Formation of short-period planets by disk migration (1903.02004v1)

Abstract: Protoplanetary disks are thought to be truncated at orbital periods of around 10 days. Therefore, origin of rocky short period planets with $P < 10$ days is a puzzle. We propose that many of these planets may form through the Type-I migration of planets locked into a chain of mutual mean motion resonances. We ran N-body simulations of planetary embryos embedded in a protoplanetary disk. The embryos experienced gravitational scatterings, collisions, disk torques, and dampening of orbital eccentricity and inclination. We then modelled Kepler observations of these planets using a forward model of both the transit probability and the detection efficiency of the Kepler pipeline. We found that planets become locked into long chains of mean motion resonances that migrate in unison. When the chain reaches the edge of the disk, the inner planets are pushed past the edge due to the disk torques acting on the planets farther out in the chain. Our simulated systems successfully reproduce the observed period distribution of short period Kepler planets between 1 and 2 $R_\oplus$. However, we obtain fewer closely packed short period planets than in the Kepler sample. Our results provide valuable insight into the planet formation process, and suggests that resonance locks, migration, and dynamical instabilities play important roles the the formation and evolution of close-in small exoplanets.

• The paper shows that groups of sub-Neptune planets in resonance can collectively migrate inward, aligning with observed short-period distributions.
• The N-body simulations reveal that disk metallicity significantly influences planet size and frequency, with higher metallicity creating more compact systems.
• The study highlights resonance chain migration as a key mechanism in short-period planet formation, suggesting revisions to traditional formation models.

Overview of "Formation of short-period planets by disk migration"

The paper by Carrera et al. presents a comprehensive examination of the formation of short-period exoplanets, particularly those with orbital periods less than 10 days. The paper offers a novel perspective that departs from typical standalone planet migration models by introducing the concept of Type-I migration in resonance chains of sub-Neptune planets. This idea effectively addresses the enigma of how such planets form within truncated protoplanetary disks. Here, I will explore the methodology, findings, and implications of this work for our understanding of planetary system formation.

Conceptual Framework and Methodology

The authors postulate that short-period planets form as a consequence of a complex interplay between disk migration and resonance locking. Primarily, instead of individual planets migrating inward and halting at the disk's edge, they hypothesize that groups of planets could be captured into chains of mean motion resonances. These resonant chains then migrate towards the star as a collective unit, which allows the inner members of the chain to be pushed inside the cavity that forms at the inner edge of the disk due to continuing torques exerted by the outer planets.

To test their hypothesis, the authors employed N-body simulations that incorporate a variety of physical processes, including gravitational interactions, collisions, and disk-planet torques. They paid particular attention to how disk properties such as metallicity influence the migration patterns and outcomes. Furthermore, instead of retroactively determining planetary occurrences, they applied a forward model of detection biases using the Kepler survey data to obtain a realistic view of observable exoplanet distributions.

Key Results

• Migration and Resonance Locks: The simulations demonstrated that once locked, the planets could effectively migrate inward as a unit, penetrating the inner edge of the disk. This behavior aligns the predicted period distribution closely with the observed distribution of short-period Kepler planets.
• Influence of Disk Metallicities: The simulations tailored for different disk metallicities showed variation in planet size and occurrence frequency, highlighting the significant impact of initial disk conditions on the final planetary system architecture. Higher metallicity environments favored more closely packed and larger planets.
• Period Ratio Distributions: While the general period distribution matched well with observations, the simulations predicted larger period ratios between adjacent short-period planets than those in the Kepler data. This discrepancy suggests potential overestimations in post-disk dynamical instabilities or that low-mass systems could undergo closer resonant lockings.

Implications and Future Directions

The resonant-chain migration mechanism introduces a plausible explanation for the observed ubiquity of short-period planets, emphasizing the collective dynamical processes over isolated planet behaviors. This model underscores the importance of considering multiple planet interactions and resonance dynamics as fundamental in planetary system formation theories.

Despite the promising results, the paper identifies areas requiring further investigation, such as the influence of stellar tides and varying disk edge locations, to account for phenomena like ultra-short-period planets. The role of metallicity and disc properties in shaping early planetary systems also warrants deeper exploration, as variabilities in these conditions might explain broader diversities in exoplanet demographics observed.

In summary, the work by Carrera et al. significantly enriches our understanding of exoplanet formation dynamics. It prompts a recalibration of theoretical models that could integrate with future exoplanet discoveries and missions, thereby refining forecasts of planetary system architectures across different star environments.