A 2 au resolution view by ALMA of the planet-hosting WISPIT 2 disk

We present deep, high spatial resolution interferometric observations of 0.88 mm continuum emission from the TYC 5709-354-1 system, hereafter WISPIT 2, obtained with the goal of detecting circumplanetary emission in the vicinity of the newly discovered WISPIT 2b planet. Observations with the most extended baseline configuration offered by ALMA, achieving an angular resolution of $25 \times 17$ mas ($3.3 \times 2.2$ au), revealed a single, narrow ring with a deprojected radius of 144.4 au and width of 7.2 au, and no evidence of circumplanetary emission within the cavity. Injection and recovery tests demonstrate that these observations can rule out point-like emission at the location of WISPIT 2b brighter than $\approx 45$ $μ$Jy at the $3σ$ level. While these data can rule out PDS 70c like circumplanetary emission, the upper limit is consistent with empirical mass-flux relationships extrapolated from the stellar regime. Visibility modeling of the continuum ring confirms that WISPIT 2b lies significantly interior to the mm dust ring, raising doubts about the ability of WISPIT 2b to be the only driver of the dust structure. Possible solutions include either another lower mass companion, residing between WISPIT 2b and the cavity edge, likely in the gap seen by SPHERE at $\sim130$ au, or that WISPIT 2b is either substantially more massive than IR-photometry based estimates ($\sim15$ $M_{\rm Jup}$) or on a moderately eccentric orbit. The combination of observations sensitive to the gas and dust distributions on larger spatial scales and dedicated hydrodynamical modeling will help differentiate between scenarios.

💡 Research Summary

In this work the authors present the deepest, highest‑resolution ALMA Band 7 observations of the protoplanetary system TYC 5709‑354‑1 (hereafter WISPIT 2), targeting the recently discovered planet WISPIT 2b. Using the most extended C‑10 configuration (baselines 132–15 238 m) they achieve an angular resolution of 25 × 17 mas (3.3 × 2.2 au at 133 pc) and a synthesized beam of 24 × 17 mas. The data consist of three execution blocks for a total on‑source integration of 125 min, with excellent weather (PWV 0.25–0.4 mm) and a maximum recoverable scale of 0.27″, well below the ∼2″ disk diameter.

The continuum image reveals a single, narrow ring of dust emission. Visibility modeling with a thin Gaussian ring (GALARIO) yields a deprojected radius of 1.086″ (144.4 au) and a radial width σ_R = 0.054″ (7.2 au). The total flux density is 151 mJy. The inclination (≈44°) and position angle (≈360°) match those derived from H‑ and K‑band scattered‑light images, indicating that the mm and near‑IR structures share the same geometry and likely a modest eccentricity.

The main scientific question is whether the planet WISPIT 2b (projected separation 0.32″, PA ≈ 242°) hosts a circumplanetary disk (CPD) that could be detected at 0.88 mm. Direct inspection of the residual image after subtracting the best‑fit ring model shows no significant point source. Because the ultra‑high resolution filters out large‑scale emission and introduces correlated noise ripples, the authors adopt an injection‑recovery test: they subtract the model visibilities, inject a synthetic point source at the planet location with fluxes ranging from 10 to 200 µJy, and re‑image with the same CLEAN parameters. The recovered flux scales linearly with the injected flux, and the rms in the cavity (14.8 µJy) leads to a 5σ upper limit of 75 µJy (≈ 45 µJy at 3σ) for a point‑like CPD. This limit rules out CPD emission comparable to that seen around PDS 70c (∼150 µJy) but is consistent with extrapolations of stellar mass‑flux relations to planetary masses. The authors note that an extended CPD with low surface brightness could remain undetected.

Because the planet lies well inside the mm dust ring (≈0.4 R_ring), it is unlikely to be the sole sculptor of the observed ring. The authors discuss two plausible scenarios: (1) an additional, lower‑mass companion resides between the planet and the ring edge, possibly associated with a faint gap seen in SPHERE scattered‑light images at ∼130 au; (2) WISPIT 2b is more massive than the IR‑based estimate (∼5 M_Jup) – perhaps ∼15 M_Jup – or follows a moderately eccentric orbit (e ≈ 0.2–0.3), thereby generating a pressure maximum at the observed ring location. Both possibilities would explain the large mm cavity (∼0.27″) and the narrow ring.

The paper emphasizes that further observations are needed. Shorter baselines (to recover larger‑scale emission) will improve sensitivity to diffuse CPD material and reduce imaging artifacts. Complementary gas observations (e.g., CO 3‑2) will map the kinematic structure and pressure profile, while high‑resolution hydrodynamical simulations can test whether a single massive/eccentric planet or a multi‑planet system reproduces the observed dust morphology. In summary, the 2 au resolution ALMA data provide a precise characterization of the WISPIT 2 dust ring, set stringent limits on CPD emission around WISPIT 2b, and suggest that additional dynamical agents are required to explain the disk’s architecture.