Physicsy

Planetology

Five pairs. The same four questions. One history of the solar system.

Each page puts two worlds side by side. The short answer is on the page. The controls only open a contrast that is already there. Where a note called something a hypothesis, or left it open, that status stays visible.

1. Formation pathway How the body was built, and what stopped it.
2. Collisions and captures The hits and the moons that were grabbed, not grown.
3. Volatiles and air Water, ice, hydrogen, and what stayed in the atmosphere.
4. A constrained future What solar evolution or a changing orbit still allows.

1. Inner rock

   Mercury and Mars

   A hit-and-run metal core beside a smaller world that kept its rock. Densities 5.43 and 3.93 g/cm³.

2. Twin start

   Earth and Venus

   A Moon-forming impact and oceans that stayed, beside collisions, volcanism, and water loss. The same red-giant ending.

3. Gas on a clock

   Jupiter and Uranus

   Runaway hydrogen for the one that grew fast. A thin gas skin, a 98° tilt, and heavy ices for the one that grew late.

4. Disk and ice

   Saturn and Neptune

   95 Earths of gas giant beside 17 Earths of ice giant. Pebbles, Triton, and rings that are already draining.

5. Dwarf planets

   Pluto, Ceres, and Eris

   Kiss-and-capture, a cratered asteroid-belt world, and an impact moon. Atmospheres too thin to warm the ice.