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• 1 2016-05-13T12:53:19+00:00 The Solar Nebula 40 image_header 2022-07-21T18:02:58+00:00 The most widely accepted model for the formation of the Solar System is the nebular hypothesis, which holds that the Solar System formed from nebular material.  In short, a supernova explosion at 4.55 billion years ago formed a nebula (an immense cloud of gas and dust in space; also called a “molecular cloud”) and the formation of the Sun and of the Solar System is the product of the condensation and gravitational collapse of the solar nebula. A Sun-like star usually takes ~1 million years to form from a nebula.  The planetary system evolves over the next 10 to 100 million years.
  
  Please watch this video for a good summary of the nebular hypothesis.  More details follow in the text below.
  
  
  
  The nebula that became our solar system began as a large irregularly shaped mass of gas and dust in space. The nebula was gravitationally unstable.  Within the nebula the pressure of the gases act outwards to cause it to expand while gravitational forces (forces that pull bodies towards each other) act to cause the nebula to collapse onto itself.
  The force of gravity prevailed over gas pressure and the nebula collapsed and began to spin.
  As the diameter of the nebula was reduced with collapse, the rate of spin increased. An analogy is a spinning figure skater who draws her arms toward her center to increase rotation speed.  (This is called conservation of angular momentum. Don't worry about the details behind the physics.)
  Due to the interaction of the pressure and gravitational forces, as the nebula was spinning it became flatter and formed a broad disk as the nebula continued to collapse.
  As the density of the centre of the disk increased along with its temperature, the core of the nebula became the protosun.  Collapse of the protosun is often accompanied by jets of dense gas, called molecular outflow, that emanate along the rotational axis of the disk .  The initial collapse to a protosun the size of the one that formed our Sun takes around 100,000 years. 
   
  Within the cloud swirling eddies developed drawing matter towards their centres to form the protoplanets.
  
  As the protosun became even hotter gases were driven off the inner region of the Solar System. The protoplanets became solid planets and continued their orbit, governed by the initial spin of the swirling nebula.
   
  The temperature gradient  within the solar nebula influenced the formation and distribution of the planets.  
  • The very high temperatures toward the centre of the solar nebula caused only the most refractory materials to condense upon initial cooling.  (Refractory refers to difficulty of melting.)
  • The very high temperatures toward the centre of the nebula also meant that the innermost planet Mercury is particularly volatile-poor (e.g., low water and carbon dioxide) and richer in more refractory phases, such as iron.  Mercury therefore has a higher density than the other planets when compression is taken into account.
  • Gas giants are thought to form beyond the frost line, which is a distance in the solar nebula from the central protostar where it is cold enough for volatile compounds such as water, ammonia, methane, and carbon dioxide to condense into solid ice grains. The presence of these ices in the gas giants causes them to have low overall densities. 
  Protoplanetary disks have been observed around young stars by the Hubble Space Telescope.
  
  The Nebular Hypothesis is attractive because it explains many features of the Solar System.  For example, the orbits of the planets lie in a plane with the sun at its center.  This plane is called the "orbital" plane or "ecliptic" plane and it is also the plane of the early disk-shaped nebula.
  The Nebular Hypothesis also explains why the planets mostly rotate in the same direction and their axes of rotation are nearly perpendicular to the orbital plane.  This direction of rotation was inherited from the direction of spin of the eddies in the spinning nebula that formed the protoplanets.
  
  Venus and Uranus do not rotate in the same direction as the other planets. Venus’ rotational axis is at right angles to the plane of the planets (the ecliptic plane) but it rotates in the opposite direction compared to the other planets. Uranus rotates about an axis that is almost parallel to the plane of the planets. Modern thinking is that the rotations of both planets were affected by major collisions with other bodies very early in their history.