Jupiter could swallow 1,300 Earths and still have room for the Moon. After Jupiter, the size rankings get unintuitive.
How to Play: Guess if the world on the right has a LARGER or SMALLER diameter than the one on the left.
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Top 10 Largest Planets and Moons
The Sun dwarfs everything. After it, four gas giants — Jupiter, Saturn, Uranus, Neptune — sit far above the four rocky planets. Several moons (Ganymede, Titan) are larger than the planet Mercury.
| # | Name | Diameter (km) | Unit |
|---|---|---|---|
| 1 | Sun | 1,392,700 | km |
| 2 | Jupiter | 139,820 | km |
| 3 | Saturn | 116,460 | km |
| 4 | Uranus | 50,724 | km |
| 5 | Neptune | 49,244 | km |
| 6 | Earth | 12,742 | km |
| 7 | Venus | 12,104 | km |
| 8 | Mars | 6,779 | km |
| 9 | Ganymede (moon of Jupiter) | 5,268 | km |
| 10 | Titan (moon of Saturn) | 5,150 | km |
How Planet Diameters Are Measured
Planet diameters in this game are equatorial, the standard astronomical reference. Gas giants are measurably wider at the equator than at the poles because of fast rotation — Jupiter’s polar diameter is 7% smaller than its equatorial. Solid bodies are approximately spherical and the difference is negligible.
Diameters are determined by direct measurement (occultations and transits, which time how long a body blocks a star’s light) and increasingly by spacecraft missions. Hubble Space Telescope, Voyager, Cassini, Juno, and New Horizons have all provided precise measurements of solar-system bodies. Pluto’s diameter was revised from 2,370 to 2,376 km after New Horizons’ 2015 flyby.
Two of Jupiter’s moons (Ganymede, Callisto) and one of Saturn’s (Titan) are larger in diameter than Mercury. Mercury is denser, so it has more mass — but a Mercury-sized moon orbiting a giant planet is striking. Ganymede has its own magnetosphere, the only moon known to.
Pluto was demoted from planet status in 2006 when the IAU adopted a stricter definition requiring planets to have ‘cleared their orbital neighborhood.’ Pluto shares the Kuiper Belt with thousands of similar icy bodies (Eris, Makemake, Haumea, Sedna), which led to its reclassification as a ‘dwarf planet.’
Frequently Asked Questions
What is the largest planet in the solar system?
Jupiter at 139,820 km equatorial diameter. It is 11 times wider than Earth.
Are some moons bigger than planets?
Yes. Ganymede (moon of Jupiter) and Titan (moon of Saturn) are both larger in diameter than Mercury, the smallest planet. Ganymede is the largest moon in the solar system.
Why isn't Pluto a planet anymore?
The IAU’s 2006 definition requires planets to have ‘cleared the neighborhood’ around their orbit. Pluto shares its region with thousands of similar Kuiper Belt objects, so it was reclassified as a ‘dwarf planet.’
How do scientists measure planet size from far away?
Multiple methods: timing how long a planet blocks a background star’s light (stellar occultation), spacecraft flyby measurements, and high-resolution imaging from telescopes like Hubble and JWST.
Note: Equatorial diameters in km from NASA fact sheets. Some bodies (especially small TNOs) carry ±10–20% uncertainty.
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