 | Origin of Matter |
| Origin of Planets |
| Survey of the Solar System |
| The Age of the Solar System |
| Extra-Solar Planets |
The universe is made almost entirely of hydrogen and helium in the proportion of 75% to 25%.
As we shall learn when we study cosmology, these elements were created within minutes of the creation of the universe.
But the heavier elements of which we are made carbon, oxygen, nitrogen and others were created within the cores of massive stars that subsequently exploded, shedding their precious cargo of elements into space.
Since the creation of the universe, two or three generations of stars have lived and died in our galaxy. The Sun, which is about 5 billion years old, is of the third or fourth generation and is built, in part, from the ashes of dead stars. Consequently, the Sun has a slightly higher concentration of heavier elements than stars of earlier generations.
Facts about our Solar System
There are many features of our solar system which we believe merit an explanation. Here are some of them
There are two kinds of planet
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terrestrial: small, rocky and dense |
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Jovian: large and gaseous. |
The small planets, with the exception of Pluto, lie nearest the Sun while the larger ones are further away.
The planets orbit the Sun in approximately the same plane.
When viewed from above the north pole of the Sun all planets revolve about the Sun in a counter-clockwise manner.
Most planets rotate about their axes in the same sense as the Sun and with axes that are tilted by less than 30o with respect to the perpendicular to their orbital plane.
The Solar Nebula Theory
The theory that accounts most readily for the observed facts about our solar system is the solar nebula theory.
While the discovery of extra-solar systems has called into question some of the details of this theory, we believe that in broad outline the theory is correct. Indeed, the discovery of these extra-solar systems confirms an important prediction of this theory:
most stars should have planetary systems.
Our solar system probably formed from a large rotating nebula (of dust and gas). Over time gravity caused the nebula to contract into a rotating disk, the hub of which contained the bulk of the material and formed the Sun, while the outlying disk condensed to form the planets.
The Hubble telescope has provided, for the first time, direct evidence of young stars at the center of proto-planetary disks of dust and gas. These disks are not hot enough to emit light but glow instead in the infra-red.
From infra-red observations of the Orion Nebula using the Hubble Telescope we now know that about half the stars in that nebula are located at the center of disks of dust and gas.
An excellent survey of the solar system can be found at the nine planets site.
The Age of the Solar System
A key prediction of the solar nebula theory is that the Sun and the planets should be about the same age. Currently, the best way to measure the age of something very old is to analyze the radioactive elements it contains. This is called radioactive dating.
A radioactive element is an element whose nuclei decay, that is, break apart, into other nuclei (called daughter nuclei) at a well-defined rate. So over time a lump of radioactive element transforms into another element.
Although we cannot predict when a given nucleus will decay, we can predict precisely what fraction of the nuclei will remain intact after a given duration.
For example, uranium 238U can decay to lead 206Pb. Given any piece of uranium you would need to wait 4.5 billion years to find that half of the uranium nuclei had been transformed into lead. That time is called the half-life of uranium.
The half-file is the time required for half the nuclei to decay into the daughter nuclei.
The basic idea of radioactive dating is that if we know the abundance of the radioactive elements in the original rock we can infer the age of the transformed rock by measuring how much of each radioactive element is left and figuring out how long one would need to arrive at that amount.
There are many other radioactive elements. For example, potassium (40K) decays to calcium (40Ca) with a half-life of 1.3 billion years.
By using this method we know that the oldest rocks on Earth are about 3.9 billion years old. So we conclude that the Earth is at least that old. From lunar rocks we know that the solar system must be at least 4.48 billion years, since this is the age of the oldest lunar rocks brought back by the Apollo astronauts. From Martian rocks that have fallen on Earth we know that the solar system is at least 4.5 billion years.
In conclusion, by measuring the ages of many different rocks from the solar system and using computer models of its formation we conclude that the solar system is about 4.6 billion years old.
Last updated October 27, 1999 Harrison
B. Prosper