About 13.8 billion years ago, the universe began in a big bang. With it was born space, time and a fiery soup of exotic subatomic particles. Of this, cosmologists are reasonably confident.
In the first fraction of a second, the super-hot universe – with a temperature of 10,000 trillion trillion degrees – inflated in size by 100 trillion trillion times. The expansion continues today but at a far more leisurely pace.
When it was one second old, the now-inflated universe contained matter, radiation and the four basic forces of nature. After, only three minutes, the expanding cosmos had cooled enough for the first three elements – hydrogen, helium and lithium – to start forming.
After a few hundred million years of pitch-black darkness – a period dubbed the cosmic Dark Ages – the great ignition began. This was when the first stars lit up the sky as galaxies assembled from vast clouds of frigid primordial matter that condensed under the embrace of gravity.
The conversion of cold, virtually inert gas into dazzlingly hot suns continued for billions of years, seeding the universe with billions upon billions of galaxies, each containing billions upon billions of stars.
One of those galaxies was our own Milky Way, its vast agglomeration of matter arranged in luminous spiral arms sprouting from a central spherical bulge.
Then, about 4.6 billion years ago, a cloudlet of interstellar gas and dust – enriched by heavy elements from previous generations of giant and very bright but short-lived stars that had exploded – started to collapse roughly halfway from the Milky Way's centre.
"Over billions of years, the original mixture of hydrogen, helium and lithium became more and more enriched with heavier elements such as carbon, nitrogen, oxygen, silicon, and heavy metals, produced by subsequent generations of massive stars," ICRAR-Curtin University astrophysicist Rob Soria says.
So far so good. But, this is where the doubt creeps in. For a start, what exactly caused the cloudlet to collapse?
"Quite possibly shock waves from the explosion of a neighbouring star or two," Dr Soria says. As it shrank, he explains, the cloudlet spun faster and flattened into the shape of a whirling disc that grew hotter and denser until the hydrogen gas at its core started fusing into helium. From a lifeless blob of gas, a star, our sun, had ignited into being.
The ravenous sun devoured more than 99 per cent of the original disc – which is why the sun's mass is 745 times greater than that of all the solar system's planets combined.
That's the sun. So, what about the planets? Over many millions of years, scientists say, our suite of orbs developed from what was left – the residual debris called the solar nebula.
According to the now widely accepted core accretion hypothesis, dust grains formed as material collided and coalesced. Hordes of grains united to produce objects up to a metre in size.
Although many of these drifted inwards towards the solar furnace, kilometre-sized objects eventually formed, with several merging to become puny planets. These, in turn, attracted more material to create bigger planets, all of them orbiting the now-blazing inferno of a sun.
The core accretion hypothesis holds that rock-ice protoplanetary cores form first of all, Oxford University physicist Leigh Fletcher says. "Once they reach a critical mass, they accrete the leftover hydrogen and helium gas from the solar nebula to create the enormous worlds we have today."
The fate of each primordial core, though, is unclear: are they well mixed throughout the whole planet – so that there's no longer a distinct high-mass core in the centre? Or has gravitational settling led to a well-defined core under excruciating pressure?
"We don't know right now – with models of the gravity of those planets consistent with both hypotheses," Dr Fletcher explains. "We cannot even be certain that these planets have cores."
Another potential candidate for planetary formation is the disc instability model. Under this scenario, gravitational perturbations cause the planets to collapse directly out of the disc.
"This idea has fallen out of favour in our solar system – but it could be relevant to giant planets discovered around other stars," Dr Fletcher notes. "Reality probably lies somewhere in between. One thing's certain, though – detection of a core, using gravitational measurements, or any other technique, would be a huge step in the right direction towards better understanding the origins of these worlds."
Solid versus gas
Why are the inner planets solid and the outer ones not? It all depended on where and how they formed, experts say.
The latest computer models suggest that the gas giants all started out beyond the frost line, where the sun is far enough away for water to exist as ice.
"Beyond the frost line, they'd be able to grow faster – because of the higher space density of solid material," Swinburne University astrophysicist Kurt Liffman says. "The current, most popular scenario, known as the Grand Tack, is that Jupiter and Saturn subsequently migrated towards the sun, breaching the frost line, and then moved out again."
Jupiter and Saturn probably moved substantial distances – first inwards across the frost line and eventually outwards to rejoin Uranus and Neptune in the solar system's freezing outer suburbs, Dr Fletcher agrees. Uranus and Neptune most likely also shifted inwards and outwards – probably swapping positions – but never actually breached the frost line.
"The outward movement of the gas giants was due to orbital resonance effects and their interaction with the solar nebula's gas," Dr Liffman says.
All four gas planets grew fast and fat on a feast of hydrogen and helium gas, along with a heady brew of icy compounds. After these giants had gorged themselves on gas, radiation from the newborn sun probably evaporated what was left of the nebula.
"During the process of shifting inwards and outwards, the gas giants helped form the current terrestrial planets – and the asteroid belt," Dr Liffman explains.
These four smaller inner planets are built mainly of rock and metal as most of the hydrogen and helium gas had already gone.
High drama
The solar system's birth was anything but peaceful: if current models are correct, its inception was chaotically violent and decidedly dramatic.
Some objects smashed into each other and fused into planets and moons, while others plunged headlong into the sun, Dr Fletcher says.
Gravitational interactions doubtless hurled some bodies completely out of the solar system.
Rotation
All this material was spinning around in the same direction, which is why the planets still orbit in what's called the ecliptic – the same almost flat plane as the sun's equator – and in the same direction that the sun rotates.
"All the matter inside the Earth rotates around the planet's centre of mass while the Earth, and the rest of the solar system, goes around the sun," Dr Soria says. "The sun, meanwhile, travels around the Milky Way."
Molecular clouds
Molecular clouds are transient agglomerations of cold gas in galaxies. Being relatively short-lived, they do not hang around for many billions of years – unless perhaps they are completely isolated, such as in intergalactic space.
The huge clouds form from diffuse interstellar gas – when it gets cold and dense enough.
"Diffuse interstellar gas exists in atomic form – in other words, each hydrogen atom is on its own," Dr Soria notes. "If the temperature of the gas plummets to between minus 250 and minus 260 degrees, and it becomes sufficiently dense, individual hydrogen atoms join up in pairs. This is when the gas becomes a molecular cloud, comprising mostly hydrogen molecules – the simplest ones in chemistry."
This is what happened 20 to 50 million years before our star was created; from a giant molecular cloud, formed from atomic gas enriched by previous generations of stars, our sun was born.
Much earlier molecular clouds, which existed 13 billion years ago, had mostly disappeared by the time our sun was formed, Dr Soria adds. "In something like 50 million years or so, the molecular clouds we observe today will have collapsed, shredded or evaporated."
Links
Read more about the sun, moon and planets on pages 347 to 354 of the textbook Heinemann, Physics 11, third edition (enhanced)
Delve into the mathematics that underlies astrophysics in the textbook Cambridge Essential Advanced General Mathematics (third edition)
Watch two Hubble animations of Milky Way molecular clouds in the process of collapsing, shredding or evaporating (the clouds in these videos would be black but are illuminated by the young stars forming inside the gas): Westerlund 2 and M16
VCAA link
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