The First Evidence of a Host Star Metallicity Cut-off In The Formation of Super-Earth Planets (2407.13821v1)

Abstract: Planet formation is expected to be severely limited in disks of low metallicity, owing to both the small solid mass reservoir and the low opacity accelerating the disk gas dissipation. While previous studies have found a weak correlation between the occurrence rates of small planets ($\leq$4R$_\oplus$) and stellar metallicity, so far no studies have probed below the metallicity limit beyond which planet formation is predicted to be suppressed. Here, we constructed a large catalog of ~110,000 metal-poor stars observed by the TESS mission with spectroscopically-derived metallicities, and systematically probed planet formation within the metal-poor regime ([Fe/H] $\leq$ -0.5) for the first time. Extrapolating known higher-metallicity trends for small, short-period planets predicts the discovery of ~68 superEarths around these stars (~85,000 stars) after accounting for survey completeness; however, we detect none. As a result, we have placed the most stringent upper limit on super-Earth occurrence rates around metal-poor stars (-0.75 < [Fe/H] $\leq$ -0.5) to date, $\leq$ 1.67%, a statistically significant (p-value=0.000685) deviation from the prediction of metallicity trends derived with Kepler and K2. We find a clear host star metallicity cliff for super-Earths that could indicate the threshold below which planets are unable to grow beyond an Earth-mass at short orbital periods. This finding provides a crucial input to planet formation theories, and has implications for the small planet inventory of the Galaxy and the galactic epoch at which the formation of small planets started.

• The paper identifies a metallicity cliff around [Fe/H] ∼ -0.5, with super-Earth occurrence limited to ≤1.67%, highlighting a key threshold in planet formation.
• It employs a catalog of approximately 110,000 metal-poor stars and utilizes a rigorous Transit Least Squares detection pipeline to validate its findings.
• The results compel a reevaluation of planet formation models by illustrating that sufficient metallicity is critical for the accretion of planetary cores in short-period orbits.

Analysis of a Potential Metallicity Threshold in Super-Earth Formation

The paper "The First Evidence of a Host Star Metallicity Cut-off In The Formation of Super-Earth Planets" presents a systematic investigation into the influence of stellar metallicity on the formation of super-Earth planets, using data from the TESS mission. This research focuses on determining if a critical metallicity threshold exists, below which the formation of super-Earths is significantly reduced or inhibited. The results provide valuable insights into planet formation mechanisms, particularly for planets in the super-Earth size range within short orbital periods.

Study Design and Methodology

The authors constructed a comprehensive catalog of approximately 110,000 metal-poor stars with metallicities [Fe/H] ≤ -0.5, utilizing data from the TESS mission. This paper represents the first large-scale analysis of this kind in the metal-poor regime, deliberately targeting an under-explored region of the metallicity spectrum where previous studies have predicted challenges for planet formation.

Key features of the methodology included:

• Spectroscopic Selection: The paper relied on spectroscopically-derived metallicities to form a robust sample from which reliable metallicity correlations could be drawn.
• Planet Detection Pipeline: Utilizing Transit Least Squares for planet detection, the paper outlines a detailed completeness analysis through rigorous injection-recovery tests, modeling detection efficiency as a function of various planetary and stellar parameters.
• Forward Modeling: A forward model approach attempted to match observed detection rates with synthetic populations generated under varying assumptions of period and radius power-law distributions.

Principal Findings

The research uncovers a 'metallicity cliff' around [Fe/H] ∼ -0.5, below which super-Earths become notably sparse or undetectable. The absence of super-Earth discoveries in regions predicted to host numerous such planets suggests the existence of a metallicity threshold, aligning with certain theoretical models of planet formation. The paper makes a statistically significant distinction, particularly within the [-0.5, -0.75] metallicity range, contradicting extrapolation trends from prior Kepler and K2 data. The derived 99.7% confidence upper limit on super-Earth occurrence in this regime is ≤ 1.67%.

Theoretical and Practical Implications

This paper's insights hold substantial implications for planet formation theories. The results shift the conversation about metallicity thresholds and prompt a reevaluation of established models, such as those involving pebble accretion and disk dissipation dynamics. The discovery suggests that the formation of super-Earths might be severely inhibited in environments below a certain metallicity, underpinning the requirement for robust metallic content for the accretion of significant planetary cores within short orbital periods.

The implications for the galactic inventory of small planets are equally intriguing. The findings suggest that super-Earth formation might have commenced later in the galactic timeline than previously inferred, suggesting a delayed formation epoch tied intrinsically to metallicity evolution across cosmic time.

Future Directions

The work prompts further inquiries into the metallicity dependence of other planetary populations beyond the short-period super-Earths. Expanding the sample size, particularly in the lower metallicity end, will fortify the constraints on critical metallicity levels. Upcoming missions like Roman and PLATO are likely to provide the requisite data for a deeper examination of the metallicity correlation extending to longer-period exoplanets.

In conclusion, this paper reinforces the necessity of considering metallicity as a fundamental parameter in planet formation models, offering significant constraints and new avenues for simulation and observational validation in the paper of planetary system architectures.