The Second Discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) Program: A Cold Wide-Orbit Exoplanet around a Young Field M Dwarf at 10.9 pc

Abstract: We present the identification of the COCONUTS-2 system, composed of the M3 dwarf L 34-26 and the T9 dwarf WISEPA J075108.79$-$763449.6. Given their common proper motions and parallaxes, these two field objects constitute a physically bound pair with a projected separation of 594$"$ (6471 au). The primary star COCONUTS-2A has strong stellar activity (H$\alpha$, X-ray, and UV emission) and is rapidly rotating ($P_{\rm rot} = 2.83$ days), from which we estimate an age of 150-800 Myr. Comparing equatorial rotational velocity derived from the TESS light curve to spectroscopic $v\sin{i}$, we find COCONUTS-2A has a nearly edge-on inclination. The wide exoplanet COCONUTS-2b has an effective temperature of $T_{\rm eff}=434 \pm 9$ K, a surface gravity of $\log{g} = 4.11^{+0.11}_{-0.18}$ dex, and a mass of $M=6.3^{+1.5}_{-1.9}$ $M_{\rm Jup}$ based on hot-start evolutionary models, leading to a $0.016^{+0.004}_{-0.005}$ mass ratio for the COCONUTS-2 system. COCONUTS-2b is the second coldest (after WD 0806$-$661B) and the second widest (after TYC 9486-927-1 b) exoplanet imaged to date. Comparison of COCONUTS-2b's infrared photometry with ultracool model atmospheres suggests the presence of both condensate clouds and non-equilibrium chemistry in its photosphere. Similar to 51 Eri b, COCONUTS-2b has a sufficiently low luminosity ($\log{(L_{\rm bol}/L_{\odot})} = -6.384 \pm 0.028$ dex) to be consistent with the cold-start process that may form gas-giant (exo)planets, though its large separation means such formation would not have occurred in situ. Finally, at a distance of 10.9 pc, COCONUTS-2b is the nearest imaged exoplanet to Earth known to date.

• The paper details the discovery and direct imaging characterization of the COCONUTS-2 system, confirming a T9 exoplanet at a projected separation of 6471 AU from its host M dwarf.
• It identifies key properties of COCONUTS-2b, including an effective temperature of 434 K, a mass of about 6.3 Jupiter masses, and unusual atmospheric features such as condensate clouds and non-equilibrium chemistry.
• The study establishes the system as a benchmark for refining atmospheric and evolutionary models of wide-separation exoplanets, guiding future spectroscopic and formation research.

Overview of the COCONUTS-2 Discovery

The paper discusses the identification and characterization of the COCONUTS-2 system, part of the COol Companions ON Ultrawide orbiTS (COCONUTS) program. This system comprises an M3 dwarf star (COCONUTS-2A) and a T9 dwarf exoplanet (COCONUTS-2b), which is one of the coldest and widest-separation exoplanets discovered through direct imaging to date.

Astrometric and Photometric Analysis

COCONUTS-2A and COCONUTS-2b are confirmed as a physically bound pair based on their common proper motions and parallaxes, with a significant angular separation of 594 arcseconds, corresponding to a projected separation of 6471 AU. This large separation from its host star provides a unique opportunity to study without interference from stellar light.

COCONUTS-2A exhibits strong stellar activity with rapid rotation and significant emissions, indicative of a young age estimated between 150-800 Myr. In contrast, COCONUTS-2b demonstrates faint photometric properties across several bands, including being notably less luminous than similar field T9 dwarfs, aligning with hot-start evolutionary models for a planetary-mass object.

Highlighted Characteristics and Anomalies

COCONUTS-2b exhibits several distinctive characteristics:

• An effective temperature of approximately 434 K and a surface gravity of $\log{g} \approx 4.11$.
• The mass is estimated at $6.3^{+1.5}_{-1.9}$ Jupiter masses, indicative of giant planet classification.
• The exoplanet demonstrates substantially cooler and redder characteristics compared to benchmark field dwarfs for its spectral type, potentially due to its youth and evolutionary history.

Spectral analysis suggests the presence of condensate clouds and non-equilibrium chemistry in its atmosphere, drawing parallels with other young exoplanets like 51 Eridani b. This combination of atmospheric conditions may contribute to its atypical photometric signatures.

Implications for Exoplanetary Studies

The COCONUTS-2 system serves as a rare benchmark for understanding the atmospheric, physical, and orbital characteristics of directly imaged wide-separation exoplanets at a young age. The detection of COCONUTS-2b extends the dynamic range of known exoplanets, supporting the concept that planetary-mass companions can form and remain in stable, distant orbits reminiscent of stellar binaries.

From a theoretical standpoint, the results offer critical data for validating and improving atmospheric and evolutionary models of giant exoplanets and brown dwarfs. This underscores the potential variability in atmospheric conditions influenced by formation history and stellar environment, emphasizing the necessity for high-quality spectroscopic follow-ups.

Future Prospects

Further investigations into COCONUTS-2b and similar systems could refine models of wide-separation planet formation and survival, informing on both accretion processes and dynamical context. Continued observations may also contribute to understanding the distinct evolutionary trajectories and atmospheric processes influencing directly imaged exoplanets. These endeavors will likely advance with more sensitive instrumentation and refined methodologies, leveraging COCONUTS-2 as a foundational discovery in the field of exoplanetary science.