Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally Unstable Ring

Abstract: We investigate the formation of icy super-Earth mass planets within a gravitationally unstable ring of solids orbiting at 250-750 AU around a 1 solar mass star. Coagulation calculations demonstrate that a system of a few large oligarchs and a swarm of pebbles generates a super-Earth within 100-200 Myr at 250 AU and within 1-2 Gyr at 750 AU. Systems with more than ten oligarchs fail to yield super-Earths over the age of the solar system. As these systems evolve, destructive collisions produce detectable debris disks with luminosities of $10<sup>{-5}$ to $10<sup>{-3}$ relative to the central star.

• The paper investigates the formation of super-Earths via pebble accretion in a gravitationally unstable disk ring 250 750 AU from a star, using coagulation calculations.
• Super-Earth formation timescales depend on distance, taking 100 200 Myr at 250 AU but 1 2 Gyr at 750 AU, while scenarios with over ten initial oligarchs fail to form super-Earths.
• The findings offer a framework for understanding distant exoplanet systems (like HR 8799/HD 106906) and suggest future observational efforts can test these pebble accretion models.

Pebble Accretion and Formation of Super-Earths in Gravitationally Unstable Disks at Large Distances from Central Stars

The paper "Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally Unstable Ring" presents a detailed investigation into the formation of icy super-Earth mass planets in outer regions of a circumstellar disk. The research explores the formation process within a gravitationally unstable ring of solids, orbiting at distances between 250 and 750 AU from a 1 solar mass (M☉) star. Using coagulation calculations, the paper examines how a few large planetary oligarchs together with a swarm of smaller pebbles can accumulate to form a super-Earth over varying timescales depending on their proximity to the star.

The findings of this study offer valuable insights into planetary formation far beyond the traditional confines of systems like our own solar system. The simulations indicate that at closer distances, such as 250 AU, a super-Earth planet can form within 100–200 Myr, whereas at 750 AU, the formation could take about 1–2 Gyr. Importantly, scenarios where more than ten oligarchs are initially present do not result in super-Earth formation within the age of the solar system.

Numerical Results and Implications

The numerical modeling within the research provides strong empirical backing for the understanding of super-Earth formation through pebble accretion in distant orbits. Notably, the simulations show that the creation of these planets depends heavily on the initial conditions, such as the number of oligarchs and the strength of the pebble counterparts. In scenarios with strong pebble accretion, super-Earths are more frequently formed, while weaker pebbles tend to dissipate and fail to accrue sufficient mass for super-Earth formation.

These results have practical implications extending beyond our solar system, offering a framework for understanding the formation and distribution of mass within distant exoplanetary systems. The research suggests that similar processes could account for the orbits of some gas giants in systems like HR 8799 and HD 106906, systems identified to have planets similar to the predicted planet nine in our solar system.

Discussion and Theoretical Implications

Theoretically, the study contributes a new perspective to planet formation theories, particularly for planets located far from their host stars. It challenges older notions that assumed planet formation predominantly occurs closer to the star, as indicated by the efficiency of the pebble accretion model at vast distances. The work builds upon previous research, expanding the scope of star-disk interactions and highlighting the need to consider broader metrics when modeling circumstellar evolution.

The understanding derived from this research could significantly impact future theoretical studies in planetary science, leading to refined models that integrate the nuances of gas dynamics and material distribution in outer disk regions. The persistence of detectable debris disks, with luminosities between $10^{-5}$ and $10^{-3}$ relative to the central star, further underscores the dynamic activity characterizing these far-off regions during planetary formation.

Future Directions

In future research, refining simulations through incorporation of more complex disk dynamics, including the interactions between gas and solids, may offer even more precise predictions. Additionally, expanding observational efforts using instruments like ALMA and the forthcoming James Webb Space Telescope will be pivotal in validating these computational models against real-world data.

Enhanced observational attempts to directly detect and characterize distant celestial objects, possibly analogous to the conjectured planet nine, will provide a meaningful test to the validity of such theoretical frameworks. Over time, such efforts could substantiate the pebble accretion theory as a primary process in diverse planetary systems, leading to a richer understanding of our cosmic neighborhood.