Our understanding of our place in the universe has been evolving over many centuries, both with the use of naked-eye observations and more recently with the aid of increasingly sophisticated scientific equipment.
To gain an awareness of what we can actually view in the universe, it’s helpful first to get a sense of what is out there. Let’s take a tour, travelling out from our inner solar system to the edge of known space. Not all the objects mentioned in this section are viewable from Earth, even with the best telescopes, but knowing what lies in the heavens will gives us a better understanding of our universe and our place in it.
THE PLANETS
We’re all familiar with our home planet, Earth, but this is just one of eight planets that are in orbit about the Sun.
MERCURY
The closest planet to the Sun is Mercury, named after the Roman messenger of the gods, which is the smallest planet in our solar system and is a lot hotter than Earth (its daytime temperature can reach 427°C/800°F). It doesn’t have a moon. Not much is known about Mercury – it’s thought that if it did ever have an atmosphere, it would have been torn away by the solar wind (charged particles that flow from the Sun’s surface).
We will soon get our first close-up glimpse of Mercury with the European Space Agency space mission, Bepi Columbo, due to be launched in 2017.
VENUS
The second planet from the Sun is Venus, and is seen as one of Earth’s twins in terms of size and mass – but there the similarities end, as Venus is a fiery planet covered in possibly active volcanoes with scorching temperatures. Most of us will have observed Venus, but many may not have recognised it as a planet. Due to its closer orbit of the Sun, Venus is often seen as a very bright ‘star’ just before sunrise or just after sunset. Historically, Venus was associated with beauty and love – but it was not until 1970, when the Russian Venera 7 robotic space probe landed on its surface, that we discovered a barren planet with a corrosive atmosphere. After a hard impact on landing due to a malfunctioning parachute, the probe valiantly transmitted for 23 minutes before succumbing to the inhospitable atmosphere, with a temperature of around 467°C (872°F), and pressure 90 times that of the Earth’s surface. Venus is even hotter than Mercury, despite the fact that it lies further from the Sun, because the Venusian atmosphere is 96 per cent carbon dioxide*. Like Mercury, Venus does not have any moons.
* A greenhouse gas found on Earth that traps infrared radiation from the Sun in the atmosphere, elevating the temperature.
The Earth is the third planet from the Sun, and has a single moon in orbit about it. From space it looks like a beautiful blue marble (in fact, this is the title of a famous photograph taken of Earth from the Apollo 17 spacecraft in 1972), thanks to its vast oceans (water covers about 71 per cent of the Earth’s surface). It is thought that it was an image like this that triggered the environmental movement, suddenly seeing our planet alone and vulnerable in space. The Earth is the densest planet in our solar system, but only the fifth largest in terms of diameter, at 12,742km (7,918 miles).
MARS
Beyond Earth lies the planet Mars, named after the Roman god of war, with its two moons, Phobos and Deimos (the Greek names for the twin sons of Venus and Mars; not surprisingly, their names are the ancient Greek for fear and terror). Mars is the fourth planet from the Sun, and the second smallest planet in our solar system. It is seen by many scientists to be the Earth’s twin in terms of its environment, as its temperature of -56°C (-70°F) is the closest of any planet in our solar system to that of the Earth’s. Scientists also think Mars may have had a large ocean at one time, which is another similarity with the Earth. Mars lies in the cusp of a so-called Goldilocks Zone, which is a potentially habitable region of space that broadly replicates the relationship between our own Sun and Earth; therefore the conditions of the planet (or planets) can support water on its surface. Evidence studied by planetary geologists indicates that Mars did indeed once have water flowing over its surface, but this is no longer the case. Water found on the planet is frozen into the ground or sits as surface ice at the northern and southern polar caps. Why the atmosphere of Mars changed so radically remains a mystery, but it is one of the questions that the many exploration orbiters and rovers on and around the planet have been sent to find an answer to.
JUPITER
At this distance from the Sun we enter the region of the gas giants, Jupiter and Saturn, so-called because they are composed of helium and hydrogen. Jupiter, named after the Roman king of the gods, is the first that we encounter. It is the fifth planet from the Sun and the largest planet of our solar system (at a whopping 142,797km (88,730 miles) in diameter it is just over 11 times the size of the Earth, and in terms of volume you can fit 1,000 Earths into one Jupiter). Jupiter can be seen as Earth’s defender: as thanks to its size and mass, it helps protect the Earth by gravitationally attracting objects that could otherwise hit our planet. Jupiter has around 65 moons in orbit about it.
SATURN
Beyond Jupiter lies Saturn, named after the Roman god of agriculture, a beautiful planet surrounded by vibrant rings, which are made of tiny particles of mainly water ice. It is the sixth planet from the Sun, and the second-largest planet in our solar system. Scientists have been learning more about this mysterious, gaseous planet through the Cassini-Huygens space mission. Saturn has over 50 confirmed moons, but more potential candidates are being investigated. The Huygens space probe landed on one of the larger moons, Titan, and sent back amazing images of the surface.
URANUS AND NEPTUNE
The two outer planets of our solar system are Uranus and Neptune, sitting seventh and eighth respectively. Uranus was the Roman god of the sky, while Neptune was the god who ruled the seas. Often known as the ice giants because of their icy composition, these sentinels of our solar system sit a long way away from where we live and not much is known about them. They have mainly been studied via the Voyager spacecraft and recently the Hubble Space Telescope. Uranus is the third largest planet in our solar system, and Neptune is the fourth largest.
BELTS AND CLOUDS
As well as planets, there are many other objects orbiting the Sun, most notably the Asteroid Belt, the Kuiper Belt, and the curious sounding Oort Cloud (named after the man who discovered it in 1950, the Dutch astronomer Jan Oort).
Just beyond Mars lies the asteroid belt. Material in this region comes in many different shapes and sizes. It consists of billions of asteroids, which range from pieces of very large rock (some of the largest are around 240km/150 miles in diameter) to clumps of rubble held together by gravity. They are thought to be part of the detritus left behind during the formation of the main planets. The gravitational pull of Jupiter is thought to have stopped these objects from forming a planet. However, the asteroid belt has one dwarf planet, Ceres. A dwarf planet is an astronomical object that orbits a sun, is large enough to be approximately spherical in shape, owing to its gravitational mass but, unlike a planet, it has not cleared all the material in its orbit. To date scientists have discovered five confirmed dwarf planets in our solar system – Pluto*, Ceres, Haumea, Makemake and Eris – but there may be as many as 50 more out there. In early 2015, Ceres caused excitement when the Dawn Space Probe took pictures of its surface, revealing two intriguing bright spots. Scientists think they may be sunlight reflected from patches of surface ice, but no one knows yet.
* Pluto was reclassified due to its small size.
KUIPER BELT
Similar to the Asteroid Belt but much wider and more massive, the Kuiper Belt sits out beyond Neptune. It is named after the Dutch-American astronomer Gerard Kuiper (1905–1973). The largest object in the Kuiper belt is Pluto, which was thought to be a planet but has been recently reclassified. The Kuiper Belt is also thought to be the origin of the comets we see in our night skies. Comets are icy bodies made of water, rock and carbon-based materials known as organics. Every so often a comet gets nudged out of position in its orbit in the Kuiper Belt by the interactions of the giant planets. It is then sent into an elliptical orbit and is eventually pulled in towards the inner solar system by the Sun’s gravity. As the comet draws closer to the Sun, it starts to melt, releasing dust and gases that form a comet’s impressive tail.
OORT CLOUD
Out beyond the Kuiper belt lies the Oort Cloud. This huge spherical cloud consists of up to 2 trillion icy bodies that stretch out to a distance nearly a quarter of the way to our Sun’s next-door neighbour Proxima Centauri, some 4.24 light years away. This cloud marks the limit of the gravitational reach of the Sun and therefore it can be considered to be the edge of our solar system.
We now enter a zone of space that lies between the stars. This region is called interstellar space. Out here there is not very much, but we do find occasional volumes of dust and gas. If these volumes are large and active enough, they are called molecular clouds, or nebulae. Stars are born in these clouds. If undisturbed, a nebula can remain stable, but if it is disturbed by a nearby gravitational event, then its matter starts to clump together due to the gravitational attraction between the particles. If the density of the material becomes high enough, a star is formed, sometimes with a planetary system around it.
THE STARS
Moving further out into space, the next thing we encounter are the stars we see in the night sky, similar to our own Sun. Some are larger and some smaller, some older and some younger. Stars, like people, go through life cycles and, just like people, their appearance changes at the different stages of that cycle. The life cycle of a star depends on its mass: i.e. the amount of stuff that it is made of. The more massive the star is the shorter its life cycle will be. Our Sun, which has an age of about 5 billion years, is of average size and is likely to be around for at least another 10 billion years. A star 10 times the mass of our sun is likely to exist only 30 million years and a star 50 times the mass of the Sun will only survive for around 5 million years.
Stars shine brightly due to a process called nuclear fusion that is happening at their core. This is when atoms fuse together to form new elements. Most stars start off as a gaseous nebula made up of mainly hydrogen and dust particles, which are drawn together by gravitational forces. Once the core of the infant star is dense enough, hydrogen atoms fuse together to make helium through nuclear fusion. This fusing process releases amazing amounts of energy and stops gravitational forces from making the star collapse in on itself.
The energy released from the surface can be detected as various forms of radiation. One type of radiation is visible light, which we can see with our eyes, and this is the reason why we can see stars as bright objects. Other energy released includes infrared, ultraviolet, radio waves and X-rays, to name but a few.
THE SUN
The life cycle of a star is very dependent on the fusion that is taking place at its core. Take our Sun – it’s about halfway though its life cycle and is consuming the hydrogen, leaving behind a growing core of denser helium. As the Sun continues to age the hydrogen will become more and more depleted and the helium core will start to collapse in on itself. When this happens the pressure at the core will increase. At some point the helium core will reach a critical pressure where further fusion of helium into other heavier elements is possible. This causes a release of energy that forces the outer layers of the Sun to expand outwards and cool. At this stage the Sun will become a red giant, so-called because the appearance of a red giant is a luminous red. The expansion of these layers will eventually become large enough to subsume all the planets of the inner solar system (Mercury, Venus, Earth and Mars), but don’t worry: this will not happen for many billions of years!
While the outer regions of the Sun continue to expand, the helium nuclei in the core will continue to fuse into carbon. Now, when carbon is made, the pressure in the core is not enough to convert it into heavier elements, so no further fusion is possible. The Sun’s core will stabilise, but the outer layers will continue to expand and will eventually sit independently of the core. At this stage the core will form a white dwarf planet, and the now free outer layers will form a planetary nebula. This marks the final stage of the Sun’s life cycle.
A white dwarf is the final phase in the life cycle of a star the size of our Sun, but for larger stars the end can be a lot more dramatic. Stars some 10 to 100 times bigger than our Sun can continue fusion past the carbon phase and make heavier elements such as neon, oxygen, silicon and eventually iron. For these massive stars, their final stages result in a huge explosion of energy known as a supernova. This release is so bright that we can detect supernova events occurring in distant galaxies. Left behind in the core of a supernova can be either a neutron star or a black hole.
NEUTRON STARS
A neutron star is an object with a mass a few times more than that of our Sun, but with a radius of only 10km (6 miles). This object is so super-dense that a teaspoon of this matter on Earth would weigh as much as a mountain!
BLACK HOLES
The alternative to a neutron star, a black hole, is one of the great enigmas of our universe. This is a body so dense, and with such a super-powerful gravitational field, that not even light can escape from its gravity. As light is not emitted from this region it should make it very hard to detect, but this is not the case. Because of their strong gravitational field black holes disturb the path of many neighbouring bodies, providing us with clues to their location.
EXOPLANETS
One of the most exciting developments in astronomy in recent years has been the detection of extrasolar planets or exoplanets. These are planets that are in orbit about distant stars. To date, approximately 2,000 exoplanets have been detected and scientists are expecting to find more. One of the questions that arises in the study of exoplanets is, ‘Is there life out there?’ So far, no life has been found, but with more of these planets being discovered, and with better telescopes available in the future, it may be possible to find indications of life on another planet.
GALAXIES
When we encounter stars in the universe we are most likely to find them in large clusters called galaxies. Our solar system exists in a galaxy called the Milky Way, which contains approximately 200 billion stars. All of the stars that we’re able to see in any detail belong to the Milky Way. Galaxies vary in size and shape; one of the most common shapes is the spiral. The Milky Way is a spiral galaxy and our Sun sits in one of its spiral arms.
The cluster of stars that make up our galaxy can be seen quite clearly in the northern night sky if the night is very clear and there is little light pollution around. It’s much easier to spot the Milky Way, in the skies of the southern hemisphere, as from this viewpoint you look straight into the centre of the galaxy where there is a greater abundance of stars. The Milky Way, as the name suggests, looks like a broad, silvery or ‘milky’ pathway across the darker night sky.
Finally, to end our journey through the firmament we need to discuss the universe. The universe contains all space, time, matter and energy. Scientists currently believe that the universe began some 13.8 billion years ago and started with something that we call the Big Bang. The theory of the Big Bang was postulated to explain the observed phenomena that most of the visible stars and galaxies in the universe seem to be moving away from us. Logically, if they are moving away from us now, going back in time they must have all converged in a single point from which they originated – the Big Bang. Although the Big Bang idea is still just a theory, it has stood the test of time by best fitting the observations scientists have made to date.
So this is our playground, the universe. In the next chapters we will get an understanding of what we can observe and the best ways to do it.
MYSTERIES AND WONDERS OF THE UNIVERSE
Big Bang: Edwin Hubble made observations of distant galaxies and noticed that the further they were from us, the faster they seemed to be travelling away from us. This means that if we reversed time, all things in the universe would coalesce into a single point. From this the theory of the Big Bang was formed, suggesting that the universe started from a single point and then expanded.
Dark energy: When we observe the universe and its expansion we have noticed that it is expanding faster than expected. The cause is unknown but it has been given the term dark energy to describe the force that is expanding the universe. We currently believe that dark energy accounts for 68 per cent of the universe.
Dark matter: When we observe the movement of large bodies in space, such as galaxies, we notice that their movement isn’t consistent with the matter that we can see within that body. Many of the galaxies we observe should fly apart if following the laws of physics. As we believe that the laws of physics are universal, we need some other explanation of what is holding these bodies together. Nothing has yet been found but we call the invisible matter that we think is there but can not see, dark matter, as it does not interact with the electromagnetic spectrum. It is of great importance as it makes up around 27 per cent of the universe.
Black holes: A black hole occurs when a truly massive object collapses in on itself due to its own gravity. The resultant body has such a strong gravitational field that not even light can escape its clutches. Black holes have been observed in space by the effect that they have on other bodies in their surroundings. Although we cannot see them, we can see other bodies rotating around them and sometimes we can see matter from other bodies being pulled into them. It is thought that at the centre of every galaxy (including our own) there is a super massive black hole.
Multiverse: The multiverse is a mind boggling idea that posits that as well as this universe there is a multiplicity of other universes out there. It seems a bit crazy but many of the scientific theories that we are working on today predict the possibility of the multiverse.
Spacetime: Spacetime was a concept first proposed by Einstein’s teacher Hermann Minkowski. The idea was to take the three dimensions of space and link these with ‘time’ as a 4th dimension to form a continuum. Einstein used spacetime in his theory of general relativity to analyse gravity, a curvature in spacetime.
Wormholes: Wormholes are the friend of science-fiction writers. They are thought to significantly cut journey times across space by bending spacetime and were a prediction of Einstein’s theory of general relativity, but none have been found yet.
Gravitational waves: Predicted by Einstein’s theory of general relativity, a gravitational wave is a ripple in the curvature of spacetime which travels outwards from its source as a wave. These waves carry gravitational radiation. To date no gravitational waves have been detected but a number of space- and ground-based experiments have been set up to detect them.
Cosmic inflation: Through the Big Bang theory we believe that the universe has been expanding. The idea behind cosmic inflation is that the universe went through a period (less than a trillionth of a second) of massive expansion, growing from the size of a subatomic particle to the size of a grapefruit. It was used to explain the unevenness of cosmic background radiation and why we have the formation of clumps of matter in the universe.
