The planets of the Solar System – key facts in one table

On Venus, one spin on its axis takes longer than a whole trip around the Sun. Facts like that are easy to miss in a list, but they jump out of a table. Here are the eight planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The table

Figures are rounded. Distances are averages, because every orbit is a slightly stretched circle (an ellipse). One astronomical unit (AU) is the Earth's average distance from the Sun, about 150 million km. "Day" here means the time for one spin relative to the stars.

Moon counts are given as minimums because astronomers keep finding small moons around the giant planets. A book with a different number was probably right when it was printed.

To remember the order, you can use My Very Easy Method Just Speeds Up Naming, though a sentence you invent yourself tends to stick better.

Four patterns in the table

1. Further out, longer year. Outer planets have further to go and move more slowly, so Neptune takes about 164 Earth years to complete one orbit.
2. The gaps grow. The four inner planets are packed close together. The gap from Saturn to Uranus is about as large as the whole distance from the Sun to Saturn.
3. Giants spin fast. Jupiter, the largest planet, has the shortest day, under 10 hours.
4. Venus is the odd one out. It spins once in 243 Earth days but orbits in 225, so its "day" is longer than its year. It also spins in the opposite direction to most planets.

Rocky inner planets

Mercury, Venus, Earth and Mars are small, dense worlds of rock and metal with solid surfaces.

• Mercury is the smallest planet and the closest to the Sun. With almost no atmosphere, it swings between extreme heat by day and extreme cold by night.
• Venus is the hottest planet, at about 465 °C on the surface, even though Mercury is closer to the Sun. Its thick carbon dioxide atmosphere traps heat. It is also the easiest planet to spot: often the brightest point in the sky after the Moon, low in the west after sunset or low in the east before dawn.
• Earth is the only planet known to have liquid water on its surface, and life.
• Mars is red because of iron oxide (rust) in its dust. It has polar ice caps and the tallest volcano in the Solar System, Olympus Mons. Robots have explored it for decades, as described in Mars rovers.

Giant outer planets

Jupiter, Saturn, Uranus and Neptune are much larger and have no solid surface to land on.

• Jupiter and Saturn are gas giants, made mostly of hydrogen and helium. More than 1,000 Earths would fit inside Jupiter, and its Great Red Spot is a storm wider than the Earth. Saturn's rings are made of countless pieces of ice and rock, and its average density is less than that of water. Deep inside both, the pressure is so high that hydrogen behaves like a liquid, so "balls of gas" is not quite right.
• Uranus and Neptune are ice giants, containing large amounts of water, ammonia and methane beneath their atmospheres. Methane absorbs red light, which is why both look blue-green. Uranus is tilted by about 98°, so it rolls around the Sun on its side.

What happened to Pluto?

Pluto was counted as the ninth planet from its discovery in 1930 until 2006, when the International Astronomical Union agreed a formal definition. A planet must:

1. orbit the Sun,
2. be massive enough for its own gravity to pull it into a round shape, and
3. have cleared the neighbourhood around its orbit of other objects of similar size.

Pluto passes the first two tests but not the third: it shares the Kuiper Belt with many other icy bodies. It is now a dwarf planet, alongside objects such as Ceres and Eris. Pluto did not change; the definition did.

Getting a sense of scale

Light travels about 300,000 km every second. Even so, sunlight takes about 8 minutes to reach the Earth and over 4 hours to reach Neptune.

Try it: make a scale model on a strip of paper, with 1 cm for every 10 million km. Mercury goes about 6 cm from the Sun and Earth at 15 cm. How long must the strip be to reach Neptune? (4,515 ÷ 10 ≈ 451 cm, about 4.5 metres.)

You can practise putting objects in order of size, from atoms up to galaxies, with this ModernScholar ranking activity.

Worked example: your weight on other worlds

Your mass, in kilograms, is the same everywhere. Your weight, in newtons, is the pull of gravity on that mass, so it changes from world to world:

weight = mass × gravitational field strength

For a pupil of mass 40 kg:

1. Earth: 40 × 9.8 = 392 N.
2. Mars: 40 × 3.7 = 148 N, well under half their Earth weight.
3. The Moon: 40 × 1.6 = 64 N, about one sixth.
4. Jupiter: 40 × 23.1 = 924 N, more than double, if there were anything to stand on.

Activity: planet fact cards

Make eight cards, one per planet, with the distance, diameter, day and year from the table plus one fact of your own. Play a comparing game with a partner: choose a category, such as "longest day", and the higher value wins the round. Watch out for Venus.

To find out what will eventually happen to the Sun at the centre of it all, read about the life cycle of stars.

Sources and further reading

• Planetary fact sheet – NASA Space Science Data Coordinated Archive – the figures used in the table, with many more for each planet.
• About the planets – NASA Science – overviews of every planet and the dwarf planets.
• Pluto and the developing landscape of our Solar System – International Astronomical Union – the 2006 definition of a planet and why Pluto was reclassified.

Read next

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