Planetary orbits?

Question

This might sound like a silly question, a very silly question... but why are the orbits of all the planets and dwarf planets alike, around the sun, in the same direction?

And as far as I can remember, I've heard of one or two em... thousands asteroids orbiting in the opposite direction. Not sure what the source was, so can't be certain about what was being said there.

Sound answer, by the way.

I know the rotation of certain planets (including venus) is opposite to that of Earth. But what struck me was that they all orbited in the same direction...

Answer 1

In physics, an orbit is the path that an object makes around another object while under the influence of a central force, such as gravity.

As discovered by Kepler, the planets orbit on ellipses with the Sun at one focus. In addition, the planets all revolve in the same direction on their orbits (direct orbital motion).

Planets move because the gas cloud that originally formed the solar system was spinning. The gas that wasn't spinning fell into the proto-Sun, and the planets formed from gas and chunks that were spinning enough that they were in orbit. Once the planets formed and were in orbit about the Sun, there was no reason for them not to stay there (objects in motion tend to remain in motion). The expansion of the universe is essentially negligible on the scale of the solar system, that is, it appears to have only a very, very small affect on the solar system.

Gravity from the Sun is what keeps the planets in orbit around the Sun, just as gravity from the Earth is what keeps the Moon and satellites and the space shuttle in orbit around the Earth. The reason the Moon doesn't hit the Earth (and the Earth and other planets don't hit the Sun) is that the Moon is moving fast enough to miss the Earth.

If you were to stand on Mt. Everest and throw a rock (and forget completely about air resistance), it would travel a certain distance before hitting the Earth. As you throw the rock faster, it travels farther. Eventually, if you throw the rock fast enough, it travels all the way around the Earth and hits you in the back. It is still falling towards the Earth, but the surface of the Earth (because the Earth is round) is falling away just as fast. The rock is now in orbit. Continue throwing it faster and faster and the orbit gets bigger (farther from the Earth) on the far side of the Earth, but still comes around to hit you in the back. Eventually you throw the rock at what is called the "escape velocity" and it breaks away from the gravity of the Earth and never falls back.

So there are three ranges of sideways velocity:

Slow enough so that the object falls down and hits

Fast enough so that the object is in orbit, but not too fast

Really fast so that the object escapes

All of the planets (and the Moon around the Earth) are in category 2. If this seems strange, it's not. When the Solar System was forming, everything that was in category 1 actually fell into the Sun, and everything that was in category 3 escaped from the Solar System, leaving only the stuff (planets, asteroids, comets, etc.) in category 2.

Basic principles of physics state that when an explosion occurs, all particles coming from it will all be spinning in the same direction, right? So if the Universe occured by explosion (i.e. Big Bang), all of the planets and such should be spinning in the same direction.

There is no physical law that states that when an explosion occurs all particles will be spinning in the same direction. Angular momentum (the physical measure of spinning) will be conserved, so if the original object was spinning (had angular momentum), the sum of the angular momentums of all the "shrapnel" will equal the original angular momentum, so more will be spinning in the original direction. But individual "shrapnel" can be spinning in the opposite direction.

In the case of Venus and Uranus, the most reasonable theory is that a massive collision while they were forming caused them to spin opposite (in the case of Venus) or at right angles (in the case of Uranus) to the other planets and the Sun.

The orbits of the planets are ellipses, as discovered by Kepler. An ellipse is defined by its major axis (D) and the distance between the two foci (F). The eccentricity (e) is the ratio of F to D, or: e = F/D. For Mars, D = 4.556 x 108 km, and e = 0.09.

We can get an idea of how our solar system was formed by observing star formation elsewhere in our galaxy. This suggest that our solar system originated from a cloud of gas and dust, called the solar nebula,which had a mass three times that of our sun. Matter in this cloud was composed of gas and small grains of ice and dust. Initially, the solar nebula is cold with temperatures less than 50K. At these temperatures most gases freeze into ice particles. However these temperatures are not cold enough to solidify hydrogen and helium, which remain as gases.
The gravitational pull of the particles and gases causes them to collect toward the center of the solar nebula. As the concentration of matter in the center increase, density and pressure rose in magnitude. The high pressure and density caused molecules to collide and thus the temperature deep in the nebula increased. The high temperatures and enormous amount of matter characterized the early stages of our sun, called the protosun. After a few million years, temperatures in the protosun reached millions of Kelvin, igniting thermonuclear reactions; from which our sun was born.

The combined effects of gravity and angular momentum caused the once shapeless solar nebula cloud to turn into a rotating disk whose center was hot, yet its outer edges cold. This disk formation explains why all the planets lie in nearly the same plane. If you could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. The reason for this motion of the planets is due to the gravitational pull exerted on them by the sun. The sun has a force of gravity so strong, that it can pull in an object as far away as Pluto. Why then, do planets not fall straight into the sun? The answer to this lies in the definition of centripetal acceleration. The planets are attracted towards the center of the sun, in a manner which constantly changes their direction, but not magnitude of tangential velocity. The planets would fall in towards the sun, but are going too fast to actually do this. If there was no centripetal force being exerted on planets, they would just go off into space in a direct line. If on the other hand the planets were halted in their motion and then let go, they would fall into the sun. Therefore, the combination of these two characteristics explains why planets revolve in a circular/elliptical path. The planets are trying to travel in a straight line, but are not able to maintain their paths due to the effects of the suns gravity.
All of the planets except Venus and Uranus rotate on their axes in this same direction. The entire system is remarkably flat. Some of the planets are distinct in their revolutions. For example, Mercury exhibits a 3 to 2 spin-orbit coupling. This means that for every two complete orbits around the sun, the planet makes three complete rotations about its axis. Planets also vary in the shape of their orbits. For example, Pluto's orbit is so elliptical that it is sometimes closer than Neptune to the Sun. Thus, it is the farthest planet from the sun only part of the time.

There are so many theories behind the formation of the solar system that perhaps one day we will physically be able to venture out to other planets to gain a better understanding about them instead of having to read about them. With the successful journey to our moon, scientists are trying to endeavor further. Plans are already underway to send a manned shuttle to Mars. The mystery behind our solar system will not be solved for a long time, but with the advent of new technology and an increased understanding of scientific knowledge, we continue to get closer to understanding our Sun and its nine planets.

When studying something as abstract as space, we must remember that there is no definitive answer to any questions, only theories. There are many theories to everything we have discussed, and throughout this chapter, we have listed many of those assumptions (such as the three theories of moon formation). In other cases, we have given the most widely accepted ideas. We must keep in mind that these are only ideas. Throughout our research, we too have come up an ideas. As mentioned before, it is unknown whether or not the two inner most planets, Mercury and Venus, have iron cores. Well we believe that they may not. If all moons are formed at the same time as the iron core (as believed for the Earth), and these two planets don't have moons, they must not contain an iron core either. We know that this is just a deduction based on ideas we have encountered. But it is these types of hypotheses that must be made and re-made over and over in order to gain a more concrete understanding of the truth behind the sun and its nine planets. Perhaps someday we will find an answer to all the questions which have baffled us since the beginning of time.

The planets in our Solar System orbit the Sun. The orbits of some planets are almost perfect circles, but others are not. Some orbits are shaped more like ovals, or "stretched out" circles. Scientists call these oval shapes "ellipses". If a planet's orbit is a circle, the Sun is at the center of that circle. If, instead, the orbit is an ellipse, the Sun is at a point called the "focus" of the ellipse, which is not quite the same as the center.

Since the Sun is not at the center of an elliptical orbit, the planet moves closer towards and further away from the Sun as it orbits. The place where the planet is closest to the Sun is called perihelion. When the planet is furthest away from the Sun, it is at aphelion. The words "aphelion" and "perihelion" come from the Greek language. In Greek, "helios" mean Sun, "peri" means near, and "apo" means away from.

When Earth is at perihelion, it is about 147 million km (91 million miles) from the Sun. When it is at aphelion, it is 152 million km (almost 95 million miles) from the Sun. Earth is about 5 million km (more than 3 million miles) further from the Sun at aphelion than at perihelion!

Some people think that this is why we have seasons, but they are wrong. Earth reaches perihelion, its closest approach to the Sun and when you might think it should be warmest, in January - the middle of winter in the Northern Hemisphere! The difference in distance is not the cause of our seasons. Instead, seasons are caused by the tilt of Earth's axis.

Some planets have very "stretched out" orbits. Pluto, for example, is much further from the Sun at aphelion than it is at perihelion. Astronomers say that a "stretched out" orbit has a high eccentricity, which means it is long and skinny, not round like a circle. Asteroids, many comets, and some spacecraft also travel around the Sun in elliptical orbits. They all have perihelion and aphelion points along their orbits. Anything following an elliptical orbit moves fastest

Perihelion is when the planet is closest to the Sun. Aphelion is when it is furthest away.

If an object orbits something other than the Sun, we don't use the terms perihelion and aphelion. Satellites orbiting Earth (including the Moon!) have a close point called perigee and a far point called apogee.

Moon - perilune (or periselene) and apolune (or aposelene)

a star - periastron and apastron

Jupiter - perijove and apojove

a generic object - periapsis or apoapsis

I know the information i have given here is drifted from the actual question but then i found this really interesting while searching for your answer. Hope you like this.

And as for your question, the universe is filled with never ending mystery. You solve one another arises!

We can only make theories, the truth will always be a secret!

And sorry if i bored you with a real long answer!

I found this interesting myself and couldnt resist sharing this information.

P.S. All planets do not move in the same direction!

:)

Answer 2

Essentially what most of these folks said are correct. I will only paraphrase somewhat for the purpose of clarification and to provide a less technical answer. When an object passes the sun, there are three possible outcomes. The object is moving too slow for its distance and gets drawn into the sun, the object is moving too fast for its distance and passes thorugh the solar system and leaves forever, or the object is moving within a range that is neither too slow nor too fast and gets caught and pulled into an orbit. The range of orbital velocities is quite large, and depending on where in that range the object is moving will determine what shape its orbit will take. Within this range is a very narrow zone of speed whereby if the object is travelling at that speed then it will get a circular orbit. Outside of that narrow zone, the orbits will all be elliptical. Some are very eccentric, others are almost circular. So the odds are that any object in orbit will have an elliptical orbit. But on very rare occasion, if the conditions are just right, the orbit will be a circle, which is also a type of an ellipse, a very special type.

Answer 3

Triton moon of Neptune the only orbiting body, that orbits in the opposite direction of its home planets rotation. The planets were captured and or created by our sun. in our solar systems early days of creation the far reaching fan spiral spin of the sun dictated all planets direction of travel or be destroyed by an head on collision with another soon to be heavenly body. The asteroid belt is proof of such a problem planet. Some theorist assume. No one can be certain. .

Answer 4

killer's answer is correct. I believe that the statement of thousands of asteroids travelling in the opposite direction is untrue. Asteroids can be knocked out of regular orbit by gravitational perturbations from Jupiter and Mars and the Kirkwood Gap principle. But not to the degree of rotating in the opposite direction.

Short period comets have orbits within 30 degrees of the solar system plane. Long period comets which can be disturbed by passing stars, can have highly irregular orbits.

Perhaps it is long period comets originating within the Oort cloud which you are thinking of.

Answer 5

They are all in the same direction because they all formed at the beginning of the solar system. The solar system started out as a sphere of gas and dust. Something, maybe a passing star, started the sphere rotating slightly. As matter slowly collapsed to the center (the future sun), the rotation speed increases (conservation of angular momentum) and the sphere flattens into a disk. This disk is all spinning in the same direction, and from this spinning disk the planets were formed, all moving in the same direction.

Answer 6

A little side fact for you: Other solar systems spin the the complete opposite direction. Also, 3 planets spin ni the opposite direction of earth.

Answer 7

Hi. I like answer 1, but don't forget comets which can have random orbits. You are speaking of the majority, not the total.

Answer 8

That's not a silly question at all!

It's very strong evidence that they all share a common origin as described in the first answer. Good thinking!

Answer 9

the reason the orbits all go in the same way could have something to do with the earths gravitional pull