What would kill the idea

Tests

A hypothesis that cannot die is not a hypothesis. Each line below is what would have to be true for that stage to come off the table.

Three tests decide more than any essay: extra mass under the Great Red Spot, or none; whether Europa’s water had to arrive after Io; whether Triton is just outer-system ice.

• Great Red Spot

  If the storm is only air all the way down, with no extra mass under the weather, this beginning comes off the table.

• Io

  If Io’s fire and sulfur are fully explained as a finished moon heated by tides, with nothing left of a first crust, Stage 1 comes off the table.

• Europa

  If Europa’s ocean is native ice from birth, with no volcanic predecessor, Stage 2 comes off the table.

• Ganymede

  If Ganymede formed already layered and magnetized, with no path through fire and a first ocean, Stage 3 comes off the table.

• Callisto

  If Callisto is only a cold first-generation ice moon that never had to become Titan, Stage 4 comes off the table.

• Titan

  If Titan has always belonged to Saturn, with no quiet ice-lid predecessor, Stage 5 comes off the table.

• Triton

  If Triton is ordinary outer-system ice and rock, captured as it is, Stage 6 comes off the table.

• Mars

  If Mars has always been an inner-system planet, assembled dry 4.5 billion years ago, Stage 7 comes off the table.

• Earth

  If everything about Earth is explained as a first-generation inner planet, with nothing left from an earlier stage, the cycle reading of Earth comes off the table.

• Venus

  If Venus never had oceans or continents, the spent-Earth reading comes off the table.

• Mercury

  If Mercury was always small and mostly iron from the start, the remnant reading comes off the table.

Missions that will actually speak

Juno closeout and later Jupiter gravity work — extra mass under 100 bars, or a storm that is only air. Europa Clipper — the chemistry of that ocean. JUICE — the insides of Ganymede and Callisto. Dragonfly — Titan’s air and ground. Mars sample return. BepiColombo at Mercury. A future look at Triton. A Venus lander or orbiter that can tell whether Venus ever had oceans and continents.