The Planets

by Read Listen Learn


What makes the sun a star and not a planet? Why does the earth have just the right climate for life? Where did the moon come from? Does our sun go around a bigger sun? What makes a planet and how is it different from an asteroid? What do we know about planets that are not in our solar system? The questions go on and on and on. But it is only now that we’re learning some of the answers.

The Earth is about one hundred and fifty million kilometres or a bit more than ninety-two million miles away from the sun. Mercury is the nearest to the sun; Venus comes next; and then the Earth. A bit further away is Mars. These four planets all have solid surfaces that are hard enough for people to walk on and atmospheres made of gases (even if human beings can’t breathe them). Then, after Mars, there is a huge gap, which is full of asteroids but nothing big enough to call a planet.

The first one after the gap is Jupiter, and it’s followed by Saturn, Uranus and Neptune. (Pluto was discovered in the 1930s and called a planet but, nowadays, we think it is too small and we call it an asteroid.) These four planets are much bigger than the four solid ones and are covered with gas. We sometimes call these ‘the gas giants’. Their centres (or ‘cores’, as we often call these) are hard and there is extra material around them, but it’s probably not fixed. We think it may move in the gases around it.

Anyway, these planets are a very long way away. Neptune is about twenty-eight times further away than the sun and so is, perhaps, 4.2 billion km from us. It is so far away that it only goes around the sun every one hundred and twenty-eight years. But that’s not the last planet or asteroid we know about: after Neptune come Pluto, Sedna (only discovered in 2004) and Qua.

So, why do all our planets go around the sun anyway? Well, the first thing to know about the sun is that it is a star, not a planet. The difference between these is that a planet has a hard centre, but the sun has nuclear reactions in the middle of it. It’s these that make heat and light. Planets don’t have any nuclear reactions but stars do. That’s why we can see light from stars in the sky at night. The only light from planets is reflected from the sun. We think that the sun is about five billion years old. It’s so large that it’s really heavy and has huge gravity. It’s this that keeps the planets from Mercury to Neptune going around it.

Scientists estimate that the Earth is about four and a half billion years old. At that time, it was probably made of liquid metal and heavy metals sank to the centre. Lighter ones stayed on top. We are very lucky that the Earth does not move further towards the sun. At this distance, the planet is warm enough for life and has enough water too. Although life started very slowly – it took billions of years to develop – it has changed fast since then. Most importantly, nothing terrible has happened to stop it. There have been no dramatic changes in climate. The worst things to happen have been when rubbish (or debris) from space has crashed into our planet. This meant the end of the dinosaurs, for example.

What was bad news for dinosaurs has been very good news for the Earth. Scientists think that a crash between our planet and a huge asteroid meant that rubbish (or debris) was thrown into space and, after a while, made the moon. Why are they so sure? Well, it’s very unusual for a planet to have a moon so close. Another thing is that the moon and the Earth are not made of the same things. The moon does not have any heavy metals at its centre. It seems then that something hit the Earth and made the lighter elements near the surface explode into space. Very slowly, they formed the moon. And lucky for us too! The moon helps with gravity.

The last thing to say is that we only know something about one single planet that goes around a different sun. It’s called 51 Pegasus and is enormous– probably about the same size as Jupiter. We also know that it goes around its sun every four days. So, it’s closer to its sun than Mercury is to ours, even though it’s a really big planet. That’s something new for us. Scientists thought that big planets naturally pulled far away from their suns. Obviously not with 51 Pegasus!

We started this article about the sun and its planets by asking whether the sun and all the planets in our solar system are moving around another sun… but the answer to that must wait for another few years – or a few hundred – because we have no idea yet.