Scientific Hypotheses
Tracking 25 hypotheses related to planet formation and disk evolution.
4
Active
3
Uncertain
15
Ruled Out
3
Superseded
Active
Dust traps solve barriers and enable planetesimal formation
Pressure maxima and dust traps in substructured disks concentrate particles, overcome growth barriers, and provide sites for planetesimal formation via streaming instability or gravitational collapse
Evidence:
Active
Pebble accretion for rapid planet growth
Planets can grow rapidly by accreting mm-cm sized pebbles that drift inward, allowing for fast formation timescales consistent with young observed planets
Evidence:
Active
Pebble accretion as dominant mechanism for rapid core growth
Pebble accretion (mm-cm sized particles) can form planetary cores much faster than planetesimal accretion alone, solving the timescale problem for giant planet formation (cores must form within ~3 Myr disk lifetime). The reservoir of pebbles plays an important role in the growth of planetary cores.
Active
Early dark energy resolves Hubble tension
A scalar field contributing ~10% energy density around matter-radiation equality reduces sound horizon and raises H0
Uncertain
Pebble accretion as dominant mechanism for terrestrial planet formation
The hypothesis that terrestrial planets (Earth, Mars) formed primarily by accreting millimeter-sized pebbles drifting inward from the outer solar system. This was proposed to explain rapid growth timescales.
Uncertain
ΛCDM is complete for H0
Standard ΛCDM correctly predicts H0 from CMB without new physics
Uncertain
Local systematics explain Hubble tension
Unresolved systematics in Cepheid/TRGB measurements account for the H0 discrepancy
Superseded
Streaming instability requires super-solar metallicity
Earlier work suggested streaming instability requires metallicities significantly higher than solar values to trigger strong particle clumping and planetesimal formation
Evidence:
Superseded
Classical planetesimal accretion as sole mechanism
Giant planet cores form exclusively through pairwise collisions of km-sized planetesimals over millions of years
Superseded by: 2012A&A...544A..32L
Superseded
Meter-sized barrier as insurmountable
Dust growth stalls at meter sizes due to rapid radial drift (the meter-sized barrier problem), preventing planetesimal formation
Superseded by: 2010A&A...513A..57Z
Ruled Out
Free mixing between inner and outer Solar System
Scenario where material from inner and outer Solar System freely mixed during the first ~4 Myr of Solar System formation
Ruled out because: NC-CC isotopic dichotomy shows prolonged spatial separation of inner/outer disk reservoirs for 1-4 Myr, incompatible with free mixing
Ruled Out
Late Jupiter formation (>4 Myr)
Hypothesis that Jupiter's core formed late (beyond 4 Myr after Solar System formation) and could not have created an early barrier
Ruled out because: NC-CC dichotomy requires early and rapid growth of Jupiter's core to inhibit material exchange between inner/outer disk, ruling out late formation scenarios
Ruled Out
Uniform isotopic composition across protoplanetary disk
Assumption that the solar protoplanetary disk had uniform isotopic composition throughout
Ruled out because: NC-CC dichotomy demonstrates fundamental isotopic heterogeneity between inner and outer Solar System reservoirs
Ruled Out
Direct growth from micron to km sizes in smooth disks
The idea that dust particles can grow continuously from micron to kilometer sizes in smooth protoplanetary disks without encountering insurmountable barriers
Ruled out because: Bouncing barrier prevents growth beyond mm-cm sizes in smooth disks; fragmentation and radial drift also halt growth
Evidence:
Ruled Out
Pebble accretion origin of Solar System terrestrial planets
Solar System terrestrial planets (Earth, Mars) formed by accreting sunward-drifting millimeter-sized pebbles from the outer solar system
Ruled out because: Isotopic compositions of Earth and Mars show limited contribution from outer solar system material (few percent), refuting pebble accretion and supporting collisional growth from inner solar system embryos
Evidence:
Ruled Out
Pebble accretion origin of terrestrial planets
Terrestrial planets (Earth, Mars, Venus) formed primarily by accreting mm-sized pebbles drifting inward from the outer solar system
Ruled out because: Isotopic compositions of Earth and Mars show
Evidence:
Ruled Out
Streaming instability works equally well for all particle sizes
The hypothesis that streaming instability efficiency is relatively uniform across different particle sizes (Stokes numbers)
Ruled out because: Li & Youdin discovered a sharp increase in the critical metallicity for small solids when Stokes number ≤0.01, meaning very small particles cannot trigger SI even at high metallicity
Evidence:
Ruled Out
Linear SI growth rates predict nonlinear clumping
Linear, unstratified streaming instability growth rates can reliably predict particle clumping outcomes in nonlinear, stratified simulations
Ruled out because: Li & Youdin found that linear, unstratified SI growth rates are a surprisingly poor predictor of particle clumping in nonlinear, stratified simulations, especially when finite resolution is considered
Evidence:
Ruled Out
Core-powered mass loss for mini-Neptunes
Core-powered mass loss models predict slow outflows (1-3 km/s) for mini-Neptune atmospheric escape
Ruled out because: Helium observations show fast outflows (10-30 km/s) consistent with photoevaporation, not slow core-powered models
Ruled Out
Jupiter gap as sole barrier for NC-CC separation
Jupiter's core opens a gap that traps dust and separates inner/outer Solar System materials, explaining NC-CC meteorite dichotomy
Ruled out because: Simulations show trapped particles rapidly fragment and leak through the gap, contaminating inner disk on timescales inconsistent with meteoritic record
Ruled Out
Spatially uniform planet formation in disks
Planets form uniformly throughout protoplanetary disks with no preferential locations
Ruled out because: ALMA observations show preferential locations for planetesimal and planet formation; significant fraction of solids doesn't grow past pebble sizes outside these locations
Ruled Out
Synchronous core formation within single disk
All planetary cores within a protoplanetary disk form at approximately the same time
Ruled out because: Review emphasizes that cores within one disk likely form at different times; growing evidence that first cores start forming early during disk buildup
Ruled Out
Pebble accretion origin of Solar System terrestrial planets
Hypothesis that Earth and Mars formed primarily by accreting millimeter-sized pebbles that drifted inward from the outer solar system
Ruled out because: Isotopic evidence shows Earth and Mars contain
Ruled Out
Gravitational instability as primary giant planet formation mechanism
The hypothesis that most giant planets form directly through gravitational instability and fragmentation of protoplanetary disks, rather than through core accretion
Ruled out because: Rafikov 2005 showed that disks must be extremely massive (>0.7 Msun) and hot (>1000K) to fragment at planet-forming distances, and Kratter 2010 demonstrated that typical disk conditions produce brown dwarfs/stars rather than planets
Evidence:
Ruled Out
High turbulence throughout disks
Protoplanetary disks are highly turbulent throughout, driving accretion via turbulent viscosity
Ruled out because: Observational measurements show disks are not as turbulent as thought; turbulence levels may be insufficient to power accretion, suggesting MHD winds may drive accretion instead