Two accreting protoplanets around the young star PDS 70 (1906.01486v1)

Abstract: Newly forming proto-planets are expected to create cavities and substructures in young, gas-rich proto-planetary disks, but they are difficult to detect as they could be confused with disk features affected by advanced image-analysis techniques. Recently, a planet was discovered inside the gap of the transitional disk of the T-Tauri star PDS 70. Here we report on the detection of strong H-alpha emission from two distinct locations in the PDS 70 system, one corresponding to the previously discovered planet PDS 70 b, which confirms the earlier H$\alpha$ detection, and another located close to the outer-edge of the gap, coinciding with a previously identified bright dust spot in the disk and with a small opening in a ring of molecular emission. We identify this second H$\alpha$ peak as a second proto-planet in the PDS 70 system. The H$\alpha$ emission spectra of both proto-planets indicate ongoing accretion onto the proto-planets, which appear to be near a 2:1 mean motion resonance. Our observations show that adaptive-optics-assisted, medium-resolution, integral-field spectroscopy with MUSE targeting accretion signatures will be a powerful way to trace ongoing planet formation in transitional disks at different stages of their evolution. Finding more young planetary systems in mean motion resonance would give credibility to the Grand Tack hypothesis in which Jupiter and Saturn migrated in a resonance orbit during the early formation period of our Solar System.

• The paper confirms that PDS 70 b and PDS 70 c exhibit strong H-alpha emissions with 11σ and 8σ detections respectively, indicating active accretion processes.
• The research employs MUSE integral-field spectroscopy combined with HRSDI to isolate spectral features and overcome common high-contrast imaging artifacts.
• The findings support resonant orbital migration models and reveal mass accretion rates near the stellar rate, refining our understanding of early planet formation.

Detection of Two Accreting Proto-Planets in the PDS 70 System

The paper by Haffert et al. presents compelling evidence for the discovery of two accreting proto-planets within the proto-planetary disk of the T-Tauri star PDS 70. The findings are substantiated by the detection of strong H-alpha emission lines, indicating active accretion processes at two distinct locations in the PDS 70 system, notably confirming the known presence of PDS 70 b and introducing PDS 70 c. These detections offer critical insights into planet formation processes and planetary system evolution.

Key Observational Results

The paper utilized the Multi Unit Spectroscopic Explorer (MUSE) at the ESO's Very Large Telescope to capture medium-resolution integral-field spectroscopy data. This approach, combined with a high-resolution Spectral Differential Imaging (HRSDI) technique, enabled the isolation of strong H-alpha emission associated with two proto-planets. Significant findings include:

• PDS 70 b: Confirmed via an 11σ H-alpha emission detection, the planet exhibits a H-alpha line-of-sight redshift of 25 ± 8 km/s and a FWHM of 76 ± 13 km/s.
• PDS 70 c: Identified through an 8σ detection located near the outer edge of the disk gap, displaying a redshift of 30 ± 6 km/s and a FWHM of 102 ± 19 km/s. The positional coherence with previous broad-band imaging bolsters its classification as a planet.

Both proto-planets are embedded within a disk demonstrating an extensive gap, likely formed through dynamical interactions with these planetary bodies.

Implications for Theoretical Models

The paper’s findings have substantial implications for the understanding of planet formation and migration:

• Accretion Dynamics: The mass accretion rates of the proto-planets are near parity with the stellar accretion rate, emphasizing that the accretion processes onto the planets are substantial, although the current rates would imply a lengthy formation timescale inconsistent with typical disk lifetimes.
• Mean Motion Resonance: The 2:1 mean motion resonance suggested between the two proto-planets reinforces theoretical models regarding resonant orbital migration, such as the Grand Tack hypothesis posited for Jupiter and Saturn’s early migration.

Methodological Contributions

The deployment of MUSE with adaptive optics in conjunction with HRSDI underscores a refined method for identifying accreting planets amidst their circumstellar environments. This paper's methodology circumvented the pitfalls of artifacts common in high-contrast imaging, ensuring robust detection and characterization of spectral features imperative for exoplanetary science.

Future Prospects

The demonstrated capability of HRSDI in conjunction with integral-field spectrographs like MUSE opens avenues for systematic searches and detailed studies of accreting planets in transition disks. Further observations could calibrate theoretical mass estimates against hydrodynamic models to refine accretion rate predictions and mass determinations. Continuous monitoring will elucidate accretion variability over temporal scales, providing insight into planetary growth processes.

In summary, this research not only provides evidential support for the presence of multiple planets in the PDS 70 system but also advances our grasp of the physical processes governing planet formation in early stellar environments. These methodological and theoretical contributions offer a basis for future studies exploring the intricacies of young planetary systems and their evolutionary trajectories towards maturity.