According to current theory, the formation of the planets of our solar system occurred as a result of collisions between increasingly large bodies of matter within a flattened cloud of matter circling the early Sun, known as a protoplanetary disk. Essentially, through accretion, these collisions led to larger and larger masses clumping together. In eight cases, enough matter was present in a given orbital region to form the masses we now call planets, which were then pulled into spherical shapes by the force of their own gravity. The process of formation was slightly different in the close-in rocky planets (sometimes called “terrestrial” planets, because of their rough resemblance to Earth) than in the more distant gas giants.
This theory is known as the nebula theory or nebular hypothesis. It was originally developed during the Enlightenment period but subsequently considered a failed theory, until new twentieth-century evidence was able to confirm a variant of the theory by observing protoplanetary disks around young stars elsewhere in the galaxy.
The birth of the planets was preceded by the birth of the Sun itself, as a result of a similar process of accretion in which an enormous amount of matter from a large, dispersed giant molecular cloud was drawn inward by its own gravity, forming large clumps of hydrogen and helium gas. Eventually, the increased heat and pressure generated at the centre of this collapse was sufficient to begin fusing hydrogen together, and the core of our Sun was born.
Beyond the Sun, the matter in the cloud flattened out into a disc shape orbiting the new star, known as a protoplanetary disk. It is within the protoplanetary disk that the planets were later born, essentially as a result of a similar process as occurred within the Sun itself, except based on collisions between dust and rocky debris. Matter in this disk, like the gas which formed the Sun, collided frequently over time and stuck together. Eventually this process of accretion continued until planet-sized formations emerged. In astronomical terms, because of the density of matter in the cloud this process occurred extraordinarily quickly, probably on the order of hundreds of thousands or millions of years. (In contrast, the total age of the solar system today is measured in billions.)
The inner rocky planets formed relatively close to the Sun, where the heat given off by the star was high enough that methane and water remained gaseous. This meant that the bodies which formed in our region could consist only of metals, with their higher melting points: iron, silicate, and so on. The current theory suggests they actually formed farther from the Sun than their current orbits, but were slowed and pulled inward as a result of drag from matter and gas within the protoplanetary disk.
Slightly farther out lies the denser body of pre-planetary material we know as the Asteroid Belt. Previously, many believed that the Asteroid Belt was the debris left over when an ancient ninth planet was destroyed. Today, the model suggests that no planet ever existed within the Asteroid Belt. Despite its dense appearance on maps of the solar system, most of the space in the belt is empty (which is why spacecraft have traversed it multiple times without difficulty) and overall it is no more massive than a large moon, rather than a planet. Jupiter’s massive gravity, moreover, probably prevented planetary formation from occurring.
The region of the Asteroid Belt is significant for another reason, however. At some point between Mars and Jupiter lies the so-called frost line, the point at which the Sun’s heat is no longer enough to keep water and methane consistently in gas form. Beyond that point planetary formation takes a quite different course.
Rather than being small rocky bodies, in the outer solar system planets can form in a process somewhat more analogous to that which created the Sun. This meant that the initial process of planetary formation involved not only rock but ice, as well – enough ice to make the planets much smaller than their inner-system neighbours. This, in turn, coupled with their distance from the Sun, allowed them to begin attracting not only matter but large amounts of hydrogen and helium gas from the remains of the nebular cloud. This process describes the two closer-in gas giants, Jupiter and Saturn, which lie closest to the frost line. Much farther out, Neptune and Uranus formed in even colder regions, and therefore possess higher proportions of ice. They are sometimes referred to as “ice giants” to mark the difference in composition.
Beyond Neptune, there was still a substantial amount of mass present in the form of gas and rocks. However, this was dispersed over a far larger region of space than the inner solar system. This area – an inner, comparatively dense population called the Kuiper Belt, and then an even more distant region called the Scattered Disk – therefore has many small objects, including comets and the equivalent of asteroids, but no large objects currently classified as planets. Only a handful of these, like Pluto and Eris, have been large enough to collapse into spherical shapes under their own gravity, but all of these are noticeably smaller than our own Moon.
