IB Astrophysics: Planets

Screen Shot 2013-11-09 at 14.10.53Astronomy is an ancient science and it was only about 5-600 years ago that we began to make sense of what the ancients saw. The bedrock for modern thinking lies with Copernicus, Tycho Brahe – the first real astronomer – Johannes Kepler , Galileo and Newton. It’s an interesting study to trace the emergence of knowledge from these few names.

Imagine a time when the Earth and its planetary siblings were nothing but cosmic dust. Yet astronomers agree that this was the state of affairs some 4.5/4.6 billion years ago. Our sun was a fledgling protostar, continually amassing more matter via gravity and steadily cranking up its internal nuclear fusion. There was no solar system, only a giant, amorphous rotating cloud of particles called the solar nebula.

To figure out how all that leftover gas and dust led to planets, astronomers have largely studied the structure of our own solar system for clues. They’ve also looked to distant, younger solar systems still in varying stages of development.

With the formation of the sun, the remaining gas and dust flattened into a rotating protoplanetary disk. Within this swirling debris, rocky particles  from long extinct supernova remnants began to collide, forming larger masses that soon attracted even more particles by gravitational attraction. Their volume contracted under gravity to create planetesimals, which collided with one another to become the solid inner planets. Meanwhile, gases froze into giant balls that would build the outer gas giants.

Why did rocky planets form closer to the sun and the gas giants farther away? One theory involves the solar wind, the steady flow of plasma that emanates from a star. When the sun first came into being, this wind was far stronger than it is today, strong enough to blast lighter elements such as hydrogen and helium away from the inner orbits. When these expelled elements reached the outer orbits, the strength of the solar wind dropped off. The gravity of the outer gas giants quickly drew these elements in, bloating them into their current forms: solid cores of rock and ice covered with gas.

This spreadsheet summarises much of our current understanding of our solar system. Kepler’s Laws do not need to be memorised but they are so fundamental they’re worth more than a look.  Study this link also my spreadsheet entitled  Solar System Data.

I want you to look for and note any patterns that you think you have found. In particular,  use the spreadsheet data to verify the Law of Periods, in other words,

Screen Shot 2013-11-09 at 14.36.01


Make sure the units are SI.

Earthlike planets?


Goldilocks Planets


Are the rules the same all over the Universe? Maybe not…

Strange Objects in Space


About John Vagabond

I have taught physics and math all over the world
This entry was posted in AS and A2 Physics and tagged , , , . Bookmark the permalink.

1 Response to IB Astrophysics: Planets

  1. John Vagabond says:

    Reblogged this on John Vagabond's Physics and Chemistry Blog and commented:

    From esfscience. Our Solar System

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