Our Solar System stamps

Mercury

Closest to the Sun, Mercury is the smallest planet in our solar system (about 4,900 km in diameter), its surface baked and wrinkled by the Sun’s intense heat.

Mercury takes just 88 Earth days to orbit the Sun, but it rotates very slowly – a “day” on Mercury is 58 days long.

It also has the largest temperature variation between day and night of any of the planets, with night time surface temperatures down to -173⁰C and daytime highs of 427⁰C. Mercury has no atmosphere and its arid surface is scarred by countless impacts from asteroids, meteors and comets.

Venus

Hidden under a blanket of clouds, Venus is a hellish world of crushing atmospheric pressure, high temperature and acid rain.

Similar in size to Earth, with a diameter of 12,100 km, Venus is the hottest world in the solar system, its thick carbon dioxide atmosphere trapping heat like a greenhouse, so that the surface temperature reaches 462°C – hot enough to melt lead.

A “year” on Venus is 225 Earth days long, but the planet spins so slowly (and in the opposite direction to the spin of most of the other planets) that it takes 243 Earth days to complete one rotation, meaning that Venus’ day is longer than its year. However, the clouds at the top of its atmosphere zip round the planet in only four days, driven by wind speeds around 360kph.

Observations using radar to “see” through the clouds have revealed that there are thousands of volcanoes on the surface, some up to 240 km in diameter, and evidence strongly suggests that volcanic activity is still occurring. The atmospheric pressure at the surface of Venus is a crushing 90 times that at sea level on Earth.

Seen from Earth, Venus is the brightest object in the night sky apart from the Moon.

Earth

Earth is the largest of the “terrestrial”, or rocky, planets (12,742 km in diameter), and has the greatest density of any planet in the solar system.

With its vast oceans and protective atmosphere, our home planet has proved just right for the development of life and is the only world in the universe where life is currently known to exist. The atmosphere is composed mainly of nitrogen (78 percent) and oxygen (21 percent) and only allows a narrow slice of the electromagnetic spectrum (mostly visible light) to reach the surface, shielding us from harmful infrared, ultraviolet, X-rays and gamma-rays.

The Earth’s powerful magnetic field also protects us from bombardment by radiation from space. With surface temperatures ranging from -88 to 58°C, Earth is the only place in the solar system where water exists in all three forms – solid, liquid and gas.

Our planet has one large moon, with a radius about one quarter of the Earth’s, making it the largest satellite compared to its parent planet (although Charon, the moon of the dwarf planet Pluto, is about half the diameter of Pluto).

Mars

At 6,799 km Mars is about half the diameter of the Earth, with a surface temperature range of -125 to -20°C. A day on Mars is about 40 minutes longer than an Earth day, and it takes 687 days to orbit the Sun.

Mars may once have been Earth-like, but has now lost its surface water and possesses only a very thin carbon dioxide atmosphere and a surface pressure less than one percent that of Earth. Most of Mars’ atmosphere was either oxidised into its iron rich surface (forming the rusty red colour we see today) or lost to space over time due to Mars’ weak gravity. Orbiting spacecraft and rovers have provided considerable evidence that there was once water flowing on the surface of Mars; water may even exist today, frozen as permafrost beneath the surface. Mars has seasons, like the Earth, and polar caps composed of carbon dioxide ice and water ice.

Mars boasts the largest volcano in the solar system, Olympus Mons, which is 25 km tall (almost three times the height of Mt Everest) and one of the biggest canyon systems, the Valles Marineris, more than 4,000 km long. Microbial life may have evolved on Mars billions of years ago when it was far more Earth-like; whether any microbial life still exists today is a matter for further exploration. Mars has two tiny moons, Phobos and Diemos, which were discovered in 1877 and appear to be captured asteroids.

Jupiter

The largest planet in the solar system, Jupiter is a “gas giant” – a huge ball of hydrogen and helium gas with possibly a small rocky or icy core. With an equatorial diameter of 141,000 km, Jupiter is so massive that 1,000 Earths could fit inside it.

A day on Jupiter is just 10 hours long, but it takes 12 Earth years to circle the Sun. Its gaseous envelope has complex cloud bands that are roiled by massive storms. One of the most recognisable is the Great Red Spot, a huge anticyclonic storm larger than the Earth that has been raging for more than 400 years. The temperature at the top of Jupiter’s clouds is -108°C. Massive lightning storms have been observed on Jupiter, as well as auroras at its north and south poles.

Jupiter has a ring system, consisting of four rings, and 67 moons have so far been discovered. Jupiter’s four largest moons, first seen by Galileo in 1610, are fascinating worlds in their own right.

Ganymede is the largest moon in the solar system and is actually bigger than the planet Mercury. It may have a very thin oxygen atmosphere and perhaps a sub-surface ocean.
Callisto, Jupiter’s second largest moon, is a ball of rock and ice.
Io, closest to the planet, is a boiling world of active volcanoes and lava lakes.
Europa is perhaps the most interesting moon of all: its cracked, icy shell is believed to cover an ocean of liquid water which may possibly harbour life.

Saturn

Famous for its glorious and complex ring system, Saturn is the sixth planet from the Sun and has an equatorial diameter of 120,536 km. It is made primarily of hydrogen, but its upper cloud layers include ammonia and water ices. Saturn rotates in 10 hours 39 minutes and takes about 29.45 years to orbit the Sun.

Like Jupiter, Saturn has complex cloud bands (with a temperature of -139°C) and massive storms, lightning and auroras. Saturn's magnificent ring system is made up of billions of pieces of water ice ranging in size from one centimetre to chunks the size of icebergs. The rings are very wide (about 100,000 km across), but not very deep, being less than a kilometre thick. There are more than 30 rings, each of which is divided into thousands of ringlets.

Saturn has 62 known moons, some of which play an important part in shaping the structure of the ring system. Saturn's largest moon, Titan, is the second largest in the solar system and has a dense atmosphere made of nitrogen and methane. Methane exists on Titan in solid, liquid and gaseous states (just like water on Earth) and there are rivers and lakes of liquid methane on its surface. Titan is considered a possible location for life to evolve, though it would be very different from life on Earth.

Other fascinating moons of Saturn include:

Enceladus, which has geysers spewing out massive plumes of water ice
Mimas, known as the "Death Star" moon, because a huge crater gives it the appearance of the Death Star space station from the Star Wars films
Hyperion, which looks like a giant sponge and may be a captured comet nucleus
Iapetus, which shows clean ice on one side and a dark, carbon-covered surface on the other, and has a huge mountain ridge running around its equator.

Uranus

The first planet to be discovered with the aid of a telescope (in 1781 by Sir William Herschel), Uranus is an unusual world, tipped on its side by an ancient impact.

With an equatorial diameter of 50,000 km, Uranus rotates every 17 and a quarter hours (but, like Venus, rotates in a counter-clockwise direction) and takes 84 Earth years to complete its orbit of the Sun.

Uranus has a smoggy atmosphere of hydrogen, helium and methane over an icy interior. It has the coldest planetary atmosphere in the solar system, with a minimum temperature of -224.2°C. A methane haze covers the upper layers of the atmosphere and hides the clouds and storm activity below. Because of its unusual axial tilt, during its orbit, one or the other of Uranus' poles will be pointing directly at the Sun and receive about 42 years of direct sunlight. The rest of the time they are in darkness. Uranus has 13 known rings, made of much darker particles than the rings of Jupiter and Saturn, perhaps the result of the destruction of an ancient moon by an impact.

Uranus has 27 known moons. Titania, less than half the size of our Moon, is the largest, but Miranda is the most interesting, with its tortured surface of deep fault canyons, terraced layers, and a chaotic variation in surface ages and features.

Neptune and Pluto

Neptune, the outermost of the major planets, was discovered by telescope (in 1846, by Urbain Le Verrier and Johann Galle) as the result of mathematical calculations. It is a little smaller than Uranus (equatorial diameter 49,528km) and similar in composition, but its atmosphere is wracked by fierce winds (measured at travelling 600m per second) and giant storms.

One of these massive storms was the Great Dark Spot, first observed in 1989, and lasting for five more years. Neptune takes 164.8 years to complete an orbit of the Sun, but its day lasts just over 16 hours. Neptune has a thin system of five rings, composed of ice particles and dust grains, and 14 known moons. Triton, the largest moon, has geysers spewing out nitrogen and dust from below the surface.

Pluto was also discovered mathematically, like Neptune, in 1930 (by Clyde Tombaugh). It was once considered a planet, but is now categorised as a dwarf planet. It is included on the Neptune stamp as a representative of the smaller bodies in our solar system. These include dwarf planets, asteroids, comets and the icy bodies of the Kuiper Belt and the Oort Cloud, at the outer fringes of the solar system. Plto has an estimated diameter of 2,370km and has five known moons.

Dwarf planet Pluto, the largest known object in the Kuiper Belt object, was finally explored in 2015. Amazing images of Pluto and its large moon Charon have revealed two very different icy worlds, with complex and unexpected geology, demonstrating that we still have much to learn about our solar system.