The Grand Tack

One of the most exciting recent things that I’ve worked on recently — with colleagues in France and the US — is a new model for how the Solar System formed that we call the Grand Tack.

“Why do we need a new model for how the Solar System formed?”, you might ask. “What was wrong with the old one?”

The old model formed the rocky planets by bashing together smaller bodies: asteroid-sized objects called “planetesimals” and Moon-sized ones called “planetary embryos”.  That part worked great.  The old model could explain a lot of features of the Solar System, like Earth and Venus’ sizes, orbits and formation timescales.  But there was one thing that it kept screwing up: Mars.

Now, messing up an entire planet is a big no-no in this business!  There is a lot we don’t know but we are pretty confident of Mars’ size so if your model can’t make a planet of Mars’ size then it doesn’t look too good.

This image shows what the real inner Solar System looks like — it’s not perfectly to scale but the relative sizes are right.  Mars is small!  It’s only about one tenth of Earth’s mass and one third of its size.


But our models kept making systems that looked like this:


Mars was as big as Earth, and there were extra planetary embryos stranded in the asteroid belt.  Mercury also doesn’t appear but we’ll talk about that in another post.

The old model didn’t work because there was simply too much rocky material at Mars’ location.  Mars couldn’t avoid growing large because rocky stuff simply couldn’t avoid bashing into it.  We tried all sorts of tricks to remove some of the building blocks from the Mars region.  But whenever we got Mars right, we would screw up a different, even harder to fix part of the Solar System.  Tricky stuff.

One day we hit upon a crazy new idea.  What if Jupiter were the reason that Mars is so small?  Jupiter’s gravity dominates the outer Solar System today, but it is so far away from the rocky planets that it’s hard to imagine it shaping Mars.  But what if Jupiter was once closer to the Sun than it is today?

Remember that Jupiter is a gas giant planet.  It formed early in the Solar System’s history while there was still a thick disk of gas and dust orbiting the young Sun.  The planets all formed from this disk.  But even though they are smaller, the rocky  planets took about ten times longer to form than Jupiter and Saturn.  And after Jupiter and Saturn formed there was a time during which their orbital evolution was affected by the gas disk.  Being far more massive than the planets, the disk could  push them around and make their orbits expand or shrink. This is called orbital “migration”.

This is where things get weird.  Jupiter by itself migrates inward.  But Jupiter and Saturn together migrate outward.  This is a neat hydrodynamical effect that occurs when the outer one is less massive than the inner one.  Here is what Jupiter and Saturn look like when they are embedded in a planet-forming disk.  (hydrodynamical simulation by Arnaud Pierens):

Jupiter and Saturn (circled) embedded in a gaseous planet-forming disk.  The colors show density waves created by the planets' gravity.  The black region is a wide ring-shaped gap containing little gas.  From a hydrodynamical simulation by Arnaud Pierens
Jupiter and Saturn (circled) embedded in a gaseous planet-forming disk. The colors show density waves created by the planets’ gravity. The black region is a wide ring-shaped gap containing little gas. From a hydrodynamical simulation by Arnaud Pierens

Let’s put together the pieces of the puzzle.  Jupiter formed before Saturn.  Jupiter migrated inward.  Saturn caught up and the two planets migrated outward.  If their turnaround point happened at about Mars’ current orbital distance, then the gas giants’ gravity would naturally create an edge in the distribution of rocky material that was busy forming Earth.  Jupiter and Saturn kept migrating outward until the disk ran out of gas and the planets were stranded somewhere not too far from their current orbits.

Since Jupiter migrated all the way to Mars’ current location, it completely cleared out the Mars zone.  So how did Mars get there?  A small Mars forms naturally as an “edge effect”.  Earth and Venus formed within a region with lots of rocky material.  But Mars was gravitationally scattered past the edge imposed by Jupiter.  Mars formed small because it was scattered into a region that was almost completely empty.

This is the Grand Tack model.  A tack is a sailing term for turning into the wind.  In this model, the tack refers to the point where Jupiter’s migration changed direction.  It works really well — it forms rocky planets that look about right!  The Grand Tack also explains the asteroid belt, which is dominated by drier processed “S-type” asteroids in the inner belt and more pristine hydrated “C-type” asteroids in the outer belt.  Even thought Jupiter and Saturn migrated through the asteroid belt twice — both coming and going — they actually  re-filled the belton their way out.  This image shows how the process works.  Here, the Sun is at the far left.  Jupiter moving to the left indicates its migration toward the Sun and moving to the right is migration away from the Sun.

How the Grand Tack emptied and re-filled the asteroid belt.  Credit: Kevin Walsh.
How the Grand Tack emptied and re-filled the asteroid belt. Credit: Kevin Walsh.

When the giant planets migrated inward they scattered a lot of S-type material outward.  When the giant planets migrated back outward, they first ran into the scattered S-types and then into more distant, presumably pristine C-types.  A small fraction of these were placed on stable orbits in the asteroid belt.  The final position of the asteroids depended on Jupiter’s location at the time of scattering.  Since Jupiter was closer-in when it scattered the S-types, the S-types dominate the inner asteroid belt and the C-types dominate the outer belt, as is indeed observed.

A last piece of awesomeness is that the Grand Tack model offers a new explanation of where Earth’s water came from.  I’m going to write a more detailed post about this in the future, but let me just say that it works!

BOOM!  It feels great when a crazy new idea actually works!  I guess that is why it was published in Nature (Walsh et al 2011).  If you’re interested in a slightly more technical explanation including some movies, please check out my Grand Tack website.


42 thoughts on “The Grand Tack

    1. There is indeed a new study out there claiming that the Grand Tack — proposed by researchers mostly in France back in 2011 (including myself) — could have driven a primordial system of super-Earths into the Sun. I think this is a very cool idea but it has two BIG holes. First, the collisional debris created by the migrating Jupiter should be rapidly accreted onto other bodies, not just spiral inward. Second, even if this debris spiraled inward and starting pushing planets inward, it can’t reach the star because the protoplanetary disk has an inner edge at something like 10 times the size of the Sun. Other researchers have used a similar idea to explain how to FORM close-in super-Earths and this paper claims that this same stuff will DESTROY it. I don’t see how it could possibly be right. (But I love that people are thinking about this stuff!)

  1. Makes sense. Mostly.

    Still can not wrap my head around how a pair of in synch Jovian’s can muster the energy to dig out of the Sub’s gravitational well and wander outward.

    Where is this angular mo9emntum coming from?

    1. Good question. The angular momentum comes from gas exterior to Jupiter and Saturn’s orbits. This gas is pushed inward by the planets and the back-reaction of this scattering pushes the giant planets outward. The balance only works for certain configurations of the gas giants, with a more massive inner planet. And FYI there are now two other models that can match the Solar System as well as the Grand Tack — we’re working to differentiate between them. See

  2. I guess the question is then…. how common would a Grand Tack scenario be in extra-solar systems?

    Must something need to occur to make a smaller than 2 Earth mass terrestrial planet in the habitable zone, if no Grand Tack, then the system gets stuck with one or more super Earths instead, which are probably not favorable for life, if too big. Or Hot Jupiters, hot Neptunes and likely no planets in the HZ.

    And if you combine the Grand Tack with the Big Splat, assuming that we needed Luna’s formation to kick start plate tectonics and strengthen our magnetic field, stabilize our axial tilt….. you get a really tiny probability of occurrence. Rare Earth?

    1. The Grand Tack scenario is consistent with being a common outcome in that no pairs of giant planets on close orbits have been found *close to other stars* when we think they should have migrated *outward*.

      We do expect an anti-correlation between close-in super-Earths and outer giant planets (see

      The “big splat” — I assume you’re thinking of the Moon-forming impact. That idea is pretty solid and is easily accomodated by many different formation models. So, no need for Rare Earth…

      1. Very interesting. I have a couple of questions. Do these sorts of models explain the observed orbital inclination angles of the planetary orbits? And do they explain the relatively low angular momentum of the sun? thanks …

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