If Jupiter had NOT migrated inward and then outward in the early solar system (as proposed by the Grand Tack hypothesis), which observation would be HARDEST to explain?
AWhy Jupiter is composed mostly of hydrogen and helium gas
BWhy Mars is significantly smaller than Earth and why the asteroid belt is so depleted
CWhy Jupiter has persistent storm systems like the Great Red Spot
DWhy Jupiter has many large moons formed from disk material
Jupiter's inward migration would have swept through and scattered the material available for inner planet growth, stunting Mars and depleting the asteroid belt. Jupiter's composition (hydrogen/helium) follows from core accretion beyond the snow line — no migration needed to explain that. Storm systems are atmospheric phenomena unrelated to migration history. Moon formation is explained by disk accretion around the growing planet.
Question 2 Multiple Choice
Why can't hot Jupiters — gas giants orbiting very close to their host stars — have formed in their current positions?
AStars emit radiation that prevents gas giants from forming within a certain distance
BClose to the star, temperatures are too high for ices to condense and solid material is too scarce for core accretion to build the ~10 Earth-mass core needed to trigger runaway gas capture
CGas giants cannot form by core accretion at all — they must form by gravitational disk instability
DGravitational forces from the star would immediately strip the gas envelope from a newly forming giant planet
Giant planets form by core accretion: icy and rocky planetesimals accumulate beyond the snow line where volatile ices condense, building a solid core of ~10 Earth masses. Close to the star, it's too hot for this ice to exist and the solid material density is too low. Hot Jupiters must have formed in the outer disk, beyond the snow line, and migrated inward through interaction with the protoplanetary disk.
Question 3 True / False
The snow line (frost line) is critical to giant planet formation because beyond it, volatile ices condense into solids, dramatically increasing the amount of material available for planetesimal building and core accretion.
TTrue
FFalse
Answer: True
Beyond the snow line, water ice, methane ice, and ammonia ice all contribute solid grains alongside rock and metal, roughly tripling the solid surface density compared to the inner disk. This abundance of building material allows cores to grow rapidly to the ~10 Earth-mass threshold needed to trigger runaway gas accretion. The snow line is not just a temperature boundary — it's the threshold that makes giant planet formation possible.
Question 4 True / False
The current positions of the planets in our solar system reflect where they formed in the protoplanetary disk — the giant planets formed in the outer system and have remained in roughly those positions ever since.
TTrue
FFalse
Answer: False
Multiple lines of evidence and theoretical models (Grand Tack, Nice model) show that giant planets migrated significantly from their formation locations. Jupiter likely migrated inward to ~1.5 AU before Saturn's formation drove them both back outward. Neptune likely moved outward into the Kuiper Belt in a later dynamical instability. The solar system's architecture is the product of violent dynamical history, not orderly in-place formation.
Question 5 Short Answer
Why does giant planet formation face a strict time deadline that rocky planet formation does not?
Think about your answer, then reveal below.
Model answer: Giant planets must accumulate their massive gas envelopes from the protoplanetary disk, but the disk dissipates within roughly 3–10 million years, driven away by stellar winds and radiation. Once the disk is gone, there is no gas to accrete. If a solid core doesn't reach the critical ~10 Earth-mass threshold in time to trigger runaway gas capture before disk dispersal, it cannot become a gas giant — it remains as a rocky or icy body. Rocky planets, by contrast, grow by collisions between solid bodies, a process that can continue for hundreds of millions of years after the gas disk is gone.
This tight deadline explains why only a few giant planets formed in our solar system (and why not all protoplanetary disks around other stars produce giant planets). The race between core growth and disk dispersal is a key constraint in planet formation theory.