The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics From 10-100 AU (1904.05358v1)

Abstract: We present a statistical analysis of the first 300 stars observed by the Gemini Planet Imager Exoplanet Survey (GPIES). This subsample includes six detected planets and three brown dwarfs; from these detections and our contrast curves we infer the underlying distributions of substellar companions with respect to their mass, semi-major axis, and host stellar mass. We uncover a strong correlation between planet occurrence rate and host star mass, with stars M $>$ 1.5 $M_\odot$ more likely to host planets with masses between 2-13 M${\rm Jup}$ and semi-major axes of 3-100 au at 99.92% confidence. We fit a double power-law model in planet mass (m) and semi-major axis (a) for planet populations around high-mass stars (M $>$ 1.5M$\odot$) of the form $\frac{d^2 N}{dm da} \propto m^\alpha a^\beta$, finding $\alpha$ = -2.4 $\pm$ 0.8 and $\beta$ = -2.0 $\pm$ 0.5, and an integrated occurrence rate of $9^{+5}_{-4}$% between 5-13 M${\rm Jup}$ and 10-100 au. A significantly lower occurrence rate is obtained for brown dwarfs around all stars, with 0.8$^{+0.8}{-0.5}$% of stars hosting a brown dwarf companion between 13-80 M$_{\rm Jup}$ and 10-100 au. Brown dwarfs also appear to be distributed differently in mass and semi-major axis compared to giant planets; whereas giant planets follow a bottom-heavy mass distribution and favor smaller semi-major axes, brown dwarfs exhibit just the opposite behaviors. Comparing to studies of short-period giant planets from the RV method, our results are consistent with a peak in occurrence of giant planets between ~1-10 au. We discuss how these trends, including the preference of giant planets for high-mass host stars, point to formation of giant planets by core/pebble accretion, and formation of brown dwarfs by gravitational instability.

• The paper finds that stars with masses >1.5 M☉ are significantly more likely (99.92% confidence) to host 2–13 M_Jup giant planets.
• It employs a double power-law model for planet mass and orbital separation, indicating a 9% occurrence rate for giant planets between 5–13 M_Jup and 10–100 AU.
• The paper also establishes a 0.8% occurrence rate for brown dwarf companions, supporting distinct formation mechanisms for giant planets and brown dwarfs.

The Gemini Planet Imager Exoplanet Survey: An Analysis of Giant Planet and Brown Dwarf Demographics

The Gemini Planet Imager Exoplanet Survey (GPIES) provides a valuable dataset for analyzing the demographics of giant planets and brown dwarfs within the 10-100 AU region. With observations on 300 stars resulting in the detection of six planets and three brown dwarfs, this research enhances our understanding of the underlying population characteristics of these substellar companions.

Key Findings and Methodology

The GPIES paper utilized the Gemini Planet Imager to obtain high-contrast imaging, enabling the detection of companions at wide separations around selected target stars. This imaging capability was particularly suited for identifying lower-mass companions that typically elude other detection methods such as radial velocity (RV) or transit searches which favor closer-in companions. Here's a focused exploration of major findings and implications derived from the survey:

• Correlation Between Host Star Mass and Planet Occurrence: Analysis indicates a strong correlation between the occurrence rate of giant planets and the mass of the host star. Stars with masses greater than 1.5 M☉ are significantly more likely (99.92% confidence) to host planets with masses between 2–13 M$_{\rm Jup}$ compared to their lower-mass counterparts. This finding aligns with the core accretion theory which suggests that these planets form more readily in more massive protoplanetary disks often found around higher-mass stars.
• Modeling Substellar Companion Distributions: A double power-law model in planet mass and semi-major axis was fitted for the planet populations around high-mass stars, yielding power law indices of α = -2.4 and β = -2.0 with an integrated planet occurrence rate of 9% between 5–13 M$_{\rm Jup}$ and 10–100 AU. This indicates a preference for lower-mass planets and those closer to their host stars, enhancing the understanding of planet distribution beyond the domains of previous RV surveys.
• Brown Dwarf Occurrence Rate: The survey elucidated a lower occurrence rate for brown dwarf companions at 0.8% between 13–80 M$_{\rm Jup}$ and 10–100 AU. Brown dwarfs tend to exhibit a mass distribution opposite to giant planets, favoring higher masses and larger semi-major axes.
• Implications on Formation Mechanisms: The data support theories suggesting that giant planets form via core or pebble accretion, a bottom-up approach, while brown dwarfs follow a top-heavy distribution indicative of formation through gravitational instability. Notably, the stellar mass dependence of giant planet occurrence is consistent with predictions from the core accretion model, which is sensitive to stellar mass and the environment of the protoplanetary disk.
• Comparison with RV and Other Surveys: The findings from GPIES are discussed against RV data, which reveals a contrasting distribution of giant planets at wider separations compared to closer orbits, offering insights into a potential turnover or "sweet spot" for giant planet formation at intermediate distances (around 3 AU).

Future Directions

The results from this subset of the GPIES highlight how direct imaging complements other detection techniques, filling in gaps in our spatial and mass coverage of exoplanets. Future imaging efforts, particularly those with upgraded sensitivity or at longer wavelengths, could provide deeper insights. Understanding whether these findings hold across different stellar environments and with the incorporation of additional detection methods, such as astrometry and IR imaging, is essential for a more comprehensive planetary formation narrative.

In summary, the GPIES data contributes markedly to the paper of giant planet demographics beyond the reach of traditional methods, shaping theoretical and observational frameworks about planetary system architecture and formation.