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Register a new userThe Occurrence and Architecture of Exoplanetary Systems
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Joshua N. Winn
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The basic geometry of the Solar System -- the shapes, spacings, and orientations of the planetary orbits -- has long been a subject of fascination as well as inspiration for planet formation theories. For exoplanetary systems, those same properties have only recently come into focus. Here we review our current knowledge of the occurrence of planets around other stars, their orbital distances and eccentricities, the orbital spacings and mutual inclinations in multiplanet systems, the orientation of the host stars rotation axis, and the properties of planets in binary-star systems.
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Earth-like planets have anelastic mantles, whereas giant planets may have anelastic cores. As for the fluid parts of a body, the tidal dissipation of such solid regions, gravitationally perturbed by a companion body, highly depends on its internal friction, and thus on its internal structure. Therefore, modelling this kind of interaction presents a high interest to provide constraints on planet interiors, whose properties are still quite uncertain. Here, we examine the equilibrium tide in the solid central region of a planet, taking into account the presence of a fluid envelope. We first present the equations governing the problem, and show how to obtain the different Love numbers that describe its deformation. We discuss how the quality factor Q depends on the rheological parameters, and the size of the core. Taking plausible values for the anelastic parameters, and examinig the frequency-dependence of the solid dissipation, we show how this mechanism may compete with the dissipation in fluid layers, when applied to Jupiter- and Saturn-like planets. We also discuss the case of the icy giants Uranus and Neptune.
We present archival Giant Metrewave Radio Telescope (GMRT) observations of two exoplanetary systems, $tau$ Bootis, and 55 Cancri, at 610 MHz and 150 MHz, respectively. Theoretical models predict these systems to have some of the highest expected flux densities at radio wavelengths. Both $tau$ Bootis and 55 Cancri have been previously observed at low frequency ($sim$ 30 MHz) with Low-Frequency Array (LOFAR) (Turner et al. 2020). $tau$ Bootis shows tentative signatures of circularly polarized emission at 30 MHz, while no emission was detected from 55 Cancri. We do not detect radio emission from both the systems, but the GMRT observations set $3sigma$ upper limits of 0.6 mJy at 610 MHz for $tau$ Bootis and 4.6 mJy at 150 MHz for 55 Cancri. The sensitivity achieved at 610 MHz in these observations is comparable to some of the deepest images of an exoplanet field.
The distribution of angular momentum of planets and their host stars provides important information on the formation and evolution of the planetary system. However, mysteries still remain, partly due to bias and uncertainty of the current observational datasets and partly due to the fact that theoretical models for the formation and evolution of planetary systems are still underdeveloped. In this study, we calculate the spin angular momenta of host stars and the orbital angular momenta of their planets using data from the NASA Exoplanet Archive, together with detailed analysis of observation dependent biases and uncertainty ranges. We also analyze the angular momenta of the planetary system as a function of star age to understand their variation in different evolutionary stages. In addition, we use a population of planets from theoretical model simulations to reexamine the observed patterns and compare the simulated population with the observed samples to assess variations and differences. We found the majority of exoplanets discovered thus far do not have the angular momentum distribution similar to the planets in our Solar System, though this could be due to the observation bias. When filtered by the observational biases, the model simulated angular momentum distributions are comparable to the observed pattern in general. However, the differences between the observation and model simulation in the parameter (angular momentum) space provide more rigorous constraints and insights on the issues that needed future improvement.
Revealing the mechanisms shaping the architecture of planetary systems is crucial for our understanding of their formation and evolution. In this context, it has been recently proposed that stellar clustering might be the key in shaping the orbital architecture of exoplanets. The main goal of this work is to explore the factors that shape the orbits of planets. We used a homogeneous sample of relatively young FGK dwarf stars with RV detected planets and tested the hypothesis that their association to phase space (position-velocity) over-densities (cluster stars) and under-densities (field stars) impacts the orbital periods of planets. When controlling for the host star properties, on a sample of 52 planets orbiting around cluster stars and 15 planets orbiting around field star, we found no significant difference in the period distribution of planets orbiting these two populations of stars. By considering an extended sample of 73 planets orbiting around cluster stars and 25 planets orbiting field stars, a significant different in the planetary period distributions emerged. However, the hosts associated to stellar under-densities appeared to be significantly older than their cluster counterparts. This did not allow us to conclude whether the planetary architecture is related to age, environment, or both. We further studied a sample of planets orbiting cluster stars to study the mechanism responsible for the shaping of orbits of planets in similar environments. We could not identify a parameter that can unambiguously be responsible for the orbital architecture of massive planets, perhaps, indicating the complexity of the issue. Conclusions. Increased number of planets in clusters and in over-density environments will help to build large and unbiased samples which will then allow to better understand the dominant processes shaping the orbits of planets.
Planets and their host stars carry a long-term memory of their origin in their chemical compositions. Thus, identifying planets formed in different environments improves our understating of planetary formation. Although restricted to detecting exoplanets within the solar vicinity, we might be able to detect planetary systems that formed in small external galaxies and later merged with the Milky Way. In fact, Gaia data have unequivocally shown that the Galaxy underwent several significant minor mergers during its first billion years of formation. The stellar debris of one of these mergers, Gaia-Enceladus (GE), is thought to have built up most of the stellar halo in the solar neighborhood. In this Letter, we investigate the origin of known planet-host stars combining data from the NASA Exoplanet Archive with Gaia EDR3 and large-scale spectroscopic surveys. We adopt a kinematic criterion and identify 42 stars associated with the Milky Ways thick disk and one halo star. The only halo star identified, BD+20 2457, known to harbor two exoplanets, moves on a retrograde and highly eccentric orbit. Its chemical abundance pattern situates the star at the border between the thick disk, the old halo, and accreted populations. Given its orbital parameters and chemical properties, we suggest that BD+20 2457 is likely formed in the protodisk of the Galaxy, but we do not exclude the possibility of the star belonging to the debris of GE. Finally, we estimate a minimum age and mass limit for the star, which has implications for its planetary system and will be tested with future Transiting Exoplanet Survey Satellite observations.
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