Planet Size Comparison Game

How to play

1. Two worlds appear side by side (stacked on a phone). The first shows its diameter; the second keeps its value hidden.
2. Decide whether the second one is larger or smaller and press Larger or Smaller.
3. The hidden value counts up and your answer is judged. When you are right, the second card moves across and a new one takes its place.
4. One wrong answer ends the run. There is no timer, and values stay fixed for the whole run. Pictures are not to scale. Two worlds are only paired when one is at least 10% wider and their measurement uncertainties do not overlap.
5. Your result is saved in this browser. Once enough people have played a game, the results also show how your score ranks, and you can copy or share an image of your result, or replay.

Understanding the comparison

Think in factors of ten. Jupiter is about 11 times as wide as Earth (12,756 km), and the Sun is about 109 Earths across. At the small end, Mars’s moons Phobos (22.2 km) and Deimos (12.4 km) and the asteroids Ryugu (896 m) and Bennu (484 m) are measured in kilometres or metres, so check the unit on each card.

How the sizes are measured

The planets use their equatorial diameter, and Saturn’s excludes its rings. The moons use their mean diameter as NASA JPL lists it: twice the radius of a sphere with the same volume.

5 small worlds are lumpy rather than round: Proteus, Psyche, Phobos, Eros and Deimos. Their figure is a mean or volume-equivalent diameter, so it is an average of a shape that is wider in one direction than another.

Planets, dwarf planets, moons and asteroids

Each world is labelled by what it is, not by its size. The 4 dwarf planets are Ceres (939 km), Eris (2,326 km), Makemake (about 1,430 km) and Pluto (2,376 km). Eris, sized when it passed in front of a star in 2010, is a little smaller than Pluto, too close to be paired in the game; NASA gives Makemake only as a rounded figure. The largest moon, Ganymede, is 5,262 km across, and the smallest moon here, Deimos, 12.4 km.

Sources

Source: NASA and NASA JPL · Sources reviewed 2026-10-01

Planet and Pluto diameters from NASA’s Planetary Fact Sheet, the Sun from NASA’s Sun Fact Sheet, moons from NASA JPL’s planetary satellite parameters (mean radius × 2) and Ceres and the asteroids from the JPL Small-Body Database, Eris from a 2010 stellar occultation (Sicardy et al. 2011, Nature) and Makemake from NASA’s rounded figure. Planet images are NASA public domain; other images come from Wikimedia Commons under the licence credited in the game.

• NASA and NASA JPL
• NASA Sun Fact Sheet: The Sun
• NASA Planetary Fact Sheet: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, Pluto
• NASA JPL Solar System Dynamics: Ganymede, Titan, Callisto, Io, The Moon, Europa, Triton, Titania, Rhea, Oberon, Iapetus, Charon, Umbriel, Ariel, Dione, Tethys, Enceladus, Miranda, Proteus, Mimas, Phobos, Deimos
• Sicardy et al. 2011, Nature 478, 493: Eris
• NASA Science: Makemake
• NASA JPL Small-Body Database: Ceres
• NASA JPL Small-Body Database: Vesta
• NASA JPL Small-Body Database: Pallas
• NASA JPL Small-Body Database: Psyche
• NASA JPL Small-Body Database: Eros
• NASA JPL Small-Body Database: Ryugu
• NASA JPL Small-Body Database: Bennu
• Every size, source quote and image credit (JSON)