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Starts With A Bang

What Was It Like When Planet Earth Took Shape?

The ‘giant impact’ that led to Earth might not have been so giant, after all.

A little over 4.5 billion years ago, our Solar System began to form. Somewhere in the Milky Way, a large cloud of gas collapsed, giving rise to thousands of new stars and star systems, each one unique from all the others. Some stars were much more massive than our Sun; most were much smaller. Some came with multiple stars in their systems; about half the stars formed all by their lonesome, like ours did.

But around practically all of them, a large amount of matter coalesced into a disk. Known as protoplanetary disks, these would be the starting points for all the planets that formed around these stars. With the advances in telescope technology that’s accompanied the past few decades, we’ve started to image these disks and their details firsthand. For the first time, we’re learning how planetary systems like our own came into existence.

20 new protoplanetary disks, as imaged by the Disk Substructures at High Angular Resolution Project (DSHARP) collaboration, showcasing what newly-forming planetary systems look like. The gaps in the disk are likely the locations of newly-forming planets. (S. M. ANDREWS ET AL. AND THE DSHARP COLLABORATION, ARXIV:1812.04040)

In theory, the process of forming planets is incredibly straightforward. Whenever you have a large mass, like a gas cloud, you can expect the following steps to happen:

  • the mass gets drawn into a central region,
  • where one or more large clumps will grow,
  • while the surrounding gas collapses,
  • with one dimension collapsing first (creating a disk),
  • and then imperfections in the disk grow,
  • preferentially attracting matter and forming the seeds of planets.

We can now look directly at these protoplanetary disks, and find evidence that these planetary seeds are present from a very early time.

The star TW Hydrae is an analogue of the Sun and other Sun-like stars. Even from its very early stages, as imaged here, it already shows evidence of new planets forming at various radii in its protoplanetary disk. (S. ANDREWS (HARVARD-SMITHSONIAN CFA); B. SAXTON (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO))

But these disks won’t last very long. We’re looking at timescales that are typically only tens of millions of years long to form planets, and that’s due to not only gravitation, but to the fact that we’ve got at least one central star shining as well.

The cloud of gas that will form our planets is made out of a mix of elements: hydrogen, helium, and all the heavier ones, going way up the periodic table. When you’re close to the star, the lightest elements are easy to blow off and evaporate. In short order, a young solar system will develop three different regions:

  1. a central region, where only metals and minerals can condense into planets,
  2. an intermediate region, where rocky and giant worlds with carbon compounds can form,
  3. and an outer region, where volatile molecules such as water, ammonia, and methane can persist.
A schematic of a protoplanetary disk, showing the Soot and Frost Lines. For a star like the Sun, estimates put the Frost Line at somewhere around three times the initial Earth-Sun distance, while the Soot Line is significantly closer in. The exact locations of these lines in our Solar System’s past is hard to pin down. (NASA / JPL-CALTECH, ANNONATIONS BY INVADER XAN)

The border between the inner two regions is known as the Soot Line, where being interior to it will destroy the complex carbon compounds known as polycyclic aromatic hydrocarbons. Similarly, the border between the outer two regions is known as the Frost Line, where being interior to it will prevent you from forming stable, solid ices. Both lines are driven by the heat of the star, and will migrate outward over time.

Meanwhile, these protoplanetary clumps will grow, accrete additional matter, and will have opportunities to gravitationally perturb one another. Over time, they can merge together, gravitationally interact, eject each other, or even hurl one another into the Sun. When we run simulations that allow planets to grow and evolve, we discover an extraordinarily chaotic history that’s unique for each and every solar system.

When it comes to our own Solar System, the cosmic story that unfolded was not only spectacular, it was in many ways unexpected. In the internal region, it’s very likely that we had a relatively large world present early on, which was possibly swallowed by our Sun in our cosmic youth. There is nothing preventing a giant world from forming in the inner Solar System; the fact that we have only the rocky worlds close to our Sun tells us that something else was likely present early on.

The largest planets probably formed from seeds early on, and there may have been more than four of them. In order to get the present configuration of gas giants, the simulations we run seem to show that there was at least a fifth giant planet that was ejected at some point long ago.

In the early Solar System, it’s very reasonable to have had more than four seeds for giant planets. Simulations indicate that they are capable of migrating inwards and outwards, and of ejecting these bodies as well. By the time we reach the present, there are only four gas giants that survive. (K. J. WALSH ET AL., NATURE 475, 206–209 (14 JULY 2011))

The asteroid belt, between Mars and Jupiter, is very likely the remnants of our initial Frost Line. The border between where you can have stable ices should have led to a large number of bodies that were a mix of ice and rock, where the ices mostly sublimated away over the billions of years that have passed.

Meanwhile, out beyond our last gas giant, the leftover planetesimals from the Solar System’s earliest stages persist. Although they may merge together, collide, interact, and occasionally get hurled into the inner Solar System from gravitational slingshots, they largely remain out beyond Neptune, as a relic from the youngest stages of our Solar System. In many ways, these are the pristine remnants from the birth of our cosmic backyard.

The planetesimals from the portions of the Solar System beyond the Frost Line came to Earth and made up the majority of what is our planet’s mantle today. Out beyond Neptune, these planetesimals still persist as the Kuiper belt objects (and beyond) today, relatively unchanged by the 4.5 billion years that have passed since then. (NASA / GSFC, BENNU’S JOURNEY — HEAVY BOMBARDMENT)

But the most interesting place of all, for our purposes, is the inner Solar System. There may have once been a large, interior planet that was swallowed, or perhaps the gas giants once occupied the inner regions and migrated outwards. Either way, something delayed the formation of planets in the inner Solar System, allowing for the four worlds that did form — Mercury, Venus, Earth, and Mars — to be much smaller than all the others.

From whatever elements were left, and we know they were mostly heavy ones from the planetary density measurements we have today, these rocky worlds formed. Each one has a core made of heavy metals, accompanied by a less-dense mantle made out of material that fell onto the core later, from beyond the Frost Line. After only a few million years of this type of evolution and formation, the planets were similar in size and orbit to how they are today.

As the Solar System evolves, volatile materials are evaporated, planets accrete matter, planetesimals merge together, and orbits migrate into stable configurations. The gas giant planets may dominate our Solar System’s dynamics gravitationally, but the inner, rocky planets are where all the interesting biochemistry is happening, as far as we know. (WIKIMEDIA COMMONS USER ASTROMARK)

But there was a huge difference: in these early stages, Earth didn’t have our Moon. In fact, Mars didn’t have any of its moons, either. In order for this to occur, something needed to create them. That would require a giant impact of some type, where a large mass struck one of these early worlds, kicking up debris that eventually coalesced into one or more moons.

For Earth, this was an idea that wasn’t taken particularly seriously until we went to the Moon and investigated the rocks we found on the lunar surface. Quite surprisingly, the Moon has the same stable isotope ratios that the Earth does, while they’re different between all the other planets of the Solar System. Additionally, the Earth’s spin and the Moon’s orbit around Earth have similar orientations, and the Moon has an iron core, all facts which point to a mutual common origin for the Earth and the Moon.

The Giant Impact Hypothesis states that a Mars-sized body collided with early Earth, with the debris that didn’t fall back to Earth forming the Moon. This is known as the Giant Impact Hypothesis, and while it is a compelling narrative, it might only have elements of the truth, rather than being the full story. It is possible that all rocky planets with large moons acquire them via collision like this. (NASA/JPL-CALTECH)

Originally, the theory was called the Giant Impact Hypothesis, and was theorized to have involved an early collision between proto-Earth and a Mars-sized world, called Theia. The Plutonian system, with its five moons, and the Martian system, with its two moons (that likely used to be three), all show similar evidence of having been created by giant impacts long ago.

But now, scientists are noticing problems with the Giant Impact Hypothesis as originally formulated for creating Earth’s Moon. Instead, it looks like a smaller (but still very large) impact, from an object originating much farther out in our Solar System, may have been responsible for the creation of our Moon. Instead of what we call a giant impact, a high-energy collision with proto-Earth could have formed a debris disk around our world, creating a new type of structure known as a synestia.

An illustration of what a synestia might look like: a puffed-up ring that surrounds a planet subsequent to a high-energy, large angular momentum impact. (SARAH STEWART/UC DAVIS/NASA)

There are four big properties of our Moon that any successful theory for its origin must explain: why there is only one large moon rather than multiple moons, why the isotope ratios for elements are so similar between the Earth and Moon, why the moderately volatile elements are depleted in the Moon, and why the Moon is inclined as it is with respect to the Earth-Sun plane.

The isotope ratios are particularly interesting for the Giant Impact Hypothesis. The similar isotopic properties between the Earth and Moon suggest that the impactor (Theia) and Earth, if they were both large, had to be formed at the same radius from the Sun. This is possible, but models that form a Moon via that mechanism don’t give the right angular momentum properties. Similarly, grazing collisions with the right angular momentum give rise to different isotopic abundances than what we see.

A synestia will consist of a mixture of vaporized material from both proto-Earth and the impactor, which forms a large moon inside of it from the coalescence of moonlets. This is a general scenario capable of creating one single, large moon with the physical and chemical properties we observe ours to have. (S. J. LOCK ET AL., J. GEOPHYS RESEARCH, 123, 4 (2018), P. 910–951)

That’s why the alternative — a synestia — is so appealing. If you have a fast, energetic collision between a smaller body that’s less massive and our proto-Earth, you’d form a large torus-shaped structure around the Earth. This structure, called a synestia, is made of vaporized material that originated from a mix of proto-Earth and the impacting object.

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Over time, these materials will mix, forming many mini-moons (called moonlets) in short order, which can stick together and gravitate, leading to the Moon we observe today. Meanwhile, the majority of the material in the synestia, particularly the inner part, will fall back to Earth. Rather than a single, contrived giant impact, we can now speak in terms of generalized structures and scenarios that give rise to large moons like our own.

Rather than a single impact from a massive, Mars-sized world in the early Solar System, a much lower-mass but still high-energy collision could have given rise to our Moon. Collisions like this are expected to be far more common, and can better explain some of the properties we see on the Moon than the traditional Theia-like scenario involving a giant impact. (NASA / JPL-CALTECH)

There was almost certainly a high-energy collision with a foreign, out-of-orbit object that struck our young Earth in the early stages of the Solar System, and that collision was required to give rise to our Moon. But it was very likely much smaller than Mars-sized, and it was almost certainly a sturdy strike, rather than a glancing collision. Instead of a cloud of rock fragments, the structure that formed was a new type of extended, vaporized disk known as a synestia. And over time, it settled down to form our Earth and Moon as we know them today.

At the end of the early stages of our Solar System, it was as promising as it could be for life. With a central star, three atmosphere-rich rocky worlds, the raw ingredients for life, and with gas giants only existing much further beyond, all the pieces were in place. We know we got lucky for humans to arise. But with this new understanding, we also think the possibility for life like us has happened millions of times before all throughout the Milky Way.

Further reading on what the Universe was like when:

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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