The Two Old Recipes for Planets
For decades, scientists have debated two main theories for how rocky planets form from the disk of gas and dust swirling around a young star. The first is called 'planetesimal accretion'. In this scenario, dust grains stick together to form kilometre-sized
bodies called planetesimals, which then collide and merge over millions of years in a chaotic process of cosmic construction. The second theory is 'pebble accretion'. This idea suggests that larger planetary 'embryos' grow much more quickly by sweeping up vast numbers of centimetre-sized pebbles, which are slowed by gas drag and pulled in by gravity. Each model presented its own set of problems and neither could fully explain the architecture of our solar system on its own.
A Trail of Volatile Clues
To solve the puzzle, researchers behind the new study looked for chemical fingerprints left over from the formation process. They focused on 'volatile elements' like zinc, potassium, and sodium. These are elements that vaporise at relatively low temperatures. Their abundance—or lack thereof—in a planet's crust and mantle today serves as a powerful record of the hot, violent conditions it experienced while it was growing. By analysing the specific composition of these elements on Earth and Mars, scientists can work backwards and reconstruct the story of their birth.
Introducing the Hybrid Model
The latest findings, published in Nature Astronomy, strongly support a 'hybrid' model of planet formation. This new framework doesn't throw out the old ideas but instead suggests that both pebble accretion and planetesimal accretion were involved, with their relative importance varying dramatically from planet to planet. The study indicates that planets are not built using a single method, but a combination of both grabbing tiny pebbles and smashing together larger rocks. This resolves the tension between the two competing theories by showing they are both crucial parts of the planetary assembly line.
What the New Study Found
By feeding data on volatile elements into sophisticated computer models, the research team came to a startling conclusion about our own neighbourhood. They found that Earth and Mars, despite being neighbours, were built in fundamentally different ways. At least 75 percent of Earth's mass appears to have come from protoplanets that grew large by accreting pebbles. The remaining 25 percent was added through collisions with larger planetesimals. For Mars, the recipe was flipped. Roughly three-quarters of its mass seems to have come from the accretion of planetesimals, with only a quarter originating from sweeping up pebbles. This explains many of the chemical differences we see between the two worlds today.
Why This Changes Our View of the Cosmos
This hybrid model provides a much more elegant and powerful explanation for the diversity of rocky worlds. It not only tells us more about the specific history of Earth and Mars, but also provides a new framework for understanding planets outside our solar system. The surprising finding that two adjacent planets could form so differently suggests that a planet's final composition and potential for habitability might depend heavily on the specific mix of formation processes it undergoes. As we continue to discover thousands of exoplanets, this new understanding will be critical in identifying which of those distant worlds might be most like our own and potentially capable of supporting life.
