How big are the planets?

About 11 Earths fit across Jupiter, and about 109 across the Sun.

Diameters

The quickest way to compare planets is by width. The table gives each diameter in kilometres and as a multiple of Earth’s 12,756 km. Divide any diameter by 12,756 and you get the last column: Jupiter is 142,984 ÷ 12,756 ≈ 11.2 Earths across, and the Sun is about 109.

Venus is almost Earth’s twin in size at 95% of its width, while Mars is only about half. Mercury, the smallest planet, is 0.38 Earths across. Among the giants, Uranus and Neptune are close to each other at about 4 Earths, and Jupiter and Saturn are in a class of their own.

These diameters are measured at the equator. Fast-spinning giants bulge there. Saturn turns once every 10.7 hours, and its equatorial radius of 60,268 km is about 11% larger than its polar radius of 54,364 km. Earth bulges too, but only by about 21 km in radius.

Size and volume are not the same

Volume grows with the cube of the diameter. Double the width of a ball and it holds 2 × 2 × 2 = 8 times as much. Jupiter is 11.2 times wider than Earth at the equator. Using the average diameters of both planets (139,822 km and 12,742 km), Jupiter has about 1,321 times Earth’s volume: you could pour about 1,300 Earths into it. NASA gives the same kind of number for the Sun: about 1.3 million Earths would fill its volume.

Big does not always mean heavy. Saturn’s average density is 687 kg per cubic metre, lower than water at about 1,000, so a lump of Saturn material would float in a bathtub large enough to hold it. Earth is the densest planet at 5,514 kg per cubic metre, because it is packed with rock and iron.

Surface gravity depends on mass and size together. NASA lists Jupiter’s gravity at its cloud tops as 23.1 m/s², about 2.4 times Earth’s 9.8 m/s². Saturn is almost as wide as Jupiter, yet its gravity is 9.0 m/s², close to Earth’s, because it is so light for its size. Mars and Mercury both have 3.7 m/s², so on either of them you would weigh a little more than a third of your weight on Earth.

A model you could walk

Shrink Earth to a 1 cm bead and keep every size and distance in the same ratio. The Sun becomes a ball about 1.09 m wide, Jupiter a ball about 11 cm wide, and Earth sits about 117 m from the Sun. Jupiter lands about 610 m away, and Neptune about 3.5 km away. Almost everything in between is empty space, which is why the true-scale view in the 3D model has to use two separate scales.