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This page answers questions about solar systems in general, and about our Solar System in particular. The questions are:

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## 1. What is a Solar System?

The Solar System is the part of space where the Sun is and everything that orbits around the Sun. The Solar System includes the Sun, the 9 planets (Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto), their moons, the asteroids, and the comets, including the comets in the Oort Cloud and Kuiper Belt.

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The boundary that most people draw around the Solar System is determined by the furthest things are orbit around the Sun. For a while, people thought that Pluto was the furthest, so then the Solar System ended at the orbit of Pluto. Nowadays we know that there are very many comets that orbit around the Sun in the Oort Cloud very far beyond the orbit of Pluto, so now the boundary of the Solar System is beyond the Oort Cloud. The Oort Cloud probably reaches to about 1 lightyear from the Sun.

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If you get more than about 1 lightyear from the Sun, then your orbit around the Sun gets so much perturbed by the gravitational attraction of neighboring stars that it does not keep going nicely around the Sun, so you can say that the boundary of the Solar System is about 1 lightyear from the Sun. The center of the Solar System is the Sun, because the Sun has more mass more than all other members of the Solar System combined.

Other stars can have planets and such as well. Such a star with its planets is sometimes also called a solar system.

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A galaxy is a collection of millions or thousands of millions of stars that together orbit around the center of that galaxy. A galaxy can contain thousands of millions of solar systems.

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## 2. Other Solar Systems

Astronomers think they have discovered over 100 planets so far that orbit around other stars, and form solar systems with those stars. Most of the stars around which planets have been discovered are similar to the Sun, though some are bigger and brighter and some are smaller and dimmer. A recently discovered extraordinary case has a planet orbiting around a white dwarf that itself orbits around a pulsar. Those kinds of stars are definitely very different than the Sun. For information about these planets near other stars you can go to //www.obspm.fr/encycl/encycl.html.

Those planets are invisible to us and were detected through the small wobble they cause in the motion of the star that they orbit, which we can see. Because we cannot see the planets themselves we don't know much about them, except for their mass (always at least comparable to that of Jupiter, so a few hundred times the mass of the Earth) and some information about their orbit around their star.

We cannot yet detect planets like the Earth of Mars around other stars, mostly because such planets have too little mass. The wobble that the Earth causes in the motion of the Sun is about 130 times smaller than the wobble that Jupiter causes.

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## 3. Rotation in the Solar System

The planets rotate around their own axis and around the Sun because of two things:

1. The planets were already rotating when they were formed.
2. There was not enough friction to stop the rotation.

Below, I explain about these things.

### 3.1. Friction

Friction is very important on Earth. You don't slide from your seat because there is friction between your pants and the chair. You can speed up or slow down or go in a different direction when you're walking because there is friction between your feet and the ground. A bicycle or car can stop because there is friction between the brakes and the wheels and between the wheels and the ground. A ball does not keep rolling forever because there is friction between the ball and the ground. Stirred chocolate milk or coffee gets back to rest because there is friction between the drink and the cup.

There is much less friction on ice, if you don't prick spikes or scates into the ice, and that's why it is not easy to change your direction on ice and why things keep sliding across ice for a long time.

In space there is even less friction. To get friction between two things the things have to scrape against each other. Space is terribly empty: there isn't even any air there. There is nothing that can scrape against the Earth or another planet and so stop the planet's rotation around its axis or around the Sun. If you're in space and you're rotating or moving, then it is not easy to stop again. That's why astronauts that make a space walk have to bring along little rockets or a jet pack or have to be tied to the space ship by a wire. If they happen to push off from the space ship without any of those things, then they cannot return to the space ship by themselves.

So, the planets still rotate around their axis and around the Sun because they already did so when they were formed and because there is no (or not enough) friction in space.

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### 3.2. Formation of the Solar System

The Solar System was formed about 5 thousand million years ago from a big rotating cloud of gas and dust. In such a cloud there is always a fight between gas pressure, which wants to make the gas expand, and gravity, which wants to bring the gas closer together. At some time gravity got the upper hand and then the cloud collapsed to a flat rotating disk of gas and dust that was thickest in the center. The center of the disk clumped together and formed the Sun. All rotation from the center of the disk ended up in the Sun and made the Sun rotate around its axis.

Further away from the Sun, smaller clumps formed in the disk, which rotated around their own axis because they contained the rotation from their part of the disk. Those smaller clumps often collided with each other and sometimes they stuck together and so formed The Earth and the other planets and the moons that orbit around them, as did all the other members of the Solar System.

Because the Solar System formed from a single rotating cloud of gas, alsmost everything in the Solar System rotates in the same direction: All planets orbit the Sun in the same direction. Most moons orbit their planet in that same direction. Most planets and the Sun rotate around their axis in the same direction.

The rotation that you can find everywhere in the Solar System comes from the rotation that was already in the cloud of gas and dust from which the Solar System formed. It was not surprising that that cloud itself rotated, because all (or at the very least almost all) stars of which we've measured if they rotate around their axis do in fact rotate, so those must have formed from rotating clouds as well.

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The Sun and the Solar System didn't form from material deriving from a single supernova explosion. The estimated original masses of two well-known open star clusters (the Pleiades = M45, and the Hyades) and a well-known star-forming nebula (the Orion Nebula = M42) are in the neighborhood of 1000 times the mass of the Sun, which is far greater than the mass involved in even the biggest supernova explosion.

Material from multiple supernovas contributed to the Sun. The periodic table shown at https://en.wikipedia.org/wiki/Supernova_nucleosynthesis#The_r-process says that the chemical elements found on Earth (and similarly in the Sun) derive from a variety of sources, including the following "explosive" ones: exploding massive stars (likely supernovas of types other than Ia), merging neutron stars, and exploding white dwarfs (which are likely type Ia supernovas). And the vast majority of the mass of the Sun consists of hydrogen and helium formed during the Big Bang, before the first supernova explosion.

The collapse of the molecular cloud that led to the formation of the Sun may have been triggered by a shock wave from a nearby supernova explosion, but that doesn't mean that any material from that particular supernova explosion must have ended up in the Sun, because shock waves pass through material, and at speeds much greater than the speed at which the material itself moves.

So, if the formation of the Sun was triggered by a supernova then that supernova most likely did not contribute any material to the Sun, and the supernova-derived material in the Sun came from multiple types of supernovas and other stellar fates.

And supernovas aren't the only possible trigger for star formation. Density waves (especially in spiral galaxies) can also trigger star formation.

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Last updated: 2021-07-19