Planetary Stats
How different is Jupiter from Earth, really? Eleven times the diameter. Two and a half times the surface gravity.
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When you hear "Jupiter is the biggest planet", that's roughly true — but "big" hides multiple separate facts. Jupiter is 11× Earth's diameter, 318× Earth's mass, but only 2.53× Earth's surface gravity. Why doesn't 318× mass mean 318× gravity? Because gravity at the surface depends on both mass and the distance from the centre — and Jupiter's surface is 11× farther out than Earth's. The two effects almost cancel. Jupiter is huge, but standing on its 1-bar level (the conventional "surface" for a gas giant) you'd weigh 2.5× what you weigh on Earth — not 300×.
Mars is the opposite trade-off. Mars has only 11% of Earth's mass, but it has 53% of Earth's diameter. That makes its surface gravity 38% of Earth's — much higher than the mass ratio alone would predict, because you're closer to a smaller centre. The relationship that makes both cases sensible: surface gravity scales as mass / radius². For a planet of average density, gravity scales linearly with radius. Saturn (9.14× Earth's diameter) has surface gravity almost equal to Earth's (1.07 g) because Saturn is much less dense than Earth — Saturn is mostly hydrogen-helium, less than 0.7× water's density.
Atmospheric pressure: huge dynamic range. Mercury essentially zero. Mars 0.006 bar (a thin CO₂ wisp). Earth 1 bar. Venus 92 bar (about the pressure 900 m down in Earth's ocean — would crush you). Gas giants don't have a true "surface pressure" because there's no surface; the convention is to use the 1-bar level inside the atmosphere as the reference. Below that, pressures grow continuously into millions of bars deep in the interior.
Diameters relative to Earth: Mercury 0.38, Venus 0.95 (nearly Earth-sized), Earth 1.0, Mars 0.53, Jupiter 10.97, Saturn 9.14, Uranus 3.98, Neptune 3.86, Pluto 0.19. Surface gravities in Earth-g: Mercury 0.38, Venus 0.91, Earth 1.00, Mars 0.38, Jupiter 2.53, Saturn 1.07, Uranus 0.89, Neptune 1.14, Pluto 0.06. Notable surprises: Mars and Mercury share the same surface gravity (0.38 g) despite very different sizes, because their density × radius products land close. Uranus and Saturn both have lower-than-Earth gravity despite their giant sizes — both are dominated by H/He, much less dense than rocky Earth.
Rotation periods (sidereal, in hours): Mercury 1407.5 (essentially 58 Earth days for one full spin), Venus −5832.5 (retrograde, 243 days), Earth 23.93, Mars 24.62, Jupiter 9.93 (the fastest rotator — its day is under 10 hours despite its size, which is why Jupiter is visibly oblate; centrifugal forces flatten it), Saturn 10.66, Uranus −17.24 (retrograde), Neptune 16.11, Pluto −153.3 (retrograde). Solar days (sunrise to sunrise) differ from sidereal periods because the planet is also moving along its orbit — Venus's solar day is 117 Earth days even though its sidereal rotation is 243 days, because the retrograde spin partially cancels the orbital motion.
The Earth-comparison ghost on /explore lets you read these ratios at a glance: Jupiter's planet image looks 11× the apparent size of the Earth thumbnail, Mars's about half, Pluto's about 1/5. The tactical-scan overlay surfaces the harder-to-imagine numbers: the gravity you'd feel on the surface, the air pressure you'd breathe (or get crushed by, or have nothing to breathe in), the rotation period that sets the planet's day length. All four numbers come from the planetary fact sheet, hard-coded into the PLANET_STATS map in /explore (Slice E.4).