The Solar System
Planet Earth from Space Shuttle
Spectacular view of our home Planet Earth and the Sun as seen from the Space Shuttle
Solar System Facts
  • The solar system is around 4.6 billion years old.
  • At the center of the solar system is the sun, a yellow dwarf star which produces vast amounts of energy.
  • There are eight major planets and over 100 moons in the solar system.
  • Mercury, Venus, Earth and Mars are the small rocky planets. Jupiter, Saturn, Uranus and Neptune are the gas giants.
  • All the planets orbit the sun in an elliptical, oval shaped path.
  • Many of the planets in the solar system are visible to the naked eye.
  • Other objects in the solar system include dwarf planets, asteroids and comets.
  • The solar system is in a galaxy known as "The Milky Way".
  • It is estimated that at least a third of the 200 billion stars in the Milky Way are orbited by one or more planets.
  • The Voyager 1 spacecraft is the furthest man-made object in the solar system, it is around 11 billion miles (18 billion km) from the sun and is still sending data back to Earth.

Birth of the Solar System

early solar system
Material accreting around the sun in the early solar system
Our solar system began as part of a massive nebula cloud of molecular hydrogen and dust around 4.6 billion years ago. In a region of this dark cloud conditions allowed gravity to begin condensing the hydrogen until a substantial mass began to grow ever larger and hotter, eventually this mass collapsed in on itself, forming the early stage of a star called a protostar. The gravitational pull of the embryonic star caused a large disc of gas and dust to begin forming around it.
Over millions of years pressure began to build up inside the protostar as it became hotter and denser until eventually nuclear fusion began in its core, giving birth to the sun as we know it. During this time the disc of gas and dust that had formed around the star had also began condensing into ever growing orbiting bodies. In time they would go on to form the planets, moons and other objects in our solar system.

Planets of the Solar System

The nearest planet to the sun is Mercury, aptly named after the swift footed messenger god it orbits the sun in only 88 days, quicker than any other planet in the solar system. Next comes Venus, often referred to as Earth’s sister planet but underneath her thick clouds lies a hellish oven baked landscape with temperatures hot enough to melt lead. Third planet from the sun lies Earth, a striking blue sphere covered with oceans of liquid water and the only planet known to us which harbors life. The last of the rocky inner planets is Mars, consisting of a thin atmosphere this red body was once covered in oceans just like Earth but is now a desert where dust storms can engulf the entire planet.

Almost 800 million kilometers from the sun we find the first of the gas giants, Jupiter, enormous in its size it would take over a 1,000 Earths to fill its volume. Saturn is the second of the gas giants and is unlike any other planet in the solar system with its spectacularly colorful rings made from dust and ice particles. Then comes the first of the planets known as ice giants, Uranus, this large turquoise ball of gas lies tilted on its side with freezing atmospheric temperatures of -371 Fahrenheit. At 4.5 billion kilometers from the sun we find the last of the planets, Neptune, a blue ice giant where winds in the atmosphere reach over 2,000 kilometers per hour.

Moons of the Solar System

As mankind explores space we have found that some of the moons in our solar system are equally and at times even more fascinating than the planets themselves. Jupiter’s wonderfully colored moon Io is the most volcanically active object in the solar system, Saturn’s moon Titan has a thick nitrogen based atmosphere which rains methane producing liquid lakes on its surface. Another of Saturn’s moons Enceladus has volcanoes which erupt with water ice and possibly has a liquid ocean underneath its icy surface. Then there is the ice world of Europa, another of Jupiter’s fascinating moons, underneath its surface lies a vast ocean of liquid water which is 100 kilometers deep and possibly teeming with life.

Other Bodies in the Solar System

At the center of the solar system is our sun, a massive ball of hydrogen and helium over a hundred times larger in diameter than Earth producing immense heat and enormous explosions which jettison solar winds millions of miles into space. Between the orbits of Mars and Jupiter lies the Asteroid Belt, millions of rocks reside in this area some as large as 60 miles (100 km) in diameter. Beyond Neptune we reach the Kuiper Belt where we find most of the dwarf planets including Pluto, at one time considered the ninth planet in our solar system this tiny body orbits the Sun at an average distance of almost 6 billion kilometers. Comets originate in the Kuiper Belt or the even more far flung Oort Cloud, a massive spherical cloud of icy bodies that surrounds the solar system.

The Planets in Scale of Size

planets in scale of size

Life in the Solar System

Life on Europa
A future probe searching for life in the underground ocean of Europa
Human kind has often pondered whether life exists elsewhere in the solar system, and if so what kind of life? Up until the mid 20th century it was thought every planet could harbor some kind of life forms, possibly even more advanced than ourselves. In the 1930’s Orson Wells radio production of Jules Verne’s famous novel “War of the Worlds” caused widespread panic as listeners mistakenly thought it was real. This is an indication of how much the belief in advanced alien life forms from neighboring planets was held in the common psyche.

Of course now we know this is just science fiction and that we're not under imminent attack from an alien civilization on Mars or Neptune. So is there life anywhere in the solar system apart from Earth and who are the suspects? Well you may be surprised to learn that there are several candidates in our varied solar system.

Let's begin with Mars, the planet that captures our imagination the most. Even though we now know there are no canals on Mars or advanced civilizations, scientists involved in the Viking lunar landings still expected to find signs of life in the Martian surface. Viking 1 landed on Mars in June 1976 and analyzed samples of the Martian soil, but found nothing. Subsequent missions have also found no signs of life but there's still hope, and it lies underground. The reason for this theory is that surprisingly methane is present in the Martian atmosphere and one way methane can be produced is biologically. In the warmer summer months on Mars the presence of methane increases dramatically giving more credence to the idea that there are organisms living deep under the surface, possibly around hot vents.

There are two fascinating moons around Saturn which could possibly support life. Both are very different from one another, they are Enceladus and Titan. Enceladus is a very small icy world with an area only slightly larger than Texas but under its surface there is believed to be a salt water ocean. How can there be warm water in such a cold area of the solar system? Well the tremendous gravitational force of Saturn pushes and pulls the small moon heating up its interior and melting the ice underneath its surface. If so this could provide an environment for micro-organisms or some other forms of life to exist.

Enceladus's near neighbor, well 600,000 miles away, is another of Saturn's moons, Titan. This moon is exceptional in the solar system as it is the only one with a significant atmosphere. What's even more surprising is that its atmosphere is comprised mainly of nitrogen, just like our own planet. Even more amazing is that it is the only object in our solar system apart from Earth to have large areas of liquid on its surface, not of water but liquid methane. Methane on Titan acts like water on Earth, there are methane clouds which produce methane rain and methane lakes. Titan is often compared to primordial Earth, but unfortunately it's in deep freeze with surface temperatures around -179C (-290F). It is still possible that methane based microbial life could exist there breathing hydrogen instead of oxygen.

Lastly we come to an icy moon orbiting around Jupiter called Europa. It is believed this moon, just slightly smaller than our own moon, presents the best possibilities for life in the entire solar system. The force of Jupiter's gravity produces tremendous tidal heating inside Europa, warming its interior and producing a salt water ocean which is 62 miles (100 km) deep. It is possible that there is twice the amount of liquid water on Europa than there is on Earth! It's been speculated that Europa's ocean could be teeming with life, not just bacteria but complex organisms could be swimming in the warm water. Around the year 2025 NASA hopes to land a probe on Europa's surface which will then melt through the ice and investigate the ocean for signs of life, maybe then we will find out that we are not alone in the solar system.

Our Galaxy and the Universe

andromeda galaxy
The Andromeda Galaxy
In the context of the size of the universe, the Earth and indeed the entire solar system is insignificant. Our sun is just one of 200 billion stars in our galaxy, and the milky way is just one of over 100 billion galaxies in the known universe. The solar system exists at the edge of our galaxy, far from the center where there is a supermassive black hole which has 4 million times the mass of our sun. The milky way is in fact a giant among galaxies, spanning 100,000 light years in diameter, there are more than 30 other galaxies in our "neighborhood" of which only Andromeda (pictured right) is larger.

The best explanation we have as to how the universe was created is The Big Bang theory. Around 14 billion years ago a superdense, superhot mass billions of times smaller than a proton began expanding, in time creating the stars, planets and galaxies we have in the universe today, indeed it is still expanding. It is not known how the initial mass came to be.



The Sun | Mercury | Venus | Earth | The Moon | Mars | Jupiter | Asteroids
Io | Europa | Ganymede | Callisto | Saturn | Enceladus
Titan | Uranus | Neptune | Pluto & Dwarfs | Comets
Learn about stars, nebulae, galaxies and exoplanets in the Universe Section