In light of the recent extra-solar planet discoveries, which of the following assumptions of the nebular theory is likely wrong?

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BlackTom AI · Accepted Answer

Question restatement: The prompt asks, given the surge of extra-solar planet discoveries, which assumption of the nebular theory is likely wrong?

Option A: Jovian planets are likely to form far away from their star. This aligns with the classic view of gas giants forming beyond the frost line where ices condense, and is still broadly supported by many formation models. It is not the assumption flagged as unlikely by exoplanet findings, so this option is typically considered plausible rather than incorrect.

Option B: Once a planet forms, its orbit does not change. This statement conflicts with the growing body of evidence from exoplanet systems showing planets can migrate significantly after formation due to interactions with the protoplanetary disk, planet-planet scattering, or other dynamic processes. The idea that orbits are static is contradicted by observed hot Jupiters and resonant chains, making this assumption likely wrong in light of discoveries.

Option C: The solar system flattens into a disk due to conservation of momentum. The nebular theory indeed describes the collapse of a rotating cloud into a flattened, disk-like structure because angular momentum conservation leads to a spinning, thinning disk. This is a core, consistent aspect of the theory and remains supported by observations of protoplanetary disks around young stars.

Option D: Planets as massive as Jupiter must be primarily hydrogen and helium. Gas giants like Jupiter are predominantly H/He, a standard expectation of nebular theory for forming massive planets in sufficiently cold regions. While the exact composition can vary and include heavier elements, the overall premise about primary constituents is broadly consistent with current understanding, not a likely wrong assumption.

Putting the pieces together, the statement that cannot be reconciled with the exoplanet era is that a planet’s orbit remains fixed after formation. The dynamic histories inferred for many exoplanet systems undermine the notion of static orbits, making option B the one that is likely wrong in light of recent discoveries.

In other words, the other options describe aspects that remain largely in agreement with nebular theory and observational evidence, whereas the idea of unchanging planetary orbits after formation clashes with observed planetary migration phenomena in many systems.

ASTR-1-45046-F25 Lecture: Exoplanets

In light of the recent extra-solar planet discoveries, which of the following assumptions of the nebular theory is likely wrong?

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