Something I've been wondering lately is how much the grand tack hypothesis stands up to present scrutiny. While the grand tack proposes that Jupiter and Saturn were caught in a 3:2 mean-motion resonance which results in migration outwards to roughly close to their present distances, simulations of dynamics of planetary migration seem to indicate that a 1:2 MMR is much more likely to occur, which most likely does not result in migration outwards.

Also, a much more recent hypothesis suggests that Jupiter formed much further out than the present location and then migrated in over a period of ~700,000 years, which would explain the distribution of Trojans in Jupiter's orbit. The starting scenario of this hypothesis is completely different from the grand tack which states that Jupiter formed around 3.5 AU distance. It also says absolutely nothing about Jupiter and Saturn migrating further inwards and then back out due to resonance.

Can the grand tack hypothesis be reconciled in any way from these newer studies?


1 Answer 1


What you have to keep in mind is that the Grand Tack was devised as a mechanism to generate a truncated planetesimal disc between 0.4 and ~2 AU during the protoplanetary disc phase, in order to explain the small masses of Mars and Mercury (Walsh et al., 2011).

The smallness of Mars and Mercury might be explained differently by pebble accretion (Lambrechts et al., 2019), which wasn't fully worked out yet during that time.

A direct test of whether Jupiter/Saturn indeed migrated during their growth phases, is their Trojan number asymmetry, which is the basis of the paper by Pirani et al., which you quoted. They find that the Trojan-to-Greek numbers should be equal only in the case of in-situ formation, but they are not.
The observed asymmetry is consistent with migration inwards during Jupiter's growth. However, from their work, no clear statements can be made on

  1. where Jupiter's core formed;
  2. how fast the inward migration proceeded; or
  3. whether there was a later outward migration.

Furthermore, the migration rates required for the Grand Tack to operate might be wrong. Recent work coming from people working on migration rates (McNally et al., 2019, Lega et al., 2020) seem to indicate that migration in low-viscosity discs works very differently than what was envisioned in earlier decades. Outwards migration, or stopping, is then easier for giant planets.

Summarizing, many of the puzzle pieces on why the Grand Tack was envisioned and how it worked are currently under active investigation. This is not an issue I expect to be solved soon, as certainty into any of those models would additionally require Trojan asymmetry numbers for Neptune and Uranus, and those are very challenging.

  • $\begingroup$ It would be nice to have a model that explained the Juno data on Jupiter's strange core (I put some relevant links here). $\endgroup$
    – PM 2Ring
    Feb 8, 2021 at 15:01
  • $\begingroup$ @PM2Ring: Jupiters fuzzy core might as well originate in core erosion. The possible link with a giant impact is unclear. But in OP's question the inwards-outwards migration is the most critical part of physics. Are you proposing there should be an influence of the direction of migration on to the the core fuzziness? $\endgroup$ Feb 8, 2021 at 15:44
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    $\begingroup$ The migration is definitely the critical aspect of the OP's question (and your answer). I only made that previous comment because you mentioned Jupiter's core, and it would be a nice bonus if an alternative to Grand Tack could also go some way to explaining the (apparently) fuzzy core. $\endgroup$
    – PM 2Ring
    Feb 8, 2021 at 16:20
  • $\begingroup$ I'm not a fan of the grand tack theory myself, and I find the idea of orbital resonance as a cause for Jupiter's movement away from the sun very far fetched. Wouldn't it be more credible that the early solar system flew through a cloud, which would make the outer orbits (like Saturn) heavier, and this would have caused the movement of Jupiter back to the outer layers of the Solar system? $\endgroup$
    – Dominique
    Jan 26, 2022 at 15:18
  • $\begingroup$ @Dominique: Can you substantiate your disbelief in outwards orbital type III migration with a study or some numbers? I don't think this part of the scenario is very widely disputed. You seem to misunderstand migration in a gas disc. The more massive part of a disc will push the planet radially away from it, due to the exchange of angular momentum. A massive outer disc will push the planet faster inward. Gravity is merely the mediating force between the disc and planet. $\endgroup$ Jan 26, 2022 at 16:09

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