About the Planet Size Explorer
This simulation draws all eight planets and the Sun on an HTML canvas, with each disc sized from its real equatorial radius — from Mercury (2,439 km) up to Jupiter (71,492 km) and the Sun (696,000 km). A fixed pixel-per-kilometre factor anchors Earth at roughly 7 pixels, so every body keeps its true size ratio. Click any disc to read its diameter, comparison to Earth, and how many Earths fit inside by volume.
The Scale slider magnifies every body together (0.3× to 2×) so small worlds stay visible, while Orbit Speed and View Tilt animate the planets around the Sun on tilted elliptical paths. Preset buttons isolate inner planets, outer planets or the gas giants. Comparative planetology like this underpins how astronomers describe exoplanets, by ranking new worlds against the familiar Earth and Jupiter benchmarks.
Frequently Asked Questions
What does this simulation actually show?
It shows the true relative sizes of the eight planets and the Sun on a single screen. Each circle is scaled from its real radius using a constant factor, so Jupiter appears genuinely huge next to Mercury, and the Sun dwarfs them all.
Are the planet sizes drawn to scale?
The body sizes are to scale relative to each other, anchored on Earth's radius of 6,371 km. The orbital distances, however, are compressed into fixed pixel rings so that all planets stay on screen rather than spreading out across an impractically wide canvas.
How many Earths fit inside Jupiter?
About 1,321. Volume scales with the cube of radius, so since Jupiter's radius is roughly 11.2 times Earth's, its volume is 11.2 cubed — close to 1,321 times larger. The simulation computes this live when you click a planet.
What do the Scale, Orbit Speed and View Tilt sliders do?
Scale (0.3× to 2×) multiplies every body's drawn radius so tiny planets remain visible. Orbit Speed (0 to 5×) sets how fast the planets revolve around the Sun. View Tilt (0° to 60°) flattens the circular orbits into ellipses to suggest a three-dimensional viewing angle.
What equation converts kilometres into pixels?
A single radius scale is used: pixels equal radius in kilometres multiplied by Earth's display radius divided by Earth's real radius, then multiplied by the Scale slider value. A minimum floor of 3 pixels keeps the smallest worlds from vanishing.
Why does the Sun look so much bigger than the planets?
Because it genuinely is. The Sun's radius of 696,000 km is about 109 times Earth's and roughly 9.7 times Jupiter's. It holds 99.86 per cent of the solar system's mass, which is why every planet orbits it rather than the other way around.
Why do Saturn and Uranus have rings in the view?
The data flags those two bodies as ringed, so the renderer adds an elliptical band tilted to match the View Tilt. Saturn's rings are mostly ice and rock and are the most prominent; Uranus has a fainter, darker ring system, shown here in a cooler tint.
What happens when I click a planet?
The click coordinates are matched against each drawn disc. The nearest hit becomes the selected body, and the info panel updates with its name, diameter in kilometres, size relative to Earth, and the number of Earths that would fit inside it by volume.
Is Mercury or Mars the smallest planet?
Mercury, with a radius of 2,439 km, is the smallest planet. Mars is next at 3,390 km, a little over half Earth's radius. Mercury is also the closest planet to the Sun, though Venus is the hottest because of its thick carbon-dioxide atmosphere.
How is this useful for understanding real astronomy?
Grasping size ratios is the foundation of comparative planetology. When astronomers find an exoplanet, they describe it as Earth-sized, super-Earth, Neptune-like or Jupiter-class — all relative to the very bodies shown here, so an intuitive feel for these scales makes that vocabulary meaningful.