Diagram showing the interior of the terrestrial planet Mars. The outermost layer is the crust, which is about 30 miles (50 km) deep on average - quite thick compared to Earth's. The convecting mantle, immediately below the crust, is a thick shell of silicate rocks. Lastly there is a substantial core which occupies up to 52 per cent of the planet's radius. Like the Earth's it's probably made up mostly of nickel-iron with about 17 percent sulfur.

Diagram showing the theoretical interior of the gas giant planet Jupiter. At the centre there is probably a rocky and icy core several times the mass of the Earth. This is surrounded by an extensive inner mantle, more than two-thirds of the planet's total radius, of liquid metallic hydrogen under great pressure. On top of this is the shell of helium-neon 'rain'. Then comes the outer mantle, a layer of liquid molecular hydrogen and helium. And finally comes an extensive hydrogen-helium atmosphere about 1000 km thick.

Diagram showing the theoretical interior of the ice giant planet Uranus. It probably looks very similar to the interior of Neptune. At the very centre is a rocky and icy core, similar and size and mass to the Earth. This is encased in a thick inner mantle, a slushy mixture of various ices including methane, ammonia and water. Above this is the outer mantle, which is made up of a mixture of liquid hydrogen and other elements. And finally there is a thick atmosphere, composed chiefly of hydrogen, helium and methane.

Formation of a gas giant. The planet is in the centre, still growing. A concentric disc surrounds it, inside of which the natural satellites are forming via the process of accretion. The Sun is at the top, itself still contracting.

Image comparing the size of Earth (left) with the planet Neptune. Neptune is the farthest planet from the Sun, with an average distance from it of 29.8 times the Earth-Sun distance. Being a fluid world of mostly hydrogen and helium, rich in ices of methane, water and ammonia, astronomers label it (along with Uranus) an ice giant. With a diameter of 3.9 times that of the Earth, Neptune is the Solar System's fourth largest planet.

The Sun as a red giant. Earth begins to melt as the red-giant Sun swells and approaches our planet's orbit. Eventually the melting may penetrate throughout Earth's crust, leaving the planet molten from surface to core. The fate of the Earth is unclear. It may become engulfed by the Sun, or it may move to a safe orbit as the Sun sheds mass at the end of its life.

Formation of moons around a gas giant. This is how the moons of Jupiter and Saturn, and possibly Uranus, are thought to have formed. The planet is in the centre, still growing. A concentric disc surrounds it, inside of which the natural satellites are forming via the process of accretion. The illustration could also show the formation of planets around a star, which was a similar process albeit on a far larger scale.

Image comparing the size of Earth (left) with the planet Saturn. Saturn is the sixth planet from the Sun, with an average distance from it of 9.4 times the Earth-Sun distance. Being a fluid world of mostly hydrogen and helium, astronomers label it (along with Jupiter) a gas giant. With a diameter of 9.4 times that of the Earth, Saturn is the Solar System's second largest planet (after Jupiter).

Image comparing the size of Earth (left) with the planet Jupiter. Jupiter is the fifth planet from the Sun, with an average distance from it of 5.2 times the Earth-Sun distance. Being a fluid world of mostly hydrogen and helium, astronomers label it (along with Saturn) a gas giant. With a diameter of 11.2 times that of the Earth, Jupiter is the Solar System's largest planet.

Computer artwork comparing the size of Mars (left) with that of the Earth. Mars' diameter is 53% that of the Earth's. It has ten percent the mass of Earth and is, on average, 1.53 times further from the Sun. While the Earth is host to oceans of liquid water and has a relatively dense water-rich atmosphere, Mars is exceedingly dry, cold and, as far as we know, sterile. It is thought to have lost most of its water to space a long time ago, and any that remains is frozen at the poles and locked deep in the ground as permafrost.

Diagram showing the theoretical interior of the gas giant planet Saturn At the centre there is probably a rocky and icy core several times the mass of the Earth. This is surrounded by an extensive inner mantle, about half of the planet's total radius, of liquid metallic hydrogen under great pressure. On top of this is a shell of helium 'rain', topped with a thick outer mantle of helium-saturated liquid molecular hydrogen. This gives way to a thick shell of hydrogen gas, and eventually, to an extensive hydrogen-helium atmosphere.

Diagram showing the theoretical interior of the gas giant planet Jupiter. At the centre there is probably a rocky and icy core several times the mass of the Earth. This is surrounded by an extensive inner mantle, more than two-thirds of the planet's total radius, of liquid metallic hydrogen under great pressure. On top of this is the shell of helium-neon 'rain'. Then comes the outer mantle, a layer of liquid molecular hydrogen and helium. And finally comes an extensive hydrogen-helium atmosphere about 1000 km thick.

Image comparing the size of Earth (left) with the planet Jupiter. Jupiter is the fifth planet from the Sun, with an average distance from it of 5.2 times the Earth-Sun distance. Being a fluid world of mostly hydrogen and helium, astronomers label it (along with Saturn) a gas giant. With a diameter of 11.2 times that of the Earth, Jupiter is the Solar System's largest planet.

Diagram showing the theoretical interior of the ice giant planet Uranus. It probably looks very similar to the interior of Neptune. At the very centre is a rocky and icy core, similar and size and mass to the Earth. This is encased in a thick inner mantle, a slushy mixture of various ices including methane, ammonia and water. Above this is the outer mantle, which is made up of a mixture of liquid hydrogen and other elements. And finally there is a thick atmosphere, composed chiefly of hydrogen, helium and methane.

Diagram showing the theoretical interior of the ice giant planet Uranus. It probably looks very similar to the interior of Neptune. At the very centre is a rocky and icy core, similar and size and mass to the Earth. This is encased in a thick inner mantle, a slushy mixture of various ices including methane, ammonia and water. Above this is the outer mantle, which is made up of a mixture of liquid hydrogen and other elements. And finally there is a thick atmosphere, composed chiefly of hydrogen, helium and methane.

Diagram showing the interior of the Earth's Moon. The outermost layer, the crust, is about 45 miles (70 km) thick. This is thicker than the Earth's crust, which cooled down at a much slower rate. Beneath the crust is a thick silicate mantle, then a zone of partial melt with a radius of 480 km. This is probably where moonquakes occur. Current thinking, based on a re-examination of Apollo lunar seismometer data, is that the core, once thought solid, is now composed of a liquid outer component (330 km radius) and a solid inner one (240 km).

Image comparing the size of Earth (right) with the planet Venus. Venus is the second planet from the Sun, with an average distance from it of 0.72 times the Earth-Sun distance. It is also the hottest planet (despite being further from the Sun than Mercury) because its thick atmosphere of carbon dioxide has created a runaway greenhouse effect. Lead, tin and zinc would all melt on Venus. With a diameter of 95% that of the Earth, Venus is the Solar System's sixth largest planet.

Diagram showing the theoretical interior of the gas giant planet Saturn At the centre there is probably a rocky and icy core several times the mass of the Earth. This is surrounded by an extensive inner mantle, about half of the planet's total radius, of liquid metallic hydrogen under great pressure. On top of this is a shell of helium 'rain', topped with a thick outer mantle of helium-saturated liquid molecular hydrogen. This gives way to a thick shell of hydrogen gas, and eventually, to an extensive hydrogen-helium atmosphere.

Image comparing the size of Earth (left) with the planet Uranus. Uranus is the seventh planet from the Sun, with an average distance from it of 19.2 times the Earth-Sun distance. A fluid world of mostly hydrogen and helium, it is rich in ices of methane, water and ammonia, causing some astronomers to label it (along with Neptune) an ice giant. With a diameter of four times that of the Earth, Uranus is the Solar System's third largest planet (after Jupiter and Saturn).

Diagram showing the theoretical interior of the gas giant planet Jupiter. At the centre there is probably a rocky and icy core several times the mass of the Earth. This is surrounded by an extensive inner mantle, more than two-thirds of the planet's total radius, of liquid metallic hydrogen under great pressure. On top of this is the shell of helium-neon 'rain'. Then comes the outer mantle, a layer of liquid molecular hydrogen and helium. And finally comes an extensive hydrogen-helium atmosphere about 1000 km thick.