The story of our Solar System is a tale of deep time and the distant past - in that what we know of the formation and early periods of our Solar System’s history is firmly rooted in evidence observed around us today. By deciphering the craters and geological histories of the rocky worlds, the orbits and influence of the gas giants, and the paths and position of the Kuiper belt and beyond, astronomers have stitched together a “story quilt” of sorts that takes us back from today to the cloud in which our Sun was born, around five billion years ago.
The story goes: Once upon a time, there was a stellar nursery, filled with abundant hydrogen and helium, enriched by elements from many prior generations of stars. In this cloud a perturbation (perhaps the shockwave of a nearby supernova or stellar wind from a brightly shining star) moved a collection of cool hydrogen gas together, just close enough that its own gravity caused it to contract until it was dense enough to form a sphere of material at the very center. As the gas collapsed, minor random motions within the cloud imparted a slight rotation and, as the gas contracted further, formed a rotating disk around the central sphere. As the sphere pulled itself tighter and tighter, hydrogen began to fuse and the newborn star fluctuated until it reached hydrostatic equilibrium. This state was achieved when the force of gravity (pulling inwards) was exactly the opposite of the pressure from fusion (pushing outwards.) This sphere became our brightly shining young Sun, and all around it was the protoplanetary disk of gas and dust. Within the disk, material began to collect and coalesce into small collections of material (almost like dust-bunnies) as these small particles moved about, they collected more material, growing larger and collecting material faster. Over time some of the particles grew from the size of a fist to the size of a car (somehow, this part seems a little “handwave-y”) and then those larger particles began to collide and accrete more material to form larger and larger particles.
This begins the period of true planet formation like a demolition derby in the young Solar System. Objects flying around, smashing into one another, growing larger and larger, this would be a dynamic and somewhat scary time period in our Solar System's history (at least for any aliens or time-travelers around to observe it). The particles grew into planetesimals, but not equally - some grew faster than others, accumulating more plentiful gaseous material that existed farther out in our Solar System, some started later and grew slower with the less plentiful metallic and silicic material that existed closer to the Sun. As the major planets formed, they swept their orbits clean, either pulling in and assimilating smaller objects or flinging them into/away from the Sun. With our eight major planets formed, you might think we were done and have reached the present, but something was amiss.
You see, the planets went Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, and Uranus! In order to become the Solar System we know today, something had to change profoundly enough to shuffle the orbits of the ice giants.
The thing that changed was Jupiter. At first, Jupiter was a single large planet surrounded by smaller objects still gathering material from their nearby portions of the protoplanetary disk.
As Jupiter grew larger, it moved inward, diminishing the available material for the later forming Mars (Walsh et al. 2012) until Saturn grew rapidly and approached its current size. Once this happened, Jupiter moved outwards until it had an orbital resonance with Saturn. For every Saturn year, two years passed on Jupiter. Over time, this was not stable and disturbed the belt of unincorporated material that existed at around 30AU, the present distance of Neptune. The shift with Jupiter and Saturn “booted” Neptune into a more distant orbit and scattered the nearby disk of asteroids and small unincorporated objects triggering the event known as the Heavy Bombardment. The period of heavy bombardment across the inner solar system is evidenced by heavy cratering visible on the surface of Mars, Mercury, and our Moon. This was a critical event in many ways and may have triggered the formation of Earth’s own Moon. When the dust settled, we had lost the majority of our debris disk population, leaving behind the asteroid belt, the Kuiper belt, and the Oort cloud and bringing the giant planets into their familiar position.
This is the prevailing story of our Solar System's formation growth and evolution over time. It is known as the Nice model (not because it is nice, per se, but due to the fact that it was formulated by astronomers working at the Observatoire de la Côte d'Azur - located in Nice, France.) The fundamental tenets of it are that the Solar System formed from a cloud of material called a proplyd (sometimes referred to as the Nebular hypothesis), that the Solar System didn’t start in its current configuration, and that the mechanism for planetary formation was a rotating protoplanetary disk.
Credit:ALMA(ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF)
We see abundant evidence for the Nice model in the size and composition of the planets of the Solar System. The planets closest to the Sun are called terrestrial planets and they have several traits in common, all are rocky/metallic and have low ratios of hydrogen and helium compared to their overall composition. Since the protoplanetary disk would be made of approximately the same material as the newborn Sun this would require some explanation. The variation of abundant materials with increasing distance from the Sun is actually predicted by the Nice/nebular hypothesis. Since the inner planets form closer than the “condensation point” of materials like hydrogen and helium, they were scarce. Since the inner four planets did not have the most abundant materials around them as they formed, they never grew to the size of the outer giants.
Another piece of conformation can be found in one of the more curious features of our Solar System - the asteroid belt. Once imagined to have been the remains of a planet that was destroyed, or perhaps never formed; the objects that make up the asteroid belt are just too small. All said and done, there is scarcely enough material there to make a small moon. The Nice model gives a potential origin for this belt. In the Walsh (et al. 2012) paper on the subject we see a description and explanation of the “move in & out” of Jupiter which obliterated and cast widely the objects that made up the outer major belt and stabilized a subset of these object between Mars and itself.
Much of the Nice model supposes a disk of material around the early Sun that would have largely disappeared long ago. How do we know it was there if it cannot be observed? It is strange to think that in order to confirm how our Solar System came into existence that we would have to resort to telescopes and observe objects very far afield. Observing our own Solar System 5 billion years ago is impossible, so observing stars that are just beginning to form planets embedded in protoplanetary disks of their own is a treasure trove of information (Lissauer, 2006). This data becomes particularly useful when we can observe similar nascent exoplanetary systems at different points in the formation process and have a set of “checkpoints” for a more accurate model.
As elegant as the Nice model is, it has some issues with current observations. It does a “nice” job of explaining the positions of the major planets but the devil is in the details. Several of the worlds within our Solar System rotate in a way that is not easily explained if all of the worlds of our system came from a common disk of material. While the history of the planet may give more insight into the rotation and how it changes over time (Keane, 2016) there are limits to what can be explained by geological shifts of major impacts. Another issue that come up with Nice is the inclination of some planetary orbits with respect to the ecliptic. Many of them are significantly offset from the plane of the ecliptic and forming from a common disk would make that unlikely. Lastly, even as evidence from other solar systems has bolstered the Nice model, it has also called up some other challenges in that the model cannot adequately explain the formation of the “Hot Jupiters” observed with great frequency at the start of the Kepler mission.
These challenges to the model are good though, for any true theory of planetary formation should be widely applicable and yet no theory will explain every possible scenario perfectly. Dr. Grinspoon spoke recently about his new book “Earth in Human Hands” and included a simple thought that seems to cut to the core of the matter - he said “planets are more like people than like protons, each is unique and they are not necessarily interchangeable” (D. Grinspoon, Benjamin Dean lecture series)
I think the same can be said of Solar Systems as well; they age and grow and change, always writing their own stories over the course of their own lives.
Keane, J.T., Matsuyama, I., Reorientation Histories of the Terrestrial Planets. American Geophysical Union Fall Meeting, San Francisco, CA, USA. 14.12.16
Lissauer, J. 2006 ASP conference proceedings vol.357 The Spitzer Space Telescope: New Views of the Cosmos, Edited by L. Armus and W.T. Reach. San Francisco: Astronomical Society of the Pacific, 2006., p.31
Walsh, Kevin J., Alessandro Morbidelli, Sean N. Raymond, David P. O'brien, and Avi M. Mandell. "A low mass for Mars from Jupiter’s early gas-driven migration." Nature 475, no. 7355 (2011): 206-09. doi:10.1038/nature10201.