What's Out There?

In this article, we'll find out about:

1.

• Where the Earth is in the universe

• Stars, planets and moons

• Gravity

• The Solar System

• Space rocks

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Where is our Earth?

The Earth we live on is one of eight planets orbiting, or moving around the Sun. The Sun and everything that orbits it is known as the Solar System. We'll get into a little more detail about the Solar System later. The Sun is a star and is one of billions of stars in the Milky Way Galaxy. A galaxy is a group of many, many stars. Finally, the universe is everything in existence. There are estimated to be two hundred billion galaxies in the universe.

Knowing this, we might feel a little small. We're just specks on the Earth and the Earth is just a speck within the whole universe. But, if we looked out into the immensity of space like the person in the above picture, we may get a sense of being part of some great and wonderful cosmic story.

Stars

Stars, like our Sun, are giant balls of incredibly hot gas. They are mostly made of hydrogen and helium. What happens is that hydrogen gets turned into helium and this releases a lot of energy. The energy travels across space and in the case of our Sun, gives light and heat to the Earth, which, among other things, allows living organisms to grow and thrive.

The Sun is so massive that its gravity is strong enough for all the planets in the Solar System to be influenced by it and orbit around the Sun.

So what is gravity?

This is a physics question. But I'll answer it here because gravity is an important part of astronomy, which is the study of stars, planets, galaxies and basically everything in the universe that's outside the Earth.

Sir Isaac Newton was a famous scientist who lived from 1643 to 1727, back when physics, and all the sciences, were in their early stages. The famous story goes that he was sitting under an apple tree and an apple fell on his head. While this didn't actually happen, it was while Newton was considering how an apple falls that he asked the question: if an apple falls, does the Moon also fall? It turns out, that the force that makes an apple fall to the ground is the same that causes the Moon to go around the Earth.

Image by Andrew Howe

Every object is attracted to one another by gravity. The more mass an object has, the greater its gravitational pull on another object. So, although the apple pulls in the Earth, it only has a very tiny effect on the planet. The Earth, which is much more massive, possesses a far greater gravitational pull and as such, the apple moves towards it and we observe it as falling to the ground. Gravity is what keeps us all stuck to the Earth's surface. We may apply a greater force, by jumping in the air, for example, and break free of the gravity for a while; however, gravity will then pull us back in once we've lost that energy. We'll cover forces in a coming physics article, but this will suffice for now.

So what about the Moon? To answer Newton's question, the Moon does indeed fall to the Earth. However, the Moon moves towards the curved surface of the planet and this makes it move, or orbit around the Earth. The same is true for the Earth, and all the other planets, which orbit the Sun as it too, is curved. It's a good thing too, or else the Earth and everything else in the Solar System would have hurtled into the Sun. I think you can guess who would win in that battle.

The force of gravity increases as an object gets closer and closer to another. So if we're on the surface of the planet the pull is very strong, but if we get far enough away, we can experience weightlessness as we orbit the Earth. This is the case for astronauts (persons who travel into space) while in the International Space Station a large satellite in orbit around the Earth, which is used for researching...well, mostly what it's like to go into space. You can watch them floating around and grabbing onto the bulwarks, or walls, and pulling to propel themselves forward. If you watch these funny folk enough, you may see that spilled liquids form into balls. Even if one tried to pour his or herself a drink, he or she would produce a load of small balls.

While air in the station is the same as on Earth: made of free floating molecules, and solids keep their shape, liquids have the density (a measure of how close particles are to each other: so solids are denser than liquids) for the molecules to bond to each other and yet be apart enough to not be a solid. Gravity then pulls the molecules together so they form a ball. This ball can then spin around and the force of the spin causes it to stretch out a little so it doesn't remain completely ball-shaped. I'm not just going on a tangent, this is an important point about gravity which will be relevant in the next section.

Planets and how stars and planets are made

Planets are large celestial bodies that orbit stars. Well, most of them do. Astronomers have found some rogue planets which are drifting around outside the gravitational pull of any star. Planets have at least a rocky core with some, like our Earth, composed mostly of rock. These rocky worlds are known as terrestrial planets. Others, much bigger than the Earth, are mostly made up of an atmosphere of various gases with a rocky core and maybe a rocky or liquid coating over the core. These are given the name gas giants.

There are many rocks in space and they come in various shapes and sizes. However, when a rock is large enough, gravity will pull it together so that it collapses into a spherical (or roughly spherical) shape, just like with balls of liquid. Now, it's not as if an object would get to being that big and then gravity forces it to become a ball: it's a ball because gravity has already been doing that to the object as it formed. So it wouldn't be that big unless it's a ball.

Image of the Orion Nebula, by Alexander Andrews.

Planets formed from smaller pieces coming together due to gravity. I'll cover this in more depth on my article on the Solar System, but basically a star forms from a cloud of gas, called a nebula, which spins fast and gravity pulls it together into a star, which is mostly made of hydrogen and helium. The other elements move to a certain distance away from the star and then come together to form planets as well as other rocky bodies. Note that nebulae will have many stars forming within them as gas coalesces in particular areas.

In order for something in the Solar System to be called a planet, it must pass three tests:

1. A planet must orbit the Sun.

2. A planet must be (roughly) spherical in shape.

3. A planet must clear its surroundings of other objects.

There are smaller celestial objects which, although may orbit the Sun, are not round in shape as they are not large enough. Then there are others which are large enough to be spherical but they don't clear their surroundings of other objects. These are known as dwarf planets or planetoids. Pluto is one such dwarf planet, which used to be considered a planet.

As they orbit the Sun, these space rocks might get close to other objects of smaller or even similar size and their orbits may change a little due to coming close to these other objects. Many small objects can even crash into each other. It's a mess trying to sort out all these objects out there so we need a way to classify them. Anything that gets close to a planet will either orbit it around, thus becoming its moon (and all the planets in our Solar System have had their moons for millions of years and aren't going to have any more in the foreseeable future), get pulled around and swing away or get 'absorbed' (fall into it). What a planet won't have is an object that hangs around it for a while but then goes off somewhere else. So there, that's a planet.

Moons

Pretty simple: moons are objects that orbit planets. They are almost all spherical too. The collective term for anything that orbits a planet is a satellite. A moon is a natural satellite. The Earth has many artificial satellites.

Planets may have a single moon or many moons. The planet Jupiter has a whopping 67 moons! It was very greedy when it came to capturing objects and making them its moons.

I've mentioned other space rocks, but I'll cover them after a brief description of the Solar System.

The Solar System

As I said, there are eight planets in the Solar System. They are each at different distances from the Sun. I'll do an article on the Solar System, which will include some data on distances as well as sizes of planets etc. This section will just give an introduction to our neighbourhood.

The four planets closest to the Sun are all relatively small. They are close together and are much nearer the Sun than the other four. These terrestrial planets, in order from closest to the Sun to furthest, are:

Mercury

Venus

Earth

Mars

The outer four planets are gas giants. Again in order, these are:

Jupiter

Saturn

Uranus

Neptune

You may want to know which gas giant is the biggest of them all. That's Jupiter and it's followed by Saturn and they are more than twice the size of the other two.

When it comes to the size of any and all objects in the Solar System, however, the Sun comes out on top. The planets are not even in the same league! You could line up 109 planet Earths along the Sun, or very nearly 10 Jupiters. I'm talking about diameter here. If you want volume, then about 1,300,000 Earths could fit inside of the Sun.

There's much more to know about planets than just their size, but how big things are tends to be a popular question in astronomy and it serves as one of the good first steps for wrapping our minds around the Solar System. Check out its own article when it goes up to familiarise yourself better with our nearest neighbours.

Space rocks

Lastly, we'll take a brief look at other rocky objects. Between the orbits of Mars and Jupiter we find the Asteroid Belt. This is a chain of rocky bodies too small to be considered planets or even dwarf planets. The larger rocks are called asteroids while the smaller ones are meteorites. They're not only found along this belt. They also make up the Kuiper Belt which is a much larger ring of rocks orbiting the sun at distances past the orbit of Neptune and the Oort Cloud, whose rocks orbit the edges of the Solar System. There are also free floating ones: asteroids and meteorites which perhaps have collided with other rocks in these belts and have drifted off. These can crash into planets and moons, leaving impact craters. This is a photo of craters on the moon:

The energy of the impact is so great that the rocks get smashed to pieces and the fragments go all over the place. The atmosphere of a planet can also break up a rock. When something flies through the atmosphere it gets resisted by air and for something that moves as fast as an asteroid or meteorite, the resistance becomes so great that the rock burns up and is usually completely destroyed. Only a large rock will maintain some of its material to hit the surface of the planet. The Moon, which has no atmosphere, gets continuously bombarded with meteorites and the occasional asteroid. On Earth, we almost never get asteroid sized rocks but we do have plenty of meteorites or meteors, as they're called when they're flying through our atmosphere. They're what we call shooting stars.

There have been large asteroid impacts, including one 65 million years ago which wiped out the dinosaurs and there may have been one a lot more recently: about 12,800 years ago which melted glaciers, or huge areas of ice formed during a very cold period of the Earth called an ice age. These glaciers then melted to cause a sea level rise. Not every scientist is in agreement about whether this happened but nor were they about the theory of an asteroid 65 million years ago. Then again, it could have been a comet that hit the Earth 12,800 years ago and not an asteroid.

A comet is made of rock and ice. It's like a big dirty snowball. A comet flies through space from the edge of our Solar System and inward towards the Sun. As it does so, the ice melts leaving a long trail. Once it gets close to the sun it then orbits around part way but doesn't get caught in a continuous orbit. Instead, it flings off again away from the Sun. We see the same comets every few decades as they make their long elliptical orbits, which are orbits that aren't circular (or mostly circular) like those of planets but 'stretched out'. Comets can fly towards a planet or moon in such a way that they impact the surface and get smashed into fragments just like an asteroid or meteorite would.

So there's your introduction to astronomy.