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Overview of the Solar System
The solar system formed about 4.5 billion years ago from a giant cloud of gas and dust Lecture 8
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Inventory Mercury, Venus, Earth, Mars, Jupiter, and Saturn were known to the ancients Uranus, Neptune, and Pluto were discovered after the invention of the telescope The masses of the planets have been known for centuries based on Newton’s Law of Gravity Each planet rotates on it own axis Most rotate in the same direction as they orbit Venus rotates slowly backward Uranus and Pluto are nearly tipped on their sides Lecture 8
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Terrestrial Planets The four inner planets are called the terrestrial planets Mercury, Venus, Earth, Mars Composed of rock and metal The Moon is often classified as a terrestrial body Lecture 8
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Composition of the Terrestrial Planets
Venus, Earth and Mars have similar compositions 1/3 iron-nickel and iron-sulfur 2/3 silicates Mercury has a higher proportion of iron The Moon has a much lower proportion of iron All terrestrial planets show differentiation While molten, dense materials sink toward the center Lecture 8
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Positions of the Inner Planets
Positions of the inner planets as of February 5, 2001 Lecture 8
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Gas Giants The next four planets are called the giant planets
Jupiter, Saturn, Uranus, Neptune Composed of lighter ices, liquids, and gases Jupiter is the largest planet comprising 0.1% of the total mass of the solar system Lecture 8
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Composition of the Gas Giants
Bulk of Jupiter and Saturn is compressed, liquefied hydrogen Liquid giants! Jupiter and Saturn have 75% hydrogen and 25% helium, much like the primordial distributions Uranus and Neptune have much less hydrogen and helium The gas giants have a small rocky central core Jupiter’s core is about times the size of Earth Lecture 8
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Positions of the Outer Planets
Positions of all the planets as of February 5, 2001 Lecture 8
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Density of Planets Density is defined as mass per unit volume
By measuring the density, one can infer the composition of the planet The terrestrial planets are much denser than the gas giants The density of gas giants implies that they must have a solid core Lecture 8
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Pluto Pluto seems to be similar to the moons of the outer planets
Lecture 8
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Geological Activity The crusts of the terrestrial planets show evidence of being hit by objects from space Earth and Venus show evidence of volcanoes and built-up mountain ranges Geological activity Planets were once molten Early crater formation erased Moon and Mercury retain crater hits Surface of Venus mapped by Magellan’s radar shows “recent” lava flows Lecture 8
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Moons All the planets except Mercury and Venus have moons orbiting them The four largest moons of Jupiter are called the Galilean satellites Several of the moons are as large as Mercury (4878 km) Lecture 8
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Rings The Gas Giants all have rings as well as moons Lecture 8
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Asteroids There are small bodies that are much smaller than the planets and their moons Asteroids Located mainly in a band between Mars and Jupiter Astronomers speculate that there was once a planet or planets that were destroyed by the gravity of Jupiter Lecture 8
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Comets Comets are remnants from the formation of the solar system
They orbit farther from the Sun than the asteroids Oort cloud, Kuiper belt Many are in very eccentric orbits that bring them close to the Sun Warmth from Sun caused the frozen material to evaporate and stream out into the distinctive tail Lecture 8
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Radioactive Dating One can determine the age of a world by determining the age of its rocks The ages of rocks can be determined using radioactive dating Radioactivity is the spontaneous splitting or decaying into of nuclei into smaller or different nuclei The decaying nucleus is changed into a different nucleus Radioactivity involves the emission of various kinds of particles Alpha particles () Electrons and positrons () Gamma rays () Lecture 8
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Half Life We cannot know exactly when a nucleus might decay
If we have a very large number of nuclei, we can define the half-life After one half life, half the nuclei will have decayed A particular nucleus may survive longer or decay earlier Lecture 8
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Decays used to Date Rocks
These decays can be used to date rocks The parent products are created in stars when the material that makes up the rocks is created Radioactive decay begins at that time We measure the relative amount daughter and parent and calculate back the age of the rock Lecture 8
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Origin of the Solar System
We saw that all the planets orbit in the same direction and almost in the same plane The planets and the Sun rotate on their axes in the same direction (except Venus and Uranus) Suggests that the solar system was formed from a spinning system of gas and dust called the solar nebula Lecture 8
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Formation of Other Solar Systems
We can look for formation of other solar systems in space Such planetary nurseries have been observed Solar nebula Circumstellar disks These system are too far away to observe planet formation directly Astronomers recently have been able to identify large Jupiter class planets around distant stars but as yet there is no evidence for terrestrial class planets Lecture 8
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Orion Nebula In the Orion Nebula star formation is visible
1500 light years away Contains new stars about 100,000 years old Young compared with our 5 billion year old Sun! Lecture 8
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