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But there were problems. No one was quite sure how planetesimals themselves formed — how they made the jump from tiny dust grains to city-size rocks, a problem known as the meter-size barrier. The presence of liquid water on Earth was confusing, as it relied on the chance arrival of water-bearing asteroids. And most troubling, planetesimal accretion would take far too long to build Saturn, Uranus and Neptune. By the time their solid cores formed — after tens of millions of years — it would be too late for them to accumulate enough gas from the protoplanetary disk to become gas giants, as “most disks go away in a few million years,” said ‪André Izidoro, a planetary scientist at Rice University.

Johansen and Lambrechts proposed a new model. Instead of multiple planetesimals colliding together, they instead suggested that a single dominant planetesimal could grow to a huge size in a short amount of time — just a few million years — by sweeping up material inside a protoplanetary disk “like a vacuum cleaner,” said Johansen. This material would consist of tiny seedlike rocks that surrounded young stars. They called the idea pebble accretion.

Pebbles are extremely small, just a few millimeters to centimeters in size, whereas planetesimals are much larger, up to hundreds of kilometers wide, like many of the asteroids we see in the solar system today. Both would be found in a star’s protoplanetary disk, with the latter occasionally smashing into one another.

In 2014, just two years after Johansen and Lambrechts published their pebble model, observations revealed that disks were indeed full of pebbles. A network of 66 telescopes called ALMA (the Atacama Large Millimeter/submillimeter Array) revealed up to 100 Earth masses’ worth of pebbles inside a protoplanetary disk surrounding a young star, including wide gaps created by growing planets carving out their orbits. Inside these disks, pebbles were everywhere. ALMA “showed that protoplanetary disks are born with enormous mass reservoirs of small pebbles, not planetesimals,” said Lambrechts.

Before long, most scientists came to agree that pebble accretion formed the giant planets. It just seemed to be the only way for them to grow fast enough. “For the cores of the giant planets there is no doubt pebble accretion is the solution,” said Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory in France.

Yet, while it seemingly explained the formation of Jupiter, Saturn, Uranus and Neptune, pebble accretion raised considerable questions about the formation of the terrestrial planets: Mercury, Venus, Earth and Mars. “In principle one could form the terrestrial planets with planetesimal accretion,” said Lambrechts. “But now there’s this invasion of the pebbles.”

In the pebble accretion model, you begin with a protoplanetary disk around a young star, just like in the planetesimal accretion model. Both models then require planetesimals to form via a phenomenon called streaming instability. Essentially, dust and pebbles experience drag as they encounter the gas surrounding the star. This causes the pebbles to clump together, until some clumps “are so massive that they become gravitationally bound, and they collapse into planetesimals” up to hundreds of kilometers wide, said Joanna Drążkowska, an astrophysicist at the Ludwig Maximilian University of Munich. The clumps may then rotate as they form, which gives them two lobes. “This is exactly what we see” in outer solar system objects such as Arrokoth, said Drążkowska. The process is expected to be incredibly quick, perhaps taking only 100 years.

From here, the two models diverge. Under planetesimal accretion, these planetesimals form everywhere in the disk, leaving few pebbles left. Over tens of millions of years, the large planetesimals collide and merge, eventually giving rise to the terrestrial planets we see today.

In pebble accretion, just a few planetesimals become dominant. These planetesimals begin to sweep up pebbles in the protoplanetary disk, which stream down onto the surface of the planetesimal in long riverlike filaments. It is an extremely energetic process, with hot magma oceans glowing on the surface as pebbles rain down. “These planets would shine,” said Lambrechts. The process is very efficient; Earth would grow to its full size in just a few million years, compared to perhaps 100 million years in planetesimal accretion.

One of the most interesting outcomes of pebble accretion is that it gives a direct prediction of how habitable planets form. Rather than relying on water-rich asteroids to haphazardly collide with protoplanets, the model suggests that incoming icy pebbles from the outer solar system could provide a steady supply of water to a planet like Earth, an idea known as pebble snow. “The nice thing about pebble snow is that it becomes predictable,” said Johansen. “The amount of water and carbon and nitrogen that comes down to Earth is something that can be calculated.”

Thus, if the pebble accretion model for terrestrial planet formation is correct, it may bode well for the prospects of other life in the universe. Whereas under planetesimal accretion the existence of water on Earth was a chance event, in pebble accretion it might be expected in a planetary system like our own. Take a proto-Earth and put it around a similar star in a similar position, and the amount of water it collects could be the same. Habitable worlds would not be chance events; their existence would be a calculable outcome if a planetary system has the right ingredients. “One can use this as a starting point for understanding prebiotic chemistry and the origin of life,” says Johansen.

The Great Architect

Pebble accretion seems like an attractive idea. It solves the problem of rapid planet growth, it explains the presence of water on Earth, and we can even observe pebbles in developing exoplanetary systems. “With ALMA we know now pebbles are concentrated in particular regions that lead to planetesimal formation and potentially planets,” said Paola Pinilla, a planetary formation scientist at University College London.

Yet, while it provides a good explanation of giant planet growth, pebble accretion has some notable issues when it comes to terrestrial planets.

First, where did the pebbles in the inner solar system come from? In recent years, planetary scientists have come to believe that Jupiter, the largest planet in our solar system, was the primary force shaping the destiny of the planets. “The emergent picture is that Jupiter was the great architect of the solar system,” said Batygin.

Soon after Jupiter’s rapid formation, it created a barrier between the inner and outer solar system, preventing material from the “mass-rich” outer regions from flowing to the “mass-starved” inner terrestrial planets, said Batygin. “The giant planets blocked the flux of dust and pebbles,” said Morbidelli. Pebbles in the inner disk may have dissipated before the terrestrial planets could form, and without more material coming in from the outer solar system, there simply would not have been enough material to make Earth.

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