How did the outer Jovian planets most likely form?

• A. Direct collapse of gas
• B. Formation from solid cores only
• C. Gravitational capture of passing planets
• D. Planetesimals of rock and ice attracting hydrogen and helium gas from the solar nebula

Answer: (D)

Formation begins with building up solid cores from icy and rocky planetesimals in the cold outer disk. When these cores reach a critical mass, their gravity draws in a vast hydrogen/helium envelope from the surrounding solar nebula, creating the massive gas envelopes that become the outer Jovian planets. This sequence explains why these giants have large amounts of gas along with heavy-element material, matching formation timescales before the nebula dissipates. Other scenarios—direct collapse of gas without a solid core, capturing a planet from elsewhere, or forming as solids only without a thick gas envelope—don’t naturally explain the combination of substantial gaseous envelopes and core-like enrichment observed in the outer giants.