From nebula to planets

Foundations

The orderly picture

The textbook diagram of the solar system is calm: a central Sun, eight planets in tidy circles, rocky ones near, gassy ones far, everything in the place it has always been. The order is real, and the calm is not. The planets built themselves out of collisions, in a few tens of millions of years, and there is good reason to think the giant planets moved a long way from where they formed, wrecking and rearranging as they went. One of the mysteries this explains is why Mars is so small.

A date, from a meteorite

Start with when. Earth’s own rocks cannot answer, because the planet recycles its crust and destroys the evidence. The answer comes from meteorites that have never been part of a planet.

Break open a primitive meteorite and you find white specks, a few millimetres across, rich in calcium and aluminium. Those minerals are the first to condense as a hot gas cools, so these are the oldest solids in the solar system: they formed directly from the nebula and were never reheated. Dating them gives 4.567 billion years, and that number is time zero for everything in this chapter. On the cosmic calendar it is the 2nd of September.

The clock is short

Lesson 2.1 left a young star surrounded by a disc of gas and dust. That disc is not permanent. Surveys of young clusters show that about half of all discs have gone within three million years, and almost all within about six. The star’s own light and wind blow the gas away.

That deadline shapes everything. Anything that needs to gather gas, meaning Jupiter and Saturn, has to be finished within a few million years. The rocky planets, which need no gas, can take much longer, and did.

From dust to worlds

The first step is the strangest. Dust grains, the same ones that were doing chemistry in lesson 3.5, collide gently and stick, building fluffy aggregates. But growth stalls at around a metre: bigger objects bounce off each other or shatter rather than sticking, and they also spiral into the star within a few thousand years. How nature crosses this metre barrier is one of the honest gaps in the theory, and the leading answer is that the disc’s turbulence gathers pebbles into dense clumps that collapse under their own gravity, skipping the difficult sizes entirely.

Past about a kilometre, gravity takes over and the process becomes straightforward. Bodies pull on each other, collide and merge. The largest body in any region grows fastest, because its gravity sweeps a wider area, so it rapidly outgrows its neighbours. Within a few hundred thousand years the disc holds dozens of Moon-to-Mars-sized embryos, moving on crossing orbits.

Then they collide with each other. That final stage takes tens of millions of years and is anything but calm. The Earth is the outcome of dozens of such impacts, the last of which is the subject of the next lesson.

The snow line

Why are the inner planets rock and the outer ones gas and ice? Because of temperature.

Close to the Sun it was too hot for anything but metal and rock to condense. Beyond a certain distance, around three times the Earth-Sun distance, water could freeze. Ice is several times more abundant than rock, so beyond that snow line there was far more solid material available, and cores grew much faster. A core of about ten Earth masses can capture gas from the disc directly, and once it starts it runs away. Jupiter got there in time. Uranus and Neptune, farther out where everything is slower, seem to have been late, which is why they have thick atmospheres but not enormous ones.

This is why the solar system is sorted. Not by the Sun sifting materials by weight, but by where ice could exist while the planets were being built.

Where Mars went wrong

Here is the problem the standard picture cannot solve. Mars sits in a region that should have had plenty of material, yet it is only about a tenth of Earth’s mass. Simulations that build the planets in fixed orbits produce a Mars several times too heavy, every time.

The leading explanation is that Jupiter did not stay put. In the Grand Tack model, Jupiter migrated inward through the disc to about where Mars now orbits, scattering and clearing material as it went, then reversed and moved back out when Saturn caught up and the two became locked together. The region that would become Mars was left stripped. It also explains why the asteroid belt is so nearly empty and why it contains two different populations of bodies, rocky ones from the inner system and icy ones from the outer.

A second, later rearrangement, called the Nice model, has the giant planets shifting again after the gas was gone, flinging small bodies across the solar system and out of it. Both models are well-motivated and neither is settled. What is no longer in doubt is that planets move.

Not the only system

Until 1995 we had one example. Now several thousand planets are known around other stars, and the most useful thing they have taught us is that our arrangement is not a template. Giant planets orbit some stars in a matter of days, closer than Mercury. Planets between Earth and Neptune in size, of which we have none, are among the commonest kinds in the galaxy. Many systems are packed far more tightly than ours.

Planets, in short, are ordinary. Whether systems like ours are ordinary is a different question, and one the surveys cannot yet answer, because the methods that find planets most easily are biased toward the large and the close.

Deeper

Dating the beginning

Calcium-aluminium inclusions are dated by the lead-lead method, which uses the decay of two uranium isotopes to two lead isotopes and requires no assumption about the initial lead. Bouvier and Wadhwa reported 4568.2 million years for an inclusion from the Northwest Africa 2364 meteorite; Connelly and colleagues obtained 4567.30 ± 0.16 million years and dated chondrules from the same and other meteorites, finding they formed from the time of the inclusions and continued for about three million years afterward. The short-lived isotope aluminium-26, with a half-life of 717,000 years, provides a relative chronometer that agrees.

The metre barrier

Grains up to millimetre size grow by sticking through van der Waals forces. Above about a centimetre, collision velocities in a turbulent disc exceed the threshold for sticking, and bouncing or fragmentation dominates. Simultaneously, gas drag causes bodies of around a metre to lose angular momentum and spiral into the star on timescales of order a hundred years at one astronomical unit. The leading resolution is the streaming instability, in which pebbles concentrate in the gas until a clump exceeds its own gravitational binding and collapses directly to a body of tens to hundreds of kilometres, bypassing the vulnerable size range. Laboratory experiments and numerical simulations support this, but the conditions required are not universal.

Oligarchic and giant-impact growth

Once bodies exceed roughly a kilometre, gravitational focusing makes the collision cross-section exceed the geometric one, and growth becomes runaway: the largest body in a feeding zone grows fastest. When an embryo dominates its zone it enters oligarchic growth, and the disc ends with tens of embryos of Moon to Mars mass at roughly regular spacings. Their mutual perturbations then cross their orbits and a chaotic phase of giant impacts follows, lasting 30 to 100 million years and producing the terrestrial planets. Hafnium-tungsten isotope systematics indicate that Earth’s core formation, and hence most of its accretion, was largely complete within about 30 to 50 million years of time zero.

Core accretion and the deadline

In the Pollack model, a solid core grows to several Earth masses, accumulating an increasingly massive gaseous envelope in hydrostatic equilibrium. When the envelope mass approaches the core mass, at around ten to fifteen Earth masses total, the envelope can no longer be supported and runaway gas accretion begins, adding hundreds of Earth masses in a hundred thousand years. The entire sequence must complete before the disc disperses, which is why the timing of the core-building stage is critical, and why the ice-rich region beyond the snow line, where solids are abundant, is where giant planets form.

The Grand Tack in detail

In the Walsh model, Jupiter forms near 3.5 astronomical units and migrates inward to about 1.5 through interaction with the gas disc. Saturn then forms, migrates inward faster, and becomes captured into a mean-motion resonance with Jupiter; the pair then migrates outward together to Jupiter’s present region. The inward passage truncates the inner disc near one astronomical unit, starving the Mars region and producing a low-mass Mars in simulations, and it also scatters both inner rocky and outer icy planetesimals into the asteroid belt, reproducing its observed mixed composition. The model reproduces several features at once, which is its strength; its dependence on specific disc conditions and timings is its weakness.

Frontier

What the models still cannot do

Morbidelli and Raymond list the open problems plainly: the metre barrier is bridged by mechanisms whose conditions are uncertain; the origin of chondrules is unknown despite them being the most abundant material in primitive meteorites; the timing and extent of giant-planet migration are model-dependent; and reproducing the terrestrial planets’ masses and orbits together, including a small Mars and a low-mass asteroid belt, remains difficult without invoking special circumstances. Planet formation is a field with excellent observations and an incomplete theory.

Watching it happen elsewhere

ALMA now resolves rings and gaps in dozens of discs, some around stars less than a million years old, which suggests planets form earlier than models assumed. Two protoplanets have been imaged directly within a disc. Each new system constrains the sequence described above, and several have already forced revisions to assumed timescales.

Transcript and shot list

Target runtime about 8 minutes. Built as building the solar system from dust upward, stopping at the one thing the build cannot explain, and showing what has to move to fix it. Shot types: ANIM is rendered from code with Manim, AI is a generated shot from Higgsfield with the shared style reference, REAL is agency imagery with credit, CAM is the narrator.

[00:00] S01 · ANIM · The textbook solar system: neat circles, tidy labels; then the orbits blur and the planets start moving inward and outward.

Here’s the picture everyone has of the solar system. A Sun, eight planets, tidy circles, rocky ones near, gassy ones far, everything where it has always been. The order is real. The calm is not. These planets built themselves out of collisions, and the big ones almost certainly moved a long way from where they formed, wrecking things as they went. That’s the leading explanation for a puzzle you may never have noticed: why Mars is so small.

[00:36] S02 · ANIM · Cosmic calendar; a marker at 2 September labelled 4.567 billion years ago.

The 2nd of September on the cosmic calendar. Four point five six seven billion years ago.

[00:43] S03 · REAL · A cut and polished slice of a primitive meteorite showing white calcium-aluminium inclusions (museum or NASA image; confirm credit line).

How do we know the date that precisely? Not from Earth’s rocks: this planet recycles its crust and destroys the evidence. From meteorites that were never part of a planet. Break one open and you find white specks, a few millimetres across, rich in calcium and aluminium. Those are the first minerals to condense as a hot gas cools, so they’re the oldest solids there are. They formed straight from the nebula and were never reheated. Date them and you get four point five six seven billion years. That’s time zero for this entire chapter.

[01:27] S04 · ANIM · A disc around a young star; a clock runs; half the discs vanish by three million years, nearly all by six.

Lesson 2.1 left us a young star with a disc of gas and dust around it. That disc is on a timer. Surveys of young clusters show half of them gone within three million years, and almost all within six. The star’s own light and wind blow the gas away. So anything that needs to grab gas, meaning Jupiter and Saturn, has to be finished within a few million years. The rocky planets, which need no gas, can take far longer. And they did.

[02:06] S05 · ANIM · Dust grains sticking into fluffy aggregates; growth stalls at a metre as bodies bounce and shatter; some spiral into the star; then a swarm of pebbles clumps and collapses directly into a large body.

The first step is the strangest one. Dust grains collide gently and stick, building fluffy clumps. But growth stalls at about a metre. Bigger objects bounce off each other or shatter instead of sticking, and they also spiral into the star within a few thousand years. How nature gets past this metre barrier is one of the honest gaps in the theory. The leading answer is that turbulence gathers pebbles into dense swarms that collapse under their own gravity, skipping the difficult sizes altogether.

[02:45] S06 · ANIM · Past a kilometre: gravitational focusing; the biggest body in each zone grows fastest; dozens of Moon-to-Mars embryos on crossing orbits.

Past about a kilometre, gravity takes over and it gets straightforward. Bodies pull on each other, collide, merge. The biggest body in any region grows fastest, because its gravity sweeps a wider area. Within a few hundred thousand years the disc holds dozens of embryos the size of the Moon or Mars, on crossing orbits. And then they hit each other. That final stage takes tens of millions of years, and it is not calm at all. Earth is the result of dozens of those impacts.

[03:24] S07 · ANIM · A temperature gradient across the disc; the snow line at 3 AU; inside, only rock and metal condense; outside, ice, several times more abundant; a core reaches ten Earth masses and runs away with gas.

Why are the inner planets rock and the outer ones gas? Temperature. Close in it was too hot for anything but metal and rock to condense. Past about three times the Earth-Sun distance, water could freeze. And ice is several times more abundant than rock, so out there, there was far more solid material and cores grew much faster. Get a core to about ten Earth masses and it starts capturing gas from the disc directly, and then it runs away. Jupiter made it in time. Uranus and Neptune, further out where everything’s slower, seem to have been late, which is why they have thick atmospheres but not enormous ones. So the solar system is sorted, but not by the Sun sifting things by weight. By where ice could exist while the planets were being built.

[04:27] S08 · ANIM · Simulations building planets in fixed orbits produce a Mars several times too heavy; a red X.

Now the problem. Mars sits in a region that should have had plenty of material, and it’s only a tenth of Earth’s mass. Simulations that build the planets in fixed orbits produce a Mars several times too big. Every time.

[04:46] S09 · ANIM · Jupiter migrates inward to 1.5 AU, clearing the disc; Saturn catches up; the pair lock and move back out; the Mars region is left stripped; the asteroid belt ends up with two populations.

The leading explanation is that Jupiter didn’t stay put. It migrated inward through the disc to about where Mars orbits now, scattering and clearing material as it went, and then reversed and moved back out when Saturn caught up and the two became locked together. The region that would become Mars was left stripped. And the same manoeuvre explains why the asteroid belt is so nearly empty, and why it contains two different populations: rocky bodies from the inner system and icy ones from the outer. It’s called the Grand Tack. A second rearrangement, later, has the giant planets shifting again after the gas was gone and flinging small bodies right across the solar system. Both are well-motivated. Neither is settled. What’s no longer in doubt is that planets move.

[05:46] S10 · ANIM · A gallery of exoplanet systems: hot Jupiters in days-long orbits, super-Earths, tightly packed systems; our own system among them looking unremarkable.

Until 1995 we had one example to go on. Now several thousand planets are known around other stars, and the most useful thing they’ve taught us is that our arrangement is not a template. Giant planets orbit some stars in a matter of days, closer in than Mercury. Planets between Earth and Neptune in size, of which we have exactly none, are among the commonest in the galaxy. Many systems are packed far more tightly than ours. Planets are ordinary. Whether systems like ours are ordinary is a different question, and the surveys can’t answer it yet, because the easiest planets to find are the big close ones.

[06:36] S11 · ANIM · Labels: ESTABLISHED for the age, the disc timer and the snow line; ACTIVE RESEARCH for the metre barrier, chondrules and migration.

The scorecard. The age of the solar system, the short life of the disc, why the inner planets are rocky, and that Earth grew by giant impacts: established. How dust gets past the metre barrier, what made the chondrules that most primitive meteorites are built from, and exactly how far the giant planets moved: active research. Planet formation is a field with excellent observations and an unfinished theory.

[07:07] S12 · AI into ANIM · A disc seen edge-on around a young star; it resolves into planets; the last frame is a molten Earth; the calendar returns.

So by the end of this, there’s a young Sun and a set of planets, one of them roughly the size of ours, still molten, still being hit. And it’s about to be hit by something the size of Mars. Next lesson: Earth’s violent birth, and where the Moon came from.

Shot list

One AI shot, ten code-rendered shots, one meteorite image. Estimated Higgsfield use: 2 to 4 generations including retries.