Scientists do not know for certain how the solar system was formed, almost 5 billion years ago. What’s more, they’re even less certain how certain important events would have happened after that date. Why Earth and Venus should have such drastically different climates, for instance (Venus suffers from a runaway greenhouse effect, while Earth is temperate and of course supports diverse life forms), is an entirely separate question from how the solar system might have formed in the first place, which some scientists hope will be answered by future space missions to Venus. It’s also a separate question from how scientists think the solar system will end – they’re pretty certain that will happen when the sun runs out of hydrogen, balloons outwards into a red giant, and finally collapses inward into a white dwarf.
How did the solar system form in the first place, however? The prevailing scientific theory, the so-called nebular hypothesis, holds that until about 4.5 billion years ago the area of space we inhabit was nothing more than an enormous, dispersed molecular cloud, debris left over from previous stars which exploded into supernovae. The mass of this cloud caused it to very slowly collapse inward – a process which accelerated as the centre accumulated more and more mass and therefore exerted more and more gravitational force on the remaining cloud. The majority of molecular material ended up directly in the centre, coalescing to form the Sun; the remainder was spun into a disc which gradually underwent its own processes of accumulation, forming today’s planets, moons, asteroids, and comets.
FORMATION OF THE SUN
That the Sun is the centre of our solar system only, and that it might have come into being according to the nebular hypothesis, was first proposed only in the 1700s, by Immanuel Kant and Pierre Laplace. After various unsuccessful attempts to refine this theory during the 19th and 20th centuries, to resolve several important flaws, Soviet scientist Victor Safronov finally published the fundamental basis of the current nebular hypothesis in the 1970s. The resulting body of scientific theory is known as the solar nebular disc model.
Over many millions of years, cold hydrogen gas gradually accumulated at the centre of the molecular cloud where our present solar system now sits, growing into a larger and larger mass and in the process growing faster and faster, as a result of its growing gravitational strength. The resulting body, referred to as a protostellar nebula, gradually begins to heat up as a result of the growing speed of the hydrogen particles falling into it. Over about one hundred thousand years, it is believed, this heat builds up to the point where it becomes so hot at the centre of the protostar that hydrogen begins to fuse together into helium. This chemical reaction lies at the heart of all stars, and is the source of the heat and light which makes life on Earth possible.
While the protostar is now performing the most fundamental reaction of a star, it continues accumulating mass. Current models predict that it would have taken hundreds of millions of years for our Sun to continue growing until it reached something similar to its present size.
FORMATION OF THE PLANETARY DISC
At the same time as the future Sun was beginning to accumulate mass in the centre, under its gravitational influence the remainder of the cloud would have been slowly collapsing together into a relatively flat disc-like shape, and beginning to revolve slowly around the centre. The lighter material would be pulled directly in, increasing the Sun’s mass still more; the rest would be pulled at varying rates into something resembling current-day orbits.
At the same time, the material within this disc would have been beginning to undergo the same process of accretion or accumulation as the Sun itself. Through the influence of gravity, some small pieces of matter would begin to stick to each other, and then to attract still more pieces of matter. Over millions of years, at least a small number of such accumulations would have progressed from the level of simple dust specks into the building blocks of planets, large rocky shapes perhaps half a mile across.
In our solar system, large numbers of these so-called planetesimals probably still survive today. We know them as asteroids (ones which are still relatively close in to the Sun, although much of the material in the Asteroid Belt itself probably has more recent origins), and as comets (ones which formed in the outer reaches of the solar system and occasionally swoop through the inner solar system as part of their long elliptical orbits). In just a few cases, however, these chunks would have collided and stuck together as well, forming ever-larger blocks of matter.
FORMATION OF PLANETS
The rock-based planets – Mercury, Venus, Earth, and Mars – are easiest to explain as outcomes of this gradual accumulation of matter. Because their growing gravitational force would have accelerated the process by which nearby matter was attracted onto the surface, the new planetesimals would have grown very quickly, probably in less than a million years. The gradual process of colliding and merging would have continued until virtually all of the material in the inner solar system had combined into these four planets. The remainder, now, is just a few relatively scattered asteroids and minor near-Earth objects. These large, stable bodies are what can finally be called true planets, and tend to have stable orbits. This is not to say that all objects have stable circular orbits like Earth’s; instead, it’s simply that most of those objects without stable orbits either fell into the Sun or escaped the solar system.
So much for the inner, rocky planets – and for Pluto, which is probably a very distant but effectively similar rocky planetoid (we’ll know more about Pluto when the New Horizons space probe reaches it in a few years). However, the formation of the four gas giants – Jupiter, Saturn, Uranus, and Neptune – is more problematic, and the part of the origin process that currently has the most scientific disagreement.
The gas giants are unlike the rocky planets in two respects: they are huge, and they are either entirely or almost entirely lacking in solid core material. The most promising theory at present holds that these planets form in an essentially similar process to the rocky ones, gradually accumulating more and more material from the planetary disc. This does not entirely solve the problem, however. The revised theory indicates that gas giants might be able to form in the orbits where we now find Saturn and Jupiter, but not much farther out, in the orbits of Uranus and Neptune. Scientists’ best guess for the moment, therefore, is that these planets formed closer in and then have gradually drifted away from the Sun over the past several billion years.
In sum, then, scientists have a generally good idea how, through the force of gravity, a large molecular cloud could have turned into the present-day solar system over the course of billions of years. What they haven’t been able to explain conclusively yet, however, is how that process would have given rise to gas giants as well as rocky planets.
