Introduction: Unraveling the Link Between Outer Giant Planets and Inner Light Planets
Understanding whether outer giant planets (OGPs) shape the formation and arrangement of inner light planets (ILPs) is central to planetary-system theory. The debate spans theoretical expectations—some models predict a positive correlation between OGPs and ILPs, while others anticipate an anticorrelation due to dynamical disruption or migration pathways. Observationally, results have been mixed, often hampered by small sample sizes, heterogeneous methods, and inconsistent stellar properties across surveys. This article is the first in a series reporting on a long-term, homogeneous observational effort using CORALIE, HARPS, and ESPRESSO to probe ILP occurrence in systems with and without OGP.
Survey Design: Striving for Homogeneity in a Heterogeneous Field
Key to reducing biases is a carefully controlled observational design. The current program emphasizes:
- Consistent stellar selection to minimize confounding factors such as metallicity, temperature, and age.
- A uniform observing cadence and data-quality thresholds across instruments (CORALIE, HARPS, ESPRESSO) to ensure comparable radial velocity precision.
- Standardized data reduction and signal-detection pipelines, with cross-instrument calibration to maintain a coherent planetary signal catalog.
- A transparent methodology for distinguishing ILPs from stellar activity and noise, enabling fair comparisons between systems with and without OGPs.
These design choices aim to produce a robust sample that can reveal subtle statistical trends, rather than isolated outliers, and to set a baseline for future, larger-scale analyses.
First Detections: ILPs in OGP Host Systems
The program’s inaugural detections add three new inner planets to the OGP host sample. The planets, measured with high-precision radial velocities, are:
- An 8.3 Earth-mass (mE) planet at a 5.75-day orbit around HD 23079.
- A 10.4 mE planet at a 4.6-day orbit around HD 196067.
- A confirmed 7.5 mE planet at a 3.98-day orbit around HD 86226.
These detections extend the catalog of ILPs in systems with known outer giants and provide valuable data points for testing formation scenarios. The detected ILPs are all short-period, low-mass worlds, a common outcome in systems that undergo specific migration or disk-planet interaction histories. While the sample size remains small, the observed ILP frequencies in OGP-hosting systems contribute important constraints to competing theories, particularly when cross-checked against similar searches in OGP-free systems.
Interpretation and Future Prospects
Although a comprehensive statistical assessment will follow in subsequent papers, the preliminary results suggest a nuanced picture. The relatively modest number of ILP detections in the OGP-hosting sample challenges earlier studies that argued for a strong correlation between outer giants and inner planets. Several interpretations are possible:
- The formation environment around young stars may favor the coexistence of ILPs with OGPs more often than previously thought, but observational biases or detection limits have hidden this in past surveys.
- Dynamic histories, including migration and resonant interactions, could produce a broad spectrum of outcomes, with some systems harboring ILPs alongside OGPs while others do not.
- Sample selection effects, such as metallicity or stellar activity, might influence the apparent relationship; a homogeneous approach helps mitigate these biases but requires larger samples to settle.
The ongoing program will expand the ILP census around HD stars and other targets, refining estimates of ILP occurrence in OGP vs. non-OGP environments. A rigorous statistical framework is being developed to quantify the correlation (or anticorrelation) with proper treatment of detection thresholds and completeness.
Why This Matters: Implications for Planetary System Architecture
Disentangling the relationship between outer giant planets and inner light planets is not a purely catalog-building exercise. It informs theories of planet formation, migration, and the dynamical sculpting of planetary systems. For instance, the presence or absence of ILPs in OGP systems can support or challenge models that rely on disk-driven migration, planet-planet scattering, or in-situ formation with varying solid content. The emphasis on homogeneous samples and long-term monitoring helps ensure that the resulting insights reflect genuine astrophysical trends rather than observational artifacts.
Looking Ahead: The Series Continues
This paper marks the opening chapter of a broader investigation into how OGPs shape inner planetary architectures. As more data accumulate, the team will perform comprehensive statistical analyses, test specific formation scenarios, and compare ILP frequencies across different stellar populations. The ultimate goal is to paint a coherent picture of planetary-system architecture that integrates inner and outer planetary components, advancing our understanding of how common, diverse, and dynamically intricate planetary systems truly are.
Authors and collaborators include researchers from multiple institutes contributing to the CORALIE, HARPS, and ESPRESSO programs, reflecting a collaborative effort to map the landscape of planetary systems.
