# What is the reason for so many of uniform circular motions even after so many irregularities in phenomena like motion of planets?

A body remains in uniform circular motion around another body due to the centripetal force between them, when the first body keeps moving with a specific velocity.

This is more possible in an isolated system. For Earth and Sun for example, say if Earth is moving with the required velocity then doesn’t the attraction of Jupiter, Mars, a random asteroid cause deviations here. And if due to that the Earth is moving away or closer to Sun, then looking at the age of the solar system shouldn’t the whole solar system have collapsed by now?(1)

And is it a meer coincidence for 8 planets to come in a specific radius with specific velocity around the Sun and the system continuing for billions of years? (2)

Because otherwise it would mean more like this-someone held Earth and the Sun at a specific distance apart and gave Earth a specific velocity through a very precise torque to initiate circular motion and with gravity as the centripetal force Earth keeps revolving around the Sun. Is that how a motion as such as the planetary revolution occur? With a precise initial torque, quickly removed and the centripetal force to carry on the uniform circular motion?(3)

In fact looking at the specific velocity requirement it should be nearly impossible for revolutions to happen in the universe. But that is not how facts are, then where is the flaw in my concept?(4)

Actually it’s not about why our solar system, it’s more like why is it so common across the universe because there are trillions of solar systems there, many within our galaxy, our solar system is just an example to state the problem, what I mean is that it should not be highly unlikely for it to happen, so there is some mistake in my theory, so given my idea, what is wrong in my concept that’s causing a clash with the facts

Also please suggest the right tags for this question(5)

• This is the N body problem, when N is > 2, orbits aren't always integral. May 4, 2021 at 16:12
• The title asks why planetary orbits are circular but the body of the post seems to want to know why the solar system is stable (and doesn't care that the orbits are in fact closer to ellipses). Which are you interested in? May 4, 2021 at 17:35
• I want the answer of the question marked (3) and on the basis of this answer I want there answer to the question marked (4), to be very specific May 5, 2021 at 17:23

I think all your questions are along the lines of "How can the Earth, Sun, planets, etc. be in the current configuration, when such a configuration is very unlikely, given all the things that have to go right for this to happen?"

To explain this, I would like you to think of the example of a lottery ticket. If 1 billion people buy a lottery ticket, then the probability for any given person to win the lottery is 1 in 1 billion. But, the probability that someone will win the lottery is 1, right?

Whoever wins the lottery could keep asking, what is the reason he won the lottery when it is highly unlikely that he would have. Or he could just accept that he was the lucky one.

Similarly, the universe is a vast place with billions and billions of galaxies, stars and planets. So, even though for any one system, it is highly unlikely that it would be the lucky one, it is highly likely that some solar system would be the lucky one.

And our solar system is that system.

In that sense, there is nothing special about our solar system. If it wasn't the solar system, then some other solar system out of all the billions and billions of suns would have been the one to be stable and have life evolve on it. And the people on that system would have been asking the question, what is the reason their solar system is so stable despite all the things that could have gone wrong.

• Actually it’s not about why our solar system, it’s more like why is it so common across the universe because there are trillions of solar systems there, many within our galaxy, what I mean is that it should not be highly unlikely for it to happen, so there is some mistake in my theory, so given my idea, what is wrong in my concept that’s causing a clash with the facts May 4, 2021 at 17:08

Well over $$99\%$$ of the mass of the Solar System is in the Sun, so it is not surprising that the planets and most of the other bodies in the Solar System orbit the Sun in fairly stable orbits.

However, these orbits are not all circular, or even close to circles. From the major planets, the eccentricity of Mercury’s orbit is over $$20\%$$ and the eccentricity of Mars’s orbit is almost $$10\%$$. For dwarf planets and asteroids, eccentricities can be even higher - Sedna, for example, has an eccentricity of over $$80\%$$.

Nevertheless, gravitational influences of one object on another can build up over time, especially if there is an orbital resonance between them. The long term stability of the Solar System over periods of tens or hundreds of millions of years is a topic of ongoing research.

• But there have been instances of asteroid collisions which would have altered the planets’ radius of revolution and thus at a moment it must be possible for a planet to collide with Sun, however Earth did not do so, why? May 4, 2021 at 16:05
• @LumbiniATambat Look up the anthropic principle . May 4, 2021 at 16:18
• @LumbiniATambat The very early solar system was chaotic and many proto-planets must have been absorbed into the Sun or been ejected from the solar system. We know that the proto-Earth was involved in at least one major collision, which created the Moon. The Earth was just fortunate to survive until the solar system became fairy stable. But if it hadn't then we wouldn't be here to wonder about it (or we would be living on some other planet). May 4, 2021 at 16:24