Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504 (1307.2886v2)

Abstract: Several exoplanets have recently been imaged at wide separations of >10 AU from their parent stars. These span a limited range of ages (<50 Myr) and atmospheric properties, with temperatures of 800--1800 K and very red colors (J - H > 0.5 mag), implying thick cloud covers. Furthermore, substantial model uncertainties exist at these young ages due to the unknown initial conditions at formation, which can lead to an order of magnitude of uncertainty in the modeled planet mass. Here, we report the direct imaging discovery of a Jovian exoplanet around the Sun-like star GJ 504, detected as part of the SEEDS survey. The system is older than all other known directly-imaged planets; as a result, its estimated mass remains in the planetary regime independent of uncertainties related to choices of initial conditions in the exoplanet modeling. Using the most common exoplanet cooling model, and given the system age of 160 [+350, -60] Myr, GJ 504 b has an estimated mass of 4 [+4.5, -1.0] Jupiter masses, among the lowest of directly imaged planets. Its projected separation of 43.5 AU exceeds the typical outer boundary of ~30 AU predicted for the core accretion mechanism. GJ 504 b is also significantly cooler (510 [+30, -20] K) and has a bluer color (J-H = -0.23 mag) than previously imaged exoplanets, suggesting a largely cloud-free atmosphere accessible to spectroscopic characterization. Thus, it has the potential of providing novel insights into the origins of giant planets, as well as their atmospheric properties.

Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504

The paper by Kuzuhara et al. presents the direct imaging discovery of a cold Jovian exoplanet, GJ 504 b, orbiting a Sun-like star. This discovery was part of the SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru) survey. GJ 504 b, with a projected separation of 43.5 AU from its host star, exhibits characteristics that distinguish it from previously imaged exoplanets, making it a compelling subject for further paper.

Key Findings and Characteristics of GJ 504 b

1. Age and Mass Estimates: GJ 504 b is associated with an older planetary system, with an estimated age of 160$^{+350}_{-60}$ Myr. This positions it as the oldest known directly imaged planet. The age and mass of GJ 504 b, estimated at 4$^{+4.5}_{-1.0}$ Jupiter masses, are less sensitive to the uncertainties that typically affect younger exoplanetary systems. This robustness against initial condition variations enhances the reliability of the mass estimation.
2. Atmospheric Characteristics: The effective temperature of GJ 504 b is approximately 510$^{+30}_{-20}$ K, which is significantly cooler than those of other directly imaged exoplanets. Its bluer $J-H$ color compared to other known exoplanets suggests a relatively clear atmosphere with reduced cloud cover. This aligns GJ 504 b more closely with T-type brown dwarfs, indicating novel atmospheric conditions not commonly observed in similar past discoveries.
3. Metallicity of Host System: The host star, GJ 504, is noted for its super-solar metallicity ([Fe/H] = 0.28). This suggests an enhanced content of solid materials and planetesimals within the protoplanetary disk, potentially influencing the formation scenarios of surrounding planetary bodies.

Implications and Formation Hypotheses

The unique properties of GJ 504 b present challenges to conventional exoplanet formation theories. Two primary hypotheses are considered:

• Core Accretion (CA): Traditionally, CA has difficulty explaining the formation of massive planets at wide separations (>30 AU) due to the time constraints on core formation in the outer regions of disks. However, theories involving planet migration and scattering suggest that GJ 504 b could have formed closer to the star and was later scattered outward by gravitational interactions with other planets. The high metallicity of the system supports a CA origin, as it correlates with efficient planetesimal accretion and subsequent dynamical evolution.
• Gravitational Instability (GI): While GI can rapidly form massive planets at larger distances, maintaining such a planet against inward migration and preventing excessive mass accretion is problematic. The effective cooling and the high opacity in metal-rich disks could inhibit GI, rendering CA with migration as the more plausible scenario in this case.

Conclusion and Future Prospects

GJ 504 b's low mass, age, and atmospheric properties make it an intriguing candidate for future paper, particularly in the context of direct imaging and spectroscopic analysis. These efforts could yield insights into the composition and dynamics of exoplanetary atmospheres and provide empirical constraints to calibrate theoretical models of planet formation and evolution. Identifying additional companions in the GJ 504 system could further elucidate the dynamical history and formation processes of wide-orbit exoplanets, contributing to a more comprehensive understanding of planetary systems with solar-like stars.