Astrochemistry

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

By Keith Cowing

Press Release

astro-ph.EP

July 23, 2024

Filed under astro-ph.EP, astrochemistry, astrogeology, exoplanet, K2, Kepler, metal-poor star, metallicity, Super Earths

The First Evidence of a Host Star Metallicity Cut-off In The Formation of Super-Earth Planets — Super-Earth occurrence for Kepler (black circles) and K2 (gray circles), TESS data assuming all candidates are real (teal square), and 99.7% confidence intervals to calculate the upper limits for TESS data assuming all candidates are false positives (teal triangles) as a function of metallicity. The [-0.25,-0.5] bin occurrence rates are offset horizontally for visual clarity. The best-fit power-law trend line for the Kepler and K2 data (yellow) is displayed and extrapolated to [Fe/H]= -0.75 showing the 1-σ uncertainties (Zink et al. 2023). We show the combined best-fit exponential trend line for Kepler, K2, and TESS (purple) (eq. 6) including the 1-σ uncertainties. Each metallicity bin is indicated by gray dashed lines. Within the [-0.75,-1] bin, there is insufficient data to further constrain the super-Earth occurrence rate as a function of metallicity (denoted by the gray hatched region). — astro-ph.EP

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 (≤4R_⊕) 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] ≤ -0.5) for the first time. Extrapolating known higher-metallicity trends for small, short-period planets predicts the discovery of ~68 super Earths 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] ≤ -0.5) to date, ≤ 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.

Kiersten M. Boley, Jessie L. Christiansen, Jon Zink, Kevin Hardegree-Ullman, Eve J. Lee, Philip F. Hopkins, Ji Wang, Rachel B. Fernandes, Galen J. Bergsten, Sakhee Bhure

Comments: Accepted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2407.13821 [astro-ph.EP] (or arXiv:2407.13821v1 [astro-ph.EP] for this version)
Submission history
From: Kiersten Boley
[v1] Thu, 18 Jul 2024 18:00:02 UTC (3,279 KB)
https://arxiv.org/abs/2407.13821
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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