Are we alone?

Planets in the solar system

All images are from NASA websites and in the public domain unless otherwise credited.  Click on them to see the originals and to access more information or other similar images.

What is the possibility that life has formed elsewhere in the universe? To think about this, we need to consider what conditions life needs to form. We will also need to think about what life might be like elsewhere. However, we will start by looking at the planets and moons in our solar system, and see if it is at all possible that life might be found elsewhere in the solar system. 

All the planets in the solar system revolve around the Sun, including our own planet, the Earth.

But there is more to the solar system than just the 8 planets. There are also:

• dozens of moons and moonlets
• hundreds of asteroids and comets
• hundreds and thousands of meteors

There are two basic types of planets. The first type are the solid rocky ones like the Earth and also Mars, Venus and Mercury. These are called the inner planets (as they are closest to the Sun) or terrestrial (Earthlike) planets, and they are solid balls of rock and metal. The Earth has one moon and Mars has two moons, but Venus and Mercury do not have any moons. 

(Copyright Calvin J. Hamilton)

They also show craters from being constantly bombarded by asteroids and meteorites during their first 600 million years of their existence. You can see craters on the Moon using binoculars or a telescope - the image on the left shows a close-up of some craters on Mercury (looking much like the moon’s surface). There are craters on the Earth, Venus and Mars also

The other kind of planets are the ‘gas giants’: Jupiter, Saturn, Uranus and Neptune. These are the outer planets which lie further out in the Solar system than the inner rocky planets, beyond the asteroid belt that lies between Mars and Jupiter. Because they are so much further away from the sun, their temperatures are much lower than the inner planets, and they take much longer to orbit the Sun. For example, it takes Neptune 165 of our earth years to orbit the sun - and as it was only discovered in 1846, has not yet gone round the sun once since it was first seen!

They are not called the gas giants for nothing - the picture on the right above shows the inner, rocky planets to the same scale. Note how much larger Jupiter and Saturn are than the Earth - you could fit 1000 earths inside Jupiter! But even Jupiter is still nowhere as big as the Sun. You could fit 1000 Jupiters inside the sun.

They’re called the gas giants because they are literally made of gases, such as hydrogen and helium, and they don’t have a solid surface as the rocky planets do. You would continue to fall through them if you tried to stand on them - all that would happen is that the gas would become more and more concentrated as you go down into the atmosphere, with perhaps a small, rocky core at the centre (the size of a few earths).

What we see when we look at a gas giant is just the tops of the clouds high in the atmosphere, not the surface. The different colours come from different layers of clouds in the atmosphere, all being swirled around in high winds and storms. The beautiful patterns, which you can see in the picture on the left, show the clouds all moving around and being stirred up by very fast winds. One huge storm we see swirling about in Jupiter’s atmosphere is the Great Red Spot - other similar but smaller spots are also known. The GRS has been observed from earth for more 300 years, and has probably lasted over a million years. It is such a huge storm, it is big enough to hold two Earths! So far, we don't know how such structures can last for so long.

All the gas giants have systems of rings about their equators, the best known being those around Saturn, which are made from chunks of ice, dust, or boulders. These particles were either left over when the planets were formed, or they may be the remains of broken-up moons. The ring systems are flat and thin - only about 10 to 200 metres thick. The particles are not spread out all round the planet, because the entire system is spinning, forcing them into a flat plane around the equator of the planet. 

The particles in the rings range in size from that of ice cubes to that of a car. It is likely that there are a few kilometre-sized objects also. When you look at the rings in close detail, you can see also that each ring is broken up into numerous ringlets. The narrowest features are about 15 kilometers wide with the overall image here about 6,000 kilometers wide. The variations in brightness of the rings are due to a combination of differences in the number of particles in a given area and the way that different particles scatter light.

The origin of the rings is uncertain. It may be that the gas giants have had rings since their formation, but the systems are not stable and must be regenerated by ongoing processes such as the breakup of smaller satellites or moons. Gaps in the rings still have particles in them - variations in the density of the particles are due to density waves caused by the gravitational effects of nearby undetected satellites.

All the gas giants have multiple moon systems. 

If we had to choose whether to live on a rocky or a gas giant planet, we'd probably choose something like the Earth, because we know that this is a good environment for us. The conditions on gas giants, such as the high pressure atmospheres and very low temperatures, don’t rule out life, but it would have to be in a form very different to from anything we recognise as life on Earth. The table below gives a comparison of the two types of planets:

You might wonder why different types of planets formed in the first place. The location of a planet in the solar system during its formation is important here. Consider how temperature varies with distance from Sun: 

Close = Hot; Far = Cold.

The temperature of a substance is really a measure of the average kinetic energy of the particles making up the substance (kinetic energy is the energy a particle has if it is moving). This energy depends on the mass and speed of the particles. The terrestrial (or rocky) planets formed close to the Sun, where the temperature was warm enough to give small, light particles enough energy to escape the gravity of the young planets - the heavier material was left behind to form these rocky terrestrial worlds. The gas giants formed far from the Sun, on the other hand, where their gravity could hold onto slower-moving particles, whether heavy or light, and so these planets contain large volumes of lightweight gas (hydrogen and helium).

The giant gas planets have many more moons than the terrestrial planets because of their larger mass, which means they can exert much larger gravitational forces. Because of these great forces, either passing asteroids have been captured to become moons, or moons were formed as the host planet formed. Larger planets have the potential to have more moons simply because their gravitational "reach" allows them to "control" more space and hold more mass around them.