The Puzzle of Terrestrial vs. Super-Earths

In the realm of our solar system, the terrestrial planets, including Earth, never exceeded Earth’s mass. Surprisingly, super-Earths orbit nearly every star resembling our Sun. But what distinguishes these super-Earth systems from those producing terrestrial planets? And what is the influence of distant gas giants in this cosmic drama?

The Role of Viscous Heating in Planet Formation

Recent research sheds light on this cosmic question, revealing that the crucial factor lies in the viscous heating within the protoplanetary disc. This heating regulates how effectively pebbles—small planetesimals—can accumulate to form larger bodies.

Pebble Accretion and Its Limitations

In a detailed parameter study, the scientists examined how pebble sizes are influenced by fragmentation and radial drift. They analyzed conditions where initial planetesimals spark the growth of planetary embryos based on the mass distribution shaped by streaming instability.

Findings: Viscous Heating Matters

Their conclusions reveal that in systems experiencing maximum viscous heating, where the heat accumulates in the disc’s midplane, the infamous pebble accretion process is suppressed in the inner regions where terrestrial planets form. However, under more realistic heating conditions at higher elevations, the formation of Earth-like planets becomes viable.

Water Iceline: Minor Yet Significant

Interestingly, the water iceline's role is minimal unless combined with extreme volatile loss and alterations in pebble fragmentation velocity. This nuanced understanding could redefine how we view planetary formation in colder planetary systems.

The Interplay Between Gas Giants and Pebbles

Moreover, in systems where gas giants exist, the interaction between outer embryos forming from pebbles proves limited—especially when inner disc development is hindered by heat or the presence of an iceline.

Implications for Planetary Diversity

These insights suggest that the diverse array of inner-disc systems is likely influenced by complex, still-elusive disc accretion dynamics, particularly within the water iceline. This could explain the continued prevalence of super-Earths in systems known to host gas giants.

What's Next?

As researchers continue to unravel these cosmic mysteries, the understanding of planetary formation is poised for significant breakthroughs, potentially rewriting our knowledge of how planets like Earth come into existence in the universe.