HR 8799 is a young A5/F0 star hosting four directly imaged giant planets at wide separations (∼16–78 au), which are undergoing orbital motion and have been continuously monitored with adaptive optics imaging since their discovery over a decade ago. We present a dynamical mass of HR 8799 using 130 epochs of relative astrometry of its planets, which include both published measurements and new medium-band 3.1 μm observations that we acquired with NIRC2 at Keck Observatory. For the purpose of measuring the host-star mass, each orbiting planet is treated as a massless particle and is fit with a Keplerian orbit using Markov chain Monte Carlo. We then use a Bayesian framework to combine each independent total mass measurement into a cumulative dynamical mass using all four planets. The dynamical mass of HR 8799 is 1.47 − 0.17 + 0.12 M ⊙ assuming a uniform stellar mass prior, or 1.46 − 0.15 + 0.11 M ⊙ with a weakly informative prior based on spectroscopy. There is a strong covariance between the planets’ eccentricities and the total system mass; when the constraint is limited to low-eccentricity solutions of e < 0.1, which are motivated by dynamical stability, our mass measurement improves to 1.43 − 0.07 + 0.06 M ⊙. Our dynamical mass and other fundamental measured parameters of HR 8799 together with Modules for Experiments in Stellar Astrophysics Isochrones and Stellar Tracks grids yields a bulk metallicity most consistent with [Fe/H] ∼ −0.25–0.00 dex and an age of 10–23 Myr for the system. This implies hot-start masses of 2.7–4.9 M Jup for HR 8799 b and 4.1–7.0 M Jup for HR 8799 c, d, and e, assuming they formed at the same time as the host star.

Debris disks are extrasolar analogs to our own Kuiper Belt and they are detected around at least 17% of nearby Sun-like stars. The morphology and dynamics of a disk encode information about its history, as well as that of any exoplanets within the system. We used ALMA to obtain 1.3 mm observations of the debris disk around the nearby F5V star HD 170773. We image the face-on ring and determine its fundamental parameters by forward-modeling the interferometric visibilities through a Markov Chain Monte Carlo approach. Using a symmetric Gaussian surface density profile, we find a 71 ± 4 au wide belt with a radius of 193 +2 −3 au, a relatively large radius compared to most other millimeter-resolved belts around late A / early F type stars. This makes HD 170773 part of a group of four disks around A and F stars with radii larger than expected from the recently reported planetesimal belt radius -stellar luminosity relation. Two of these systems are known to host directly imaged giant planets, which may point to a connection between large belts and the presence of long-period giant planets. We also set upper limits on the presence of CO and CN gas in the system, which imply that the exocomets that constitute this belt have CO and HCN ice mass fractions of < 77% and < 3%, respectively, consistent with Solar System comets and other exocometary belts.

We analyzed 20 s cadence Transiting Exoplanet Survey Satellite time-series photometry of the exoplanet host star HR 8799 collected in Sector 56. The amplitude spectrum shows Gamma Doradus (γ Dor) pulsations consistent with previous space-based photometry from MOST. Assuming that HR 8799 is a representative of γ Dor stars in the Kepler sample, the dominant dipole mode at 1.98 cycles day−1 implies a core rotation period of ∼0.7 day, which combined with v sin i and stellar radius measurements would result in a preliminary stellar inclination of ∼28° assuming rigid rotation. We find no significant residual photometric variation after removing the pulsation signal aside from a ∼9 days trend that is likely a systematic effect or an artifact from performing aggressive frequency subtraction in the presence of red noise.

51 Eri is well known for hosting a directly imaged giant planet and for its membership to the β Pictoris moving group. Using 2 minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS), we detect multiperiodic variability in 51 Eri that is consistent with pulsations of Gamma Doradus (γ Dor) stars. We identify the most significant pulsation modes (with frequencies between ∼0.5 and 3.9 cycles day−1 and amplitudes ranging between ∼1 and 2 mmag) as dipole and quadrupole gravity modes, as well as Rossby modes, as previously observed in Kepler γ Dor stars. Our results demonstrate that previously reported variability attributed to stellar rotation is instead likely due to γ Dor pulsations. Using the mean frequency of the ℓ = 1 gravity modes, together with empirical trends of the Kepler γ Dor population, we estimate a plausible stellar core rotation period of 0.9 − 0.1 + 0.3 days for 51 Eri. We find no significant evidence for transiting companions around 51 Eri in the residual light curve. The detection of γ Dor pulsations presented here, together with follow-up observations and modeling, may enable the determination of an asteroseismic age for this benchmark system. Future TESS observations would allow a constraint on the stellar core rotation rate, which in turn traces the surface rotation rate, and thus would help clarify whether or not the stellar equatorial plane and orbit of 51 Eri b are coplanar.