an autodidact meets a dilettante…

one impression of our proto-sun and solar nebula

Jacinta: I’d like to know how we got in this position.

Canto: What position?

Jacinta: Here, on Earth.

Canto: We?

Jacinta: Humans.

Canto: That’s a very long story, which I suspect nobody’s really qualified to tell. But maybe we can report on the best speculations. First, in order to understand how we got here we have to understand how the Earth got here.

Jacinta: And so on, infinitely regressing. So let’s just start with the Earth.

Canto: Needless to say we don’t know all the details and there are doubtless competing theories, and new data is being regularly uncovered, but it obviously has to do with how our entire solar system was formed.

Jacinta: I’ve heard that all the heavy metals like iron and whatnot are forged within stars, like when they go supernova, but our star hasn’t done that, all it seems to produce is light, yet Earth is full of heavy elements. I really don’t get it.

Canto: I recall reading years ago a theory that the Earth was formed from an accretion of planetesimals, little planets…

Jacinta: Planettes?

Canto: Yes, but how those little things came into being themselves I’m not sure.

Jacinta: Well we have lots of rocky bits and bobs called asteroids floating about in the solar system…

Canto: Yes, but not randomly. there’s a whole big asteroid belt between Jupiter and Mars, where they’re coralled, sort of.

Jacinta: But comets are different, they seem to have their individual eccentric orbits.

Canto: I suppose the point is that they also have heavy elements, and how were those elements formed?

Jacinta: Heat and pressure, I’m guessing, so things must’ve been hugely different in earlier times.

Canto: Well, this BBC site gives us some of the latest speculations. They reckon that the Earth probably formed from planetesimals, so that’s still the best hypothesis it seems, though it’s very light on details:

The Earth is thought to have been formed about 4.6 billion years ago by collisions in the giant disc-shaped cloud of material that also formed the Sun. Gravity slowly gathered this gas and dust together into clumps that became asteroids and small early planets called planetesimals.

Jacinta: Yes, that’s extremely vague. How do they know there was a disc-shaped cloud here? How can they investigate that far back?

Canto: Well don’t forget that looking out over huge distances means looking back in time.

Jacinta: Yes but a huge distance away isn’t here. Is it?

Canto: Well it might be here then.

Jacinta: Effing Einstein. But they’re also searching for extra data on the past, like checking out meteorites, which might contain material older than anything on Earth. Can they reliably date material that’s say, 5 billion years old? The Earth’s only about 4.5 billion years old, right?

Canto: I think 4.6 billion, give or take a few minutes. About a third of the age of the universe. And here’s the thing, we’ve dated all the meteorites and asteroids we can get to and they’re all round the same age, within a narrow range of a few hundred million years. So our date for the beginnings of the solar system is the oldest date for these floating and landing rocks, which is also our date for the Earth, about 4.6 billion.

Jacinta: So is our dating system completely accurate, and what by the way are carbonaceous chondrites?

Canto: Well, yes, radioactive decay provides a very accurate clock, and these meteorites have radioactive material in them, just as the core of our planet does. All the evidence so far suggests that things happened very quickly, in terms of accretion and formation of planets, once all this heavy and radioactive material was created. Carbonaceous chondrites are a type of meteorite. They’re amongst the oldest meteorites but relatively rare – they make up less than 5% of our meteorites. I mean the ones that land here. Why do you ask?

Jacinta: I’ve heard about them as being somehow important for research, and maybe dating?

Canto: Well there are different types of C chondrites as they’re called, and some of them, most interesting to us of course, are rich in organic compounds and water. This fact apparently shows that they haven’t been subjected to high temperatures, unlike for example the early Earth. But let me return to that BBC quote above. The theory goes that a supernova explosion, or maybe more than one, created all the heavy elements we have now – iron, carbon, silver, gold, uranium and all the rest, heat and pressure as you say, and these elements swirled around but were gravitationally attracted to a centre, which evolved into our sun. This was the spinning disc-shaped cloud mentioned above, known as the solar nebula.

Jacinta: Would you call that a theory, or a hypothesis, or wild desperate speculation?

Canto: I’d call it ‘the best we can do at the present moment’. But be patient, it’s a great time to be young in astronomy today. What we need is data, data, data, and we’re just starting to collect more data than we can rightly deal with on planets within and especially outside our solar system. Kepler’s just the beginning, girlie.

Jacinta: Je suis tout à fait d’accord, boyo. I think many of the astrophysicists are looking forward to having their cherished models swept aside by all the new telescopes and spectroscopes and what else and the data they spew back to Earth.

Canto: Uhh, well anyway let’s get back to our ‘best scenario for the moment’ scenario. So you have all this matter spinning around and the force of gravity causes accretion. It’s a messy scenario actually because everything’s moving at different velocities and angular momentums if that’s a thing, upwards, forwards, sideways down, and sometimes there’s accretion, sometimes fragmentation, but overall the movement is towards coalescence due to gravity. Particles grow to the size of monuments and then different sized planetesimals, fewer and bigger and farther between. And the smaller, gaseous elements are swept out by the solar wind into the great beyond, where they accrete into gas giants.

Jacinta: Right, but isn’t the data from Kepler and elsewhere already starting to play havoc with this scenario? Gas giants within spitting distance of their suns and the like?

Canto: Well, you need liquid to spit, but maybe you have a point, but I think it’s wise not to be too distracted by exoplanets and their systems at this stage. I think we need to find an internally coherent and consistent account of our own system.

Jacinta: What about the Juno probe, will that help?

Canto: Well I’m sure it will help us learn more about gas giants, but let’s just focus on the Earth now.

Jacinta: Okay, stay focussed.

Canto: These larger planetesimals became bigger gravitational attractors, each accumulating matter until we had four rocky planets in different, sufficiently distant orbits around their sun.

Jacinta: Oh yes, and what about the moons? Why didn’t they coalesce as neatly as all the other minor rocky bits?

Canto: Mmmm, well there’s nothing neat about all this, but mmmm…

Jacinta: How many moons are there?

Canto: For the inner planets? Only three, ours and two for Mars. So the question is, how come some of those rocks, or at least three, didn’t get stuck to the bigger rocks i.e. planets, via gravity, but instead started circling those planets, also due to gravity.

Jacinta: Yes, which might be the same question as why do the planets orbit around this massive gravitational attractor, the sun, instead of getting sucked into it, like what happens with those supermassive supersucking black holes?

Canto: Well first let me talk about our moon, because the most currently accepted theory about how our moon came into existence might surprise you.

Jacinta: It was a lot closer to the Earth at the beginning, wasn’t it? So it’s slowly spiralling away from us?

schematic of tidal forces affecting moon’s orbit and earth’s rotation

Canto: Yes. Tidal forces. The moon’s tidally locked to the Earth, it’s the same face she shows us always, but let’s keep on track, it was formed in the very early days, when things were still very chaotic. A pretty large planetesimal, or planetoid, slammed into Earth, which was somewhat smaller then, and it stuck to it and coalesced with it – the Earth was pretty-well molten in those days – and a lot of debris was thrown out into space, but this debris didn’t quite escape Earth’s gravitational field, instead it coalesced to form our moon. This theory was first put forward a few decades ago, after moon rocks brought back from the Apollo missions were found to be younger than the oldest Earth rocks, and composed of much the same stuff, which came as a great surprise. But now the theory is well accepted, as it accounts for a number of other factors in the relationship between the two bodies.

the hypothesised Thea impact, which enlarged the Earth and created the Moon

Jacinta: Okay, so is that it on how the Earth was formed?

Canto: Well, yes, but the bigger question is your original one – how did we get here. And that means we have to look at how life got started here. Because we’re only up to about 4.5 billion years ago – with the moon being formed about 50 million years after the Earth. And at that point the Earth was like a sea of hot magma, hot from all the collisions on the surface, and hot from the radiation bursting out from its core. Hardly great conditions for life.

Jacinta: Well there might’ve been life, but not as we know it boyo.

Canto: I’m skeptical, but we’ll talk about that next time.

Some sources:

on the relation between moon and earth

formation of the moon

the solar nebula theory and its problems