The Inward Collapse Of The Clumps

Our Earth revolves around the Sun, along with myriad other objects, from tiny atomic particles to giant gaseous planets. The expanse in which the Sun exerts a dominant role defines our immediate cosmic neighborhood, the Solar System. It’s a question that has crossed countless people’s minds throughout the ages and has been pondered by the most renowned philosophers and scientists. While an exhaustive answer is still beyond our grasp, pieces of the puzzle are gradually coming together. All the atoms we find on Earth and other objects in our Solar System come from this tenuous cloud, and yet very little is known about it. Astronomers gain insights by observing similar clouds in our galaxy. Under the right conditions, these clouds of gas and dust start to contract due to their own gravity. As our cloud shrank, the density of the gas increased dramatically. This gradient in temperature along the disk, and the increased gas density as the spinning disk becomes compressed into a thin layer, had important consequences for the next stage of its evolution. A dense disk is an ideal environment in which gas can begin to condense into solid grains, as molecules of water vapor coalesce to the tiny droplets making up fog. Further out along the disk, volatile elements could also condense, to form icy dust grains. The next is characterized by the growth of dust grains, but just as a sand grain is only the beginning of a pearl, the process is long and tortuous.

That Day Is Done

Under the right conditions, dust grains orbiting the Sun would have come close enough to collide gently and stick together. At this early stage, the relative speed of the grains can be just a few centimeters per second, about a hundredth of a walking pace. The exact size of the resulting dust aggregates, called pebbles, depends on the local conditions in the protoplanetary disk, such as the density of dust and gas, which in turn depends on the distance from the star. So the size of the pebbles isn’t uniform across protoplanetary disks. Let us pause here for a moment. The sticking together of tiny dust grains provides our first example of a collision, a vital process in planet formation, and many more examples spanning a wide range of energies will follow as our story unfolds. One can imagine that pebbles could grow further by colliding and sticking together, but this is problematic for several reasons. For a start, larger objects are less prone to stick together, as can be seen if we consider the difference between dust particles, which can easily stick to a wall, and a football, which if thrown at the wall falls to the ground. This is because the tiny forces among grains that cause sticking are less effective at attracting more massive objects. Also, as the size of the pebbles increases, they disturb nearby particles, and their collisions tend to become more energetic, and so more likely to break apart the fragile dusty aggregates rather than accreting them. Ultimately, the growth of larger particles is stalled by disruptive collisions or by scarcity of pebbles as they are lost to breakup and other processes. Indeed, larger pebbles can be efficiently removed from the disk, as the headwind exerted by the gas can perturb their orbits, causing them to spiral inward and end up falling into the Sun.

Push Comes To Shove

The first time I asked myself this question, I was a novice to astrophysics, finishing my degree at Pisa University. It seemed extraordinary to me that the great physicist Albert Einstein could have formulated his grand theory of the universe, General Relativity, decades before we had a clue about how our Earth formed. Simply put, while the expansion of the universe under certain assumptions can be rationalized with a single equation, there is no single equation that describes the formation of the Earth. So it is no surprise that, even though the idea of a nebular origin of the Solar System was first proposed by the Prussian philosopher Emmanuel Kant in 1755, it took more than 200 years to make it a viable scenario. The missing piece to the puzzle of how pebbles of just under a meter in diameter can continue to grow turned out to be down to a subtle effect. As gas and pebbles orbit the central star in unison, they perturb each other. The perturbation is an imperceptible one that arises from the fact that gas and dust revolve around the central star at slightly different velocities. In essence, dust grains experience slight but persistent headwinds that push them to accumulate in denser regions, called clumps. Eventually, these overdense regions tend to attract more material onto them gravitationally. Clumps grow in size by accreting streams of nearby gas and pebbles until their gravity is high enough to trigger a rapid collapse. This inward collapse of the clumps quickly produces myriad small planets called planetesimals scattered around the disk. According to computer simulations, the size of these planetesimals could range from 10 to 1000 km or more.

Are You Ready?

So, with a giant leap, pebbles gain an additional four to six orders of magnitude in diameter, overcoming the forces acting against their growth. The appearance in the disk of sizable solid objects is a game changer. They are localized centers of gravitational attraction and further grow in size by deflecting and accreting smaller pebbles passing by. So in a mere tenth of a percent of Solar System evolution, planetary embryos were popping out from a gaseous and dusty protoplanetary disk. Up until now, the formation of planetesimals has been inextricably linked to the presence of gas. The gas has a calming role in the earliest evolution. As planetesimals swirl through gas, they experience air drag, which helps to maintain them on nearly circular orbits. Nearby objects can occasionally cross paths and collide at a relatively low speed, because of their similar orbits. This is equivalent to traveling on a busy highway at a constant speed that is only a few kilometers per hour higher than that of a leading car.