Stars, moons, and planets

Answers to popular questions on stars, moons, and planets.

By using my knowledge of our solar system and the basic needs of living things, I can produce a reasoned argument on the likelihood of life existing elsewhere in the universe – SCN 3-06a.

By researching developments used to observe or explore space, I can illustrate how our knowledge of the universe has evolved over time – SCN 4-06a.

Stars are mainly made of the lightest type of gas called hydrogen. They are giant balls of hydrogen gas, and that gas is very, very hot in the core of a star – millions of degrees, in fact. It’s so hot that a certain type of nuclear reaction is able to occur that turns hydrogen into the next lightest gas called helium (like what we put in balloons to make them float, as helium is lighter than air. We could also put hydrogen in balloons, but that would be very dangerous as hydrogen can burn easily – so there’s a risk that your nice, floating balloon could explode.  In 1937 there was an airship, called the Hindenburg, filled with hydrogen that crashed and burst into flames after travelling between Germany and the United States; you can find out more about that Hindenburg disaster at

When hydrogen turns into helium, we call this nuclear reaction nuclear fusion – and every second inside the heart of a star like the Sun 600 million tonnes of hydrogen is fused into 596 million tonnes of helium. The other 4 million tonnes of hydrogen gets turned into energy – and that’s what produces the heat and light we see from the star.  Eventually, once all the hydrogen in the heart of the star has been used up and turned into helium, the star begins to change: its core shrinks and gets even hotter – hot enough so that other chemical elements can start to form – including elements like carbon and oxygen that we are made of!  Meanwhile the outer part of the star expands and cools down, and the star becomes what we call a red giant.

So stars are like vast factories for making different chemical elements.  In a way, we are all made out of stars because the chemical elements in our planet Earth – in the soil, the oceans, the atmosphere, the rocks, and in all the life on the planet – all of those elements originally came from stars out there in space!

The Sun is a star: a big ball of hydrogen gas, so the reason that the Sun glows – just like all the other stars – is that it is turning hydrogen into helium and releasing lots of energy as heat and light.  The temperature of the core of the Sun is about 20 million degrees, and even its surface is about 6000 degrees! Fortunately, the Sun has enough hydrogen gas to keep shining for billions of years: we estimate that it is about halfway through its supply of hydrogen but it will keep on shining for about another 5 billion years before, eventually, it turns into a red giant star. When that happens it will probably grow so big that it will swallow up the Earth – but don’t worry, remember this isn’t going to happen for about 5 billion years so we have plenty of time to find another place to live!

This is a question that astronomers have only been able to answer since the middle of the 1990s.  Up until then, we knew about our own Solar System, and that the Sun has a lot of planets – including both small, rocky planets close to the Sun and gas giant planets further away from the Sun.  We also knew that there is more than 100 billion stars, many of them just like our Sun, within our Milky Way galaxy.  So it seemed very likely that many other stars had planets too: since there was nothing particularly special about our Sun, there seemed no reason why lots of those other stars would have planets as well.

The problem was that spotting planets orbiting other stars (what we call “extra-solar planets”, “exoplanets” for short) is very tricky!  Planets don’t shine by themselves; they just reflect light from their parent star. And since other stars are much, much further away than our Sun is, seeing planets that are going round these stars is very challenging.  Nevertheless, since 1995 we have discovered thousands of planets orbiting other stars, and in some cases multiple planets going around a single star – just like the eight planets that orbit the Sun in our Solar System.  (You can find out about the latest exoplanet discoveries at http://exoplanets.org/.)  In most of these cases we haven’t yet been able to directly image the planets themselves: their light is too faint because they are so far away. However, we are sure that the planets are there because of the effect the planets have on their parent stars. We can detect either because their gravity makes their parent star “wobble”, or because the planets sometimes block out a little of their parent star’s light.  And as we build bigger and better telescope, we expect to be able to find many more planets, and even to work out what gases their atmospheres are made of.  The bottom line seems to be that lots of stars have planets, and there are probably many billions of planets orbiting the stars in our Milky Way galaxy.

You can find out lots more about exoplanets and how we find them at https://spaceplace.nasa.gov/all-about-exoplanets/en/.

Most of the planets in our Solar System have moons, and some of the planets – like Jupiter and Saturn – have lots and lots of moons, which is partly to do with their larger mass and stronger gravity and the fact that Jupiter and Saturn formed further away from the Sun.  Although Mars is about one third the size of the Earth, it has two moons – Phobos and Deimos – while the Earth has only one Moon. On the other hand, Phobos and Deimos are much, much smaller than our Earth’s Moon, and might be captured asteroids rather than moons which were formed at the same time as Mars, when the Solar System began.  And while Mercury and Venus don’t have any moons, the dwarf planet Pluto has five moons that we know about – including its largest moon Charon which is about half the size of Pluto.

And it’s very likely that, of the thousands of exoplanets that astronomers have discovered orbiting other stars, many of those exoplanets will also have moons.  Although we haven’t yet been able to confirm that any such exomoons exist, there are a few possible candidates that have already been found, and as telescopes improve we can expect to find lots more in the future.

This is all to do with gravity. We think of gravity as a force that acts between all massive bodies in the Universe. This is the picture of gravity that Isaac Newton came up with more than 300 years ago – the story goes that what inspired was seeing an apple fall from a tree and wondering what it was that made the apple fall. (In fact, when Albert Einstein came along, about 100 years ago, that made us realise that Isaac Newton’s picture of gravity as a force isn’t really the full story either – but maybe we’ll talk about that another time!…)

Thinking of gravity, acting as a force that attracts matter together, we can understand why the moon and the planets should be round. Think about making a snowball, or a ball made out of plasticine: if we start off with an unevenly shaped lump of snow and squeeze it with our hands we’ll be pushing the snow together from all directions – and it will wind up in a round shape. That’s what gravity does when a moon or planet forms, only it’s pulling the matter together from the inside instead of pushing it from the outside like when we squash the snow with our hands.

Now if a moon or an asteroid is quite small, then it doesn’t have strong enough gravity to pull the matter together evenly – so if it starts out an uneven shape then it will just stay that way. There are lots of examples of unevenly shaped asteroids and moons like this – but the biggest and most massive ones are round, just like planets are.

The main effect that the Moon has on the Earth is tidal gravity, which causes the tides in the Earth’s seas and oceans.

Some scientists think that the effect of the tides, causing water to “slosh” back and forth in shallow pools by the edge of the sea, might have created good conditions for early life to evolve. So if there hadn’t been a Moon then the tides would have been much lower, and perhaps that might even mean that life evolved differently on Earth! And the effect of Moonlight, illuminating the night – particularly around the time of full Moon – may also have had a big effect on how life evolved too.

They were formed when something, like an asteroid or meteor or comet, hit the Moon. The force of the impact made the crater – sometimes a really big crater! Most of the Moon’s craters were formed billions of years ago, when the Solar System was much younger and there was a lot of “debris” left over. There were probably lots of craters formed on the Earth back then too – but the Earth has an atmosphere, so the action of the weather and the wind and rain over millions and billions of years eroded away all the signs of the craters. The Moon doesn’t have an atmosphere so the craters on the Moon have been preserved as they were when they were formed.

Sometimes things do still hit the Earth, and can leave behind a crater. Fortunately, this doesn’t happen very often, but astronomers do try to work out when asteroids are heading near to the Earth – and make sure their orbits are going to miss us by a safe distance, like with the asteroid 1994PC1 which passed within about 1 million miles from the Earth on January 18th 2022.

Yes, absolutely! For example, if Ceres (a dwarf planet) ever did collide with Jupiter then it would be completely destroyed. Depending on how fast it was going – if it wasn’t a head-on collision, but instead Ceres was captured by Jupiter’s gravity and it orbited around the planet, gradually getting closer and closer – then it’s possible Ceres would get torn apart by what we call Jupiter’s tidal gravity. Tides are all about differences in forces: because the bit of Ceres that was closer to Jupiter would experience a stronger gravitational pull than the side of Ceres that was further away. So over time that would gradually tear Ceres apart, and give Jupiter a ring system – similar to Saturn’s rings. (In fact Jupiter already has a ring system, although it’s not as big and easy to spot through a telescope as Saturn’s, so that suggests moons or asteroids or comets have got too close to Jupiter in the past. We even saw this happen in 1994, when a comet called Shoemaker-Levy 9 got torn apart just before it collided with Jupiter.