PLANETPLANET

You wouldn’t know it from the asteroid fields in Star Wars, but our asteroid belt is basically empty.  If you flew a spaceship through the belt, the odds of crashing into an asteroid are pretty much zero.  Plus, no giant space-slugs (we haven’t found them yet, anyway…).

The asteroid belt is huge.  It extends from Mars’ orbit all the way to Jupiter, and just the main belt  — where most of the asteroids are — has more than twice the surface area of the Solar System interior to Mars’ orbit.  But add up all of the asteroids and you get less than a thousandth of Earth (about 0.05% of an Earth to be precise).

My new paper (published Sept 13 in Science Advances) proposes that asteroids are all refugees.  They grew across the Solar System but were kicked out of their homes, left to travel the voids of space and finally settle in the asteroid belt.

To explain, I need to discuss how the Solar System formed.  So let’s rewind the clock four and a half billion years, back to when the planets were forming from a swirling disk of gas and dust in orbit around the young Sun….

Given how near-empty the asteroid belt is right now, there are two ways to think about its origin. The first is to imagine that the belt once contained a lot more mass than it does now. Say, 1-2 Earth masses in rocky bodies.  To arrive at the present-day Solar System, the vast majority of that mass must have been lost.

The second approach envisions an empty primordial asteroid belt.  If the belt were born completely empty, then the challenge is to understand how the belt was populated. (Don’t worry, we’ll get to why it’s not crazy to imagine it was born empty.)

Let’s go through these two ideas.

Almost all previous work has focused on a massive primordial asteroid belt.  This is the foundation of the “classical model” of rocky planet formation (see animation here). That model has a fatal flaw: it tends to produce gigantic Marses and over-populated asteroid belts (often with normal-sized Marses). There are some other issues, like the lack of planets that look like Mercury (see possible solution here).  Here is a cartoon like the planetary systems generally produced by the classical model:

There are ways to rescue the classical model.  The most successful to date is the Grand Tack, which proposes that Jupiter’s orbit shifted drastically after its growth.  First, Jupiter “migrated” inward to close to Mars’ current orbit, then back outward to near its current orbit.  In the process, the asteroid belt was mostly cleared and Mars’ growth was stunted.  The model works quite well at matching the inner Solar System, although there are still some issues (see below).

The opposite end of the spectrum is to assume an empty primordial asteroid belt.  Before we go any further the question on your mind is probably, why would the asteroid belt have been born empty?  Planet-forming disks are mostly gas, which should be nice and smooth.  Now remember: planets grow from dust, not gas.  Here is a high-resolution image of a nearby disk that we think is forming planets as we speak:

This image only shows the dust (the gas distribution is smooth).  As you can see, the dust is clumped into rings.  There are gaps — belts — with less dust than the bright rings.  This happens because dust grows and drifts through the disk. Asteroid-sized “planetesimals” form in dust pileups.  This is a rich-get-richer process: the clumps make lots of planetesimals whereas the gaps can end up with none.

Back to the asteroid belt.  It is entirely possible that dust piled up to form planetesimals in the Earth-Venus zone as well as the Jupiter-Saturn region but not in the asteroid belt.  Some of the best models we have find exactly this. Punchline: the assumption of an empty primordial asteroid belt is actually not crazy.

Then where did the asteroids come from?

This is the distribution of asteroids in the present-day belt. Almost all of the mass is in the C-types and S-types, so let’s focus on them.

The S-types and C-types have very different chemical properties.  The main difference is that the S-types are dry rocks whereas the C-types are rich in water, carbon and other volatiles.  S- and C-types are so different that they kind of look like they came from completely different places (foreshadowing….)

In a previous post I explained how the C-types were implanted into the belt during Jupiter and Saturn’s growth.  To summarize: planetesimals orbiting near the growing Jupiter and Saturn were gravitationally scattered across the Solar System.  A fraction was implanted in the asteroid belt, preferentially in the outer parts.  Another portion was launched past the asteroid belt and delivered water to the growing Earth.  Here is an animation of the process — you can see Jupiter grow from 100-200 kyr (1kyr = 1000 years) and Saturn from 300-400 kyr.

Now come the S-types.  The S-types may simply be leftovers from the inner parts of the Solar System.  The terrestrial planets grew from planetesimals, and during the late phases of growth many planetesimals are kicked outward.  They often end up on asteroid belt-crossing orbits but these are generally pretty stretched-out (elliptical). Some scattered bodies are transferred onto stable, more circular orbits by gravitational kicks from “rogue planetary embryos“, the Moon- to Mars-sized head honchos of rocky planet growth. Most rogue embryos are on their way to encountering Jupiter and being launched into interstellar space, but on their way out rogue embryos often kick some scattered planetesimals onto stable orbits.

Below is an animation of the implantation of S-types.  Since this is a low-probability thing, I’ve combined 50 different simulations into one movie.  So this movie shows 50 parallel universes at once!  (The black circles are big planetary embryos and the red dots are planetesimals.  The grey shaded area is the asteroid belt — you can see when planetesimals get trapped.  The embryos at high-eccentricity above the belt are the “rogue embryos” I’ve been talking about.)

By this multiple scattering process, rocky planetesimals are trapped into the main asteroid belt with an efficiency of about 0.1% (1 in 1000).  That seems awfully tiny, right?  But remember that the asteroid belt is almost empty.  The total mass in S-types is only a few hundred-thousandths of an Earth mass — about 0.004% of an Earth mass.  And there were a few tenths of an Earth mass in planetesimals at this time.  Our simulations usually implanted 3-10 times more S-types than there are today, starting from zero.  Over the Solar System’s lifetime the belt has been depleted by about that much, so it’s a match.

Here is what it looks like when we put together the C-type and S-type stories:

This provides a good match to the real belt. S-types dominate the inner main belt and C-types the outer main belt.

This means that the asteroid belt would look like just it does today even if it formed completely empty!  The S-types are implanted as byproducts of the growth of the terrestrial planets. Likewise, the C-types (as well as Earth’s water) are just splatter from the growth of the giant planets.

This means that asteroids can be thought of as refugees.  They were born in a specific part of the Sun’s planet-forming disk.  Then, during the growth of the planets they were kicked out, gravitationally launched into inter-planetary space.  Finally, they were trapped onto stable orbits in the belt. The asteroid belt may be a cosmic refugee camp. 

The implantation processes for S- and C-types are both very robust.  In fact, they are each unavoidable, and simply byproducts of planet formation.  S-types are implanted from the rocky planet region and C-types from the giant planet zone even if the asteroid belt was not born empty.  No matter the Solar System’s formation history, there must be remnants of the Earth’s building blocks trapped in the asteroid belt, and also planetesimals from the Jupiter-Saturn region and beyond. Since different asteroids condensed across the Solar System, this means that we can explore our entire planetary system within the belt!

Where do we go from here?  This is not the final solution to how the Solar System formed. The empty primordial asteroid belt model is on par with the Grand Tack model, which assumes a massive primordial asteroid belt. Each of these models does a good job matching the rocky planets and asteroid belt.  And each has a weakness, a feature that, if disproven, would make the whole theory crumble like a pile of wooden blocks. For the Grand Tack model, the weakness is Jupiter’s outward migration.  It is not clear whether Jupiter’s orbit really could have expanded from roughly Mars’ present-day orbit out to its current one (technical details here).  For the empty primordial asteroid belt model, the weakness is the “empty” part.  To get a handle on that we need to better understand how dust drifts and piles up to form planetesimals, and whether that process forms a gap like the one envisioned in this idea.  We need to come up with tests to falsify these models, and keep our eyes out for new ones that work even better.

Questions, comments, words of wisdom?

 

---

The paper, “The empty primordial asteroid belt”, was published on Sept 13 in Science Advances and can be downloaded here (official) or here (arxiv).

Many thanks to my trusty coauthor Andre Izidoro!