Planet Size Explorer: Why the Solar System Never Fits One Scale

A solar system that does not fit on one scale

The single hardest thing to convey about the solar system is that size and distance use wildly different scales. If you shrink Earth to the size of a marble (about 1.3 cm), the Sun becomes a beach ball roughly 1.4 m across sitting about 150 m away - and Neptune, the outermost planet, ends up more than 4.5 km from that beach ball. Almost every classroom poster and app that shows planets next to each other at a size you can actually see is, by necessity, wildly wrong about the distances between them, because a true-to-scale model would need a corridor longer than most schools.

The size numbers, and how lopsided they are

By equatorial radius, Jupiter (about 69,911 km) is about 11.2 times Earth's radius (about 6,371 km) and could contain roughly 1,300 Earths by volume. Saturn (about 58,232 km), Uranus (about 25,362 km) and Neptune (about 24,622 km) form the rest of the gas and ice giants. The four inner, rocky worlds are all far smaller: Mercury (about 2,440 km), Venus (about 6,052 km), Earth, and Mars (about 3,390 km). The Sun dwarfs all of them - its radius, about 696,000 km, is roughly 109 Earth radii, and its volume could hold about 1.3 million Earths.

body      radius (km)   × Earth radii
Mercury      2,440          0.38
Venus        6,052          0.95
Earth        6,371          1.00
Mars         3,390          0.53
Jupiter     69,911         10.97
Saturn      58,232          9.14
Uranus      25,362          3.98
Neptune     24,622          3.86
Sun        696,000        109.2

Why the planets ended up this shape

The size split between small rocky planets and huge gas giants traces back to the frost line - the distance from the young Sun beyond which water, ammonia and methane could freeze into solid ice grains rather than staying as vapour. Inside that line, only metals and silicate rock condensed, so the inner planets built up from a comparatively small budget of solid material and stayed small. Beyond the frost line, ice roughly quadrupled the mass of solid material available, letting protoplanets grow past about ten Earth masses - the threshold at which a core's gravity is strong enough to pull in and hold onto the surrounding hydrogen and helium gas directly from the solar nebula, a runaway process called core accretion. That is why Jupiter and Saturn are mostly hydrogen and helium, while Uranus and Neptune, which formed more slowly farther out with less gas left to capture, ended up as smaller 'ice giants' dominated by water, ammonia and methane ices around a rocky core.

Rings, moons and what scale hides

Even a size-accurate model needs care with rings and moons. Saturn's ring system spans roughly 280,000 km across but is only about 10 m thick in most places - a ratio so extreme that on almost any physical model the rings would be thinner than a sheet of paper. Moons introduce their own scale problem: Ganymede, Jupiter's largest moon, is bigger than Mercury, and Titan, Saturn's largest, is bigger than Mercury too, which routinely surprises people who assume 'moon' implies 'small'.

Reading a scale model correctly

When you compare planet sizes in a model like the one on this page, the two numbers to keep in your head are separate: diameter ratio tells you how big a planet looks next to Earth, while true orbital distance - almost always compressed or omitted in a size demo - tells you how far apart they really are. Confusing the two is the single most common misconception about the solar system, and it is worth deliberately checking both scales whenever you look at any diagram, including this one.

Frequently asked questions

Try it live

Everything above runs in your browser — open Planet Size Explorer and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.