A tour of the solar system

Light from the Sun takes about 8 minutes 19 seconds to reach Earth, and more than 4 hours to reach Neptune.

What is in the solar system

The Sun holds about 99.8% of all the mass in the solar system. Everything else orbits it: eight planets and their moons, at least five dwarf planets such as Pluto and Ceres, tens of thousands of known asteroids, and a huge number of comets and icy bodies far beyond Neptune. The Sun’s gravity is what keeps all of them on their paths.

In order from the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are small and rocky, with solid surfaces of rock and metal. Earth is the largest of them at 12,756 km across. The last four are giants. Jupiter and Saturn are made mostly of hydrogen and helium, and Uranus and Neptune hold a large share of water, ammonia and methane under thick atmospheres. None of the four giants has a solid surface you could stand on.

English speakers often remember the order with a sentence whose words start with the same letters as the planets: My Very Educated Mother Just Served Us Nachos. Korean students say the first syllables in a row instead. Try the memory box below until you can say the whole order without looking.

The asteroid belt and the dwarf planets

Between Mars and Jupiter lies the asteroid belt, a wide ring of rocky bodies. Its largest member is Ceres, which Giuseppe Piazzi spotted in 1801. Ceres holds about a quarter of the belt’s total mass, and in 2015 NASA’s Dawn spacecraft made it the first dwarf planet ever visited. Even so, the asteroids in the belt are spread so thinly that spacecraft pass through without steering around them.

Measuring with AU

Distances inside the solar system are measured in astronomical units (AU). One AU is the average distance from Earth to the Sun, about 149.6 million km. Light covers that distance in about 499 seconds, which is where the 8 minutes 19 seconds in the lead comes from. To turn AU into kilometres, multiply by 149.6 million. Neptune orbits at 30.07 AU, so it is about 30.07 × 149.6 ≈ 4,498 million km from the Sun, roughly 4.5 billion km.

Look at how the gaps grow. All four rocky planets sit within 1.52 AU of the Sun. The step from Mars to Jupiter alone is more than 3.6 AU, and Neptune is about 20 times farther out than Mars. The distances above are average values: every orbit is a slightly stretched circle, so each planet moves a little closer to the Sun and a little farther away during its year.