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What makes a planet “giant”? - Ravit Helled
2025-02-27 16:00 - 17:00 UTC | Hoyle Lecture Theatre, Institute of Astronomy
The formation history of giant planets, both within and beyond our solar system, remains an open question. I propose a new pathway for giant planet formation, where runaway gas accretion begins only when a planet reaches about 100 Earth masses and occurs only after a few million years (Myrs). This suggests that the transition to a gas giant—defined as a planet primarily composed of hydrogen and helium—occurs at around Saturn’s mass. The delay in runaway gas accretion to later stages and higher planetary masses results from an intermediate phase of efficient heavy-element accretion. This process generates enough energy to hinder rapid gas accretion. Consequently, Saturn may never have undergone runaway gas accretion, classifying it as a "failed giant planet."
This proposed transition to a gas giant planet above Saturn’s mass naturally explains the distinct bulk metallicities and internal structures of Jupiter and Saturn, as well as the unique characteristics of Uranus and Neptune. In the context of giant exoplanets, postponing runaway gas accretion to planets exceeding Saturn’s mass explains the transitions in the mass-radius (M-R) relations of observed exoplanets, the relatively low occurrence of gas giants, and the high metallicity of intermediate-mass exoplanets.
