Did the Solar System form inside-out?

It is hard to know where things come from. If I drink a Belgian-style beer brewed in Colorado using hops from Washington state and I live in France, where did the beer actually come from?  And what does that mean anyway?

The cosmic situation is just as confusing.  We know where the planets are right now; we can measure their orbits to high precision.  What we don’t know is where they came from.  Did the planets form where they are today or did they get their start somewhere else?  Or, like my beer, did they pick up material in a sort of in-between way?

Here we will investigate a new idea: that Jupiter, the biggest planet in the Solar System, started its growth very close to the Sun, even closer than the planet Mercury.  It’s kind of a crazy idea, but it might actually solve a couple long-standing Solar System mysteries…  (Plus, it’s my idea and I like it!)

Planets don’t just sit still as they grow.  The planets’ birthplace — disks around young stars — are essentially giant conveyor belts.  These disks are made of gas and dust,most of which spirals inward and falls onto the star. To survive, planets must get off the conveyor belt.

Let’s think about Jupiter.  Jupiter dominates the Solar System.  It is more massive than all the other planets combined.  Jupiter is the Solar System’s Google, Darth Vader, and The Rock at the same time.

Just as every giant was once a kid, every gas giant was once a rocky or icy core.  We think that Jupiter first grew a core five to ten times as massive as Earth, then vacuumed up gas from the disk to grow to its current mass of 318 Earths.  Of course, throughout this process Jupiter was on the disk’s conveyor belt.

Earth-sized or larger planets launch spiral waves in the planet-forming disk.  Watch out, they are sort of hypnotic (….you are feeling sleepy and want to send me all your money….):

A planet somewhat more massive than Earth embedded in a gas-dominated planet-forming disk launches spiral waves. The planet is located to the left of center. The Sun is not shown in this image. Credit: Arnaud Pierens.

These waves push back on the planet and change its orbit.  Sometimes the planet’s orbit shrinks and sometimes it grows.  This is called planetary migration.  Whether a planet migrates inward or outward and how fast depends on how massive and how hot the disk is, how massive the planet is, and where the planet is (see here for more detail than you probably want to know).

Let’s rewind the clock to when Jupiter was just a puny core, only three times as massive as Earth (that is, only 1% as massive as it is today).   Models tells us that there is a special location in the disk where a planet can stop migrating.  Sort of a place where everything balances and the planet stays in the same place (on the moving conveyor belt).  Let’s call this the “no-migration zone”.

There are two solutions.  Maybe Jupiter’s core formed far away and migrated inward to the no-migration zone.  That is what the textbooks say.  But there is another possibility: maybe Jupiter’s core formed much closer to the Sun and migrated outward.  That is the foundation of our crazy idea of the day.


How could Jupiter’s core grow so close to the Sun?  The rocky planets are tiny compared with Jupiter, so shouldn’t Jupiter’s core have been tiny too?  Maybe not.  Small “pebbles” are thought to continually drift inward through the disk.  Pebbles drift so fast that they zoom along the disk’s conveyor belt in the same direction.

Pebbles don’t grow much by bashing into each other.  When two small rocks collide, do they stick?  (Of course not).  But if you get enough pebbles together in one spot, they can coalesce as a group to form a big body.  It’s like the opposite of Legos — two pebbles won’t stick; you need a garage full of them before you can build anything (sort of like a live-in Lego house).

If even a fraction of the pebbles falling into the Sun were captured and coalesced into a larger body, Jupiter’s core could form quickly.  Then, it would migrate outward, pushed along by those hypnotizing density waves.  And it could make it all the way out to where Jupiter is now!  This migration fits naturally into the fanciest models we have right now.

So the crazy story holds up — Jupiter’s core might indeed have formed really close to the Sun!  But what do we get out of this idea?  Is there any signature of this process, any sign that this actually happened?

I’m glad you asked!  One way in which the Solar System is weird is that we have no big planets close to the Sun.  About half of all Sun-like stars have a “hot super-Earth” or a “mini-Neptune”, a planet a little bigger than Earth on an orbit shorter than Mercury’s.  These planets are really common and we don’t have one.

Comparison between the Solar System’s planets (at the bottom) and a number of known systems with hot super-Earth planets.  The Solar System is unusual in its lack of close-in super-Earths.  Credit: K. Schlaufman (2014, Astrophysical Journal).

The Solar System’s closest rocky planet is Mercury and it’s pretty puny.  If the Sun’s planet-forming disk extended in to the Sun (as it should have), then it’s natural to imagine that large rocky planets should have formed close to the Sun, as illustrated in this image:


Let’s connect the dots: Jupiter’s migrating core might explain the absence of “mega-Mercuries” in the Solar System.

Here is how it works.  As Jupiter’s core migrated outward, it plowed into the rocky building blocks of the inner planets.  But those objects didn’t just collide with the core.  Jupiter’s core pushed them and caused their orbits to grow.  The “shovel” that Jupiter’s core used to “plow” these objects are called orbital resonances and they are pretty powerful.

But Jupiter’s core couldn’t just push all the rocky stuff out of the way as it migrated.  The orbital “snowplow” effect is much stronger close to the Sun, where the planet-forming disk is denser.  Farther away it is weak.  So, close to the Sun Jupiter’s core should have snowplowed away rocky stuff, but beyond some distance the core would simply migrate past rocky bodies.  It turns out that the division happens at around the orbit of Venus.  So, Jupiter’s migrating core would have emptied out the rocky stuff very close to the Sun but not the building blocks of Venus and the other actual rocky planets.  Here is the cartoon version of the story :


There are a couple of bonuses!  As Jupiter’s core migrates through and clears out rocky stuff, it can actually help coalesce those rocks into another core.  That could plausibly represent the “seed” of Saturn’s core.

Some of the rocky bodies that Jupiter’s core snowplows can be transported from very close to the Sun out to what is now the asteroid belt.  This might also solve the mystery of why there are some very iron-rich bodies in the asteroid belt.  It makes a lot more sense for these iron asteroids to have formed very close to the Sun, and perhaps they hitched a ride with Jupiter’s migrating core!

Quick summary.  Jupiter now sits in a colder region of the Solar System, but its core may have formed very close to the Sun.   Jupiter’s migrating core may have cleared out rocky material close to the Sun and could explain why there are no large rocky planets (no “hot super-Earths”) close to the Sun. 

Here is the actual scientific paper if you want the gory details. It was a group effort: the team included Andre Izidoro, Bertram Bitsch, Seth Jacobson, and myself.  (And here is a Science News article about the paper).

A last short note for the astute reader.  I’ve written before about the Solar System’s lack of super-Earths (see here).  The idea in this post is a different point of view but it is actually compatible with the “Jupiter barrier” idea (which I still like).


Finally, I wanted to use the concept of “migration” to bring up something extra important. This is the final sentence in the paper:

dedication_marisa.pngI wouldn’t be an astronomer if it wasn’t for my wife Marisa.  She has sacrificed a lot for me and my job.  We have lived in 4 different places chasing my dreams of being an astronomer.  Marisa is highly educated — she had masters degrees in Public Health Genetics and in Genetic Counseling — but cannot use those degrees here in France (where I landed a permanent job) because of administrative and language hurdles.  If it weren’t for Marisa’s support and the sacrifices she has made I wouldn’t have the luxury of spending my days pondering planets.  I couldn’t do it alone. Any success I have is thanks to her.  Thank you Marisa.

Being a scientist comes with a big set of hurdles (see here and here for some perspectives).  If you are lucky enough to get a permanent job you first have to spend 5+ years getting a PhD, do a postdoc or two or three (usually in different cities/countries), and travel to lots and lots of meetings and conferences.  It’s no picnic for a partner to tag along on this roller coaster. A large fraction of scientists’ careers are made possible by sacrifices made by their partners. These partners are absolutely essential in making science happen.  This study is dedicated to them.






12 thoughts on “Did the Solar System form inside-out?

  1. From the paper

    “Finally, we wonder: why would the Solar System only
    have formed a single core in the innermost parts of the disk?’

    What happens If an addition core forms and also migrates outward?

    1. Good question! Well, the dynamics would be the same: the migrating core would clear out much of the rocky objects close to the star. The core would migrate out to a zero-torque location (the right hand side of the eggs in Figs 1 and 6 in the paper). The first core should already be hanging out there and the two would likely become trapped in resonance (see this paper for details: http://goo.gl/6Yu9sy).

      So, basically, the net effect would be to clear out the rocky stuff from the inner disk a bit more effectively, and maybe also to transport some material to the asteroid belt. It’s unlikely that the second core would collide with the first one but the two could plausibly be the cores of Jupiter and Saturn!

  2. The key thing for Jupiter being bigger than Saturn is when it formed. Since Jupiter and Saturn are both mostly made of gas, and the disk is the source of gas, we think Jupiter got bigger because it formed earlier when the disk was denser. The migration of the cores probably doesn’t matter too much for determining Jupiter and Saturn’s relative final masses.

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