Molecules with ALMA at Planet-forming Scales (MAPS): A Circumplanetary Disk Candidate in Molecular-line Emission in the AS 209 Disk

We present selected ¹²CO channel maps in Figure 1 (see Figure 7 in Appendix A for additional channel maps). The ¹²CO emission reveals a 78 au-wide gap (determined by the distance between the adjacent peaks in the radial intensity profile; Law et al. 2021a) around ≃200 au, which is most clearly seen along the semimajor axis of the disk (see 5.6–6.0 km s⁻¹ in Figure 1). The radial location of the gap is consistent with what is previously reported in Guzmán et al. (2018) and Teague et al. (2018) using independent ¹²CO data sets, and in Avenhaus et al. (2018) using near-infrared scattered-light images. Teague et al. (2018) showed that the ¹²CO gap is associated with deviations in the rotational motion of the gas with variations of up to 5% relative to the background Keplerian rotation, which arises from the steep radial gas pressure gradient at the edges of the gap. While we defer more comprehensive kinematic studies to a forthcoming paper (M. Galloway-Sprietsma et al. 2022, in preparation), comparable rotational velocity deviations are found around the ¹²CO gap using the MAPS ¹²CO data.

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In addition to the annular gap, ¹²CO reveals a strong localized (both in space and velocity) perturbation on the southern side (see 4.6–5.0 km s⁻¹ in Figure 1), reminiscent of a so-called velocity kink induced by planets (Pinte et al. 2018). Similar velocity perturbations are found at the same location and velocity in independent archival ¹²CO data sets presented in Guzmán et al. (2018) and Teague et al. (2018). The velocity perturbation is seen near the semiminor axis of the disk, suggesting that the perturbation is associated with radial and/or vertical motions rather than rotational motions (Teague et al. 2019b). We note that a similar velocity perturbation is not seen on the northern side of the disk, highlighting the localized nature of the perturbation.

Figure 2 presents selected ¹³CO channel maps (see Figures 8 and 9 in Appendix A for additional channel maps). In addition to the characteristic butterfly pattern arising from the Keplerian rotation of the AS 209 disk, we find a point-source emission at a projected distance of 14 and a position angle of 161° (east from north). The offset from the star (assumed to be at the center of the concentric continuum rings) is (ΔR.A., Δdecl.) = (0450 ± 0.004, −1332 ± 0.003). Assuming that the point source is embedded in the midplane of the AS 209 disk, the deprojected distance between the point source and the central star is 17 (≃206 au). It is worth pointing out that the ¹³CO point source is located at the center of the ¹²CO annular gap and is coincident in both radius and azimuth with the localized velocity perturbations seen in ¹²CO. In Section 4.1, we will discuss the potential (common) origin of these features.

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The ¹³CO data are limited by the 200 m s⁻¹ velocity resolution set by the MAPS program. In the fiducial image cube, the point source is found in two adjacent channels, 4.7 km s⁻¹ (Figure 2, left) and 4.9 km s⁻¹ (Figure 2, right). The peak intensities at the two channels are 5.75 and 5.85 mJy beam⁻¹, respectively. While we cannot image at a finer channel spacing than the velocity resolution, we produce an additional image cube with the same channel spacing of 200 m s⁻¹, but with the starting velocity offset by half of the velocity resolution (100 m s⁻¹). The middle panel in Figure 2 shows the channel map at 4.8 km s⁻¹ from the half-channel-shifted image cube. The point source is more clearly seen, with the peak intensity of 9.62 mJy beam⁻¹ (rms noise = 0.543 mJy beam⁻¹). Simply comparing the peak intensity with the rms noise, the point source in the three velocity channels is detected above the 12σ, 18σ, and 13σ levels, respectively.

Although the ¹³CO emission from the AS 209 disk is less extended than the ¹²CO emission, ¹³CO emission is detected out to ≃2'' (in deprojected distance), beyond the radial location of the point source. If the point source is embedded in the midplane within the ¹²CO gap, it is thus possible that some of the emission toward the point source originates from the residual gas within the gap. To assess the potential contamination from the gas in the gap, in Figure 3 we present a radial profile of the integrated intensity averaged over the azimuthal region ±2° around the point source. We compare that to an azimuthally averaged, radial integrated intensity profile excluding the ±2° region around the point source. At the deprojected radial distance of 17, the integrated intensity of the point source is 2.54 mJybeam⁻¹ km s⁻¹, while the integrated intensity excluding the point source is 1.15 mJy beam⁻¹ km s⁻¹ with a standard deviation of 0.33 mJy beam⁻¹ km s⁻¹.

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The brightness temperature of the ¹³CO emission within the gap (excluding the point source) is ≲8 K. For a reasonable gas temperature of ≳20 K (so that CO does not freeze), the gap must be optically thin (τ_gap ≲ 0.5) because the brightness temperature is lower than the gas temperature. The fact that the integrated intensity toward the point source does not increase with the beam size (see Appendix B) further supports that the gap is optically thin in ¹³CO and the point source dominates the emission. If we assume that a similar amount of gas is present around the point source to elsewhere within the gap, the contamination from the gas in the gap is thus expected to be 1.15/2 = 0.575 mJy beam⁻¹ km s⁻¹ (because only half of the gas in front of the point source would contribute). Subtracting this from the integrated intensity toward the point source, we find that the point source is detected at a 6.0σ significance. We carry out the same analysis using the JvM-uncorrected image and find that the point source is detected at a 4.4σ significance (see Appendix C).

In order to determine whether or not the point source is spatially resolved, we follow Isella et al. (2019) and Benisty et al. (2021) and image the data using various robust parameters ranging from −1 to 2. The synthesized beam size, rms noise, and Gaussian fit to the point-source emission are summarized in Appendix B. We find that the peak and integrated intensities are constant within uncertainties when the semiminor axis of the beam is ≲120 mas, indicating that the point source is spatially unresolved at those scales. For larger beam sizes, the peak and integrated intensities increase because the point source is no longer spatially separated from the AS 209 disk.

In Figure 4, we present C¹⁸O J = 2−1 emission at 4.8 km s⁻¹ and the continuum image. No C¹⁸O emission at a >3σ significance is detected at the location of the ¹³CO point source. Similarly, no emission coincident with the ¹³CO point source is detected at a >3σ significance in the continuum image. We further inspected the continuum data by modeling the continuum emission in the visibility plane following Andrews et al. (2021), but no point-source emission coincident with the ¹³CO point source (and elsewhere) is found. In Section 4.2, we place upper limits on the CPD gas and dust masses based on the null detection of C¹⁸O and continuum emission.

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We find that the ¹³CO point source lies at the center of the ¹²CO gap and that it is colocated with the ¹²CO velocity kink (Figure 5). The coincidence suggests that the most likely explanation is that we are witnessing a planet and its CPD embedded in the AS 209 disk. In fact, the compact nature of the ¹³CO emission separated from the circumstellar disk bears a striking resemblance to what is predicted in numerical simulations of a CPD embedded in a circumstellar disk presented in Perez et al. (2015). There is no obvious counterpart to the ¹³CO point source in ¹²CO, but this is likely because the annular gap is optically thick in ¹²CO. Similarly, we do not find a counterpart to the ¹²CO velocity kink in ¹³CO or C¹⁸O due to their low optical depth within and beyond the circumstellar disk gap (see, e.g., Figure 5(d)).

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Recent numerical simulations have shown that circumplanetary materials form a rotationally supported disk when the cooling is efficient, but when cooling is inefficient, a pressure-supported envelope is instead formed (Szulágyi et al. 2016; Fung et al. 2019). The point source is spectrally unresolved in the existing data set, and whether the emission is associated with a disk or an envelope has to be confirmed in the future using high-velocity-resolution molecular-line observations that can probe the kinematics of the point source. For the rest of this paper, we opt not to distinguish between the CPD and envelope, but simply use the term CPD to refer to the two collectively.

What is the mass of the planet candidate? We can estimate the planet mass using the empirical relation between the gap width and planet mass derived from numerical simulations. The width of the gap, determined by the distance between the adjacent peaks in the ¹²CO radial intensity profile, is 78 au (Law et al. 2021a). The scaling relation between the gap width normalized by the planet's orbital semimajor axis Δ_gap/R_p , disk aspect ratio (h/r)_p , disk viscosity α, and planet mass M_p suggests ${{\rm{\Delta }}}_{\mathrm{gap}}/{R}_{p}=0.41{({M}_{p}/{M}_{* })}^{0.5}(h/r{)}_{p}^{-0.75}{\alpha }^{-0.25}$ (Kanagawa et al. 2016; see also similar relations in, e.g., Zhang et al. 2018; Yun et al. 2019). Adopting the disk aspect ratio at the midplane (h/r)_p = 0.118 from the circumstellar disk temperature constrained using CO isotopologues (Law et al. 2021b), Δ_gap/R_p = 0.38 converts to a planet mass of ${M}_{p}=1.3\,{M}_{\mathrm{Jup}}\cdot {(\alpha /{10}^{-3})}^{1/2}$. The inferred mass is comparable to those suggested to explain velocity kinks in other circumstellar disks (e.g., Pinte et al. 2018, 2019).

4.2.1. CPD Size and Temperature

As shown in Appendix B, the CPD is spatially unresolved. When the CPD is optically thick and the disk gas is in local thermodynamic equilibrium (LTE), the actual CPD gas temperature, T_CPD (assumed to be uniform across the CPD for simplicity), and the observed brightness temperature of the CPD, T_obs, are related as T_obs = T_CPD where is the beam dilution factor. ³⁰ Adopting a thin-disk geometry, the beam dilution factor is given by

$$\begin{eqnarray}&&\epsilon =\displaystyle \frac{f\,{D}_{\mathrm{CPD}}^{2}\cos i}{{\theta }_{\mathrm{maj}}{\theta }_{\min }/\mathrm{ln}2},\end{eqnarray} \tag{ 1 }$$

where f is a dimensionless parameter introduced to account for the fraction of the emitting area in a given velocity channel compared to the full geometric area of the CPD (see below), D_CPD is the diameter of the CPD, i is the inclination of the CPD, and θ_maj and ${\theta }_{\min }$ are the semimajor and semiminor axis of the synthesized beam, respectively. We can then relate the temperature and diameter of the CPD as

$$\begin{eqnarray}&&{T}_{\mathrm{CPD}}={T}_{\mathrm{obs}}\displaystyle \frac{{\theta }_{\mathrm{maj}}{\theta }_{\min }/\mathrm{ln}2}{f\,{D}_{\mathrm{CPD}}^{2}\cos i}.\end{eqnarray} \tag{ 2 }$$

Without the knowledge of the emitting area, we opt to adopt f = 1, although it is reasonable to expect f < 1 because the channel spacing of the data (200 m s⁻¹) is smaller than the FWHM of the line emission arising from the CPD (300–400 m s⁻¹; see Appendix B). With f = 1, our analysis thus offers a lower limit on the CPD temperature. We use the brightness temperature in the 4.8 km s⁻¹ channel (≃19 K) because it is likely that the channel contains the largest emitting area. We adopt i = 35° (Öberg et al. 2021) assuming that the CPD and AS 209's circumstellar disk are coplanar.

Figure 6(a) shows the CPD temperature required to explain the observed ¹³CO emission as a function of the CPD diameter, calculated with Equation (2). We note again that the CPD temperature in Figure 6(a) presents a lower limit because a higher CPD temperature is needed to explain the observed ¹³CO emission (1) when attenuation by the gas in the circumstellar disk gap is not negligible, (2) when the emitting area in the central channel is smaller than the full geometric area of the CPD (f < 1), and/or (3) when the CPD is optically thin. We find that the CPD temperature is ≳35 K, higher than the circumstellar disk gas temperature at the radial location of the CPD, 22 K (Law et al. 2021b). This suggests that additional heating sources localized to the CPD are required to explain the observed ¹³CO emission, such as the planet's thermal/accretion heating, the CPD's internal viscous/turbulent heating, or shock/compressional heating by infalling circumstellar disk material (Szulágyi et al. 2016; Szulágyi & Mordasini 2017).

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In the discussion above (and also below in Section 4.2.2), we assume that the CPD and AS 209's circumstellar disk are coplanar. This is a reasonable assumption to start, but numerical simulations show that it is possible that CPDs and their parent circumstellar disk can be misaligned if the planet is formed in a turbulent environment (Jennings & Chiang 2021). In fact, based on the observationally constrained large obliquity of a planetary-mass (M_p = 12–27 M_Jup) companion 2M0122b, ${48}_{-21}^{+28}$ deg, Bryan et al. (2020) proposed that the CPD of 2M0122b would have to be tilted out of the circumstellar disk plane (see also Bryan et al. 2021). For our analysis, the CPD temperature would be minimized when the CPD is face on (i = 0°; see Equation (2)). However, even when the CPD is face on, we infer the CPD temperature to be ≳29 K, higher than the circumstellar disk temperature, suggesting that additional heating sources localized to the CPD are still required. Future high-spatial/velocity-resolution observations would constrain the CPD geometry from its rotation, allowing us to place more accurate constraints on the CPD temperature.

4.2.2. CPD Mass

For a given CPD size and the corresponding lower limit on the CPD temperature (black curve in Figure 6(a)), we can place a lower limit on the ¹³CO mass. This is possible because the upper-state energy of the ¹³CO J = 2−1 transition (15.8 K) is below the lower limit of the CPD temperature; the population in the J = 2 rotational level would always drop if the CPD temperature is higher than the lower limit, requiring a larger CPD mass than for cooler temperatures.

To obtain the total ¹³CO number density in the CPD, we first calculate the number density of ¹³CO molecules in the J = 2 level as

$$\begin{eqnarray}&&{n}_{{}^{13}\mathrm{CO},J=2}=\displaystyle \frac{4\pi }{{{hcA}}_{21}}\int {Idv}\int {d}^{2}d{\rm{\Omega }},\end{eqnarray} \tag{ 3 }$$

where h is the Planck constant, A₂₁ = 6.04 × 10⁻⁷ s⁻¹ is the spontaneous emission coefficient from molecular data from the LAMDA database (Schöier et al. 2005), ∫Idv = 2.50 mJy beam⁻¹ km s⁻¹ is the integrated line intensity, d is the distance to AS 209, 121 pc (Gaia Collaboration et al. 2021), and Ω is the solid angle of the beam. We then calculate the total ¹³CO number density under the assumption that all energy levels are populated under LTE:

$$\begin{eqnarray}&&{n}_{{}^{13}\mathrm{CO},\mathrm{total}}={n}_{{}^{13}\mathrm{CO},J}\displaystyle \frac{Z}{2J+1}\exp \left[\displaystyle \frac{{{hB}}_{e}J(J+1)}{{kT}}\right],\end{eqnarray} \tag{ 4 }$$

where $Z={\sum }_{J=0}^{\infty }(2J+1)\exp [-{{hB}}_{e}J(J+1)/{kT}]$ and B_e =55.10101 GHz is the rotational constant (Müller et al. 2005). We convert the ¹³CO number density to the total gas mass enclosed in the CPD, adopting the interstellar CO abundance [¹²CO]/[H₂] = 10⁻⁴ and ¹³CO-to-¹²CO abundance ratio [¹³CO]/[¹²CO] = 1/60 (Wilson & Rood 1994). The resulting CPD mass is presented in Figure 6(b). The CPD gas mass is ≃0.095 M_Jup (≃30 M_⊕), when D_CPD = 14 au, and increases with decreasing CPD size because the population in the J = 2 level decreases with higher CPD temperature. The CPD-to-planet mass ratio is M_CPD/M_p ≳ 0.02.

In the discussion above, we assumed a CO-to-H₂ ratio inferred from the local interstellar medium (ISM). However, near the radial location of the CPD candidate, CO in the AS 209's circumstellar disk is inferred to be depleted in the gas phase by about an order of magnitude compared with the local ISM (Zhang et al. 2021). If the CO in the CPD is depleted at a comparable level, the CPD-to-planet mass ratio would be M_CPD/M_p ≳ 0.2, a level at which the CPD may be gravitationally unstable.

We can place upper limits on the CPD gas mass from the null detection of C¹⁸O emission (3σ ≃ 1.011 mJy beam⁻¹) following the same approach to ¹³CO. To do so, we assume that ¹³CO and C¹⁸O share the same line width (≃400 m s⁻¹) and temperature (presented in Figure 6(a)). We adopt A₂₁ = 6.01 × 10⁻⁷ s⁻¹ (Schöier et al. 2005), B_e = 54.89142 GHz (Müller et al. 2005), and [C¹⁸O]/[¹²CO] = 1/560 (Wilson & Rood 1994). The resulting upper limit on the CPD gas mass is presented in Figure 6(b). Interestingly, the null detection of C¹⁸O places a tight upper limit that is only about 50% larger than the lower limit obtained with ¹³CO. When the CPD temperature is larger than the lower limit presented in Figure 6(a), both the ¹³CO lower limit and C¹⁸O upper limit would increase. However, because the rotational constant of the two lines is comparable, the ratio between the upper limit inferred by C¹⁸O and the lower limit inferred by ¹³CO would remain nearly constant (see Equation (4)) and the null detection of C¹⁸O would still provide tight constraints.

We can also place constraints on the CPD dust mass from the nondetection of continuum emission (3σ ≃ 26.4 μJy beam⁻¹). Adopting the DSHARP dust opacity (Birnstiel et al. 2018) with a maximum grain size of 1 mm (κ_ν = 2.0 cm² g⁻¹ at 240 GHz) and the CPD temperature constrained as in Figure 6(a), the CPD dust mass in the optically thin regime is ≲0.006 M_⊕ ≃0.47 lunar masses. If instead the CPD has only small, micron-sized grains, κ_ν = 0.42 cm² g⁻¹ (Birnstiel et al. 2018) and the CPD dust mass is ≲0.027 M_⊕ ≃ 2.2 lunar masses. These results are presented in Figure 6(c).

Using the constraints on the CPD gas and dust mass, we infer the dust-to-gas mass ratio to be ≲2 × 10⁻⁴ when the maximum grain size in the CPD is 1 mm and ≲9 × 10⁻⁴ when the maximum grain size in the CPD is 1 μm (Figure 6(d)). This suggests that the CPD is likely lacking dust at a level below that in the typical ISM environment, presumably due to a limited dust supply (recall that AS 209's continuum disk is confined within the inner ≃140 au) and/or the rapid radial drift of dust within the CPD.

Based on the width of the ¹²CO gap, we infer that the planet's mass is around a Jupiter mass. How did a giant planet form at an orbital radius of 200 au?

One possibility is that the AS 209 disk was gravitationally unstable in the past and the planet formed via gravitational instability (GI; Boss 1997). Limitations of the GI scenario often mentioned in the literature are that the masses of GI-induced fragments are generally large, often in the regime of brown dwarfs, and that GI-induced fragments suffer from tidal disruption and/or rapid radial migration (Baruteau et al. 2011; Zhu et al. 2012). However, recent magnetohydrodynamic simulations including the disk's self-gravity have shown that magnetic fields can limit the mass of the GI-induced fragments to less than a few Jupiter masses, and also can prevent fragments from being tidally disrupted (Deng et al. 2021). It is also shown that orbital migration can stall when the planet actively accretes and carves a deep gap (Fletcher et al. 2019).

Despite recent theoretical developments, one challenge to the GI scenario is that the AS 209 disk has a very small mass of 0.003–0.0045 M_⊙ ≃ 3.1–4.7 M_Jup (Favre et al. 2019; Zhang et al. 2021). According to the inferred gas surface density profile from Zhang et al. (2021) and the midplane temperature constrained by CO isotopologues from Law et al. (2021b), the Toomre Q parameter (Toomre & Toomre 1972) is >100 at all radii in the AS 209 disk with Q ≃ 300 at 200 au, indicating that the disk must be gravitationally stable presently. While it is not impossible that the disk was sufficiently massive in the past, the small present-day disk mass implies that the disk should have lost its mass very efficiently since then.

An alternative to the GI scenario is pebble accretion (Johansen & Lacerda 2010; Ormel & Klahr 2010). In order for the pebble accretion scenario to work, a few things have to be reconciled. First, the continuum emission of the AS 209 disk is confined within the inner ≃140 au presently. How did the core grow and how did the planet not trap millimeter grains beyond its orbit? We propose that in this scenario, the core of the planet never reached the pebble isolation mass (Morbidelli & Nesvorny 2012) and pebbles could freely drift inward crossing the planet's orbit. Based on scaling relations from hydrodynamic simulations (e.g., Lambrechts et al. 2014; Bitsch et al. 2018), the pebble isolation mass at 200 au in the AS 209 disk (h/r = 0.118; Law et al. 2021b) is expected to be ≳100 M_⊕, much more massive than the critical core mass of M_crit = 10–20 M_⊕, at which point runaway gas accretion can start (Mizuno 1980). We can thus envision a scenario in which the pebbles that existed beyond the orbit of the core migrated inward, leaving the core behind.

Second, we need to check if sufficient pebbles existed to form a core of the giant planet. To do so, we first estimate the total mass of pebbles necessary to drift toward the core to grow to 10 M_⊕ following Lambrechts & Johansen (2014). We obtain M_peb ≈ 130 M_⊕ $\cdot {({M}_{c}/20{M}_{\oplus })}^{1/3}{(r/5\,\mathrm{au})}^{1/2}$ (St/0.05)^1/3 ≈650M_⊕ assuming that the dominant pebble size has a Stokes number of 0.05. The total dust mass in the AS 209 disk is currently about 300 M_⊕ (Sierra et al. 2021), but given that it is very unlikely that all the millimeter grains currently inward of the planet's orbit were once beyond 200 au, more efficient pebble accretion than the standard pebble accretion model of Lambrechts & Johansen (2014) is likely required. A few possibilities include the presence of pressure bumps (Pinilla et al. 2012) or a vortex that might have formed early on from the infalling flows from the protostellar envelope (Bae et al. 2015), and changes in sticking properties of grains that lead to the traffic jam effect around snow lines (Drążkowska & Alibert 2017), which can slow down or even halt the radial drift of grains. In fact, it is interesting to note that the midplane gas temperature at the current radial location of the planet (T ≃ 22 K; Law et al. 2021b) is close to the expected CO and N₂ freezeout temperature (T_frz,CO = 19–24 K, ${T}_{\mathrm{frz},{{\rm{N}}}_{2}}=17\mbox{--}21$ K; Huang et al. 2018).

Third, we need to make sure that there had been sufficient time for a core to form via pebble accretion. The timescale for pebble accretion to form a core is given by t_PA ≈ 4 × 10⁴ yr ·${({M}_{\mathrm{crit}}/10{M}_{\oplus })}^{1/3}(r/5\,\mathrm{au})$ (Lambrechts & Johansen 2012). Adopting a critical core mass of 10 M_⊕, at 200 au the pebble accretion timescale is t_PA ≈ 1.6 Myr although, as mentioned earlier, slower radial drift or particle trapping can enhance the pebble accretion efficiency and shorten the required time to form the core. Given that the estimated age of AS 209 is 1–2 Myr (Andrews et al. 2009; Öberg et al. 2021), the long core accretion timescale might indicate that the planet could have entered the runaway accretion phase in the last ≲1 Myr. If the planet has been accreting ≃1 M_Jup over the last million years or so, the average accretion rate would be ≃10⁻⁶ M_Jup yr⁻¹, orders of magnitude larger than the accretion rate of PDS 70b and c measured from Hα line emission (10^−8±1 M_Jup yr⁻¹; Wagner et al. 2018; Haffert et al. 2019). Observations of the Hα line can confirm if the planet is indeed undergoing runaway accretion, although long-term monitoring observations are likely required to distinguish steadily high accretion from episodic burst-type accretion (Lubow & Martin 2012).

We conclude the discussion by listing a few future directions from the observational perspective. While we could only place upper/lower limits from the existing data, future higher-level transition observations of ¹³CO (together with the existing ¹³COJ = 2 − 1 data) will allow us to determine whether ¹³CO is optically thick or thin, and thus place stronger constraints on the CPD temperature. As shown in Figure 6(b), the null detection of C¹⁸O already placed a tight upper limit on the CPD gas mass that is only about 50% larger than the lower limit inferred by ¹³CO. Deeper C¹⁸O observations (by a factor of ≃2 in sensitivity assuming that isotope-selective photodissociation is not important) should thus detect the CPD and place stringent constraints on the CPD gas mass. The existing data are limited by a 200 m s⁻¹ velocity resolution and we presently cannot conclude whether the point-source emission is consistent with a rotationally supported disk or a pressure-supported envelope that is potentially heated by an embedded planet (e.g., Alves et al. 2020). Future observations with higher velocity resolution will enable us to distinguish the two scenarios and to dynamically constrain the planet mass from the rotation of the CPD. High-spatial/velocity-resolution observations will also enable us to constrain the geometry of the CPD (e.g., position angle, inclination) from the disk rotation. Knowing the CPD geometry will allow us to place more accurate constraints on the CPD temperature and mass. In addition, estimating the obliquity of the CPD can help us to better understand the formation mechanism and environment of the CPD-hosting planet, in particular, whether the planet had formed in a turbulent environment that would induce large obliquities (see, e.g., Bryan et al. 2020; Jennings & Chiang 2021). The inferred CPD temperature suggests that heating sources localized to the CPD must be present. Such planetary/CPD heating can produce chemical asymmetries in the circumstellar disk, which are shown to be detectable with high-sensitivity molecular-line observations using ALMA (Cleeves et al. 2015). Besides additional ALMA observations, future observations in the infrared wavelengths using the James Webb Space Telescope and ground-based telescopes would independently confirm the planet/CPD and allow us to probe the thermal emission from the planet, which can help to constrain the mass of the planet. In addition, searching for accretion signatures (e.g., Hα line emission) would yield invaluable constraints on the current position of the planet in its evolutionary stages.