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The furthest any human has made it from Earth is just beyond the moon – some 400,000 kilometres away, or 1.3 seconds at light speed. The edge of the observable universe lies 46.5 billion light years away.

Our understanding of the vast expanse of the cosmos is perhaps still in its infancy. Yet thanks to powerful telescopes and some inspired theorising there is already much we can discern about how the universe works and what it contains.

What you’ll find below is a concise guide to everything we know of in the universe, from stars, planets and moons to galaxy clusters, dark energy and more. You will discover hot Jupiters, super-Earths, interstellar space rocks, the biggest stars in the cosmos, including one that could be an alien megastructure (but probably isn’t), supermassive black holes, the universe’s first light and the cosmic web.

You will, in other words, come away knowing a lot more about the incredible diversity of objects and phenomena in the universe than you do now.

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PLANETS AND PLANETARY SYSTEMS

We start close to home. Given the vastness of the universe and the number of stars it contains, it would take a peculiarly blinkered view to believe that our solar system, with its ordered retinue of eight planets, is the only such collection of worlds in our galaxy. Only in the past three decades, however, have our telescopes been able to see planets orbiting other stars, known as exoplanets. We have now , and most don’t look anything like home.

Hot Jupiters

The surprises with exoplanets started in 1995 with 51 Pegasi b, the first planet to be discovered orbiting a main sequence star, the most common type of star in the galaxy, other than the sun. At about half the mass of Jupiter, but orbiting closer to its star than Mercury does to our sun, it was the first so-called hot Jupiter – a gas giant that orbits scorchingly close to its host star.

We have since found over 1500 hot Jupiters, and the consensus is these planets have migrated in towards their star after forming in a more distant orbit. But while this is a large proportion of the 5000-odd exoplanets found so far, we shouldn’t overestimate how common they are. Most exoplanet detections happen by looking at how a star’s light dips or changes colour as a planet passes in front of it, which biases discoveries towards large, fast-orbiting planets that transit across the face of their star more often. “Hot Jupiter planets are relatively rare,” says at the University of Edinburgh, UK.

Rocky planets

We have found far fewer rocky exoplanets, most of which are less than twice the mass of Earth, compared with any other kind: fewer than 200 so far. This may be due to bias caused by the way we detect exoplanets, or it could be that our own solar system is unusual compared with others in our galaxy. But in the search for alien life, these are exciting planets, especially those that orbit in the “habitable zone”, at a distance from their star that would allow liquid water to exist on the surface. A particularly promising location is the TRAPPIST-1 system, which has seven rocky planets orbiting the same star.

Neptune-like

These are around the same size as Neptune and Uranus, with similar hydrogen and helium atmospheres and rocky cores. So far, we have discovered over 1700 of these Neptune-like exoplanets. In 2017, astronomers detected water vapour in the atmosphere of a planet called HAT-P-11b.

Super-Earths

Of the exoplanets that have been discovered, only a handful are rocky planets like Earth or Mars. Just under a third are gas giants, like Jupiter and Saturn, and just over a third are Neptune-like. The rest are mostly super-Earths, planets that are unlike any in our solar system. These are between twice and 10 times the mass of Earth. Some are made of gas, some are rock and some are formed of a mix of both.

Rogue planets

Some planets exist entirely without stars. These lonesome objects are often called sub-brown dwarfs because they are thought to have emerged from the collapse of clouds of dust and gas in a similar way to the formation of stars and brown dwarfs – but they were too small for any fusion to occur.

One of the handful we know about, WISE 0855, sits by itself in space just over 7 light years away from Earth. Its mass is estimated to be between three and 10 times the mass of Jupiter, water ice has been found in its atmosphere and its temperatures can reach as low as -48°C. That is the coldest atmosphere we have detected outside our solar system, perhaps giving us insights into what the atmospheres of other similarly cold planets look like.

Space rocks

In our solar system, the processes of planet formation left quite a few offcuts. These cluster in two known regions and one hypothetical place. The known regions are the asteroid belt between the orbits of Mars and Jupiter, whose 20,000-odd rocky bodies range from just a few metres across to the dwarf planet Ceres, which is almost 950 kilometres in diameter; and the Kuiper belt beyond Neptune, whose most famous resident is the dwarf planet Pluto, which is some 1200 kilometres across. There is also a small group of co-orbital asteroids, which share their orbits with planets. One even shares Jupiter’s orbit despite going the opposite way around the sun.

The hypothetical home of space rocks is the Oort cloud, a spherical cloud of icy rocks encircling the solar system, whose existence is hypothesised as a source for the long-period comets that sometimes swing by the sun on their highly elliptical orbits.

A suspicion that similar regions exist in other solar systems has been increased by the identification in recent years of two interloping bodies from outside the solar system, Oumuamua and the comet 2I/Borisov. There are hints that a further 17 such interlopers may reside in our cosmic neighbourhood.

Oddballs

A selection of anomalous cosmic objects and phenomena

Wasp-76b

This huge hot Jupiter is twice the size of our Jupiter and hot enough to melt iron – at least on the side that always faces its host star. On this "day" side, it reaches 2200°C, turning iron into a gas. When this gas eventually reaches the nightside of the planet, it cools back to liquid and rains down on the surface.

Earth

Earth has an unusually large moon compared with the size of the planet. It is also the only planet known to sustain life.

The solar system

Our solar system is home to eight planets: four rocky planets close in, then two gas giants, and finally two ice giants. When we first started exploring exoplanetary systems, we assumed all solar systems would look like ours. But the more systems we discover, the more we realise our own, with its peculiar configuration and formation history, is the unusual one.

Trappist-1

Of the seven planets we know about in the TRAPPIST-1 system, three orbit the star in the habitable zone, where liquid water might exist on their surface, and some of these may have atmospheres. What makes the system particularly unusual is that all of its planets are made from rock. Since Earth is also rocky, TRAPPIST-1 is a promising candidate in the hunt for alien life.

Unknowns

A selection of some of the things we are still trying to figure out

Exomooons

We know of 213 moons – all of them in our solar system and none elsewhere. Our gas giants have the most: Jupiter and Saturn both have 53 official moons, although Saturn also has 29 small "provisional" moons awaiting confirmation. Saturn's largest moon, Titan, is bigger than the planet Mercury and is the only body other than Earth in our solar system where water cycles between clouds, rain and seas. This makes exomoons promising candidates in the search for life beyond Earth.

When it comes to moons in other planetary systems, however, none has been detected with confidence. One study in 2020 identified , but they need to be observed for a little longer to say for sure that they exist. Exomoons are detected in the same way planets are detected – for example, the transit method that looks for a dip in the light from a star as a planet moves in front of it. But these dips are much smaller for moons than planets. "Ultimately, it is the smallness of exomoons that makes them so hard to detect," says at Western University in Ontario, Canada.

STARS

To our eyes, stars are the most obvious feature of the universe, and they are undoubtedly important. These nuclear fusion reactors created the elements that make up our bodies and the planet we live on, and one produces the heat and light that life on Earth needs to survive. But to fixate on just our own sun would be to deny the magnificent diversity of stars in all their stages of birth, life and death.

Main sequence stars

Not all stars shine as brightly or are the same colour. They are classified by the relationship between their temperature and the amount of light they give out. Hotter stars shine bluer, while cooler stars shine redder. Across this spectrum, however, stars have a huge range of brightness, from one ten-thousandth the sun's luminosity to a million times brighter than the sun. The biggest factor in a star's luminosity is its mass, which depends on the amount of material present when it formed.

Astronomers plot these variables on a Hertzsprung-Russell diagram (see "The diversity of stars", page 40), with stars appearing in different places at different stages of their life. Stars in the prime of life, during which time they are burning by fusing hydrogen nuclei into helium, are called main sequence stars and fall on a diagonal line from massive, hot blue stars to small, cool red ones. The very smallest and coolest main sequence stars are red dwarfs, with masses less than a tenth that of our sun.

Giants and supergiants

Once a star begins to exhaust its primary hydrogen fuel, it starts to fuse heavier elements in its core, while still fusing hydrogen into helium in its outer regions. This causes the star to expand, and leave the main sequence. What happens next depends on how large it was to begin with. When the Danish astronomer Ejnar Hertzsprung started categorising stars, he realised expanding stars fell into two categories: main sequence stars and other, distinctly larger stars, which he called giants. A few years later, an even larger category appeared, and was given the name supergiants. A few stars under a fourth category of hypergiant have since been identified, including UY Scuti, which is around 1700 times larger than our sun.

Binary star systems

A binary star system is a pair of stars that are gravitationally bound to and in orbit around one another. 55 Cancri, found 41 light years from Earth in the constellation Cancer, is one example. The fifth planet orbiting 55 Cancri is twice the size of Earth, and made of diamond.

White dwarfs

After a giant star has run out of hydrogen to burn, it begins to fuse helium to make carbon and oxygen. As these elements begin to build up, for most stars, the core temperature isn't high enough to take fusion any further and so it stops. At this point, the gravitational pull inwards is no longer balanced by an outward pressure from the nuclear reactions in the core and the star implodes to become a white dwarf. These remnants of dead stars are dim and dense, with hundreds of thousands of Earth masses packed into the volume of our planet. They only shine thanks to leftover heat.

Supernovae remnants

The very biggest supergiant stars don't become dim white dwarfs. Their core temperatures are sufficient to fuse nuclei right up to iron, and they end their lives in dramatic explosions – called supernovae – to leave behind black holes or neutron stars.

At the heart of the Crab Nebula sits a relatively young neutron star, left over from a supernova that was seen from Earth in AD 1054. At the time, Chinese astronomers noted it as a "guest star" near the constellation Taurus that became four times as bright as Venus in the sky, before disappearing. Astronomers realised in 1939 that this supernova must have been where the Crab Nebula is and began hunting for the star. In 1968, when astronomers finally found the neutron star, called the Crab Pulsar, it was the first known supernova remnant.

Neutron stars and pulsars

We know of around 2000 neutron stars, collapsed remnants of supergiant stars that aren't quite large enough to form black holes. The heaviest neutron star is 2.1 times the mass of the sun. They are the densest stars ever seen, with the mass of our sun packed into a sphere around 10 kilometres across.

Some neutron stars spin incredibly fast on their axis, with a jet of intense radio emissions whirling round with them like the beam from a lighthouse. These are called pulsars, and were discovered by astrophysicist Jocelyn Bell Burnell in 1967.

Black holes

Stellar-mass or astrophysical black holes are created when a massive star, with at least 20 times the mass of the sun, runs out of fuel in its core. If the core is above three solar masses, it collapses to form a black hole. Only around a couple of dozen such black holes have been observed in our galaxy, but astronomers believe hundreds of millions exist in the Milky Way alone. We can't see them directly: their gravitational pull is so strong nothing, not even light, can escape. Instead, we infer their presence from watching how nearby stars and galaxies move, and by signals produced by their collisions, called gravitational waves.

The existence of black holes was predicted by Albert Einstein's general theory of relativity as entities whose immense gravity would cause space-time to curve infinitely, creating what became known as a singularity. But here, general relativity breaks down: the solutions to the equations go to infinity. Many physicists believe these singularities don't describe what is happening inside real black holes, and instead are a sign that we need to amend our theories.

The question is how. A theory that would accurately describe black holes would need to blend general relativity with quantum theory, our description of matter on the smallest scales – a trick no one, as yet, has been able to pull off.

GALAXIES

Every star we see in the night sky is part of just one galaxy – our own Milky Way. Up until around 100 years ago, astronomers believed this was all there is. Now we know the Milky Way is just one of billions of galaxies in the universe, if not more: NASA estimates there could be 2 trillion.

Spiral galaxies

The vast majority of the 300 million or so galaxies we have observed have been looked at using ground-based telescopes, and appear mainly as unresolved blobs. Where we can make out some detail, we see that around 60 per cent take on the distinctive form of a spiral galaxy: a flat disc of stars made up of a central bulge surrounded by arms arranged in a spiral shape. In normal spiral galaxies, these arms extend directly from the galaxy's core; barred spirals, meanwhile, have a central bar and the spiral arms stretch out from its ends (see "Galaxies galore", page 42). Spiral arms only form in galaxies that are disc-shaped, but exactly why some galaxies are spirals and some aren't isn't fully understood. Some form arms because of a nearby source of gravity, but not all spiral galaxies have such a mass nearby. It is thought that spiral galaxies evolve into elliptical galaxies.

Elliptical galaxies

About a third of all the galaxies we have seen are classified as elliptical. These usually form when spiral galaxies merge together, so their shape can vary depending on the ways they merge and collide – some look almost circular, while others are much more elongated. The stars in elliptical galaxies tend to be older than in spirals because of this.

Irregular galaxies

Galaxies that don't have a clear spiral or elliptical structure are called irregular galaxies. Most of these are dwarf galaxies, small galaxies composed of a few billion stars, and they are more easily pulled apart by external gravitational forces. However, some regular-sized galaxies are irregular in shape, too, which is usually as a result of collisions with other galaxies.

Satellite galaxies

Some dwarf galaxies are held within the gravitational field of a nearby, larger galaxy. The Milky Way has 14 confirmed satellite galaxies, including the Large Magellanic Cloud, the Small Magellanic Cloud and the Sagittarius dwarf galaxy. The last of these is thought to have collided with the Milky Way at least three times in its history, and one of these events may even have triggered the formation of our solar system.

The interstellar medium

The vast space between stars within a galaxy isn't totally empty, even though parts of it are the closest thing to a vacuum that we know of in the universe. In the interstellar medium there is an average of one atom in every cubic centimetre of space, a tiny fraction of the 90 million trillion atoms found in the same volume of air at ground level on Earth.

Most of the atoms in the interstellar medium – about 99 per cent – are hydrogen atoms from dying stars. But over the past decade or so, we have spotted an ever-growing menagerie of other atoms and molecules, including helium hydride, one of the first molecules predicted to form in the universe from reactions between hydrogen ions and neutral helium atoms, and argonium, formed of hydrogen and the normally unreactive noble gas argon.

One striking fact is that the interstellar medium is everywhere, indicating that galaxies don't form new stars at a high enough rate to deplete its diffuse contents. Studying processes within it can therefore help us understand how stars form – and how they don't.

Supermassive black holes

Most galaxies are believed to host a black hole millions of times the mass of the sun at their centre. These supermassive black holes are thought to be created by the merger of smaller astrophysical black holes (see page 40). But we have spotted some in the early universe, when it was just 700 million years old. This is a problem because, according to our models, supermassive black holes shouldn't have been able to form so fast.

Mostly, we infer the presence of supermassive black holes from the gravitational effects on their surroundings. In 2019, however, astronomers at the Event Horizon Telescope released the first ever picture of a black hole, the one at the centre of the M87 galaxy. In May 2022, the same group released a picture of Sagittarius A*, the black hole at the centre of the Milky Way.

It is easy to portray supermassive black holes as voracious monsters consuming all matter within reach, but that isn't necessarily the case. "Very often supermassive black holes don't do much, sitting passively in the heart of their host galaxies for billions of years," says Philippa Hartley at the SKA Observatory, a project to build the world's largest radio telescope.

Active galactic nuclei, quasars and blazars

Sometimes matter spirals into supermassive black holes, where it is promptly shredded and superheated. This results in active galactic nuclei, some of which throw out giant jets of charged particles that stretch beyond the host galaxy. They make for a dramatic show, if we happen to be in the firing line.

When they were originally discovered as bright, point-like objects in radio frequencies, active galactic nuclei received the name "quasi-stellar radio object" or quasar. Now, this refers to any particularly bright active galactic nucleus, while those with jets angled towards Earth are called blazars.

THE UNIVERSE BEYOND

Galaxies don’t exist in isolation. Taking a more expansive view reveals the overall structure of stuff in the universe – providing clues as to its origin and evolution.

Cluster, supercluster

If you could zoom out of the Milky Way, you would start to see the Local Group – a collection of at least 80 galaxies set in a dumb-bell shape. At one end is the Milky Way and its satellite galaxies, and at the other is our closest large neighbour, Andromeda, and its satellites.

Zoom out a bit further and you would see that the Local Group is next door to a cluster of thousands of galaxies called the Virgo Cluster. That cluster and the Local Group are both, in turn, part of a much larger structure that is more than 100 million light years across and contains another 100 groups of galaxies, called the Virgo Supercluster.

Astronomers believe there to be some 10 million such superclusters in the observable universe. And yet a study in 2014 indicated that the Virgo Supercluster is part of an even bigger supercluster called Laniakea, showing that the universe is ordered on a much larger scale than we originally thought.

The cosmic web

The structure of the universe doesn't stop at superclusters. Zooming out from the Virgo Supercluster, you would see the Pisces-Cetus Supercluster Complex. This is a galaxy filament, or a vast thread of superclusters. At its largest scales, the universe is made up of these filaments, which spread out like a web with voids of space in-between.

The cosmic web is the largest known structure in the universe, and we observed it directly for the first time in 2019. Its structure is thought to have been governed by strings of dark matter that attracted normal matter, in the form of superclusters, to congregate along the filaments. The voids in-between contain few or no galaxies, and can stretch for distances of 30 to 300 million light years.

Lyman-alpha blobs

While filaments in the cosmic web are the biggest structures in the universe, they can be broken down into smaller galaxy clusters and galaxies. The biggest individual objects in the universe are huge clouds of gas, some of which are more than 400,000 light years across. Called Lyman-alpha blobs after a spectral line of hydrogen seen in the light they emit, the process that makes these huge clumps of hydrogen gas is a mystery.

The cosmic microwave background

After the big bang, the universe was so hot that all the electrons escaped their protons and the universe therefore became a huge lump of plasma. At this time, photons couldn't go very far without being scattered by electrons, making the universe "opaque". Then, about 370,000 years after the big bang, the universe cooled enough that the electrons became bound to protons, creating neutral hydrogen atoms in a process called recombination. After this happened, the photons could travel freely. We still see those photons today, in a faint source of radiation called the cosmic microwave background.

Fast radio bursts

In 2012, astronomers using the Arecibo radio telescope on Puerto Rico spotted a strange signal – a short, milliseconds-long shower of radio waves coming from outside the galaxy. The signal, which in 2018 was confirmed to be coming from a quasar, was one of the first detected fast radio bursts. Since then, hundreds more have been spotted. Some flash once, others are "repeaters". Explanations for these strange signals have ranged from aliens to black holes and moving clouds of dust.

Gamma-ray bursts

These blasts of high-energy radiation make their way towards Earth from all directions and are thought to be triggered by certain stars exploding in supernovae. Gamma-ray bursts are the most energetic explosions since the big bang, and were first spotted in the 1960s when the US was searching for signs that the Soviet Union was attempting secret nuclear tests.

Oddballs

The Great Attractor

A few decades ago, astronomers looking for elliptical galaxies noticed that the Milky Way and other galaxies were being pulled towards a concentration of mass called the Great Attractor, which lies at the centre of the large Laniakea Supercluster. It is estimated to contain more than 10 million billion times the mass of the sun and lies about 150 million light years away.

Estimated, because the Great Attractor lies in what is known as the Zone of Avoidance – a portion of the sky that is obscured from view by the dust and stars within our own galaxy. This makes the Great Attractor a tricky object to study, and what it is remains a mystery.

The Great Wall

Also known as the Hercules-Corona Borealis Great Wall, the Great Wall is the largest known structure in the observable universe that is made up of groups of galaxies. It is 10 billion light years in length, more than 10 per cent of the width of the observable universe itself.

It is the largest of several such walls observed in recent years that challenge the cosmological principle. This says that, viewed on a large enough scale, the universe looks the same on average. It was introduced as a simplifying assumption to tame the fiendishly complex equations of Albert Einstein's general relativity, which govern the make-up and evolution of the universe. If it were to fall, the consequences would be huge. "The cosmological principle is the foundation upon which the theoretical framework of cosmology is assembled," says at the University of Central Lancashire, UK, who discovered two of the biggest walls we have ever observed.

Unknown

Dark matter

More than 80 per cent of the matter in the universe comes in a form we can't see. The existence of this "dark matter" was postulated in 1922 when astronomer Jacobus Cornelius Kapteyn studied the velocities of stars. Then, in the 1930s, astronomer Fritz Zwicky came to a similar conclusion when studying galaxy clusters. Observations by Vera Rubin in the 1970s confirmed something similar about the rotations of individual galaxies, suggesting galaxies are kept together by an encircling halo of dark matter.

There are lots of suggestions for what dark matter could be, from an invisible sea of WIMPS, or weakly interacting massive particles, to MACHOS. These massive astrophysical compact halo objects would be much meatier bodies made of ordinary matter, perhaps primordial black holes – a theoretical type of black hole thought to have been created in the universe's first few moments. But dark matter's true nature remains a mystery. "We know a bunch of things that it isn't," says Seshadri Nadathur at the University of Portsmouth, UK.

Dark energy

Dark matter far outweighs normal matter, but dark energy outguns both of them together. The latest figures suggest it accounts for 68 per cent of all the stuff in the universe. Just don't ask what it is, because we have no idea.

What we can say is what dark energy does. If dark matter is an invisible glue holding galaxies and galaxy clusters together, dark energy works in the opposite way, accelerating the growth rate of the universe and making it expand ever faster as time goes on.

Dark energy was discovered in the late 1990s through observations that showed distant supernovae were further away than we expected, and its effects have been confirmed by, for example, studying temperature patterns in the earliest radiation in the universe, known as the cosmic microwave background. One possibility is that dark energy is created by quantum particles popping in and out of existence, while another is that it is caused by a "quintessence" – a completely new force of nature we haven't yet discovered. A project called the Dark Energy Survey hopes to find some answers by better mapping distant galaxies and the history of the universe's expansion.