How did our Solar System form?

Note: As you work through this section, keep in mind how the earlier parts of this module fit together.
The chemistry measured at comets (Rosetta), the mineralogy in asteroid samples (Ryugu and Bennu), and the orbital structure of small-body populations all provide essential clues that help scientists validate — or challenge — different formation models.

The question of how our Solar System formed is still an active area of research. We have a strong general framework: collapse of a rotating nebula, formation of a disk, growth of planetesimals, and eventual migration of the giant planets. That framework — once purely theoretical — is now supported by direct evidence from meteorites and sample-return missions, though many of the details are still being refined as new data arrive.

One of the most important recent advances is the concept of pebble accretion, in which small particles of dust and ice — millimetres to centimetres across — drift inward through the protoplanetary disk and are efficiently swept up by growing planetesimals. This process allows planetary cores to form far more rapidly than previously thought, explaining how the giant planets could build up enough mass to capture their thick atmospheres before the surrounding gas dispersed.

The samples returned from asteroids (Ryugu and Bennu), the close-up observations of Comet 67P by Rosetta, and new dynamical simulations all continue to reshape the story.

In the following video, two Rosetta scientists discuss this problem and the scientific goals that missions like Rosetta aim to address:

In the video, Claudia Alexander and Edward Young mention that a large amount of debris appears to have entered the inner Solar System between about 3.5 and 4 billion years ago. This period — traditionally called the Late Heavy Bombardment — was first inferred from the lunar samples dated during the Apollo program (Module 6).

More recent work, including a 2013 study of Vesta using Dawn mission data, shows that Vesta also experienced heavy impacts around this time.

However, scientists now think this “bombardment” may have been a more extended or staggered period of impacts rather than a single sharp spike. Understanding whether the bombardment was sudden or prolonged remains one of the key tests for modern models of Solar System evolution.

To explain the later rearrangement of the young Solar System, the most influential framework is the Nice model (pronounced “niece”). Originally proposed by a group of researchers in Nice, France, the model has been refined several times.

Its core idea is that the giant planets did not form where we find them today. Instead, they underwent a period of orbital migration driven by gravitational interactions with a disk of leftover planetesimals.

The three classic papers describing the original model show how this migration:

• sent planetesimals into highly eccentric orbits,
• allowed Jupiter to capture its Trojan asteroids, and
• triggered a phase of rapid migration that scattered debris throughout the Solar System.

Modern versions of the model — including the “Jumping-Jupiter” refinement — help explain many features of today’s Solar System: the structure of the Kuiper Belt, the distribution of asteroid types, the eccentric orbits of the giant planets, and even why Mars is so small.

The following video provides an excellent visual explanation and simulation of how the Nice model works:

Together, these physical and chemical clues — from Rosetta’s comet dust to Bennu’s returned samples — show that understanding the Solar System’s origin is a truly interdisciplinary challenge, combining astronomy, physics, chemistry, and planetary geology. The next decade of missions and modelling — from Lucy at the Trojan asteroids to Psyche at a metallic protoplanet — will continue refining this story.