From dust to planetesimals

This is Chapter 2 in the Solar System’s story. This one is about the first step — how the building blocks of planets (called planetesimals) form.


Dust growth and drift

Earth is a big rock. But the disk that spawned the planets was 99% gas and only 1% solids. When the Sun formed, those ever-so-important solids were just tiny little dust grains. They were helpless, blown along with the gas in the disk in orbit around the young Sun, kind of like babies strapped to their parent being carried around the supermarket. When one dust grain happened to run into a neighboring dust grain, they usually stuck together. The collision speeds were so slow that the dust grains could gently fuse together into long fractal-shaped dust bunnies.

Artist’s view of dust within a gas-dominated planet-forming disk. Credit: SOKENDAI/NAOJ.

These dust bunnies could not grow too big. After a certain size, collisions between dust no longer leads to growth. This is no big surprise — toss two clumps of sand together and they don’t grow, they just disintegrate.

It turns out that there are a lot of different possible outcomes when two dust grains bash into each other. Just look at them all:

Illustration of different outcomes of collisions between dust grains. Credit: Zsom et al (2010).

Even if their growth is limited, big dust grains become harder to push around — they gain inertia. They are no longer completely at the mercy of the gas, “blowing in the wind,” so to speak. Rather, large grains still feel the wind but end up losing orbital energy because of it, causing them to spiral inward. It’s kind of like when babies grow into toddlers, who crawl all over the place. Dust grains large enough to drift through the gas in this way are called pebbles — they are usually about the size of a grain of sand (about 1 mm).

It was shown back in the 1970s that pebbles drift inward, and fast. This led to a famous “catastrophe” in planet formation-land: how could planets form if their building blocks all drifted inward, to presumably fall onto the Sun?

The solution to this catastrophe took decades to come to light. It turns out that if enough drifting pebbles can get together, they can clump into much larger objects that don’t drift so fast. The required concentration of pebbles is not that high — if the amount of solids (pebbles) relative to gas increases to a few to ten percent (rather than the original 1%), things start to get interesting.

Planetesimal formation from clumps of pebbles

Rather than being pushed around by the gas, a concentration of pebbles acts collectively to slow down the gas. This allows more pebbles to enter the fray, which further slows down the gas and allows even more pebbles to join in. This effect is called the streaming instability. Through the streaming instability, grains of sand-sized pebbles become so concentrated that they clump directly into mountain-sized planetesimals.

Planetesimals are the protagonists of almost every planet formation story. They are the building blocks of the planets. They were typically about the size of a big city (10-100 km across). The present-day populations of asteroids and comets are leftover planetesimals — or fragments of planetesimals — that didn’t make it into planets.

We used to assume planetesimals formed in a broad disk. But that thinking has changed. Images of disks around young stars show that dust is not usually found in nice smooth disks. Instead, large dust grains (pebbles) tend to be concentrated in rings. We can’t zoom in and inspect what exactly is going on, but it’s possible that the streaming instability is operating and forming planetesimals within the dust concentrations but not elsewhere. This would form rings of planetesimals, whose characteristics will be come up again in later chapters of our story.

HL Tau‘s disk as seen by the ALMA telescope. Bright regions show concentration of pebble-sized dust (within the gas, which is not seen directly).

Why should pebbles become concentrated into rings? Well, it turns out that pebbles don’t just drift inward. Rather, they simple go from areas of low- to high-pressure within the gas. If there is a ‘bump’ in the gas pressure it acts as a pebble trap and can just gather all the pebbles that drift inward. Pressure bumps may be extra-special places for planetesimals to grow.

Planetesimal compositions

Planetesimals have the same compositions as their constituent pebbles. Close to the Sun, where it’s hot, they’re made of iron and rock. But farther away from the Sun, where it is cooler, other more volatile molecules exist as solids. The most important is water, which in the very low-pressure environment of the disk, condenses at a temperature of about 170 Kelvin (or -100 degrees Celsius). Beyond the “snow line” water exists as solid ice and any planetesimals that form are about 50-50 ice-rock mixtures (with a little iron included in the “rocky” part). At even colder temperatures other ices exist, like carbon dioxide, ammonia, and carbon monoxide, but they are not abundant enough to have a big impact on planetesimal compositions.

The “snow line” is the distance beyond which water freezes. Credit: astronoo.com

The meteorite dichotomy

Meteorites have a lot to tell us about where and when planetesimals formed, although not everyone agrees on exactly what that is.

What are meteorites, anyway? Any rock that crashes down on Earth is called a meteorite. (If it burns up in the atmosphere it’s a meteor). Most meteorites are small pieces of asteroids that were smashed apart. For instance, iron meteorites are pieces of the cores of asteroids, whereas some stony meteorites are pieces of the outer layers of similar asteroids.

Chondrites are meteorites that may remember the conditions when the planets formed. Chondrites are a mix of iron and rock, and we think that their parent asteroids never got hot enough to separate out, or to “differentiate“, into iron cores and rocky outer layers. These meteorites are thought of as pristine, mostly unchanged since when planetesimals formed.

Illustration of some of the different meteorite types. Credit: Phil Metzger.

Meteorite science took a giant leap forward when it was discovered that there are really only two fundamental classes of meteorites. When I was in graduate school (in the early 2000s), I had to learn about a whole bunch of different meteorite types. But recent analyses of meteorite isotopes show that all of these types fall into just two classes, that we now call carbonaceous (or CC) and non-carbonaceous (or NC).

Let’s break out a scientific graph about meteorites. Each of the points on this graph refers to a single meteorite. Two measurements have been made: the approximate formation time of its parent body (vertical axis), and its Molybdenum isotopes (horizontal axis — the units represent deviation from some standard). The lines are uncertainties, or “error bars”, associated with each measurement; they are pretty big simply because these measurements are really difficult.

The meteorite ‘dichotomy’. Non-carbonaceous (red) meteorites have different Molybdenum isotope ratios than carbonaceous (blue) ones. Credit: Kleine et al (2020)

The first takeaway from this graph is that the red points go almost to zero on the vertical axis. That means that planetesimals started forming really quickly once the Sun formed. The second point is that there are two fundamentally different groups of meteorites: the CC (carbonaceous, in blue) and NC (non-carbonaceous, in red). The main thing that separates the CC and NC meteorites is their different isotope distributions. But the vertical axis indicates that their ages overlap. That means that planetesimals of each of these types were forming at the same time in the Sun’s disk, just in different places.

Carbonaceous meteorites are associated with objects in the outer parts of the asteroid belt, and non-carbonaceous meteorites with the inner asteroid belt. Most (but not all) carbonaceous meteorites have a lot more water than non-carbonaceous ones. This all points toward planetesimals in the outer Solar System being CC-like and in the inner Solar System NC-like.

Why did the two types of meteorites stay separate? We will come back to this issue in a later chapter (spoiler: it might have been Jupiter).

Meteorite measurements can help us figure out the building blocks of the planets. The planets must have grown from planetesimals that were similar to each of these types of meteorites. Earth’s isotopic composition is closer to the NC group, but our ocean water is a good match to CC meteorites. This leads to the idea that Earth grew mostly from (mostly dry) NC meteorite-like planetesimals with just a sprinkling of CC planetesimals to deliver the water.

The Solar System’s planetesimals

This is all pretty vague. We all want to know: exactly where and when did the Solar System’s planetesimals form? This is one of the most active areas of planetary science right now and there are a lot of ideas out there.

One new model predicts that the Solar System formed from three separate rings of planetesimals. Each ring of planetesimals formed at a pressure bump where the gas reached the evaporation temperature of silicate rocks, water and carbon monoxide. Here is the cartoon version:

Cartoon version of a model in which the Solar System formed from three rings of planetesimals. Credit: Rajdeep Dasgupta and Andre Izidoro.

This model is appealing because each ring can explain a different part of the Solar System. It can match the planets, asteroids and comets. And the rings of planetesimals didn’t mix (explaining the NC/CC meteorite dichotomy) because the pressure bump stopped drifting pebbles. And, put together, it tells a pretty nice story.

But, bear in mind that this is just one of several models out there right now.

Wrap-up

The TL;DR version of this post: planetesimals are mountain-sized rocks (sometimes with ice) that grow from clumps of drifting dust (“pebbles”). The two types of meteorites come from planetesimals that formed in different parts of the Solar System and remained separate.


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