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Unformatted text preview: FREQUENTLY ASKED QUESTIONS February 15, 2012 Content Questions How do you determine the mass of a planet if there are no moons? If a planet has a natural moon, and assuming the mass of the moon m is less than the mass of the planet such that m << M p , one can easily use Keplers third law to find the mass of the planet M p given measured a and P values. However, if theres no moon, the planet will gravitationally attract its neighbors and perturb their orbits in a way that depends on the planets mass. Using the measured perturbations, the planets mass can be determined. This was how masses of Mercury and Venus (which have no moons) were determined in the days before spacecraft were sent to those planets. Nowadays, better measurements for those planets are available using interplanetary probes as artificial moons. How come the planets rotate in the same plane? This general behavior is a prediction of the nebular theory of planet forma- tion. A cloud of gas, before it collapses, has some net angular momentum, in some particular direction: on average the matter in the cloud will spin in that direction. As the cloud collapses, it spins faster and faster by conservation of angular momentum (like a skater pulling in her arms). To start out, particles (chunks of matter) will be moving in many direc- tions, but after a while the particles will tend to all have the same rotational motion, close to the average. To see this, consider a particular chunk of matter: if it happens to be going opposite to direction of the average flow, its more likely than not to hit something thats going in the average flow direction, which will nudge it towards the average direction. Something mov- ing much faster than average will typically smack into slower moving things and slow down; something moving much slower than average will tend to get kicked to a higher speed. After a while, the average motion will prevail,get kicked to a higher speed....
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