PLANETPLANET

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):

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.

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.