Bob the Alien's Tour of the Solar System
Bob the Alien's Tour of the Solar System

The Solar System

The Solar System is the Sun and the many thousands of objects that travel around it. These objects include eight planets, at least five dwarf planets plus thousands of asteroids, meteoroids and comets. It is your neighbourhood in space!

All of the objects in the Solar System travel around the Sun in paths called orbits. Some of these orbits are quite round, such as those of the main planets, whereas others are more oval, or ellipitcal, in shape, such as those of comets and dwarf planets. The Sun keeps its surrounding objects in its orbit by its pull of gravity which has an influence for many many millions of miles. Orbiting most planets and dwarf planets are objects called moons. Some planets have a small number of moons, others have lots. The Solar System is believed to be about 4.6 billion years old. It is situated in a galaxy called the Milky Way. So, without further delay, let's meet your neighbours...

The Sun

The Sun

First up is the Sun. This is a star, a large ball of gas situated slap bang in the middle of the Solar System. The Sun produces heat and light and is the only natural source of light in the Solar System. Without it, we'd all be pretty much scuppered. The Sun is by far the largest object in the Solar System, being 109 times wider than Earth. About 98% of the Solar System's total mass is contained within the Sun. It generates its power by converting hydrogen into helium in a process called nuclear fusion. Temperatures at the surface of the Sun reach a balmy 5,500 °C (9,900 °F), although there are darker and cooler patches on the Sun called sunspots where temperatures are anywhere between 2,700 °C to 4,200 °C (4,900 °F to 7,600 °F). Every now and again, the Sun spews out Solar Flares, rapidly sending out bursts of energy across the Solar System. On Earth, Solar Flares cause auroras near the North and South Poles, which are beautiful light displays known as the Northern Lights and the Southern Lights. The Sun is a faily average yellow dwarf star, and should remain as one for about 9 billion years. Over 4 billion of those years have already passed, so it is nearly half way through its main life. Eventually, as its energy supplies begin to run out, it will get even larger and even hotter and become a red giant. By then, it is likely that it will become large enough to absorb some of the planets closest to it. That'll be my cue for introducing the planets....

The Planets

There are eight regular planets in the Solar System. In order of distance from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four planets closest to the Sun are known as the Inner Planets, and the four furthest from the Sun are the Outer Planets. The Inner Planets are also sometimes known as the Terrestrial Planets because they all have solid surfaces, whereas the Outer Planets are sometimes called the Gas Giants because they are big and gassy. Must have been something they ate. All four of the Inner Planets are smaller than all four of the Outer Planets.


Mercury is the smallest of all of the eight regular planets in the Solar System. It has a cratered surface resembling that of Earth's Moon, and not a great deal of atmosphere. Not the kind of place you'd like to take your family out for lunch. As the closest planet to the Sun, temperatures reach a scorching 427 °C (800 °F), but over on the side away from the Sun, it's a mighty chilly -183 °C (-297 °F). Most definitely scarf and gloves weather. Some of Mercury's craters never ever see sunlight, but spacecraft that have been sent to observe Mercury have discovered that lurking within these craters is frozen water. Mercury whizzes around the Sun, taking only 88 days to get around it. Mercury has no moons.


Venus is the second closest planet to the Sun and the sixth largest of all of the Solar System's planets. It is the brightest object in the sky after the Sun and the Moon, appearing to be a beautiful and tranquil world from a distance. Some have even believed it to be Earth's twin. But get up close if you dare, and you'll discover that Venus has poisonous clouds of sulphuric acid and carbon dioxide, a surface of volcanic ruins, and suffocating temperatures of 480 °C (896 °F). All day, every day. Interestingly though, Venus's day is longer than its year, with the planet taking 243 days to turn once on its axis, but 224 days to complete an orbit of the Sun. Like Mercury, Venus is also a bit of a loner and has no moons orbiting it.


Earth is the next planet in order of distance from the Sun. It is the largest of the Inner Planets, and fifth largest of all of the Solar System's planets. Earth is quite a special little world, being the only place in the Solar System where life is known to exist. It is your home planet. 70% of Earth's surface is covered in water, and its atmosphere contains oxygen which is essential for the survival of many lifeforms, humans included. Earth is situated in what is referred to as the Goldilocks Zone, an area where bears live in cottages and eat porridge for breakfast. Ignore the bit about bears. Earth is just the right distance away from the Sun that it isn't too hot and it isn't too cold, so the planet's climate and chemical make-up is ideal for the development and ongoing existence of life. For a long time, dinosaurs were the dominant spieces on Earth, surviving on the planet for about 150 million years before they were wiped out, possibly by the collision of a large asteroid with Earth. Nowadays, human beings share Earth with countless other spieces of life, which exist in the seas, in the sky, underground, overground. As Earth takes its 365 day long trip around the Sun, it is kept company by one moon. Known by most as The Moon, and by some as Luna, it is the only other surface in the Solar System that has been stepped on by human beings, when 12 astronauts took 6 journeys to it in the late 1960s and early 1970s.


Mars is the final of the four Inner Planets and the second smallest of all planets. It is about a quarter of the size of Earth and is a popular destination for unmanned space missions from Earth. This is because, one, it isn't quite as inhospitable as Venus and two, it's possible that life could survive on it, either now or sometime in its dark and distant past. We know that water did once flow freely on Mars, and that volcanoes once erupted happily there too. That's not saying that Mars is an ideal holiday destination. Its average surface temperature is -63 °C (-81 °F), and its atmosphere is mostly Carbon Dioxide, which most Earth-based lifeforms don't particularly like to breathe in. Despite this, Mars is undoubtedly next in the list of places that Man (or Woman) will visit in the Solar System. When this will happen, nobody knows, but it will happen one day. Mars has a red surface and pink skies, and can easily be observed from Earth. Mars takes 687 to complete an orbit of the Sun, and has two small moons orbiting it. These moons are named Phobos and Deimos.


Jupiter is the first of the Outer Planets and is the largest planet in the Solar System. It is mostly made up of hydrogen and helium gas, but contains a load of other gasses too. When observing Jupiter, we can only really see its upper atmosphere which is divided into bands which are different temperatures, contain different chemicals and have winds that go in opposite directions to the ones next to them. There is one particular area of Jupiter where a storm has been raging for hundreds of years. This is the Great Red Spot which is twice as large as Earth. Nobody really knows why it is red! Jupiter has at least 79 moons orbiting it, including the Solar System's largest moon (Ganymede), the most volcanic moon (Io) and one that may be entirely covered with water under a surface of ice (Europa). Jupiter takes 9 hours and 55 minutes to turn on its axis, meaning that its day is the shortest of all planets, but takes almost 12 years (11.8 years) to get around the Sun.


Saturn is the second largest planet in the Solar System. It is similar in make-up to Jupiter, but isn't quite as stormy. What makes Saturn stand out though are its rather splendid rings. These rings are layers of rock and ice that orbit the planet. Like Jupiter, Saturn also has lots of moons (82 at the last count), including Titan which has a thick atmosphere, and Enceladus which erupts plumes of water particles, creating one of Saturn's rings and suggesting that there is lots of water on it. Saturn takes 29 years to orbit the Sun.


Uranus is the third largest planet, a featureless world which orbits on its side. It has 27 moons which are mostly named after characters in Shakespeare's plays. Uranus was the first planet to be discovered as all of the other closer planets have been observed from Earth for thousands of years. It was discovered in 1781 by English astronomer, William Herschel. Uranus takes 84 years to orbit the Sun.


Neptune is the most distant of the eight planets in the Solar System. It was discovered in 1846 by German astonomer, Johann Gottfried Galle. Neptune is a deep blue colour, with darker patches representing stormy areas that come and go over time. Neptune is thought to be the windiest planet in the Solar System. Neptune has 14 known moons and takes 165 years to get around the Sun, meaning that since it was discovered it has only completed one journey around the Sun.

The Asteroid Belt

Separating the Inner Planets from the Outer Planets is a region of thousands of asteroids called The Asteroid Belt. Asteroids are rocks, some small, some very large. Most asteroids are irregularly shaped, appearing like fragments of rock. It is thought that the Asteroid Belt and its contents may be the result of a planet that didn't form. Planets form as rocks collide with each other, increasing their pull of gravity to attract more rocks to them, and eventually growing to be fully fledged planets, meaning one that is spherical and clear of all of the bits of rock around them. For whatever reason, possibly something to do with Jupiter's own formation, the objects in the Asteroid Belt couldn't really get together to form a planet. So they remained as rocks floating around.

Some objects nearly did get there, such as Vesta, Pallas and Hygiea, asteroids which are fairly large but nowhere near planet-sized or planet-shaped. Vesta, the largest, is only 525 kilometres (326 miles) in diamater, which is tiny even when compared with the Solar System's smallest planet, Mercury, which is 4,878 km (3,031 miles).


However, one object in the Asteroid Belt which was once considered a planet, then an asteroid, and now a dwarf planet, is Ceres. Ceres, discovered in 1801 by Italian astronomer Giuseppe Piazzi, is a round object and the largest in the Asteroid Belt. It is 950 km (590 miles) in diameter. Ceres accounts for about a third of the total mass of the Asteroid Belt. A recent visit to Ceres by the Dawn spacecraft observed a couple of bright spots on its surface which might be ice volcanoes, which sounds rather exciting, or possibly some sort of salt, which isn't quite as exciting.

Not all Asteroids in the Solar System exist in the Asteroid Belt. Some Asteroids can be found travelling around other parts of the Inner Solar System in their own unique orbits. Other ones may get disturbed from the Asteroid Belt, either through a collision with another one, or by the effects of gravity of other planets. Of course, when asteroids go rogue,  there is a risk that they could collide with other planets, including Earth. Astronomers keep a look out for Near Earth Asteroids with the hope that none of them have Earth in their sights. If they do, it's either a case of sending Bruce Willis up to it, or life on Earth going the way of the dinosaur.

Dwarf Planets

There are five objects in the Solar System which are classed as dwarf planets. These are Ceres, Pluto, Haumea, Makemake and Eris. dwarf planets are usually spherical worlds which orbit in regions of other objects. Although there are are currently five dwarf planets, there are lots of other objects which are likely to fall into the same category. All we need are two more and then we have the seven dwarfs.

Ceres, mentioned above, is the only dwarf planet that orbits within the orbits of the main planets in the Solar System, which means that it is the closest dwarf planet to Earth. It is situated in the Asteroid Belt and takes 4 years and 220 days to orbit the Sun. Ceres is the smallest of the dwarf planets.


Pluto, the largest of the dwarf planets, was discovered in 1929 by American astronomer Clyde Tombaugh. It was initially classified as a planet and remained the ninth planet in the Solar System until 2006 when it was reclassified as a dwarf planet. It is so far away that it takes a whopping 248 years to get around the Sun. Although Pluto is further out than Neptune, it has an elliptical orbit which means that its orbit is sometimes closer to the Sun than Neptune's. The last time this happened was between 1979 and 1999. Pluto has 5 moons, including Charon which is almost as large as Pluto itself. Pluto was visited by the New Horizons spacecraft in 2015.

Haumea was discovered in 2004 by a team of astronomers at Palomar University in sunny California, USA. Haumea is shaped more like a rugby ball, or an American football, than a sphere like other dwarf planets. Haumea spins on its axis very quickly, taking only 4 hours to do so. It takes 284 to complete an orbit of the Sun. It is known to have two moons.

Makemake was discovered in 2005 by astronomers at Palomar University. A moon of Makemake was discovered in 2015. Not a lot is known about Makemake because it is so far away and quite small, probably up to about 1,900 km (1,180 miles) in diameter. It is known to have one moon.

Pluto, Haumea and Makemake all exist in a region called the Kuiper Belt. This is an area beyond the orbit of Neptune containing lots of rocks. It is similar the Asteroid Belt but is about 20 times wider.

Eris was discovered in 2005 and is the second largest of the dwarf planets. When it was discovered it was thought to be larger than Pluto, although Pluto was discovered to be larger when New Horizons measured it in 2015. It was the discovery of Eris that led astronomers to create the dwarf planet category. This is because they didn't really want to have to call Eris a planet seeing as it didn't quite fit in. But they needed to call it something, so they decided to call it a dwarf planet. And because it shared similarities to Pluto, Pluto too became a dwarf planet. Eris' orbit reaches an incredible 14,634,000,000 km (9,088,000,000 miles) from the Sun. It takes 557 years to get around the Sun. Eris has just one moon. Eris exists in the scattered disc, a region beyond Neptune that stretches out even further than the Kuiper Belt but contains a smaller number of objects which tend to have highly eccentric orbits, orbits that can be disturbed the gravitational influences of the planets. It's all very confusing.


Hale Bopp

Sometimes called dirty snowballs, comets are balls of rock, dust and ice that orbit the Sun in all kinds of weird and wonderful orbits. They are thought to originate from the deepest depths of the Solar System, from an area known as the scattered disc, or even further out in the Oort Cloud. They travel in elliptical orbits, usually without being visible to most observers. But, as they get closer to the Sun and heat up, the ice and dust starts to melt and burn away, producing a tail. From Earth, comets can look like blurred stars. Comets like Halley's Comet appear quite regularly and are known as short period comets. Halley's Comet last came close enough to the Sun to be visible in 1986, and will do the same again in 2061. Others take hundreds, even thousands of years, to get around the Sun. These are known as long period comets. Comets can collide with other objects in the Solar System. Comet Shoemaker-Levy 9's journey through the Solar System ended in 1994 when Jupiter got in its way. Whoops!

Meteoroids, Meteors and Meteorites

Meteoroids are small rocks that have broken off from other objects like comets or asteroids, and sometimes even planets or the Moon. They are usually no wider than 1 metre but can be as small as a grain of rice. Meteoroids spend most of their time happily travelling through space, but, as there are so many of them, they regularly collide with planets and moons. Meteoroids heading for Earth enter the atmosphere at great speeds. As they travel through the air, they get hotter, start glowing and begin burning away. They appear like a star and can leave a trail. When they are in this state, they are called meteors, but you may also know them as shooting or falling stars. Sometimes Earth passes through a region of lots of meteoroids , which results in a lot of them entering the atmosphere and causing a meteor shower. Most meteoroids completely burn up before reaching the surface of Earth, but some do manage to reach solid ground. When they do, they leave an impact crater. And what remains of the rock from space is then known as a meteorite.

Meteoroids don't just hit Earth. The impact craters that can be seen on the surfaces of most moons and on planets like Mercury and Mars are caused by meteoroids. The lack of atmosphere on such objects means that the craters remain there for millions, even billions, of years. Meteroids colliding with other planets can cause parts of that planet to be blasted off into space, and for those bits of debris to become meteoroids. In facts, some parts of Mars have managed to reach the surface of Earth, giving scientists an opportunity to observe Martian rock without even going there.

Very very tiny bits of space rock are called micro-meteoroids or space dust. Although quite harmless to Earth, they can be dangerous to satellites and spacecraft in space as they can cause numerous very small impacts called micrometeorites.

Anything Else?

The summary of the Solar System on this page is what is currently known about it. There is still so much that isn't yet known about the Solar System and many discoveries still to be made. When this website first began in 2000, Pluto was still a planet, Jupiter was thought to have only 13 moons (it is now known to have at least 67!) and the dwarf planets Haumea, Makemake and Eris hadn't even been discovered. The spacecrafts Spirit, Opportunity, Phoenix and Curiosity have all successfully landed on Mars, Cassini-Huygens entered orbit of Saturn, and Huygens even sent pictures back from the surface of Titan. Messenger went to Mercury, New Horizons went to Pluto and Venus Express went to Venus. What discoveries lie in the future? There may be more planets in the Solar System, Earth may be found not to be the only place where life can exist. Keep your eyes on the skies because who knows what lies out there?

Of course, your solar system is just one of potentially billions of solar systems in the universe. The Solar System is just the family of objects that orbit the Sun. The Sun is just one star, one very ordinary star. But when you look into the night sky, you can see thousands of stars. And there are billions more that you can't see. Billions of stars just like your own. And that's just in your own galaxy, the Milky Way. Beyond the Mily Way galaxy are billions of other galaxies, each containing billions of stars. The search for other solar systems is a relatively new thing as it requires state of the art technology to be able to observe stars which are millions of miles away from Earth. The first detected exoplanet, a planet in orbit of another star, was recorded in 1992, but it is already known that almost 3,000 other stars have planets orbiting them, and so far almost 4,000 exoplanets have been detected. Some of these planets are believed to exist in the Goldilocks Zone, that region around stars where conditions are thought to be just right for life to exist. Is there another Earth out there?

Bob the Alien's Tour of the Solar System
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