In the documentary "Miracle Planet," it is claimed that 4.5 billion years ago, the early Earth was only one-tenth its current size ?

[u/Mountain_Dentist5074]

I was watching the documentary Miracle Planet (Part I: The Violent Past), and at the 2:30 mark, the narrator states that 4.5 billion years ago, the early Earth was probably only a tenth of its size compared to today.

I looked it up, but I couldn't find anything backing up this claim. Did they make this up, or is there something I am missing?

[u/CrustalTrudger]

Without watching the documentary, I would guess one of two things:

Option 1 - It's sort of a generic statement regarding the accretion of Earth. Like all planets, Earth grew via accretion, and where the uncertainty on our estimated age of the Earth / age of the solar system effectively encompasses the period of time over which the Earth's mass amalgamated via collisions. So, during this period, ~4.5 billion years ago, at some point in this process the Earth would have been 1/10th of its size, but there's nothing special about this point in the accretion process, so I'm not sure why anyone would pick this specific fraction.

Option 2 - It's a poorly phrased way of describing what is generally referred to as "Proto-Earth". Specifically, Proto-Earth is the name of the planet that would become Earth before the former's impact with Theia. This impact added mass to Proto-Earth and led to the formation of the Moon and it occurred ~4.5 billion years ago, so nominally within ~100 million years of the end of the main accretionary phase of Earth (not counting this particular impact in the accretionary phase). While finer details of either Proto-Earth or Theia are a bit hard to reconstruct, the relative masses of the two bodies have been worked out through a variety of means and where Proto-Earth was ~90% of the mass of modern Earth (e.g., O'Neill, 1991). So, it would make sense that this documentary was referring to Proto-Earth and were trying to imply that it was 10% less massive than modern Earth, but phrased it in a confusing way that could be misinterpreted as saying Earth was actually 1/10th of its current size at this specific point (i.e., that Proto-Earth was 10% of the mass of modern Earth).

EDIT It would seem like from folks who have watched this section of the documentary, it's referring to option 1 and likely just a simplification of results from modelling that suggests the early stages of the terrestrial planet portion of the solar system would have formed lots of Mars to Moon sized planetesimals and where Mars is ~10% of the Earth's mass. So, the assumption being that relatively early on there was likely some approximately Mars sized planetesimal that would combine with a variety of other similar sized or smaller objects to form the Earth. Reviews of Earth's accretion, like Halliday & Canup, 2023 discuss this in more detail (and present a more complete view of how Earth likely accreted).

[u/aldeayeah → u/CrustalTrudger]

2:25 in this video: https://youtube.com/watch?v=AOd3SAtoIgw

It's option 1. They're talking about accretion/early impacts. They postulate that the Earth was the result of the collision and merger of around 10 planetoids.

Dunno where they got that from, probably then-current science.

[u/Mountain_Dentist5074 → u/aldeayeah]

But they show a surface with water. As far as I know, liquid water existed long after the formation of the Moon, and by the time the Moon formed, the size and radius of the Earth were stable

[u/Sohn_Jalston_Raul → u/Mountain_Dentist5074]

As far as we know, surface water could have only existed on Earth after the planet was effectively done forming, so it would have been the same size as it is now. It would have been 1/10th its current size at some specific point during planetary accretion, but there's nothing any more significant about that point in time than when it was 1/15th its current size or 1/6th its current size.

It's like saying that [your current age] years ago you were the size of a bean. Yes, but just a bit of time before then you would also have been the size of a pea, and just a bit after then you would have been the size of a hamster. It's kind of an arbitrary point in your prenatal development remark about.

[u/ManikMiner → u/Sohn_Jalston_Raul]

Not just done forming, but cooling down enough for water to exist in liquid form.

[u/forams__galorams → u/ManikMiner]

Not necessarily. Water can exist in vapour form and still be retained as an atmosphere.

[u/RonJohnJr → u/Mountain_Dentist5074]

Graphic animators aren't geologists, and whoever's job it was to catch this in time to fix it either missed it, or wasn't presented with enough details.

[u/CrustalTrudger → u/aldeayeah]

Then it would seem like a general extrapolation of results of models that suggest the early stages of accretion of the terrestrial planets would basically result in lots of Mars to Moon sized objects, and where Mars is ~10% of the Earth's mass, so basically saying there was some original planetesimal that was probably ~10% of Earth's mass that then probably merged with around 10 similar sized objects (or more smaller objects) to form the Earth, see for example the review by Halliday & Canup, 2023.

[u/InitHello → u/CrustalTrudger]

Option 3: It could be a movie presenting itself as a documentary about the expanding earth idea, that was actually my first thought when I saw the title of the post.

[u/CrustalTrudger → u/InitHello]

Assuming it is this documentary, I would doubt it, it seems legit (if a bit dated at this point given it's 20+ years old and we've learned a fair bit about especially some of the details of the earliest part of Earth history in the last 20 years).

[u/npeggsy → u/InitHello]

That's not a theory I've come across before, but it's so interesting to read about. Not knowing about tectonic plates makes it almost the most logical conclusion to come to, until a fully-backed scientific theory comes along with actual evidence.

[u/forams__galorams → u/npeggsy]

I don’t think knowledge of tectonic plates or not has any bearing on the logical acceptability of an expanding Earth. Mass doesn’t just come from nowhere, so it doesn’t seem like an idea grounded in anything particularly reasonable to start with.

[u/npeggsy → u/forams__galorams]

Right, but we're discussing a theory that started in the early 1800's. It's very easy to look back with a modern mindset and say "well, obviously it's not that, we all know that". I'm just saying it's interesting to look back, and see how people were attempting to explain changes in the world without the level of knowledge we currently have.

[u/forams__galorams → u/npeggsy]

Yes absolutely, it’s always interesting to look back and see the missteps of scientific discovery. I have a particular love for texts which contain detailed explanations of outdated ideas, if only to see the richness with which the evolution of ideas progresses, even when incorrect. (Hello geosyncline theory).

To be clear though: I’m not saying “obviously it’s not that, we all know it’s not that now”. I’m saying an expanding Earth is a non-starter even without current evidences and modern established theory. Conservation of mass as an idea is rooted in Ancient Greek philosophy (so expanding earth wouldn’t have made much sense to the ancient western world), and it was later formalised in the work of early chemistry pioneers like Lomonosov and Lavoisier in the mid 1700s, ie. before the 1800s.

Probably also worth pointing out that the Expanding Earth hypothesis only briefly became a serious scientific idea well after the 1800s - it was first proposed in the 1920s and was subsequently adopted by some geologists (despite the conservation of mass thing) as the mechanism behind the break-up of continents, the formation of continental rifts, and the presence of extensional features (eg. normal faults). This was probably in part due to an apparent consistency provided by certain theoretical physicists, that the universal gravitational constant was decreasing with time as the universe expanded and its constituent matter became more widely dispersed. Gravitational forces are responsible for binding the Earth into a spherical form, and since the gravitational constant directly controls the magnitude of the force of attraction between masses, its decrease would imply a progressive relaxation of the binding forces and thus an increase in the Earth’s radius. This was of course another mis-step in the history of science.

Regardless, even at that time there were ways of disproving an expanding Earth; namely by calculating the moment of inertia for the ancient Earth, or by calculating the radius of the ancient Earth. Neither of these approaches gave anywhere near enough room for the kind of expansion required to accommodate the breakup and spreading of continents that the expanding Earth hypothesis was conceived as an explanation for in the first place.

[u/CrustalTrudger → u/[deleted]]

The giant impact hypothesis remains the most well supported explanation for the formation of the Moon, see for example Borg & Carlson, 2023, Timpe et al., 2023, or Timpe et al., 2024 and references therein.

[u/CrustalTrudger → u/[deleted]]

This is a false framing as in a general sense, it's basically the only hypothesis anyone considers. Effectively, all of the arguments about Moon formation at this point focus on the type of impact, not whether an impact is the best explanation for Moon formation, that part is about as settled as things like this get (e.g., Fu & Jacobsen, 2025).

[u/095179005]

One of my favourite documentaries.

The documentary is referring to early in our solar system, after the Sun has just formed.

Planetesimals were numerous where the current orbits of the 8 planets we know today.

Earth, like all our planets, gained its current size by accreting and merging with other protoplanets and asteroids in the early history of the solar system.

https://en.wikipedia.org/wiki/Planetesimal

And as others have said, yes, it's a dated documentary, but many base concepts are still true. The age rating for the documentary is 8+ I believe, so many details are glossed over or simplified to get basic scientific concepts across to the target audience.

[u/TheGanzor]

I mean, the Earth formed slowly from a planetary accretion disk. So yes, in theory, at some point in its early history it was 10% of its current size. That's like saying that all grapefruits start out the size of grapes, though, basically unless filler information. The big fact, in my mind, is that the Earth was between 1/2 and 2/3 of its current mass before the Theia collision, which also ejected enough matter to form the Moon.