An Excess of Jupiter Analogs in Super-Earth Systems (1806.08799v2)

Abstract: We use radial velocity observations to search for long-period gas giant companions in systems hosting inner super-Earth (1-4 R_Earth, 1-10 M_Earth) planets to constrain formation and migration scenarios for this population. We consistently re-fit published RV datasets for 65 stars and find 9 systems with statistically significant trends indicating the presence of an outer companion. We combine these RV data with AO images to constrain the masses and semi-major axes of these companions. We quantify our sensitivity to the presence of long-period companions by fitting the sample with a power law distribution and find an occurrence rate of 39+/-7% for companions 0.5-20 M_Jup and 1-20 AU. Half of our systems were discovered by the transit method and half were discovered by the RV method. While differences in RV baselines and number of data points between the two samples lead to different sensitivities to distant companions, we find that occurrence rates of gas giant companions in each sample are consistent at the 0.5$\sigma$ level. We compare the frequency of Jupiter analogs in these systems to the equivalent rate from field star surveys and find that Jupiter analogs are more common around stars hosting super-Earths. We conclude that the presence of outer gas giants does not suppress the formation of inner super-Earths, and that these two populations of planets instead appear to be correlated. We also find that the stellar metallicities of systems with gas giant companions are higher than those without companions, in agreement with the well-established metallicity correlation from RV surveys of field stars.

Overview: An Excess of Jupiter Analogs in Super-Earth Systems

This paper, authored by Bryan et al., investigates the occurrence of long-period gas giant planets, specifically Jupiter analogs, in systems that host inner super-Earth planets. Using radial velocity (RV) observations, the researchers aimed to constrain the formation and migration scenarios pertinent to these planetary systems. Through meticulous refitting of published RV datasets and combining them with adaptive optics (AO) imaging, they provided substantial statistical evidence of outer companions in several systems. This paper further enhances our understanding of planetary system architectures, highlighting the potential correlation between super-Earths and distant gas giants.

Key Findings

• Detection of Long-Period Companions: The team analyzed RV data for 65 stars with known super-Earths, detecting statistically significant trends indicating outer companions in nine systems. Moreover, ten systems contained resolved outer gas giants.
• Occurrence Rate of Jupiter Analogs: The analysis led to an estimated occurrence rate of 39±7% for gas giant companions with masses between 0.5-20 M$_{\text{Jup}}$ and semi-major axes between 1-20 AU. This rate exceeds previously reported occurrences in field star surveys, suggesting a link between super-Earths and Jupiter analogs.
• Metallicity Correlation: Consistent with established correlations seen in RV surveys, the paper found that systems with outer gas giants featured higher stellar metallicities than those without.

Theoretical and Practical Implications

The statistical excess of Jupiter analogs in super-Earth systems challenges theoretical models that posit outer gas giants might hinder super-Earth formation. Contrary to some models suggesting that gas giants disrupt solid accretion in the inner disk, the findings suggest these giant planets do not prevent inner planet formation. This indicates a potentially complex interaction in protoplanetary disks where super-Earths and gas giants might form concurrently or sequentially without long-range migration affecting inner planets adversely.

The frequent association of metallicity with these systems underpins theories of planet formation where higher solid content in the natal disks accelerates core formation leading to giant planet formation. The apparent correlation signifies that systems favorable for forming super-Earths are also conducive to gas giant formation, hinting at shared or aligned formation pathways.

Future Research Directions

The paper prompts several avenues for future research:

• Refinement of Occurrence Rates: Refinement of mass and semi-major axis distributions for these long-period companions could better align statistical conclusions across different detection methods.
• Enhanced Observational Campaigns: Further longitudinal studies, combining transiting planet data with RV monitoring, can offer comprehensive dynamical models of these systems, revealing insights into their formation histories and evolution.
• Direct Imaging and Improved AO Techniques: Improved imaging techniques to rule out stellar companions more effectively and to observe directly these distant giants, allowing for validation of RV trends.

Conclusion

Bryan et al.'s research prominently impacts the understanding of exoplanetary system architectures, particularly those involving super-Earths and distant Jupiter-like companions. Through rigorous statistical analysis and a combination of RV and AO methodologies, the paper challenges preconceived notions regarding planet formation dynamics and migration mechanisms. This work sets a precedent for integrating multispectral observational techniques to unravel complex planetary systems and understand their genesis in enriched protoplanetary environments.