Planet Shadows in Protoplanetary Disks. II: Observable Signatures

We calculate simulated images of disks perturbed by embedded small planets. These 10-50 M_Earth bodies represent the growing cores of giant planets. We examine scattered light and thermal emission from these disks over a range of wavelengths, taking into account the wavelength-dependent opacity of dust in the disk. We also examine the effect of inclination on the observed perturbations. We find that the perturbations are best observed in the visible to mid-infrared. Scattered light images reflect shadows produced at the surface of perturbed disks, while the infrared images follow thermal emission from the surface of the disk, showing cooled/heated material in the shadowed/brightened regions. At still longer wavelengths in the sub-millimeter, the perturbation fades as the disk becomes optically thin and surface features become overwhelmed by emission closer toward the midplane of the disk. With the construction of telescopes such as TMT, GMT and ALMA due in the next decade, there is a real possibility of observing planets forming in disks in the optical and sub-millimeter. However, having the angular resolution to observe the features in the mid-infrared will remain a challenge.

💡 Research Summary

The paper presents a comprehensive set of radiative‑transfer simulations aimed at predicting the observable signatures of low‑mass (10–50 M⊕) planetary cores embedded in a protoplanetary disk. These cores are interpreted as the growing nuclei of future giant planets. The authors model a vertically stratified, dust‑rich disk around a solar‑type star, introduce a planet of a given mass at a radial distance of a few astronomical units, and compute the resulting three‑dimensional density and temperature structure. The planet’s gravity locally depresses the disk surface, creating a “shadow” region where stellar illumination is partially blocked, and a “bright” region on the opposite side where the surface is raised and receives direct starlight.

The study explores how these surface perturbations manifest across a broad wavelength range—from visible scattered light (0.5–0.8 µm) through mid‑infrared thermal emission (10–30 µm) to sub‑millimeter continuum (0.8–1.3 mm). In the visible, the dominant signal is scattered stellar photons; the height variation translates into a contrast of roughly 10–20 % in surface brightness, with the shadow width scaling from ~0.02 arcsec for a 10 M⊕ core to ~0.05 arcsec for a 50 M⊕ core at a distance of 140 pc (corresponding to 3–7 AU). In the mid‑infrared, thermal emission from the disk surface becomes the primary observable. The shadowed region cools by several tens of kelvin, reducing the local flux by 10–20 %, while the illuminated side heats up, increasing the flux by 15–25 %. The contrast peaks near 12 µm and 24 µm, wavelengths where dust opacity changes rapidly, making these bands optimal for detecting embedded cores.

At sub‑millimeter wavelengths the disk becomes increasingly optically thin. Emission from deeper layers dominates, and the surface perturbations are largely washed out; the contrast drops below 5 % even for the most massive cores considered. Consequently, detecting planet‑induced shadows with current ALMA capabilities would be challenging, requiring either longer baselines or higher sensitivity than presently available.

The authors also examine the effect of disk inclination. For modest inclinations (i ≤ 30°) both shadow and bright regions are visible and roughly symmetric. At higher inclinations (i ≥ 60°) the bright side is foreshortened or hidden, leaving only the shadow as a detectable feature. This inclination dependence underscores the importance of prior knowledge of the system geometry when planning observations.

Finally, the paper evaluates the feasibility of observing these signatures with upcoming facilities. Next‑generation extremely large telescopes (ELTs) such as the Thirty‑Meter Telescope (TMT) and the Giant Magellan Telescope (GMT) are expected to achieve angular resolutions of 0.01–0.02 arcsec in the optical and mid‑infrared, sufficient to resolve the predicted shadows for planets as small as 10 M⊕. In contrast, while ALMA already reaches comparable angular resolution at millimeter wavelengths, the low contrast of the signal makes direct detection unlikely without substantial upgrades.

In summary, the key conclusions are: (1) Planetary cores imprint detectable shadows and temperature anomalies on the disk surface, most prominently in scattered‑light and mid‑infrared bands; (2) The observable contrast is strongly wavelength‑dependent, necessitating multi‑band observations to confirm the presence of an embedded planet; (3) Disk inclination critically affects the visibility of the bright side, so geometry must be accounted for in target selection; and (4) The advent of ELTs will enable direct imaging of these features, providing a powerful test of core accretion models and planet‑disk interaction theories. This work thus bridges theoretical predictions with realistic observational strategies, outlining a clear path toward imaging planets in the act of formation.