I just want a solid reasoning that why Mercury is the first planet away from the Sun, then Venus, then Earth, then Mars and so on.
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3$\begingroup$ A question more suited to Astronomy Stack Exchange? $\endgroup$– FarcherJan 20 at 11:05
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3$\begingroup$ The "solid" reason is that it is named after the messenger of the gods, since it moves faster than the other planets, since it has a higher orbital speed due to being innermost. If another planet had been in that orbit it might likely also have been named after something or someone fast. This is an explanation of the name. Why there is a planet there is just a brute fact, like that Madagascar exists - there are astronomical/geological causes, but in the end they only say how a lump of mass ended up there. There is no deeper "why" as far as we know. $\endgroup$– Anders SandbergJan 20 at 11:11
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2$\begingroup$ Are you actually asking "Why is the first planet in the solar system sitting at that particular distance, having that particular mass with that particular density?" $\endgroup$– AtmosphericPrisonEscapeJan 20 at 12:23
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2 Answers
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We know the order of the planets in distance from the Sun by observing them and measuring their orbital periods (the time taken for each planet to travel once around the Sun). Kepler's Third Law of planetary motion says
The ratio of the square of an object's orbital period with the cube of the semi-major axis of its orbit is the same for all objects orbiting the same primary.
This allows us to determine the size of a planet's orbit (its "semi-major axis") from its orbital period. The orbital periods of the planets can be measured quite accurately by observing where they are in the sky over a long enough period of time.
The planet with the shortest orbital period is Mercury, which has an orbital period of about 88 days, so it is the planet that is closest to the Sun. Next comes Venus, with an orbital period of 224 days. Both of these orbital periods are less than a year, so Mercury and Venus are closer to the Sun than Earth (this is also clear from the fact that the positions of Mercury and Venus in the sky are never far from the Sun).
The orbital period of Mars, however, is 687 days, which is greater than a year, so it is further away from the Sun than the Earth is. And so on ...
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$\begingroup$ Kepler's law is qualitative not quantitative. Unless Newton came with his theory of universal gravitation, orbital distance couldn't be calculated. Still there was problem with Newton's law, to determine gravitational constant, which was suspiciously obtained by Cavendish. Then that used for Moon which was excluded by Kepler's law. $\endgroup$ Jan 20 at 13:52
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$\begingroup$ Kepler used the data about periods and distance to derive the third law. The third law is not how he (or us) knew (know) the distances from the Sun. $\endgroup$– nasuJan 20 at 14:21
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$\begingroup$ Another way is that we never see Mercury get very far from the Sun. We always have to look in the general direction of the Sun to see it. Venus gets farther. The other planets are farther from the Sun than Earth is, so we can see them nowhere near the Sun. $\endgroup$ Jan 20 at 17:10
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The formation of the solar system isn't simple and what we see now are the result of multiple collisions and chaotic orbital motions over billions of years. There isn't really a straightforward reason for the result we see today. The orbits which are stable over a long period are in many respects dominated by the motion of Jupiter and Saturn as they comprise the largest objects which can perturb the orbits of smaller planets like Earth. Jupiter and Saturn are also, by virtue of their large mass, less perturbed by the motion of smaller planets.
Wikipedia has a page on the formation and evolution of the solar system which I'd recommend to start understanding this. You might also want to consider that we've studied other solar systems and it might be useful for you to see how things work in those systems.