Scientists have discovered a lot about space, how it works and the things in it, but there are still loads of mysteries left to solve.

Our solar system is in a galaxy called the Milky Way. On dark nights we can see part of the Milky Way as a big, curved band of light across the sky. © sripfoto/ Shutterstock
Space is the vast expanse of the universe where energy and matter – stuff that takes up room and has mass – exist. So, it’s not a totally empty void, but did you know that less than 5% of the whole universe is normal, visible matter? That includes all the solids, liquids, gases and other tiny particles that make up the things we can see and interact with, such as stars and planets.
While we can’t see or detect them yet, we think that dark matter and dark energy are a huge part of the universe and how it works.
Space is full of radiation. Some of it is left over from the beginning of the universe. Cosmic microwave background (CMB) radiation tells us that our universe is 13.8 billion years old and started with the Big Bang.
Since the Big Bang, space has been expanding, going from a single point to the size it is now. In fact, it’s still expanding and at an accelerating rate. We don’t know why expansion is getting faster, but scientists think it has something to do with dark energy.

The higher you go, the thinner Earth’s atmosphere gets until it becomes space. © Alones/ Shutterstock
Where does space begin?
For official records, we often say that outer space begins at 100 kilometres above Earth’s sea level. We call this the Kármán Line.
But there actually isn’t an exact point where the sky ends and space begins. Instead, the higher you go, the thinner Earth’s atmosphere gets until it becomes space. The outermost layer is called the exosphere and ends at least 10,000 kilometres from Earth.
Material spewing from the Sun provides a protective bubble around our solar system called the heliosphere. This material’s effects end at a boundary called the heliopause. Beyond this is interstellar space – the space between stars.
Light years
Space is huge! In fact, we don’t really know just how big it is.
We measure the extremely long distances between things in space by light years. A light year is the distance that light travels in one Earth year.
Light travels at about 300,000 kilometres per second. So, one light year is about nine trillion kilometres.
Nebulae
A nebula is a cloud of dust and gas in space.
Nebulae – meaning more than one nebula – are often where new stars form as the dust and gas gets pulled together by gravity. Dying stars can also create nebulae when they throw out their contents in the final stages of their life.
Some nebulae emit their own light while others reflect the light of nearby stars. Dark nebulae obscure light, creating dark patches in the sky.

The Boomerang Nebula is the coldest place found in the universe so far. It’s around 5,000 light years from Earth and a freezing -272°C. Image © NASA, ESA and The Hubble Heritage Team STScI/AURA, licensed under CC BY 4.0
Stars
Stars are giant balls of superheated gas. Atoms are squeezed together at their centre creating elements and energy that’s released as light and heat.
Baby stars – called protostars – form when a nebula clumps together, collapses under its own gravity and starts to generate heat.
When protostars get hot and dense enough to cause nuclear fusion, converting hydrogen into helium, we call them main-sequence stars. This phase can last billions of years.
Eventually stars run out of hydrogen for fuel and their core collapses under gravity. As the core is squeezed, it gets hotter and the pressure increases, making the star expand.
Small stars, such as our Sun, expand from a dwarf into a red giant. In about five billion years the Sun will engulf the planets closest to it, possibly including Earth. Five billion years later, it’ll shrink, becoming a white dwarf that’ll cool down over billions of years.

Our Sun is a type of star known as a yellow dwarf. It’s not big enough to turn into a black hole at the end of its life, although you could still fit roughly 1.3 million Earths inside it. Image © NASA Goddard
Giant stars go out with an explosion called a supernova. As their core collapses it causes massive shockwaves. These make the outer parts of the star blow off. What’s left is a dense core known as a neutron star. If the star was really enormous, it may turn into black hole after exploding.
When stars go supernova, the elements they made get swept out into space. Nearly every element we know of was made in stars. Different combinations of elements creates gases, minerals, water and other ingredients for life. That means you’re made from stardust.

The Crab Nebula was formed by a supernova. This colourful image was made by combining images from telescopes that pick up different wavelengths of light.
Image © NASA, ESA, G. Dubner (IAFE, CONICET-University of Buenos Aires) et al.; A. Loll et al.; T. Temim et al.; F. Seward et al.; VLA/NRAO/AUI/NSF; Chandra/CXC; Spitzer/JPL-Caltech; XMM-Newton/ESA; and Hubble/STScI, licensed under CC BY 4.0
Black holes
Black holes affect physics in ways we don’t experience on Earth.
Black holes have extremely strong gravitational fields. Anything – matter, energy and even light – that gets too close can’t escape. This point is called the event horizon. At the centre of a black hole is the singularity. This is where our laws of physics break down, meaning we can’t predict what happens there.
They don’t emit light or other radiation, so we can’t look for black holes in the same way we search the sky for stars or planets. But we can still find them. We can detect the gravitational waves black holes make or look for light-emitting accretion disks of dust and gas around them.
We find stellar mass black holes throughout galaxies. These have masses a few times greater than our Sun. Supermassive black holes have masses hundreds of thousands to millions of times greater than our Sun and are found at the centre of galaxies.

In 2019, the Event Horizon Telescope captured the first ever photo of a black hole. This supermassive black hole is at the centre of the galaxy Messier 87, around 55 million light years from Earth. Image © EHT Collaboration, licensed under CC BY 4.0
Planetary systems and galaxies
Stars have strong gravity. Everything near to one gets pulled towards it. This stops objects, such as planets, asteroids and comets, from flying off into space.
These objects travel really fast – so fast that they counteract the gravity pulling them towards the star. This means they orbit around the star rather than spiralling towards it. Most objects that travel around stars have slightly elliptical orbits, meaning they follow an oval-shaped path.
We call a star with planets orbiting it a planetary system. So, our solar system is a planetary system – we call it the solar system because solar refers specifically to our Sun. It formed when a dusty disk called the solar nebula collapsed under its own gravity about 4.6 billion years ago. Ours is one of thousands of planetary systems that have been discovered in our galaxy, the Milky Way.
Galaxies are made of stars, the objects orbiting them, dust and gas, all pulled together by gravity.
Galaxies come in different shapes but most, including the Milky Way, are spiral galaxies. These are flat, rotating disks with arms that spiral out from the centre.
Most large galaxies have a supermassive black hole at their centre. The Milky Way’s is called Sagittarius A* and is more than 25,000 light years away from Earth.

This spiral galaxy is called NGC 2008. It’s 435 million light years away from Earth. Image © ESA/Hubble & NASA, A. Bellini, licensed under CC BY 4.0
Planets
For an object to be a planet, it must:
- orbit a star
- be big enough that gravity has forced it into a spherical shape
- have used its gravity to clear other similarly sized objects from its path
There are eight planets in our solar system. We group them into three types – terrestrial planets, gas giants and ice giants.
Mercury, Venus, Earth and Mars are terrestrial planets. They have a solid surface and are mostly made of metals or silicate minerals – where the rocks are composed of silicon, oxygen and other elements.
Jupiter and Saturn are gas giants. These don’t have a solid surface and are mostly made of swirling gases, such as helium and hydrogen.
Uranus and Neptune are ice giants. They’re made from hydrogen and helium too but are orbiting really far away from the Sun, so their gases are mostly frozen.

Gas giant Jupiter is the largest planet in the solar system. It’s 11 times bigger than Earth.
Image © NASA, ESA, A. Simon (Goddard Space Flight Center), and M. H. Wong (University of California, Berkeley) and the OPAL team, licensed under CC BY 4.0
There are also a few dwarf planets in our solar system, including Pluto and Ceres. These are similar to planets but don’t meet all three of the key criteria to be counted as one.
Exoplanets are planets outside our solar system. These can be similar types to planets in our solar system or entirely different types. For example, we’ve found super-Earths, which are more massive than Earth but lighter than our ice giants and hot Jupiters, such as HD 189733 b where it may rain glass.
Earth and its nearest neighbours orbit the Sun, which is a star, but there are also rogue planets that wander freely through interstellar space. There could be billions of rogue planets in the Milky Way.
Moons
Moons orbit planets and some asteroids. While gravity has forced our Moon into a spherical shape, other moons are more irregular.
Our Moon formed 4.5 billion years ago. It’s relatively benign now, but the dark patches on its surface – called lunar maria – were once seas of molten lava that have solidified into a rock called basalt. It also has its own atmosphere and some water trapped in its rocks.
Other planets’ moons are more volatile. For example, Io, which orbits Jupiter, has more than 400 active volcanoes across its surface.
Not all planets have moons – Mercury and Venus are moonless. Other planets have many more moons than Earth – Saturn has an incredible 274 moons and we keep discovering new ones!

Europa orbits the planet Jupiter. Scientists think it might be possible for life to exist in the liquid ocean under this moon’s icy exterior. Image © NASA/JPL-Caltech/SETI Institute, licensed under CC BY 4.0
Asteroids and comets
Asteroids are rocky or metallic objects that orbit our Sun. They’re left over from when the solar system formed. Asteroids range in size from a few metres to hundreds of kilometres in diameter. Some are material that’s loosely scooped together, while others may be more solid with metallic cores.
Around 37,000 asteroids orbit the Sun at a similar distance to Earth or closer. We call these near-Earth asteroids. The majority, however, are found further away, in the asteroid belt between Mars and Jupiter.
Sometimes bits of asteroids break off. We give these space rocks different names depending on whether they’re still in space or have entered Earth’s atmosphere.
Comets are like big, dirty snowballs. They’re balls of dust, rock and ice that come from the Kuiper Belt beyond Neptune or the Oort Cloud at the edge of our solar system. Some have extreme oval-shaped orbits.
Short-period comets complete a full orbit of our Sun in less than 200 years whereas long-period comets take more than 200 years. When their orbits bring them close to the Sun, the warmth makes them release gases, which sometimes form a visible tail behind them.

Bennu is a near-Earth asteroid with a diameter of about 500 metres. Rocks collected from its surface indicate that asteroids may have delivered some of the ingredients for life to Earth. Image © NASA's Scientific Visualization Studio via Wikimedia Commons, Public Domain
What about extraterrestrial life?
We’ve not located extraterrestrial life yet, but some think we might find evidence of it in our own solar system, perhaps on Mars or an icy moon.
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