Discovery of a planetary-mass companion within the gap of the transition disk around PDS 70

Abstract: Young circumstellar disks are of prime interest to understand the physical and chemical conditions under which planet formation takes place. Only very few detections of planet candidates within these disks exist, and most of them are currently suspected to be disk features. In this context, the transition disk around the young star PDS 70 is of particular interest, due to its large gap identified in previous observations, indicative of ongoing planet formation. We aim to search for the presence of planets and search for disk structures indicative for disk-planet interactions and other evolutionary processes. We analyse new and archival near-infrared (NIR) images of the transition disk PDS 70 obtained with the VLT/SPHERE, VLT/NaCo and Gemini/NICI instruments in polarimetric differential imaging (PDI) and angular differential imaging (ADI) modes. We detect a point source within the gap of the disk at about 195 mas (about 22 au) projected separation. The detection is confirmed at five different epochs, in three filter bands and using different instruments. The astrometry results in an object of bound nature, with high significance. The comparison of the measured magnitudes and colours to evolutionary tracks suggests that the detection is a companion of planetary mass. We confirm the detection of a large gap of about 54 au in size within the disk in our scattered light images, and detect a signal from an inner disk component. We find that its spatial extent is very likely smaller than about 17 au in radius. The images of the outer disk show evidence of a complex azimuthal brightness distribution which may in part be explained by Rayleigh scattering from very small grains. Future observations of this system at different wavelengths and continuing astrometry will allow us to test theoretical predictions regarding planet-disk interactions, planetary atmospheres and evolutionary models.

• The paper reports the discovery of PDS 70b, a co-moving planetary-mass object within the gap of PDS 70's transition disk.
• The study employed advanced techniques like ADI and PDI across five epochs to confirm the companion’s presence and motion.
• The analysis characterizes PDS 70b as a 5–9 M_Jup object with a dusty atmosphere at ~1200 K, supporting disk-planet interaction models.

Discovery of a Planetary-Mass Companion Within the PDS 70 Transition Disk

This paper presents the discovery of a point source within the transitional disk of the young star PDS 70, using a series of high-contrast imaging observations. The observations, conducted with instruments including VLT/SPHERE, Gemini/NICI, and VLT/NaCo, suggest the presence of a planetary-mass companion residing at a separation of approximately 22 astronomical units (au) from the host star. This detection, confirmed over multiple epochs, opens important avenues for studying planet formation and disk-planet interactions in situ.

The study of transition disks, characterized by their substantial gaps often suspected to be carved by forming planets, provides critical insights into the physical processes governing the final stages of planet formation. PDS 70, a K7-type young star located in the Upper Centaurus-Lupus subgroup, presents such a transition disk, making it an ideal candidate for direct imaging efforts to detect young planets.

Observational Evidence and Methodology

Across five different observational epochs using instruments operating at H, K, and L' bands, the authors report a consistent detection of a point source within the disk's gap. The astrometric measurements reveal minimal angular displacement consistent with a co-moving planetary object, as opposed to a stationary background source.

The observations have been processed using advanced image post-processing algorithms including Angular Differential Imaging (ADI) and Polarimetric Differential Imaging (PDI). These methodologies help enhance the detectability of faint objects by leveraging their movement relative to the star as a function of rotation and polarization characteristics, respectively. Given the challenges of imaging in such dense environments where stellar light often outshines planetary companions, these techniques are essential.

Characterization of PDS 70b

The photometric analysis places the planetary candidate, dubbed PDS 70b, within a mass regime of 5 to 9 Jupiter masses (M_Jup) if assumed to be in a 'hot start' formation scenario—an inference backed by evolutionary tracks. The candidate shows notably redder infrared colors than typical L-type dwarf stars, hinting at a potentially dusty or cloudy atmosphere. This atmospheric characteristic could either be intrinsic to the companion or due to surrounding material, such as a circumplanetary disk, which remains to be further explored.

Spectral Energy Distribution (SED) modeling, combined with the companion's photometric data, supports a temperature estimate of approximately 1200 K, characteristic of a warm L-type dwarf. Notably, the rotational alignment of the disk and companion supports the hypothesis of a coherent planet-forming environment.

Implications and Future Prospects

The detection of PDS 70b provides a compelling case study for understanding the properties of planets forming within protoplanetary disks. It allows for empirical testing of theoretical models concerning disk clearing mechanisms, observed disk structures (e.g., gaps and azimuthal asymmetries), and planet-disk interactions. The finding also underlines the significance of using high-contrast imaging techniques to directly image young exoplanets, which is indispensable for observationally exploring planetary formation in various circumstellar environments.

Moving forward, continued astrometric monitoring of PDS 70b will enable precise orbit characterization, offering insights into its dynamical interaction with the surrounding disk. Additionally, further multi-wavelength observations, particularly at longer wavelengths such as those achievable with ALMA, would be instrumental in probing any circumplanetary material, enhancing our understanding of the companion's atmosphere and formation history. Such comprehensive studies contribute to a broader understanding of planet formation processes, potentially influencing theories on the evolution of planetary systems both within and beyond our own.