Origin of the Solar System

The Big Bang (The Origin of Everything)

1917 Albert Einstein found that his General Theory of Relativity predicted that the universe could not exist in a steady state.   Specifically, the universe must be either expanding or contracting. This was a new idea, and Einstein consulted with astronomers to see if there was any evidence that the stars are moving together or apart.  The astronomers noted that the stars move randomly. Since the Universe was apparently in a steady state, Einstein inserted a correction (a.k.a. fudge factor) into his equations.

1917 (ironically) Vesto Slipher, an American Astronomer, documented the first evidence that the universe is indeed expanding. Slipher had been studying the spiral nebulae (nebula = "cloud"), which we now understand to be galaxies (vast conglomerations of stars), but were then considered to be dust clouds in the process of forming solar systems like our own. Slipher was using the Doppler effect to document that the spiral nebulae were spinning.  What he found was not only that the nebulae were spinning, but that most of them were also receding away from the earth (and therefore each other) at a very rapid rate (over 2 million miles per hour), as much as, based on the fact that their light was severely redshifted. Since it was believed at the time that these spiral nebulae were rather close (as close as the other stars), this observation by itself did not mean that the universe is expanding, but it was intriguing.

1927 Georges Lemaitre, a Belgian priest/mathematician connected Slipher's redshift observations to Einstein's General Relativity. He published in an obscure journal that no one read, so no one knew that he had made the connection

1929 Edwin Hubble, an American Astronomer, demonstrated that objects that are further away are also moving away more quickly, which means that the universe is expanding.  Starting in 1919 he made detailed observations of the spiral nebulae.  The first breakthrough was when he recognized that there are cepheid variable stars in the arms of the spiral nebulae, proving that the nebulae actually are galaxies of billions of stars, rather than mere dust clouds. A cepheid variable star is a star that varies in light intensity; calculations had shown that the brightness of such a star is directly related to the frequency with which it varies. Because the actual brightness of the star can be calculated and the apparent brightness can be observed, the distance to the star can be deduced--lights that are farther away look dimmer than lights that are closer--by the square of the distance. Hubble calculated the distance to several galaxies and compared the distance to the redshifts such as those observed by Slipher. he found that there was a direct linear relationship between distance and redshift. In other words, if one galaxy is twice as far away as another galaxy, it also is moving away from us twice as fast as the other galaxy.  This is consistent with the idea that the entire universe is expanding (place a number of spots on a balloon and then blow it up--you can observe this relationship), though Hubble himself was reluctant to make this conclusion.

1930 (January) Arthur Stanley Eddington and Willem De Sitter and other theoretical cosmologists met together and tried in vain to tie general relativity to the Hubble expansion.

1930 (February) George DeMaitre reminded Eddington that he had already mathematically connected universal expansion to general relativity. Eddington conveyed this to De Sitter and to the world.  In the following years, DeMaitre explored the idea of a beginning and postulated that the universe began as a primordial atom, and that it explosively expanded into the universe.  This event he called "the big noise"

Fred Hoyle, somewhat derisively, dubbed it the "big bang", which is the name that stuck.

In the early thirties many of the theoretical cosmologists and physicists fled Europe in general and Germany in particular because many of them were Jews or other "undesirables" and were personally endangered by Nazi fascism. This placed the theoretical scientists in the United states with the astronomers who had vindicated their theories.

1933 LeMaitre presented his ideas about the origin of the Universe at the Mt Wilson Observatory office in Pasadena. Einstein loved it. The idea was not well developed, and since that time many have worked out the details of what went on in the first few seconds of the formation of the Universe.

Georges Gamow, a Russian emigrant, calculated that the original light energy emitted by the big bang should be redshifted into the microwave range, and that this radiation should be emanating from all parts of the universe.

This predicted cosmic background radiation was discovered by Arno Penzias and Robert Wilson at Bell laboratories, who were trying to figure out why their microwave satellite communication equipment returned a persistent "hiss". This hiss was the cosmic background radiation predicted by Gamow.

The Origin of the Elements

At the end of the Big Bang the only elements that could have been present were mostly hydrogen and some helium. Hydrogen and Helium are still the most prevalent elements in the universe, making up more than 96% of the known matter in the universe, but the rest is made up of much heavier elements.

Elements are differentiated by the number of protons in their atoms.  The number of protons is the atomic number. Hydrogen has an atomic number of 1 (1 proton), helium 2, lithium 3, .. carbon 6, ... neon 10,... etc. Protons are massive, and atoms with higher atomic numbers are more massive than atoms lower atomic numbers; thus, elements with higher atomic numbers are referred to as heavier elements. the process of making elements heavier than hydrogen is a process of combining lighter elements called fusion.

In 1957, after much work (and based in part on the work of others), Fred Hoyle, Willy Fowler, and Geoffrey and Margaret Burbidge outlined how every element could be formed by fusion within stars. In their scenario, a star starts out by "burning" (fusion of) hydrogen atoms to form helium atoms. As the hydrogen runs out, the star's core contracts, and the additional heat of contraction starts the fusion of helium to form carbon, oxygen and neon. When the helium starts to run down the core contracts even more and the fusion of helium and neon to form magnesium, silicon, sulfur and calcium. From this stage the star may sort out into multiple concentric layers with gaseous iron in the core, and surrounding shells where silicon, oxygen, neon, carbon, helium and hydrogen are "burned" (again--undergoing fusion reaction).

Iron is something of a fusion dead end in that fusion reactions that build up from hydrogen to iron release energy, while fusion reactions beyond iron actually consume energy. While it is clear that a small amount of elements heavier than iron are formed in stars (some heavy elements with very short half lives have been observed in stars, so they had to be formed there), a star cannot be fueled by fusion of iron.  This explains why some of the heavier elements are so rare--consider gold, silver, uranium, etc. But the buildup of iron leads to an explosion that forms the heavier elements

As the iron builds up, fusion slows down and the core contracts rapidly to form a neutron star. the neutron star is as massive as a regular star like the sun, is smaller than a city. The contraction is rapid enough to cause a powerful (the most powerful known) shockwave or explosion that is known as a supernova. This shockwave lends its energy to produced most of the elements heavier than iron, and blows the outer layers of the star into space.

After such an explosion, the material is available to form other stars and planets.

The Origin of the Solar System.

Characteristics of the Solar System

  • It is a Part of a Nebula
  • Most of the mass of the Solar System is concentrated in the sun.
  • 98 percent of the angular momentum is found in the planets m*v*d
  • heavy noble gases (xenon, neon, Krypton) are rare on earth as compared to space and the sun.
  • The Earth is layered with a thin crust, overlying a heavier mantle, and centered with a nickel-iron core, the outer part of which is molten, and the inner part of which is solid
  • The planets and sun each have a somewhat different density suggesting different time and/or temperatures of origin.

Theories of Origin of the Solar System and of the Earth

The planets were spun off of the sun this theory suggests that the planets were spun off of the sun, and are thus essentially daughters of the sun. This requires a near collision with a large body.

Does it fit the facts? If this were the way that the solar system formed, then:

  • the sun would have most of the angular momentum
  • The sun would be less massive than it is.

 The nebular cloud hypothesis suggests that the solar system started out as a nebular cloud an that the planets and sun were concentrated from the dust and gasses in the cloud by gravitational attraction.

  • cloud would have been 30-40 light years across
  • mass of cloud would have been 2 -10 times the present Solar System mass.
  • originally extremely thin
  • coalesced by gravity and magnetic attraction
  • Collision heated sun until it started spontaneous fusion

Under this hypothesis, the earth and other planets formed by a process called planetary accretion, the process of smaller particles colliding and sticking to each other to build larger objects, including planets.   As more and more material was accreted into the planets at the early stage, the repeated energy of impact (gravitational energy) was converted to heat until the planets were entirely melted. The melted planets then developed internal layering as the heavier materials sank to their centers, and the lighter materials floated to their surfaces.

Current Research: New Planets in Space

 Scientific American; A Parade of New Planets
Evolution of the Solar System and the Planets