A universal brown dwarf desert formed between planets and stars

Abstract

Giant planets and brown dwarfs play a crucial role in star and planet formation, as they are situated at the boundary between planets and stars with uncertain formation mechanisms. Previous observational searches for the formation boundary were hampered by the lack of large unified samples of wide-orbit giant planets and substellar companions. A combined analysis of radial velocity and astrometry mitigates this problem and has significantly enlarged the sample. Here we present a rigorous statistical analysis of the sample of 55 giant planets, brown dwarfs and low-mass stellar companions orbiting FGK stars. We quantitatively analyze the occurrence rates of brown dwarfs and identify a distinct brown dwarf desert at approximately $30\,M_\mathrm{J}$, with no evidence of disappearance up to 20 au. Unlike previous studies that predicted a declining planet occurrence rate beyond the water-ice line, we identify a new population of giant planets and low-mass brown dwarfs in this region. The metallicity and eccentricity trends in our sample suggest that these are the consequences of two different formation scenarios. Our combined population synthesis model successfully accounts for the observed brown dwarf desert, supporting the dual formation hypothesis.

Figures (26)

• Figure 1: The Hertzsprung-Russell (HR) diagram of our final selected stars color-coded by the stellar metallicity. The circle size is proportional to the stellar mass.
• Figure 2: Occurrence rates in the $a\text{--}m$ space. The text in each cell indicates the average occurrence as a percentage (number of planets per 100 stars) along with the associated uncertainty. Text in cells with an occurrence of less than 0.3% is displayed in white for clarity. The color of each cell indicates the density (${\mathrm{d}^2N}/(\mathrm{d}\ln a \cdot \mathrm{d}\ln m)$). The color scale is truncated at 4% to enhance visibility.
• Figure 3: Boundaries of the brown dwarf desert. The upper and lower boundaries, along with their associated uncertainties, are indicated in blue and green, respectively.
• Figure 4: Occurrence rate density and associated errors for the mass range $5 < m < 20\,M_\mathrm{J}\xspace$, shown by the black stepped line. Comparative data from a previous study is overlaid with a dashed blue line.
• Figure 5: Metallicity and eccentricity distributions for sample of different masses. (A), Detection efficiency-weighted cumulative distribution of host stars' metallicity and the sample of all known exoplanets collected from NASA exoplanet archive. (B), Detection efficiency-weighted KDE of metallicities. (C), Similar to (A), but for eccentricity. (D), Similar to (B), but for eccentricity. Selected distributions from previous resultsbowlerPopulationlevelEccentricityDistributions2020 are overlaid as dashed lines for reference. The y-axis of (B) and (D) are scaled.