptic?
• why are there a set of small inner planets (Mercury, Venus, Earth and Mars), and a separate set of large outer planets (Jupiter, Saturn, Uranus and Neptune)?
• why is there an asteroid belt between the inner and outer planets?