First detection of orbital motion for HD 106906 b: A wide-separation exoplanet on a Planet Nine-like orbit

Abstract: HD 106906 is a 15 Myr old short-period (49 days) spectroscopic binary that hosts a wide-separation (737 au) planetary-mass ($\sim11\,M_{\rm Jup}$) common proper motion companion, HD 106906 b. Additionally, a circumbinary debris disk is resolved at optical and near-infrared wavelengths that exhibits a significant asymmetry at wide separations that may be driven by gravitational perturbations from the planet. In this study we present the first detection of orbital motion of HD 106906 b using Hubble Space Telescope images spanning a 14 yr period. We achieve high astrometric precision by cross-registering the locations of background stars with the Gaia astrometric catalog, providing the subpixel location of HD 106906 that is either saturated or obscured by coronagraphic optical elements. We measure a statistically significant $31.8\pm7.0$ mas eastward motion of the planet between the two most constraining measurements taken in 2004 and 2017. This motion enables a measurement of the inclination between the orbit of the planet and the inner debris disk of either $36_{-14}^{+27}$ deg or $44_{-14}^{+27}$ deg, depending on the true orientation of the orbit of the planet. There is a strong negative correlation between periastron and mutual inclination; orbits with smaller periastra are more misaligned with the disk plane. With a periastron of $510_{-320}^{+480}$ au, HD 106906 b is likely detached from the planetary region within 100 au radius, showing that a Planet Nine-like architecture can be established very early in the evolution of a planetary system.

• The paper reports the first orbital motion detection of HD 106906 b, noting a significant eastward displacement of 31.8 ± 7.0 mas from 2004 to 2017.
• The study uses 14 years of HST astrometry combined with Gaia DR2 to derive detailed orbital parameters including an eccentricity of ~0.44 and a semi-major axis of ~850 au.
• The analysis uncovers a strong covariance between the planet’s mutual inclination with the debris disk and its eccentricity, suggesting implications for planet-disk interactions.

Overview of the Detection of Orbital Motion for HD 106906 b

This paper by Nguyen, De Rosa, and Kalas investigates the orbital motion of the exoplanet HD 106906 b, a distant planetary-mass object located at 737 au from its host binary star system HD 106906. The study presents significant astrometric data collected over a 14-year span using the Hubble Space Telescope (HST) to delineate the trajectory of HD 106906 b. This exoplanet's wide separation and peculiar orbit have been likened to the hypothetical Planet Nine in our solar system.

Observational Data and Methods

The authors utilized HST observations from three instruments: the Advanced Camera for Surveys (ACS), the Space Telescope Imaging Spectrograph (STIS), and the Wide Field Camera 3 (WFC3). By combining astrometric positions calibrated using the Gaia Data Release 2 (DR2) catalog, the study achieves enhanced precision in determining the relative positions of background stars and HD 106906, thus indirectly defining the position of the planet that is often obscured by the coronagraphic mask or saturation in direct observations.

Key Findings

• Astrometric Motion: The exoplanet shows a statistically significant eastward motion of $31.8 \pm 7.0$ mas from 2004 to 2017. This motion, inferred from precise measurements, permits initial constraints on its orbital elements.
• Orbital Characteristics: The derived orbital parameters indicate an eccentricity of $0.44_{-0.31}^{+0.28}$ and a semi-major axis of $850_{-260}^{+560}$ au. The inclination of the orbit relative to the system's debris disk is estimated to be about 36 to 44 degrees, indicating a significant tilt.
• Mutual Inclination and Periastron Correlation: There is a strong covariance between the mutual inclination of the planet's orbit with the debris disk and the eccentricity, suggesting that orbits with smaller periastra are more inclined to the disk plane.

Implications for Exoplanet Studies

The investigation into HD 106906 b informs our understanding of planet-disk interactions and the architectural dynamics of exoplanetary systems. The observed inclination and misalignment with the debris disk may offer insights into the dynamical history of the system, analogous to the hypothesized effect of Planet Nine on the detached Kuiper Belt objects in the solar system.

Future Directions

This study suggests the potential for HD 106906 b to gravitationally perturb the debris disk, a relationship that can be explored further with high-fidelity simulations that align with the observed parameter space. Furthermore, measuring the radial velocity of HD 106906 b could provide additional constraints on its orbit and confirm its gravitational binding to the host binary.

In summary, this research provides a detailed astrometric analysis of HD 106906 b, contributing to the broader discourse on the formation and evolution processes of distant planetary bodies. The study underscores the importance of continued surveillance and modeling to elucidate the role of similar exoplanetary systems within the context of galactic dynamics and evolution.