Dynamical origin of Theia, the last giant impactor on Earth

https://arxiv.org/abs/2507.01826

Discussion

nehal3m
The last giant impactor so far.
shadowgovt
So the boil-down on this is "Here's a theory that says about 5-10% of Earth's mass was mostly carbon and came all at once, like if Theia was mostly carbon and we got hit by it, so we did some simulation and the idea Theia was mostly carbon isn't ruled out by our current understanding of how our solar system might have formed?"

If so, cool. It's a wise step to check the hypothesis to make sure it isn't immediately contradicting what we already understand.

readthenotes1
The moon has 1.2% the mass of earth, so earth still got embiggened
kridsdale1
What an epically cromulent day it was.
echelon
Imagine if life had evolved on Earth or Theia prior to impact. Imagine if it was intelligent and played witness to the giant cataclysm.

Given that intelligence took an awfully long time to emerge from LUCA, that seems implausible. But it's fun to imagine pre-Theia "Silurians". That sort of impact would have scorched earth of any trace or remnant of their existence. It feels as though there must be sufficiently advanced civilizations out there witnessing this exact scenario play out without the necessary technology to stop it. Though that fate would be horrifying.

Another thing to think about is that shortly after the Big Bang (if there was one, Lamda-CDM or similar models holding up), was that shortly after the Big Bang the temperature of the early universe was uniformly 0-100 degrees Celsius. It may have been possible for life to originated in this primordial interstellar medium without even so much as needing a host planet or star! Just life coalescing in space itself.

That early primordial soup, if it existed, could have seeded the whole universe. Most aliens might have matching molecules and chirality if those decisions predate our galaxy.

MarkusQ
That early warm interval would have been a soup of 75% H, 25% He, and 0.0000000% or so Li, with nothing heavier.

Not much to start life with.

echelon
No metals whatsoever. That's unfortunate.
GuB-42
Metal and hard rock are essential for life.
Towaway69
As Douglas Adams wrote, life is about banging the rocks together.
labster
I think they were referring to metals like carbon and oxygen, but I’m sure Led Zeppelin is important too.
andrewflnr
> It feels as though there must be sufficiently advanced civilizations out there witnessing this exact scenario play out without the necessary technology to stop it. Though that fate would be horrifying.

I suspect this is not actually that common. Giant impacts are more common in early solar systems; things eventually settle into nice circular orbits like we have now. Whereas intelligent life does seem to take a while to evolve, so probably more common later in a solar system's life cycle.

dylan604
> Whereas intelligent life does seem to take a while to evolve

basing that theory on an anecdotal story of 1.

andrewflnr
Depends how you count it. One planet, but a solid handful of mass extinctions with big adaptive radiations, most with several million years of development, and tons of reasonably intelligent social animals, only one of which produced industrial civilization. But yes, working with the best evidence we have...
kaliqt
While true, the opposite is true that we have zero evidence of any life whatsoever, anywhere, ever, except with us.
nkrisc
You got better evidence?
echelon
Uncommon for sure.

Our sun and earth won't last long enough, but Mercury's orbit is potentially unstable.

A red dwarf might harbor live bearing planets long enough to see its long-lived orbits eventually destabilize. Or perhaps witness the even rarer interstellar collision or destabilization from rogue planets, etc.

motoboi
We certainly cannot know if a previous intelligent civilization was present on earth before the impact.

And is not even that the impact erased their trace from earth, is that if they got extinct long enough (more than 100 million years ago), with today's technology, we cannot infer their presence, given the fossil (and other types of) record.

A great write up about this is: https://archive.is/https://pacificklaus.com/the-silurian-hyp...

somanyphotons
This animation makes it appear that proto-Earth was very comprehensibly torn apart, stunningly so

https://en.wikipedia.org/wiki/Theia_(hypothetical_planet)#Co...

shadowgovt
I remember there being some modeling done to determine whether the Theia impact blew a chunk off Earth or basically re-liquified the planet. If I recall correctly, the resulting hypothesis was that the thermal load would have re-melted at least the crust (evidence for this was stacking of density in the moon, suggesting it formed out of a basically completely-liquified ball, which would have implied the crust was also liquified).

There is some interesting evidence suggesting the deeper layers remained intact, in the form of a region under the Pacific that might be the impact scar. It's an inexplicably-dense zone that causes hot-spots at its corners resulting in increased surface volcanism, like how the edges of a leaf burn before the middle in a fire.

... but on the surface? Yeah, no hiding place.

whycome
Would oceans have remained at all?
ahazred8ta
According to physics simulations, the atmosphere was composed of vaporised rock for several centuries after the impact until it cooled down below 1000 degrees. The entire surface was magma. https://en.wikipedia.org/wiki/Synestia
shadowgovt
No; I don't remember the article saying specifically but I would assume if there is no solid land left, there is no liquid water left either. Water molecules would have been blasted into the "crust soup" and eventually re-condensed into gaseous water and eventually liquid water via atmospheric regeneration after the surface settled down a bit (because the chemicals that could be gaseous would have tended to float to the top of the soup as it settled down).
belinder
Temperature on its own wouldn't be enough for life would it? Isn't everything moving around way too fast after the Big bang and therefore too far apart for whatever life there would be to find food (or whatever equivalent source of energy)
kulahan
Temperature isn’t even close to being enough. If we didn’t have a moon, despite everything else being so good for life, we may have been stuck at the bacterial phase if we didn’t have tides, or life may have never formed at all due to minerals not being recycled, tide pools not concentrating amino acids, and constant wet-dry phases driving evolutionary pushes.

Edit: beyond that, there’s the need for a stable orbit, a stable axial tilt, a stable star (few mega flares), some kind of galactic shield a la Jupiter, and more.

kbelder
"Need" may be overstating it. Those things certainly all seem to have helped, though.
nntwozz
The pre-Theia "Silurians" as you call them, depending on technological level could have left traces in the solar system like our Parker Solar Probe or something in the Lagrange points.

Then again, how well do we know of stuff in these spaces today? It seems to me we barely have a clue of the space junk we ourselves sent up orbiting in our backyard.

hbrav
That second idea (cosmic primordial soup causing universal similarity / compatibility of life) could be a great component of a sci-fi story.