Search for: All records

The characteristic orbital period of the innermost objects within the galactic census of planetary and satellite systems appears to be nearly universal, withPon the order of a few days. This paper presents a theoretical framework that provides a simple explanation for this phenomenon. By considering the interplay between disk accretion, magnetic field generation by convective dynamos, and Kelvin–Helmholtz contraction, we derive an expression for the magnetospheric truncation radius in astrophysical disks and find that the corresponding orbital frequency is independent of the mass of the host body. Our analysis demonstrates that this characteristic frequency corresponds to a period ofP∼ 3 days although intrinsic variations in system parameters are expected to introduce a factor of a ∼2–3 spread in this result. Standard theory of orbital migration further suggests that planets should stabilize at an orbital period that exceeds disk truncation by a small margin. Cumulatively, our findings predict that the periods of close-in bodies should spanP∼ 2–12 days—a range that is consistent with observations.

 

Abstract Compact nonresonant systems of sub-Jovian planets are the most common outcome of the planet formation process. Despite exhibiting broad overall diversity, these planets also display dramatic signatures of intrasystem uniformity in their masses, radii, and orbital spacings. Although the details of their formation and early evolution are poorly known, sub-Jovian planets are expected to emerge from their natal nebulae as multiresonant chains, owing to planet–disk interactions. Within the context of this scenario, the architectures of observed exoplanet systems can be broadly replicated if resonances are disrupted through postnebular dynamical instabilities. Here, we generate an ad hoc sample of resonant chains and use a suite of N -body simulations to show that instabilities can not only reproduce the observed period ratio distribution, but that the resulting collisions also modify the mass uniformity in a way that is consistent with the data. Furthermore, we demonstrate that primordial mass uniformity, motivated by the sample of resonant chains coupled with dynamical sculpting, naturally generates uniformity in orbital period spacing similar to what is observed. Finally, we find that almost all collisions lead to perfect mergers, but some form of postinstability damping is likely needed to fully account for the present-day dynamically cold architectures of sub-Jovian exoplanets. 

Recent advances in submillimeter observations of young circumstellar nebulae have opened an unprecedented window into the structure of protoplanetary disks that has revealed the surprising ubiquity of broken and misaligned disks. In this work, we demonstrate that such disks are capable of torquing the spin axis of their host star, representing a hitherto unexplored pathway by which stellar obliquities may be generated. The basis of this mechanism is a crossing of the stellar spin precession and inner disk regression frequencies, resulting in adiabatic excitation of the stellar obliquity. We derive analytical expressions for the characteristic frequencies of the inner disk and star as a function of the disk gap boundaries and place an approximate limit on the disk architectures for which frequency crossing and the resulting obliquity excitation are expected, thereby illustrating the efficacy of this model. Cumulatively, our results support the emerging consensus that significant spin–orbit misalignments are an expected outcome of planet formation.

 

Motivated by recent visits from interstellar comets, along with continuing discoveries of minor bodies in orbit of the Sun, this paper studies the capture of objects on initially hyperbolic orbits by our solar system. Using an ensemble of ∼500 million numerical experiments, this work generalizes previous treatments by calculating the capture cross section as a function of asymptotic speed. The resulting velocity-dependent cross section can then be convolved with any distribution of relative speeds to determine the capture rate for incoming bodies. This convolution is carried out for the usual Maxwellian distribution, as well as the velocity distribution expected for rocky debris ejected from planetary systems. We also construct an analytic description of the capture process that provides an explanation for the functional form of the capture cross section in both the high- and low-velocity limits.

 

The detection of satellites around extrasolar planets, so called exomoons, remains a largely unexplored territory. In this work, we study the potential of detecting these elusive objects from radial velocity monitoring of self-luminous, directly imaged planets. This technique is now possible thanks to the development of dedicated instruments combining the power of high-resolution spectroscopy and high-contrast imaging. First, we demonstrate a sensitivity to satellites with a mass ratio of 1%–4% at separations similar to the Galilean moons from observations of a brown-dwarf companion (HR 7672 B;K_mag= 13; 0.″7 separation) with the Keck Planet Imager and Characterizer (R∼ 35,000 in theKband) at the W. M. Keck Observatory. Current instrumentation is therefore already sensitive to large unresolved satellites that could be forming from gravitational instability akin to binary star formation. Using end-to-end simulations, we then estimate that future instruments such as the Multi-Object Diffraction-limited High-resolution Infrared Spectrograph, planned for the Thirty Meter Telescope, should be sensitive to satellites with mass ratios of ∼10⁻⁴. Such small moons would likely form in a circumplanetary disk similar to the Jovian satellites in the solar system. Looking for the Rossiter–McLaughlin effect could also be an interesting pathway to detecting the smallest moons on short orbital periods. Future exomoon discoveries will allow precise mass measurements of the substellar companions that they orbit and provide key insight into the formation of exoplanets. They would also help constrain the population of habitable Earth-sized moons orbiting gas giants in the habitable zone of their stars.

 

Exoplanet systems with multiple transiting planets are natural laboratories for testing planetary astrophysics. One such system is HD 191939 (TOI 1339), a bright (V= 9) and Sun-like (G9V) star, which TESS found to host three transiting planets (b, c, and d). The planets have periods of 9, 29, and 38 days each with similar sizes from 3 to 3.4R_⊕. To further characterize the system, we measured the radial velocity (RV) of HD 191939 over 415 days with Keck/HIRES and APF/Levy. We find thatM_b= 10.4 ± 0.9M_⊕andM_c= 7.2 ± 1.4M_⊕, which are low compared to most known planets of comparable radii. The RVs yield only an upper limit onM_d(<5.8M_⊕at 2σ). The RVs further reveal a fourth planet (e) with a minimum mass of 0.34 ± 0.01M_Jupand an orbital period of 101.4 ± 0.4 days. Despite its nontransiting geometry, secular interactions between planet e and the inner transiting planets indicate that planet e is coplanar with the transiting planets (Δi< 10°). We identify a second high-mass planet (f) with 95% confidence intervals on mass between 2 and 11M_Jupand period between 1700 and 7200 days, based on a joint analysis of RVs and astrometry from Gaia and Hipparcos. As a bright star hosting multiple planets with well-measured masses, HD 191939 presents many options for comparative planetary astronomy, including characterization with JWST.