Building Planets Close to Home — Can Pebble Accretion Form Hot Worlds?
Jayashree Narayan and collaborators begin their paper by comparing two dominant theories of how planets form from dust in space: the classical planetesimal accretion model and the more recent pebble accretion model. In the classical model, planets grow by colliding and merging with kilometer-sized planetesimals. However, this approach often takes too long—especially for forming large gas giants before the surrounding gas dissipates. Pebble accretion offers a faster path by allowing small, drifting solids called pebbles to be captured efficiently by growing planetary cores. This process may explain how even giant planets like Jupiter can form quickly enough to gather a gaseous envelope.
Methods: Simulating a Planet-Forming Disc
To test whether close-in exoplanets (those within 1 astronomical unit of their stars) could have formed through pebble accretion, the authors used a detailed numerical model. This model included the physics of how gas and dust behave in a protoplanetary disc and how these materials interact with forming planets. They simulated growth in three phases: from dust to planetesimals, from planetesimals to planetary cores, and finally, from cores to full-sized planets by pebble accretion. The model focused on three key parameters: disc turbulence, pebble fragmentation velocity, and stellar metallicity. These parameters were varied to understand their effects on planet growth, while other values (like disc mass and temperature) were kept constant.
Results: Which Conditions Help Planets Grow?
The simulations showed that turbulence plays the most important role in determining whether planets can grow. Low turbulence allows pebbles to settle into a thin layer, improving the chances of them being accreted by a planetary embryo. In this calm environment, nearly 94% of planets in the dataset reached the pebble isolation mass—the point where they stop growing by pebble accretion. Higher turbulence values made it harder for planets to grow, as pebbles failed to settle and were less likely to be captured. Pebble fragmentation velocity also mattered: higher values generally helped, allowing pebbles to grow larger before breaking apart. However, if pebbles grew too fast and drifted away before the planet was ready to accrete them, this could also hinder growth.
Metallicity Matters—But Not As Much
While earlier studies have suggested that stars with higher metallicity are more likely to host giant planets, this study found that metallicity is a secondary factor for close-in planet formation via pebble accretion. Within each model, planets orbiting more metal-rich stars did have slightly higher success rates, but this effect was weaker compared to the impact of turbulence and fragmentation. This finding supports other research that has found a weak or no correlation between small planet occurrence and stellar metallicity.
Timing Is Everything
Another critical factor is the starting time of planetesimal formation. If planetesimals form late (e.g., after 100,000 years), then much of the pebble material may have already drifted away from the disc. This delay reduces the available mass and makes it harder for planetary embryos to grow. The authors found that early formation of planetesimals is key—ideally before the gas disc evolves significantly—to ensure that pebbles can be accreted efficiently. When planetesimals formed immediately, planet growth was much more successful.
Why Some Planets Don't Make It
Despite testing 40 combinations of model parameters, none of them could reproduce every planet in the dataset. The planets that failed to grow tended to orbit low-mass stars, be extremely close to their stars, or form in systems with low metallicity. In these cases, either the starting mass was too small or the time needed to reach the classical isolation mass was too long. These findings suggest that some planetary systems might require additional processes—like migration or interactions with other planets—to explain their existence.
Discussion and Future Directions
The paper concludes by acknowledging that, while pebble accretion is a promising model, several complexities need to be addressed. Real protoplanetary discs may have higher turbulence levels than those in the most successful models. Moreover, the study ignored planet migration, assumed constant planetary density, and did not include the effect of other planets blocking pebble flow. Future research could add these missing pieces, explore a wider range of parameters, and better align the model with observations of disc lifetimes and compositions. Observations of exoplanet atmospheres might also help confirm whether pebble accretion is the dominant formation pathway for close-in planets.
Source: Narayan
