I’ve previously mentioned concepts like the Solar System, galaxies and the Universe, but I’ve never properly explained what each of them actually is. I’m sure most people have a vague idea, but probably aren’t 100% sure.
Read on and I’ll explain what a solar system is. Pretty sure you already know? You may be surprised.
Then in future posts I’ll tackle what a galaxy and the Milky Way actually is, and then what the Universe is.
Actually, for the Universe I’ll try my best, but it turns out the Universe is probably a much weirder place than people previously thought.
Solar System, the name pretty much says it all. Solar means sun, so solar system means sun system, which means a star and everything that’s within its sphere of influence. For Earthlings, our solar system is our Sun, its eight planets; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (no, Pluto is not a planet), all of the moons orbiting these planets, like Titan and Europa, all the asteroids orbiting our Sun, most of them in the asteroid belt between Mars and Jupiter, all of the comets orbiting our Sun, and other objects such as clouds of gas and dust.
In our Solar System, after its last planet, Neptune, comes another belt of asteroid-like objects, called the Kuiper Belt. Asteroids are primarily made of rock, but Kuiper Belt Objects are made from a mixture of rock and ice, as they formed further from the Sun in a cooler region in which ice could freeze into a solid in space. Like asteroids, Kuiper Belt Objects range in size and shape, and some of them are massive enough to have enough gravity to compress themselves into spheres, this is what Pluto is. Pluto may look like a planet, but actually it’s basically a large asteroid made out of ice and rock floating within the Kuiper Belt.
Pluto is pretty far away. To measure large distances in space astronomers use the Astronomical Unit (or AU for short). 1 AU is the average distance between the Earth and the Sun, which is 149,597,870.7 km if you’re interested. Pluto is 49 AU at the most distant point of its orbit from the Sun, which means it’s almost 50 times further away from the Sun than the Earth is. Lots of diagrams of our Solar System, like the one I remember in school, usually begin with the Sun and end at Pluto, so you’d probably think that our Solar System is roughly 100 AU across (i.e. the distance of Pluto’s orbit either side of the Sun) but our Solar System is much bigger than this.
Something I certainly never heard of in school was the Oort Cloud. This is another belt-like region in our Solar System, this time populated by a trillion cometary bodies, made mostly of ice, with a little rock. It’s not known for sure whether the Oort Cloud really exists, as its so far away and the Oort Cloud bodies are thought to be quite small, so the Oort Cloud has never actually been seen by astronomers. Its existence is based on some pretty good scientific theory though, so I’ve no good reason to doubt it. The Oort Cloud is actually thought to be two separate clouds, one disk shaped inner Oort Cloud, sometimes called the Hills Cloud, thought to be at a distance of 1,000 to 20,000 AU (yes, you read that distance correctly), and an outer spherical Oort Cloud, extending from 20,000 possibly up to 100,000 AU. So our Solar System is much larger than 100 AU across! But where does it actually end?
You may have seen above that I defined a solar system as a star and everything that’s within its sphere of influence. Well you have two options as to how you define a star’s sphere of influence. One is to define it as the distance to which the solar wind extends.
Our Sun is like a giant magnet, and like all magnets, has a region of magnetic influence, called a magnetic field. If you ever did that experiment in school where you poured iron filings around a bar magnet, you’ll have seen that they arrange themselves in lines, called magnetic field lines. Our Sun has these too, and they extend deep into space, and become twisted into a beautiful pattern as the Sun rotates. This is called the Interplanetary Magnetic Field.
Our Sun also emits a constant stream of charged particles from its surface (mostly electrons and protons) that fly out into space along the Interplanetary Magnetic Field lines, which is what we call the Solar Wind. This ‘wind’ extends out to a distance of around 100 to 200 AU around of the Sun and forms a region called the Heliosphere. Using this definition, our Solar System is pretty big, extending around four times further than the orbit of Pluto, but the sharp-eyed among you will notice that this is nowhere near as distant as the outer edge of the second Oort Cloud, hence I prefer to use the alternative definition of our Sun’s sphere of influence, our Sun’s gravitational influence.
Gravity is the name used for a weird law of nature. Almost everything in the Universe has mass (I for instance have a mass of around 75 kg, on a good day), and gravity is the phenomena in which two objects with mass will attract each other, with the strength of this attraction based on the combined mass of the two objects, and the distance between them. You stick to the Earth’s surface because there is a gravitational attraction between your mass and the Earth’s. Newton discovered the law of gravity, but Einstein found out how it actually works. It turns out that mass warps space-time like a heavy object placed on a rubber sheet; the Earth actually warps the fabric of space around it so that you fall towards it along curved space. This sounds insane, but it appears to be true, which is one of the reasons Einstein was such a genius; he realized that the Universe is a much weirder place than Newton had us believe.
The Sun is absolutely massive (roughly 333,000 times the Earth’s mass) and its gravitational field extends for a long distance into space, approximately 230,000 AU (Oort Cloud included!) So although in school I was led to believe that Pluto was on the outskirts of our Solar System, and I suspect many other kids were told the same, our Solar System actually extends out almost 5,000 times further (depending on your definition). Take a moment to savour that, our Solar System could have a radius of 230,000 AU in either direction from the Sun. That’s absolutely enormous; 430,000 times the distance from the Sun to the Earth in total.
Remember too that our Sun is a star, and that there are many other stars out there in space, and many of these stars may have their own solar systems with planets, asteroids, comets and all that good stuff. Astronomers have only really begun to find planets around other stars, called exoplanets, but already the first solar systems have been found, including a six planet solar system found by the Kepler space telescope in early 2011.
In the next post in this series I’ll look at what a galaxy actually is, including the Milky Way, see you there.