Architecture Of Planetary Systems Research Articles

Multiwavelength modelling of the β Leo debris disc: one, two or three planetesimal populations?★

In this paper we present a model of the β Leo debris disc, with an emphasis on modelling the resolved Photodetector Array Camera and Spectrometer (PACS) images obtained as a part of the Herschel key programme DEBRIS. We also present new Spectral and Photometric Imaging REceiver (SPIRE) images of the disc at 250 μm, as well as new constraints on the disc from SCUBA-2, mid-infrared (mid-IR) and scattered light imaging. Combining all the available observational constraints, we find three possible models for the β Leo (HD 102647) debris disc: (i) a two-component model, comprised of a hot component at 2 au and a cold component from 15 to 70 au; (ii) a three-component model with hot dust at 2 au, warm dust at 9 au and a cold component from 30 to 70 au, is equally valid since the cold emission is not resolved within 30 au; (iii) a somewhat less likely possibility is that the system consists of a single very eccentric planetesimal population, with pericentres at 2 au and apocentres at 65 au. Thus, despite the wealth of observational constraints significant ambiguities remain; deep mid-IR and scattered light imaging of the dust distribution within 30 au seems to be the most promising method to resolve the degeneracy. We discuss the implications for the possible planetary system architecture, e.g. the two-component model suggests that planets may exist at 2–15 au, while the three-component model suggests planets between 2 and 30 au with a stable region containing the dust belt at 9 au, and there should be no planets between 2 and 65 au in the eccentric planetesimal model. We suggest that the hot dust may originate in the disintegration of comets scattered in the cold disc, and examine all A stars known to harbour both hot and cold dust to consider the possibility that the ratio of hot and cold dust luminosities is indicative of the intervening planetary system architecture.

The diversity of planetary system architectures: contrasting theory with observations

We develop a semi-analytical model for computing planetary system formation with the aim of explaining the observed diversity of planetary systems architectures and relate this primordial diversity with the initial properties of the disc where they were born. We adopt different initial conditions based on recent results in protoplanetary discs observations, to generate a variety of planetary systems and analyze them statistically. We explore the relevance of the mass and size of the disc, its metallicity, the mass of the central star and the time-scale of gaseous disc dissipation, in defining the architecture of the planetary system. We also test different values of some key parameters of our model, to find out which factors best reproduce the diverse sample of observed planetary systems. According to this, we predict which systems are the most common in the solar neighbourhood. Our results show that planetary systems with only terrestrial planets are the most common, being the only planetary systems formed when considering low metallicity discs and which also represent the best environment for the developing of rocky, potentially habitable planets. We also found that planetary systems like our own are not rare in the solar neighbourhood, being its formation favoured in massive discs where there is not a large accumulation of solids in the inner region of the disc. Regarding the planetary systems that harbor hot and warm Jupiter planets, we found that this systems are born in very massive, metal-rich discs. Also a fast migration rate is required in order to form these systems. According to our results, most of the hot and warm Jupiter systems are composed by only one giant planet, which is also a tendency of the current observational data.

Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems

Context. Low-mass extrasolar planets are presently being discovered at an increased pace by radial velocity and transit surveys, opening a new window on planetary systems. Aims. We are conducting a high-precision radial velocity survey with the HARPS spectrograph which aims at characterizing the population of ice giants and super-Earths around nearby solar-type stars. This will lead to a better understanding of their formation and evolution, and yield a global picture of planetary systems from gas giants down to telluric planets. Methods. Progress has been possible in this field thanks in particular to the sub-m s−1 radial velocity precision achieved by HARPS. We present here new high-quality measurements from this instrument. Results. We report the discovery of a planetary system comprising at least five Neptune-like planets with minimum masses ranging from 12 to 25 M⊕, orbiting the solar-type star HD 10180 at separations between 0.06 and 1.4 AU. A sixth radial velocity signal is present at a longer period, probably due to a 65-M⊕ object. Moreover, another body with a minimum mass as low as 1.4 M⊕ may be present at 0.02 AU from the star. This is the most populated exoplanetary system known to date. The planets are in a dense but still well-separated configuration, with significant secular interactions. Some of the orbital period ratios are fairly close to integer or half-integer values, but the system does not exhibit any mean-motion resonances. General relativity effects and tidal dissipation play an important role to stabilize the innermost planet and the system as a whole. Numerical integrations show long-term dynamical stability provided true masses are within a factor ∼3 from minimum masses. We further note that several low-mass planetary systems exhibit a rather ”packed” orbital architecture with little or no space left for additional planets. In several cases, semi-major axes are fairly regularly spaced on a logarithmic scale, giving rise to approximate Titius-Bode-like (i.e. exponential) laws. These dynamical architectures can be interpreted as the signature of formation scenarios where type I migration and interactions between protoplanets play a major role. However, it remains challenging to explain the presence of so many Neptunes and super-Earths on non-resonant, well-ordered orbits within ∼1-2 AU of the central star. Finally, we also confirm the marked dependence of planet formation on both metallicity and stellar mass. Very massive systems are all found around metal-rich stars more massive than the Sun, while low-mass systems are only found around metal-deficient stars less massive than the Sun.

Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems

Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems

The diverse origin of exoplanets' eccentricities & inclinations

AbstractRadial velocity surveys have discovered over 400 exoplanets. While measuring eccentricities of low-mass planets remains a challenge, giant exoplanets display a broad range of orbital eccentricities. Recently, spectroscopic measurements during transit have demonstrated that the short-period giant planets (“hot-Jupiters”) also display a broad range of orbital inclinations (relative to the rotation axis of the host star). Both properties pose a challenge for simple disk migration models and suggest that late-stage orbital evolution can play an important role in determining the final architecture of planetary systems. One possible formation mechanism for the inclined hot-Jupiters is some form of eccentricity excitation (e.g., planet scattering, secular perturbations due to a distant planet or wide binary) followed tidal circularization. The planet scattering hypothesis also makes predictions for the population of planets at large separations. Recent discoveries of planets on wide orbits via direct imaging and highly anticipated results from upcoming direct imaging campaigns are poised to provide a new type of constraint on planet formation. This proceedings describes recent progress in understanding the formation of giant exoplanets.

Direct imaging of massive extrasolar planets

AbstractThe direct detection of an extrasolar planet can provide accurate measurements of its orbit, mass and composition, greatly improving our understanding of how planets form and evolve. Recent advances in ground-based and space-based imaging techniques have now produced the first direct images of extrasolar planets. Typically these are many-Jupiter-mass planets on wide orbits. Direct imaging therefore probes the outer architecture of planetary systems and it is highly complementary to other techniques sensitive to inner architectures. This brief review summarizes the properties of the currently imaged exoplanets, provides an update on the orbit of Fomalhaut b, and highlights the emerging phenomenon of circumplanetary disks.

Planetary systems formation and the diversity of extrasolar systems

AbstractWith the end of answer questions as, how common are planetary systems like our own in the Universe? and What is the diversity of planetary systems that we could find in the universe?, we develop a semi-analytical model for computing planetary systems formation and consider different initial conditions for generating a large sample of planetary systems, which is analysed statistically. We explore the effects in the planetary system architecture of assuming different initial disc profiles and planetary migration rates.

STABILITY ANALYSIS OF SINGLE-PLANET SYSTEMS AND THEIR HABITABLE ZONES

We study the dynamical stability of planetary systems consisting of one hypothetical terrestrial mass planet ($1 $ or $10 \mearth$) and one massive planet ($10 \mearth - 10 \mjup$). We consider masses and orbits that cover the range of observed planetary system architectures (including non-zero initial eccentricities), determine the stability limit through N-body simulations, and compare it to the analytic Hill stability boundary. We show that for given masses and orbits of a two planet system, a single parameter, which can be calculated analytically, describes the Lagrange stability boundary (no ejections or exchanges) but which diverges significantly from the Hill stability boundary. However, we do find that the actual boundary is fractal, and therefore we also identify a second parameter which demarcates the transition from stable to unstable evolution. We show the portions of the habitable zones of $\rho$ CrB, HD 164922, GJ 674, and HD 7924 which can support a terrestrial planet. These analyses clarify the stability boundaries in exoplanetary systems and demonstrate that, for most exoplanetary systems, numerical simulations of the stability of potentially habitable planets are only necessary over a narrow region of parameter space. Finally we also identify and provide a catalog of known systems which can host terrestrial planets in their habitable zones.

Formation and tidal evolution of hot super-Earths in multiple planetary systems

Hot super-Earths are exoplanets with masses < 10 Earth masses and orbital periods < 20 days. Around 8 hot super-Earths have been discovered in the neighborhood of solar system. In this lecture, we review the mechanisms for the formation of hot super-Earths, dynamical effects that play important roles in sculpting the architecture of the multiple planetary systems. Two example systems (HD 40307 and GJ 436) are presented to show the formation and evolution of hot super-Earths or Neptunes.

Dynamical architectures of planetary systems induced by orbital migration

The aim of this talk is to present the most recent advances in establishing plausible planetary system architectures determined by the gravitational tidal interactions between the planets and the disc in which they are embedded during the early epoch of planetary system formation. We concentrate on a very well defined and intensively studied process of the disc-planet interaction leading to the planet migration. We focus on the dynamics of the systems in which low-mass planets are present. Particular attention is devoted to investigation of the role of resonant configurations. Our studies, apart from being complementary to the fast progress occurring just now in observing the whole variety of planetary systems and uncovering their structure and origin, can also constitute a valuable contribution in support of the missions planned to enhance the number of detected multiple systems.

PLANET-PLANET SCATTERING IN PLANETESIMAL DISKS

We study the final architecture of planetary systems that evolve under the combined effects of planet-planet and planetesimal scattering. Using N-body simulations we investigate the dynamics of marginally unstable systems of gas and ice giants both in isolation and when the planets form interior to a planetesimal belt. The unstable isolated systems evolve under planet-planet scattering to yield an eccentricity distribution that matches that observed for extrasolar planets. When planetesimals are included the outcome depends upon the total mass of the planets. For M tot 1 MJ the final eccentricity distribution remains broad, whereas for M tot 1 MJ a combination of divergent orbital evolution and recircularization of scattered planets results in a preponderance of nearly circular final orbits. We also study the fate of marginally stable multiple planet systems in the presence of planetesimal disks, and find that for high planet masses the majority of such systems evolve into resonance. A significant fraction leads to resonant chains that are planetary analogs of Jupiter's Galilean satellites. We predict that a transition from eccentric to near-circular orbits will be observed once extrasolar planet surveys detect sub-Jovian mass planets at orbital radii of a 5-10 AU.

The dynamical architecture and habitable zones of the quintuplet planetary system 55 Cancri

We perform numerical simulations to study the secular orbital evolution and dynamical structure of the quintuplet planetary system 55 Cancri with the self-consistent orbital solutions by Fischer and coworkers. In the simulations, we show that this system can be stable for at least 108 yr. In addition, we extensively investigate the planetary configuration of four outer companions with one terrestrial planet in the wide region of 0.790 AU ≤ a ≤ 5.900 AU to examine the existence of potential asteroid structure and Habitable Zones (HZs). We show that there are unstable regions for orbits about 4:1, 3:1 and 5:2 mean motion resonances (MMRs) of the outermost planet in the system, and several stable orbits can remain at 3:2 and 1:1 MMRs, which resembles the asteroid belt in the solar system. From a dynamical viewpoint, proper HZ candidates for the existence of more potential terrestrial planets reside in the wide area between 1.0 AU and 2.3 AU with relatively low eccentricities.

Long-wavelength observations of debris discs around sun-like stars

<i>Context. <i/>Tracing the evolution of debris discs is essential for our understanding of the architecture of planetary system. Even though the evolution of their inner discs has been recently studied with the Spitzer Space Telescope at mid- to far-infrared wavelengths, the outer discs are best characterised by sensitive millimetre observations. <i>Aims. <i/>The goal of our study is to understand the evolution timescale of circumstellar debris discs, and the physical mechanisms responsible for such evolution around solar-type stars. In addition, we make a detailed characterisation of the detected debris discs.<i>Methods. <i/>Two deep surveys of circumstellar discs around solar-type stars at different ages were carried out at 350 <i>μ<i/>m with the CSO and at 1.2 mm with the IRAM 30-m telescope. The dust disc masses were computed from the millimetre emission, where the discs are optically thin. Theoretically, the mass of the disc is expected to decrease with time. To test this hypothesis, we performed the generalised Kendall's tau correlation and three different two-sample tests. A characterisation of the detected debris discs has been obtained by computing the collision and Poynting-Robertson timescales and by modelling the spectral energy distribution.<i>Results. <i/> The Kendall's tau correlation yields a probability of 76% that the mass of debris discs and their age are correlated. Similarly, the three two-sample tests give a probability between 70 and 83% that younger and older debris systems belong to different parent populations in terms of dust mass. We detected submillimetre/millimetre emission from six debris discs, enabling a detailed SED modelling.<i>Conclusions. <i/>Our results on the correlation and evolution of dust mass as a function of age are conditioned by the sensitivity limit of our survey. Deeper millimetre observations are needed to confirm the evolution of debris material around solar-like stars. In the case of the detected discs, the comparison between collision and Poynting-Robertson timescales supports the hypothesis that these discs are dominated by collisions. All detected debris disc systems show the inner part evacuated from small micron-sized grains.

Terrestrial planet formation in low-eccentricity warm-Jupiter systems

<i>Context. <i/>Extrasolar giant planets are found to orbit their host stars with a broad range of semi-major axes 0.02 <i>≤<i/> <i>a<i/> <i>≤<i/> 6 AU. Current theories suggest that giant planets orbiting at distances between AU probably formed at larger distances and migrated to their current locations via type II migration, disturbing any inner system of forming terrestrial planets along the way. Migration probably halts because of fortuitously-timed gas disk dispersal.<i>Aims. <i/>The aim of this paper is to examine the effect of giant planet migration on the formation of inner terrestrial planet systems. We consider situations in which the giant planet halts migration at semi-major axes in the range 0.13-1.7 AU due to gas disk dispersal, and examine the effect of including or neglecting type I migration forces on the forming terrestrial system.<i>Methods. <i/>We employ an <i>N<i/>-body code that is linked to a viscous gas disk algorithm capable of simulating gas loss via accretion onto the central star and photoevaporation, gap formation by the giant planet, type II migration of the giant, optional type I migration of protoplanets, and gas drag on planetesimals.<i>Results. <i/>Most of the inner system planetary building blocks survive the passage of the giant planet, either by being shepherded inward or scattered into exterior orbits. Systems of one or more hot-Earths are predicted to form and remain interior to the giant planet, especially if type II migration has been limited, or where type I migration has affected protoplanetary dynamics. Habitable planets in low-eccentricity warm-Jupiter systems appear possible if the giant planet makes a limited incursion into the outer regions of the habitable zone (HZ), or traverses its entire width and ceases migrating at a radial distance of less than half that of the HZ's inner edge.<i>Conclusions. <i/>Type II migration does not prevent terrestrial planet formation. A wide variety of planetary system architectures exists that can potentially host habitable planets.

Observable consequences of planet formation models in systems with close-in terrestrial planets

To date, two planetary systems have been discovered with close-in, terrestrial-mass planets (� 5‐10 M ⊕ ). Many more such discoveries are anticipated in the coming years with radial velocity and transit searches. Here we investigate the different mechanisms that could form ‘hot Earths’ and their observable predictions. Models include: (1) in situ accretion; (2) formation at larger orbital distance followed by inward ‘type 1’ migration; (3) formation from material being ‘shepherded’ inward by a migrating gas giant planet; (4) formation from material being shepherded by moving secular resonances during dispersal of the protoplanetary disc; (5) tidal circularization of eccentric terrestrial planets with close-in perihelion distances and (6) photoevaporative mass-loss of a close-in giant planet. Models 1‐4 have been validated in previous work. We show that tidal circularization can form hot Earths, but only for relatively massive planets (� 5M ⊕) with very close-in perihelion distances (� 0.025 au), and even then the net inward movement in orbital distance is at most only 0.1‐0.15 au. For planets of less than ∼70 M ⊕ , photoevaporation can remove the planet’s envelope and leave behind the solid core on a Gyr time-scale, but only for planets inside 0.025‐0.05 au. Using two quantities that are observable by current and upcoming missions, we show that these models each produce unique signatures, and can be observationally distinguished. These observables are the planetary system architecture (detectable with radial velocities, transits and transit timing) and the bulk composition of transiting close-in terrestrial planets (measured by transits via the planet’s radius).

Modelling the Diversity of Extrasolar Terrestrial Planets

As we prepare to undertake the observational search for extrasolar terrestrial planets, theoretical modeling studies can help to prepare us for the likely diversity of the extrasolar terrestrial planets. This diversity may arise as a function of planetary system architecture and formation history, which results in a variety of initial planetary properties, as well as stellar, planetary and biological evolutionary processes. Modeling of the physical and chemical processes of the planetary environment, and their interaction with the parent star, allows us to understand the nature of the planetary characteristics that indicate habitability and life, and how these manifest in the planetary spectra. Here we present disk-averaged spectra of planets in our own solar system, and models of the Earth through several eons to understand the types of planetary characteristics that are likely to be observed by planned planet detection and characterization missions.

Imaging protoplanetary disks with a square kilometer array

The recent detections of extrasolar giant planets has revealed a surprising diversity of planetary system architectures, with many very unlike our Solar System. Understanding the origin of this diversity requires multi-wavelength studies of the structure and evolution of the protoplanetary disks that surround young stars. Radio astronomy and the square kilometer array (SKA) will play a unique role in these studies by imaging thermal dust emission in a representative sample of protoplanetary disks at unprecedented sub-AU scales in the innermost regions, including the “habitable zone” that lies within a few AU of the central stars. Radio observations will probe the evolution of dust grains up to centimeter-sized “pebbles”, the critical first step in assembling giant planet cores and terrestrial planets, through the wavelength dependence of dust emissivity, which provides a diagnostic of particle size. High resolution images of dust emission will show directly mass concentrations and features in disk surface density related to planet building, in particular the radial gaps opened by tidal interactions between planets and disks, and spiral waves driven by embedded protoplanets. Moreover, because orbital timescales are short in the inner disk, synoptic studies over months and years will show proper motions and allow for the tracking of secular changes in disk structure. SKA imaging of protoplanetary disks will reach into the realm of rocky planets for the first time, and they will help clarify the effects of the formation of giant planets on their terrestrial counterparts.

Particle Pileups and Planetesimal Formation

Solid particles in protoplanetary disks that are sufficiently super-solar in metallicity overcome turbulence generated by vertical shear to gravitationally condense into planetesimals. Super-solar metallicities result if solid particles pile up as they migrate starward as a result of aerodynamic drag. Previous analyses of aerodynamic drift rates that account for mean flow differences between gas and particles yield particle pile-ups. We improve on these studies not only by accounting for the collective inertia of solids relative to that of gas, but also by including the transport of angular momentum by turbulent stresses within the particle layer. These turbulent stresses are derived in a physically self-consistent manner from the structure of marginally Kelvin-Helmholtz turbulent flows. They are not calculated using the usual plate drag formulae, whose use we explain is inappropriate. Accounting for the relative inertia of solids to gas retards, but does not prevent, particle pile-ups, and generates more spatially extended regions of metal enrichment. Turbulent transport hastens pile-ups. We conclude that particle pile-up is a robust outcome in sufficiently passive protoplanetary disks. Connections to observations of circumstellar disks, including the Kuiper Belt, and the architectures of planetary systems are made.

Diversity of planetary systems from evolution of solids in protoplanetary disks

We have developed and applied a model designed to track simultaneously the evolution of gas and solids in protoplanetary disks from an early stage, when all solids are in the dust form, to the stage when most solids are in the form of a planetesimal swarm. The model is computationally ecient and allows for a global, comprehensive approach to the evolution of solid particles due to gas-solid coupling, coagulation, sedimentation, and evaporation/condensation. The co-evolution of gas and solids is calculated for 10 7 yr for several evolution regimes and starting from a comprehensive domain of initial conditions. The output of a single evolutionary run is a spatial distribution of mass locked in a planetesimal swarm. Because swarm's mass distribution is related to the architecture of a nascent planetary system, diversity of swarms is taken as a proxy for a diversity of planetary systems. We have found that disks with low values of specic angular momentum are bled out of solids and do not form planetary systems. Disks with high and intermediate values of specic angular momentum form diverse planetary systems. Solar-like planetary systems form from disks with initial masses0.02 M and angular momenta3 10 52 gc m 2 s 1 . Planets more massive than Jupiter can form at locations as close as 1 AU from