Search

PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.

In 2022 the DART mission spacecraft impacted the asteroid Dimorphos, the secondary body of the binary Didymos system, ejecting a large number of dust particles, rocks and boulders. The ESA Hera mission will reach the system in 2026 for post--impact studies and possible detection of orbiting fragments. We investigate the long term dynamics of the large boulders ejected by DART to test if any of these objects survive in orbit until the arrival of the Hera mission. To model the dynamics of the boulders we use a numerical model which includes the gravity of non-spherical Didymos and Dimorphos, the solar gravity and the radiation pressure. The SPICE kernels are used to define the correct reference frame for the integration. The dynamics of the boulders is highly chaotic and 1% of the initial boulders survive at least for 4 years on quasi--stable orbits. These orbits are characterised by wide oscillations in eccentricity in antiphase with those in inclination (including spin flips), a mechanism similar to the Kozai one. This behaviour may protect these bodies from close encounters with both asteroids. We also compute the distribution on the surfaces of the asteroids of sesquinary impacts which may influence the dust emission, after the initial DART impact, and the surface composition of the asteroids. The probability of observing boulders by the mission Hera is small but not negligible and an almost constant flux of escaping boulders is expected in the coming years since their lifetime after the DART impact covers a large time interval. Most of re--impacts on Dimorphos occur in the hemisphere opposite to the impact site, preferentially close to the equatorial plane., Comment: Accepted in A&A

In evolved and dusty circumstellar discs, two planets with masses comparable to Jupiter and Saturn that migrate outwards while maintaining an orbital resonance can produce distinctive features in the dust distribution. Dust accumulates at the outer edge of the common gas gap, which behaves as a dust trap, where the local dust concentration is significantly enhanced by the planets outward motion. Concurrently, an expanding cavity forms in the dust distribution inside the planets orbits, because dust does not filter through the common gaseous gap and grain depletion in the region continues via inward drifting. There is no cavity in the gas distribution because gas can filter through the gap, although ongoing gas accretion on the planets can reduce the gas density in the inner disc. Such behaviour was demonstrated by means of simulations neglecting the effects of dust diffusion due to turbulence and of dust backreaction on the gas. Both effects may alter the formation of the dust peak at the gap outer edge and of the inner dust cavity, by letting grains filter through the dust trap. We performed high resolution hydrodynamical simulations of the coupled evolution of gas and dust species, the latter treated as pressureless fluids, in the presence of two giant planets. We show that diffusion and backreaction can change some morphological aspects of the dust distribution but do not alter some main features, such as the outer peak and the expanding inner cavity. These findings are confirmed for different parametrizations of gas viscosity., Comment: Accepted for publication on MNRAS

NASA's Double Asteroid Redirection Test (DART) is the first full-scale test of an asteroid deflection technology. Results from the hypervelocity kinetic impact and Earth-based observations, coupled with LICIACube and the later Hera mission, will result in measurement of the momentum transfer efficiency accurate to ~10% and characterization of the Didymos binary system. But DART is a single experiment; how could these results be used in a future planetary defense necessity involving a different asteroid? We examine what aspects of Dimorphos's response to kinetic impact will be constrained by DART results; how these constraints will help refine knowledge of the physical properties of asteroidal materials and predictive power of impact simulations; what information about a potential Earth impactor could be acquired before a deflection effort; and how design of a deflection mission should be informed by this understanding. We generalize the momentum enhancement factor $\beta$, showing that a particular direction-specific $\beta$ will be directly determined by the DART results, and that a related direction-specific $\beta$ is a figure of merit for a kinetic impact mission. The DART $\beta$ determination constrains the ejecta momentum vector, which, with hydrodynamic simulations, constrains the physical properties of Dimorphos's near-surface. In a hypothetical planetary defense exigency, extrapolating these constraints to a newly discovered asteroid will require Earth-based observations and benefit from in-situ reconnaissance. We show representative predictions for momentum transfer based on different levels of reconnaissance and discuss strategic targeting to optimize the deflection and reduce the risk of a counterproductive deflection in the wrong direction., Comment: 30 pages, 7 figures. Planetary Science Journal, in press, accepted 2022 September 22

Observational data on the dust content of circumstellar disks show that the median dust content in disks around pre-main sequence stars in nearby star forming regions seem to increase from about 1 Myr to about 2 Myr, and then decline with time. This behaviour challenges the models where the small dust grains steadily decline by accumulating into larger bodies and drifting inwards on a short timescale (less than about 1 Myr). In this Letter we explore the possibility to reconcile this discrepancy in the framework of a model where the early formation of planets dynamically stirs the nearby planetesimals and causes high energy impacts between them, resulting in the production of second-generation dust. We show that the observed dust evolution can be naturally explained by this process within a suite of representative disk-planet architectures., Comment: 9 pages, 2 figures, 1 table. Accepted for publication on ApJL

Planet Planet scattering is a leading dynamical mechanism invoked to explain the present orbital distribution of exoplanets. Many stars belong to binary systems, therefore it is important to understand how this mechanism works in presence of a companion star. We focus on systems of three planets orbiting the primary star and estimate the timescale for instability finding that it scales with the keplerian period for systems that have the same ratio between inner planet and binary semimajor axes. An empirical formula is also derived from simulations to estimate how the the binary eccentricity affects the extent of the stability region. The presence of the secondary star affects the Planet Planet scattering outcomes causing a broadening of the final distribution in semimajor axis of the inner planet as some of the orbital energy of the planets is absorbed by the companion star. Repeated approaches to the secondary star causes also a significant reduction in the frequency of surviving two planet systems in particular for larger values of the inner planet semimajor axis. The formation of Kozai states with the companion star increases the number of planets which may be tidally circularized. To predict the possible final distribution of planets in binaries we have performed a large number of simulations where the initial semimajor axis of the inner planets is chosen randomly. For small values of the binary semimajor axis, the higher frequency of collision alter the final planet orbital distributions which, however, beyond 50 au appear to be scalable to wider binary separations., Comment: Accepted for publication in MNRAS

Observations indicate that large, dust-laden protoplanetary discs are common. Some features, like gaps, rings and spirals, suggest they may host young planets, which can excite the orbits of nearby leftover planetesimals. Energetic collisions among these bodies can lead to the production of second-generation dust. Grains produced by collisions may have a dynamical behaviour different from that of first-generation, primordial dust out of which planetesimals and planets formed. We aim to study these differences for the HD 163296 system and determine whether dynamical signatures in the mixture of the two dust populations can help separate their contributions. We use three-dimensional (3-D) hydrodynamic models to describe the gaseous disc with three, Saturn- to Jupiter-mass, embedded planets. Dust grains, of sizes 1um-1mm, are treated as Lagrangean particles with resolved thermodynamics and mass loss. Initial disc and planet configurations are derived from observation-based work, which indicates low gas viscosity. The 3-D approach also allows us to detect the formation of vortices induced by Rossby waves, where dust becomes concentrated and may contribute to planetesimal formation. We find that the main differences in the dynamical behaviour of first- and second-generation dust occur in the vertical distribution. The two populations have similar distributions around the disc mid-plane, although second-generation dust shows longer residence times close to the radial locations of the planets' gas gaps. Sedimentation rates of um-size grains are comparable to or lower than the production rates by planetesimals' collisions, making this population potentially observable. These outcomes can be extended to similar systems harbouring giant planets., Comment: 13 pages, 1 table, 13 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

Close binaries ($a_{bin} < 20$ au) are known to harbor planets, yet planet formation is unlikely to succeed in such systems. Studying the dynamics of disks in close binaries can help to understand how those planets could have formed. We study the impact that numerical and physical parameters have on the dynamics of disks in close binaries. We use the $\gamma$-Cephei system as an example and focus on disk quantities such as disk eccentricity and the precession rate as indicators for the dynamical state of the disks. We simulate disks in close binaries by performing two-dimensional radiative hydrodynamical simulations using a modified version of the Fargo code. First, we perform a parameter study for different numerical parameters to confirm that our results are robust. In the second part, we study the effects of different masses and different viscosities on the disks' dynamics. Previous studies on radiative disks in close binaries used too low resolutions and too small simulation domains, which impacted the disk's dynamics. We find that radiative disks in close binaries, after an initialization phase, become eccentric with mean eccentricities between 0.06 to 0.27 and display a slow retrograde precession with periods ranging from $4 - 40T_{bin}$ which depends quadratically on the disk's mean aspect ratio. In general, the disks show a coherent, rigid precession which can be broken, however, by changes in the opacity law reducing the overall eccentricity of the disk., Comment: Accepted in A&A; 14 Pages, 23 figures

Both young stars and multi-planet systems are primary objects that allow us to study, understand and constrain planetary formation and evolution theories. We validate the physical nature of two Neptune-type planets transiting TOI-942 (TYC 5909-319-1), a previously unacknowledged young star (50+30-20 Myr) observed by the TESS space mission in Sector 5. Thanks to a comprehensive stellar characterization, TESS light curve modelling and precise radial-velocity measurements, we validated the planetary nature of the TESS candidate and detect an additional transiting planet in the system on a larger orbit. From photometric and spectroscopic observations we performed an exhaustive stellar characterization and derived the main stellar parameters. TOI-942 is a relatively active K2.5V star (logR'hk = -4.17+-0.01) with rotation period Prot = 3.39+-0.01 days, a projected rotation velocity vsini=13.8+-0.5 km/s and a radius of ~0.9 Rsun. We found that the inner planet, TOI-942b, has an orbital period Pb=4.3263+-0.0011 days, a radius Rb=4.242-0.313+0.376 Rearth and a mass upper limit of 16 Mearth at 1-sigma confidence level. The outer planet, TOI-942c, has an orbital period Pc=10.1605-0.0053+0.0056 days, a radius Rc=4.793-0.351+0.410 Rearth and a mass upper limit of 37 Mearth at 1-sigma confidence level.

Any population of asteroids, like asteroid families, will disperse in semi-major axis due to the Yarkovsky effect. The amount of drift is modulated by the asteroid spin state evolution which determines the balance between the diurnal and seasonal Yarkovsky force. The asteroid's spin state is, in turn, controlled in part by the YORP effect. The otherwise smooth evolution of an asteroid can be abruptly altered by collisions, which can cause impulsive changes in the spin state and can move the asteroid onto a different YORP track. In addition, collisions may also alter the YORP parameters by changing the superficial features and overall shape of the asteroid. Thus, the coupling between YORP and Yarkovsky is also strongly affected by the impact history of each body. To investigate this coupling we developed a statistical code modeling the time evolution of semi--major axis under YORP-Yarkovsky coupling. It includes the contributions of NYORP (normal YORP), TYORP (tangential YORP) and collisions whose effects are deterministically calculated and not added in a statistical way. We find that both collisions and TYORP increase the dispersion of a family in semi-major axis by making the spin axis evolution less smooth and regular. We show that the evolution of a family's structure with time is complex and collisions randomize the YORP evolution. In our test families we do not observe the formation of a 'YORP-eye' in the semi-major axis vs. diameter distribution, even after a long period of time. If present, the 'YORP-eye' might be a relic of an initial ejection velocity pattern of the collisional fragments.

Super-Earths can form at large orbital radii and migrate inward due to tidal interactions with the circumstellar disk. In this scenario, convergent migration may occur and lead to the formation of resonant pairs of planets. We explore the conditions under which convergent migration and resonance capture take place, and what dynamical consequences can be expected on the dust distribution surrounding the resonant pair. We combine hydrodynamic planet--disk interaction models with dust evolution calculations to investigate the signatures produced in the dust distribution by a pair of planets in mean-motion resonances. We find that convergent migration takes place when the outer planet is the more massive. However, convergent migration also depends on the local properties of the disk, and divergent migration may result as well. For similar disk parameters, the capture in low degree resonances (e.g., 2:1 or 3:2) is preferred close to the star where the resonance strength can more easily overcome the tidal torques exerted by the gaseous disk. Farther away from the star, convergent migration may result in capture in high degree resonances. The dust distribution shows potentially observable features typically when the planets are trapped in a 2:1 resonance. In other cases, with higher degree resonances (e.g., 5:4 or 6:5) dust features may not be sufficiently pronounced to be easily observable. The degree of resonance established by a pair of super-Earths may be indicative of the location in the disk where capture occurred. There can be significant differences in the dust distribution around a single super-Earth and a pair of super-Earths in resonance., Comment: Accepted for publication on Astronomy and Astrophysics

The recent discovery of rings and massive satellites around minor bodies and dwarf planets suggests that they may often coexist, as for example around Haumea. A ring perturbed by an oblate central body and by an inclined satellite may disperse on a short timescale. The conditions under which a ring may survive are explored both analytically and numerically. The trajectories of ring particles are integrated under the influence of the gravitational field of a triaxial ellipsoid and (a) massive satellite(s), including the effects of collisions. A ring initially formed in the equatorial plane of the central body will be disrupted if the satellite has an inclination in the Kozai Lidov regime (39.2 < i < 144.8). For lower inclinations, the ring may relax to the satellite orbital plane thanks to an intense collisional damping. On the other hand, a significant J2 term easily suppresses the perturbations of an inclined satellite within a critical semimajor axis, even in the case of Kozai Lidov cycles. However, if the ring is initially inclined with respect to the equatorial plane, the same J2 perturbations are not a protective factor but instead disrupt the ring on a short timescale. The ring found around Haumea is stable despite the rise in the impact velocities that is due to the asymmetric shape of the the body and the presence of a 3:1 resonance with the rotation of the central body. A ring close to an oblate central body should be searched for in the proximity of the equatorial plane, where the J2 perturbations protect it against the perturbations of an external inclined satellites. In an inclined configuration, the J2 term is itself disruptive., Comment: Accepted for publication on Astronomy and Astrophysics

Binary systems are very common among field stars. While this relatively small number of planets in binaries is probably partly due to strong observational biases, there is, however, statistical evidence that planets are indeed less frequent in binaries with separations smaller than 100 au, strongly suggesting that the presence of a close in companion star has an adverse effect on planet formation. It is indeed possible for the gravitational pull of the second star to affect all the different stages of planet formation, from proto-planetary disk formation to dust accumulation into planetesimals, to the accretion of these planetesimals into large planetary embryos and, eventually, the final growth of these embryos into planets. For the crucial planetesimal accretion phase, the complex coupling between dynamical perturbations from the binary and friction due to gas in the protoplanetary disk suggests that planetesimal accretion might be hampered due to increased, accretion hostile impact velocities. Likewise, the interplay between the binary secular perturbations and mean motion resonances lead to unstable regions, where not only planet formation is inhibited, but where a massive body would be ejected from the system on a hyperbolic orbit. The amplitude of these two main effects is different for S and P type planets, so that a comparison between the two populations might outline the influence of the companion star on the planet formation process. Unfortunately, at present the two populations (circumstellar or circumbinary) are not known equally well and different biases and uncertainties prevent a quantitative comparison. We also highlight the long term dynamical evolution of both S and P type systems and focus on how these different evolutions influence the final architecture of planetary systems in binaries., Comment: Published on Galaxies, vol. 7, issue 4, p. 84

Pairs of planets in a system may end up close to their host star on eccentric orbits as a consequence of planet planet scattering, Kozai or secular migration. In this scenario, general relativity and secular perturbations have comparable timescales and may interfere with each other with relevant effects on the eccentricity and pericenter evolution of the two planets. We explore, both analytically and via numerical integration, how the secular evolution is changed by general relativity for a wide range of different initial conditions. We find that when the faster secular frequency approaches the general relativity precession rate, which tipically occurs when the outer planet moves away from the inner one, it relaxes to it and a significant damping of the proper eccentricity of the inner planet occurs. The proper eccentricity of the outer planet is reduced as well due to the changes in the secular interaction of the bodies. The lowering of the peak eccentricities of the two planets during their secular evolution has important implications on their stability. A significant number of two planet systems, otherwise chaotic because of the mutual secular perturbations, are found stable when general relativity is included., Comment: Accepted for publication on MNRAS

The aim of this paper is to underline conditions necessary for the emergence and development of life. They are placed at local planetary scale, at Galactic scale and within the cosmological evolution, as pointed out by the Anthropic Cosmological Principle. We will consider the circumstellar habitable zone (CHZ) for planetary systems and a Galactic Habitable Zone (GHZ) including also a set of strong cosmological constraints to allow life (cosmological habitability (COSH)). Some requirements are specific of a single scale and its related physical phenomena, while others are due to the conspired effects occurring at more than one scale. The scenario emerging from this analysis is that all the habitability conditions here detailed must at least be met. Then life asks something which may appear as "a monstrous sequence of accidents" as Hoyle (1959) thought, or as providential collaborations leading to realize how finely tuned is the architecture within which Life, as precious stone, is embedded.

We investigate via numerical modeling the effects of two planets locked in resonance, and migrating outward, on the dust distribution of the natal circumstellar disk. We aim to test whether the dust distribution exhibits peculiar features arising from the interplay among the gravitational perturbations of the planets in resonance, the evolution of the gas, and its influence on the dust grains' dynamics. We focus on the 3:2 and 2:1 resonance, where the trapping may be caused by the convergent migration of a Jupiter- and Saturn-mass planet, preceding the common gap formation and ensuing outward (or inward) migration. Models show that a common gap also forms in the dust component -- similarly to what a single, more massive planet would generate -- and that outward migration leads to a progressive widening of the dust gap and to a decoupling from the gas gap. As the system evolves, a significantly wider gap is observed in the dust distribution, which ceases to overlap with the gas gap in the inner disk regions. At the outer edge of the gas gap, outward migration of the planets produces an over-density of dust particles, which evolve differently in the 3:2 and 2:1 resonances. For the 3:2, the dust trap at the gap's outer edge is partly efficient and a significant fraction of the grains filters through the gap. For the 2:1 resonance, the trap is more efficient and very few grains cross the gap, while the vast majority accumulate at the outer edge of the gap., Comment: 16 pages, 12 figures. Accepted for publication

The phase function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images \citep{Bertini17}. This function show a deep minimum at phase angles near 100$^\circ$, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly-oriented aggregate particles. We show, however, that an ensamble of oriented elongated particles of a wide variety of aspect ratios, with radii $r \gtrsim$10 $\mu$m, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed phase function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60-70\% range., Comment: Accepted by Astronomical Journal, September 26th, 2018. 21 pages, 5 figures

Binary stars constitute a large percentage of the stellar population, yet relatively little is known about the planetary systems orbiting them. Most constraints on circumbinary planets (CBPs) so far come from transit observations with the Kepler telescope, which is sensitive to close-in exoplanets but does not constrain planets on wider orbits. However, with continuous developments in high-contrast imaging techniques, this population can now be addressed through direct imaging. We present the full survey results of the Search for Planets Orbiting Two Stars (SPOTS) survey, which is the first direct imaging survey targeting CBPs. The SPOTS observational program comprises 62 tight binaries that are young and nearby, and thus suitable for direct imaging studies, with VLT/NaCo and VLT/SPHERE. Results from SPOTS include the resolved circumbinary disk around AK Sco, the discovery of a low-mass stellar companion in a triple packed system, the relative astrometry of up to 9 resolved binaries, and possible indications of non-background planetary-mass candidates around HIP 77911. We did not find any CBP within 300 AU, which implies a frequency upper limit on CBPs (1--15 $M_{\rm Jup}$) of 6--10 % between 30-300 AU. Coupling these observations with an archival dataset for a total of 163 stellar pairs, we find a best-fit CBP frequency of 1.9 % (2--15 $M_{\rm Jup}$) between 1--300 AU with a 10.5 % upper limit at a 95 % confidence level. This result is consistent with the distribution of companions around single stars., Comment: 27 pages, 13 Figures, 7 Tables. Accepted for publication in A&A

From the results of a comprehensive asteroid population evolution model, we conclude that the YORP-induced rotational fission hypothesis can be consistent with the observed population statistics of small asteroids in the main belt including binaries and contact binaries. The foundation of this model is the asteroid rotation model of Marzari et al. (2011), which incorporates both the YORP effect and collisional evolution. This work adds to that model the rotational fission hypothesis and the binary evolution model of Jacobson & Scheeres (2011). The asteroid population evolution model is highly constrained by these and other previous works, and therefore it has only two significant free parameters: the ratio of low to high mass ratio binaries formed after rotational fission events and the mean strength of the binary YORP (BYORP) effect. We successfully reproduce characteristic statistics of the small asteroid population: the binary fraction, the fast binary fraction, steady-state mass ratio fraction and the contact binary fraction. We find that in order for the model to best match observations, rotational fission produces high mass ratio ($> 0.2$) binary components with four to eight times the frequency as low mass ratio ($< 0.2$) components, where the mass ratio is the mass of the secondary component divided by the mass of the primary component. This is consistent with post-rotational fission binary system mass ratio being drawn from either a flat or a positive and shallow distribution, since the high mass ratio bin is four times the size of the low mass ratio bin; this is in contrast to the observed steady-state binary mass ratio, which has a negative and steep distribution. This can be understood in the context of the BYORP-tidal equilibrium hypothesis, which predicts that low mass ratio binaries survive for a significantly longer period of time than high mass ratio systems.

The Search for Planets Orbiting Two Stars (SPOTS) survey aims to study the formation and distribution of planets in binary systems by detecting and characterizing circumbinary planets and their formation environments through direct imaging. With the SPHERE Extreme Adaptive Optics instrument, a good contrast can be achieved even at small (<300 mas) separations from bright stars, which enables studies of planets and disks in a separation range that was previously inaccessible. Here, we report the discovery of resolved scattered light emission from the circumbinary disk around the well-studied young double star AK Sco, at projected separations in the ~13--40 AU range. The sharp morphology of the imaged feature is surprising, given the smooth appearance of the disk in its spectral energy distribution. We show that the observed morphology can be represented either as a highly eccentric ring around AK Sco, or as two separate spiral arms in the disk, wound in opposite directions. The relative merits of these interpretations are discussed, as well as whether these features may have been caused by one or several circumbinary planets interacting with the disk., Comment: 10 pages, 3 figures, accepted for publication in ApJ Letters. Minor (proof-level) corrections implemented in this version

We explore the dynamical evolution of a planet embedded in a disk surrounding a star part of a binary system where the orbital plane of the binary is significantly tilted respect to the initial disk plane. Our aim is to test whether the planet remains within the disk and continues to migrate towards the star in a Type I/II mode in spite of the secular perturbations of the companion star. This would explain observed exoplanets with significant inclination respect to the equatorial plane of their host star. We have used two different SPH codes, vine and phantom, to model the evolution of a system star+disk+planet and companion star with time. After an initial coupled evolution, the inclination of the disk and that of the planet begin to differ significantly. The period of oscillation of the disk inclination, respect to the initial plane, is shorter than that of the planet which evolves independently after about 10^4 yr following a perturbed N-body behavior. However, the planet keeps migrating towards the star because during its orbital motion it crosses the disk plane and the friction with the gas causes angular momentum loss. Disk and planet in a significantly inclined binary system are not dynamically coupled for small binary separations but evolve almost independently. The planet abandons the disk and, due to the onset of a significant mutual inclination, it interacts with the gas only when its orbit intersects the disk plane. The drift of the planet towards the star is not due to type I/II with the planet embedded in the disk but to the friction with the gas during the disk crossing., Comment: 8 pages, 8 figures, accepted by Astronomy & Astrophysics

The Trojan asteroids provide a unique perspective on the history of Solar System. As a large population of small bodies, they record important gravitational interactions and dynamical evolution of the Solar System. In the past decade, significant advances have been made in understanding physical properties, and there has been a revolution in thinking about the origin of Trojans. The ice and organics generally presumed to be a significant part of Trojan compositions have yet to be detected directly, though low density of the binary system Patroclus (and possibly low density of the binary/moonlet system Hektor) is consistent with an interior ice component. By contrast, fine-grained silicates that appear to be similar to cometary silicates in composition have been detected, and a color bimodality may indicate distinct compositional groups among the Trojans. Whereas Trojans had traditionally been thought to have formed near 5 AU, a new paradigm has developed in which the Trojans formed in the proto-Kuiper Belt, and they were scattered inward and captured in the Trojan swarms as a result of resonant interactions of the giant planets. Whereas the orbital and population distributions of current Trojans are consistent with this origin scenario, there are significant differences between current physical properties of Trojans and those of Kuiper Belt objects. These differences may be indicative of surface modification due to the inward migration of objects that became the Trojans, but understanding of appropriate modification mechanisms is poor and would benefit from additional laboratory studies. Many open questions remain, and the future promises significant strides in our understanding of Trojans. The time is ripe for a spacecraft mission to the Trojans, to turn these objects into geologic worlds that can be studied in detail to unravel their complex history., Comment: Chapter for Asteroids IV book (UA Press), accepted for publication, 33 pages, 10 figures

Direct imaging surveys for exoplanets commonly exclude binary stars from their target lists, leaving a large part of the overall planet demography unexplored. To address this gap in our understanding of planet formation and evolution, we have launched the first direct imaging survey dedicated to circumbinary planets: SPOTS, the Search for Planets Orbiting Two Stars. In this paper, we discuss the theoretical context, scientific merit, and technical feasibility of such observations, describe the target sample and observational strategy of our survey, and report on the first results from our pilot survey of 26 targets with the VLT NaCo facility. While we have not found any confirmed substellar companions to date, a number of promising candidate companions remain to be tested for common proper motion in upcoming follow-up observations. We also report on the astrometry of the three resolved binaries in our target sample. This pilot survey constitutes a successful proof of concept for our survey strategy and paves the way for a second stage of exploratory observations with VLT SPHERE., Comment: 12 pages, 4 figures, 3 tables. Accepted for publication in A&A

Aims. With the purpose of determining the orbital parameters of exoplanetary systems from observational data, we have developed a software, named TRADES (TRAnsits and Dynamics of Exoplanetary Systems), to simultaneously fit observed radial velocities and transit times data. Methods. We implemented a dynamical simulator for N-body systems, which also fits the available data during the orbital integration and determines the best combination of the orbital parameters using grid search, $\chi^2$ minimization, genetic algorithms, particle swarm optimization, and bootstrap analysis. Results. To validate TRADES, we tested the code on a synthetic three-body system and on two real systems discovered by the Kepler mission: Kepler-9 and Kepler-11. These systems are good benchmarks to test multiple exoplanet systems showing transit time variations (TTVs) due to the gravitational interaction among planets. We have found that orbital parameters of Kepler-11 planets agree well with the values proposed in the discovery paper and with a a recent work from the same authors. We analyzed the first three quarters of Kepler-9 system and found parameters in partial agreement with discovery paper. Analyzing transit times (T0s) covering 12 quarters of Kepler data, that we have found a new best-fit solution. This solution outputs masses that are about 55% of the values proposed in the discovery paper; this leads to a reduced semi-amplitude of the radial velocities of about 12.80 m/s., Comment: 14 pages, 13 figures, 6 tables; accepted for publication in Astronomy & Astrophysics, and corrected by the Language Editor

Stellar flybys in star clusters are suspected to affect the orbital architecture of planetary systems causing eccentricity excitation and orbital misalignment between the planet orbit and the equatorial plane of the star. We explore whether the impulsive changes in the orbital elements of planets, caused by an hyperbolic stellar flyby, can be fully damped by the circumstellar disk surrounding the star. The time required to disperse stellar clusters is in fact comparable to circumstellar disk's lifetime. We have modelled in 3D a system made of a solar type star surrounded by a low density disk with a giant planet embedded in it approached on a hyperbolic encounter trajectory by a second star, of similar mass and with its own disk. We focus on extreme configurations where a very deep stellar flyby perturbs a Jovian planet on an external orbit. This allows to test in full the ability of the disk to erase the effects of the stellar encounter. We find that the amount of mass lost by the disk during the stellar flyby is less than in 2D models where a single disk was considered due to the mass exchange between the two disks at the encounter. The damping in eccentricity is slightly faster than in 2D models and it occurs on timescales of the order of a few kyr. The only trace of the flyby left in the planet system, after about 10^4 yr, is a small misalignment, lower than 9 degrees, between the star equatorial plane and the planet orbit. In a realistic model based on 3D simulations of star--planet--disk interactions, we find that stellar flybys cannot excite significant eccentricities and inclinations of planets in stellar clusters. The circumstellar disks hosting the planets damp on a short timescale all the step changes in the two orbital parameters produced during any stellar encounter. All records of past encounters are erased., Comment: 9 pages, 11 figures, accepted for publication in A&A

The size distribution of small asteroids in the Main Belt is assumed to be determined by an equilibrium between the creation of new bodies out of the impact debris of larger asteroids and the destruction of small asteroids by collisions with smaller projectiles. However, for a diameter less than 6 km we find that YORP-induced rotational disruption significantly contributes to the erosion even exceeding the effects of collisional fragmentation. Including this additional grinding mechanism in a collision evolution model for the asteroid belt, we generate size-frequency distributions from either an accretional (Weidenschilling, 2011) or an "Asteroids were born big" (Morbidelli, 2009) initial size-frequency distribution that are consistent with observations reported in Gladman et al. (2009). Rotational disruption is a new mechanism that must be included in all future collisional evolution models of asteroids., Comment: 5 pages, 3 figures, accepted in MNRAS letters

Circumstellar disks in binaries are perturbed by the companion gravity causing significant alterations of the disk morphology. Spiral waves due to the companion tidal force also develop in the vertical direction and affect the disk temperature profile. These effects may significantly influence the process of planet formation. We perform 3D numerical simulations of disks in binaries with different initial dynamical configurations and physical parameters. Our goal is to investigate their evolution and their propensity to grow planets. We use an improved version of the SPH code VINE modified to better account for momentum and energy conservation. The energy equation includes a flux--limited radiative transfer algorithm and the disk cooling is obtained via "boundary particles". We model a system made of star/disk + star/disk where the secondary star (and relative disk) is less massive than the primary. The numerical simulations performed for different values of binary separation and disk density show that the disk morphology is substantially affected by the companion perturbations. Trailing spiral shock waves develop when the stars approach their pericenter. Strong hydraulic jumps occur at the shock front creating breaking waves and a consistent mass stream between the two disks, significantly heating them. The high gas temperature may prevent the ice condensation by moving outward the "snow line". The hydraulic jumps may slow down or even halt the dust coagulation process. At apocenter these perturbations are reduced and the disks are cooled down and less eccentric. The strength of the hydraulic jumps, disk heating, and mass exchange depends on the binary separation, and for larger semi-major axes, the tidal spiral pattern is substantially reduced., Comment: 15 pages, 17 figures, accepted for publication in A&A

Recent observations from NASA's Kepler mission detected the first planets in circumbinary orbits. The question we try to answer is where these planets formed in the circumbinary disk and how far inside they migrated to reach their present location. We investigate the first and more delicate phase of planet formation when planetesimals accumulate to form planetary embryos. We use the hydrodynamical code FARGO to study the evolution of the disk and of a test population of planetesimals embedded in it. With this hybrid hydrodynamical--N--body code we can properly account for the gas drag force on the planetesimals and for the gravitational force of the disk on them. The numerical simulations show that the gravity of the eccentric disk on the planetesimal swarm excites their eccentricities to values much larger than those induced by the binary perturbations only within 10 AU from the stars. Moreover, the disk gravity prevents a full alignment of the planetesimal pericenters. Both these effects lead to large impact velocities, beyond the critical value for erosion. Planetesimals accumulation in circumbinary disks appears to be prevented close to the stellar pair by the gravitational perturbations of the circumbinary disk. The observed planets possibly formed in the outer regions of the disk and then migrated inside by tidal interaction with the disk., Comment: Accepted for publication in A&A

Most stars form in embedded clusters. Stellar flybys may affect the orbital architecture of the systems by exciting the eccentricity and causing dynamical instability. Since, incidentally, the timescale over which a cluster loses its gaseous component and begins to disperse is comparable to the circumstellar disk lifetime, we expect that closer, and more perturbing, stellar flybys occur when the planets are still embedded in their birth disk. We investigate the effects of the disk on the dynamics of planets after the stellar encounter to test whether it can damp the eccentricity and return the planetary system to a non-excited state. We use the hydrodynamical code FARGO to study the disk+planet(s) system during and after the stellar encounter in the context of evolved disk models whose superficial density is 10 times lower than that of the Minimum Mass Solar Nebula. The numerical simulations show that the planet eccentricity, excited during a close stellar flyby, is damped on a short timescale (~ 10 Kyr) in spite of the disk low initial density and subsequent tidal truncation. This damping is effective also for a system of 3 giant planets and the effects of the dynamical instability induced by the passing star are quickly absorbed. If the circumstellar disk is still present around the star during a stellar flyby, a planet (or a planetary system) is returned to a non-excited state on a short timescale. This does not mean that stellar encounters do not affect the evolution of planets, but they do it in a subtle way with a short period of agitated dynamical evolution. At the end of it, the system resumes a quiet evolution and the planetary orbits are circularized by the interaction with the disk., Comment: 7 pages, 8 figures, accepted for publication in Astronomy & Astrophysics

The outward migration of a pair of resonant-orbit planets, driven by tidal interactions with a gas-dominated disk, is studied in the context of evolved solar nebula models. The planets' masses, M1 and M2, correspond to those of Jupiter and Saturn. Hydrodynamical calculations in two and three dimensions are used to quantify the migration rates and analyze the conditions under which the outward migration mechanism may operate. The planets are taken to be fully formed after 1e6 and before 3e6 years. The orbital evolution of the planets in an evolving disk is then calculated until the disk's gas is completely dissipated. Orbital locking in the 3:2 mean motion resonance may lead to outward migration under appropriate conditions of disk viscosity and temperature. However, resonance locking does not necessarily result in outward migration. This is the case, for example, if convergent migration leads to locking in the 2:1 mean motion resonance, as post-formation disk conditions seem to suggest. Accretion of gas on the planets may deactivate the outward migration mechanism by raising the mass ratio M2/M1 and/or by reducing the accretion rate toward the star, hence depleting the inner disk. For migrating planets locked in the 3:2 mean motion resonance, there are stalling radii that depend on disk viscosity and on stellar irradiation, when it determines the disk's thermal balance. Planets locked in the 3:2 orbital resonance that start moving outward from within 1-2 AU may reach beyond 5 AU only under favorable conditions. However, within the explored space of disk parameters, only a small fraction - less than a few per cent - of the models predict that the interior planet reaches beyond 4 AU., Comment: 24 pages, 22 figures. To appear in The Astrophysical Journal. Updated with corrections added in proof

The flux of ISM neutral atoms surrounding stars and their environment affects the motion of dust particles in debris disks, causing a significant dynamical evolution. Large values of eccentricity and inclination can be excited and strong correlations settle in among the orbital angles. This dynamical behaviour, in particular for bound dust grains, can potentially cause significant asymmetries in dusty disks around solar type stars which might be detected by observations. However, the amount of orbital changes due to this non--gravitational perturbation is strongly limited by the collisional lifetime of dust particles. We show that for large values of the disk's optical depth the influence of ISM flow on the disk shape is almost negligible because the grains are collisionally destroyed before they can accumulate enough orbital changes due to the ISM perturbations. On the other hand, for values smaller than $10^{-3}$, peculiar asymmetric patterns appear in the density profile of the disk when we consider 1-10 mum grains, just above the blow-out threshold. The extent and relevance of these asymmetries grow for lower values of the optical depth. An additional sink mechanism, which may prevent the formation of large clumps and warping in the disks is related to the fast inward migration due to the drag component of the forces. When a significant eccentricity is pumped up by the ISM perturbations, the drag forces (Poynting-Robertson and in particular ISM drag) drive the disk particles on fast migrating tracks leading them into the star on a short timescale. It is then expected that disks with small optical depth expand inside the parent body ring all the way towards the star while disks with large optical depth would not significantly extend inside., Comment: accepted for publication in MNRAS

Debris disc analysis and modelling provide crucial information about the structure and the processes at play in extrasolar planetary systems. In binary systems, this issue is more complex because the disc should in addition respond to the companion star's perturbations. We explore the dynamical evolution of a collisionally active debris disc for different initial parent body populations, diverse binary configurations and optical depths. We focus on the radial extent and size distribution of the disc at a stationary state. We numerically follow the evolution of $10^{5}$ massless small grains, initially produced from a circumprimary disc of parent bodies following a size distribution in $dN \propto s^{-3.5}$ds . Grains are submitted to both stars' gravity as well as radiation pressure. In addition, particles are assigned an empirically derived collisional lifetime. For all the binary configurations the disc extends far beyond the critical semimajor axis $a_crit$ for orbital stability. This is due to the steady production of small grains, placed on eccentric orbits reaching beyond $a_crit$ by radiation pressure. The amount of matter beyond acrit depends on the balance between collisional production and dynamical removal rates: it increases for more massive discs as well as for eccentric binaries. Another important effect is that, in the dynamically stable region, the disc is depleted from its smallest grains. Both results could lead to observable signatures. We have shown that a companion star can never fully truncate a collisionally active disc. For eccentric companions, grains in the unstable regions can significantly contribute to the thermal emission in the mid-IR. Discs with sharp outer edges, especially bright ones such as HR4796A, are probably shaped by other mechanisms., Comment: accepted for publication in A&A

The analysis of non-radiative sources of static or time-dependent gravitational fields in the Solar System is crucial to accurately estimate the free-fall orbits of the LISA space mission. In particular, we take into account the gravitational effects of Interplanetary Dust (ID) on the spacecraft trajectories. The perturbing gravitational field has been calculated for some ID density distributions that fit the observed zodiacal light. Then we integrated the Gauss planetary equations to get the deviations from the LISA keplerian orbits around the Sun. This analysis can be eventually extended to Local Dark Matter (LDM), as gravitational fields are expected to be similar for ID and LDM distributions. Under some strong assumptions on the displacement noise at very low frequency, the Doppler data collected during the whole LISA mission could provide upper limits on ID and LDM densities., Comment: 11 pages, 6 figures, to be published on the special issue of "Celestial Mechanics and Dynamical Astronomy" on the CELMEC V conference

Recent studies have shown that alpha Centauri B might be, from an observational point of view, an ideal candidate for the detection of an Earth-like planet in or near its habitable zone (0.5-0.9AU). We study here if such habitable planets can form, by numerically investigating the planet-formation stage which is probably the most sensitive to binarity effects: the mutual accretion of km-sized planetesimals. Using a state-of-the-art algorithm for computing the impact velocities within a test planetesimal population, we find that planetesimal growth is only possible, although marginally, in the innermost part of the HZ around 0.5AU. Beyond this point, the combination of secular perturbations by the binary companion and gas drag drive the mutual velocities beyond the erosion limit. Impact velocities might later decrease during the gas removal phase, but this probably happens too late for preventing most km-sized objects to be removed by inward drift, thus preventing accretion from starting anew. A more promising hypothesis is that the binary formed in a crowded cluster, where it might have been wider in its initial stages, when planetary formation was ongoing. We explore this scenario and find that a starting separation roughly 15 AU wider, or an eccentricity 2.5 times lower than the present ones are required to have an accretion-friendly environment in the whole HZ., Comment: accepted for publication in MNRAS

The five planets discovered around the main-sequence star 55 Cnc may represent a case of stable chaos. By using both the Frequency Map Analysis and MEGNO we find that about 15 % of the systems that can be build from the nominal orbital elements of the system are highly chaotic. However, in spite of the fast diffusion rate in the phase space, the planetary system is not destabilized over 400 Myr and close encounters between the planets are avoided., Comment: 4 pages, 3 figures, 1 table, accepted for publication in MNRAS Letters (May 23, 2008)

We numerically explore planet formation around alpha Cen A by focusing on the crucial planetesimals-to-embryos phase. Our code computes the relative velocity distribution, and thus the accretion vs. fragmentation trend, of planetesimal populations having any given size distribution. This is a critical aspect of planet formation in binaries since the pericenter alignment of planetesimal orbits due to the gravitational perturbations of the companion star and to gas friction strongly depends on size. We find that, for the nominal case of a MMSN gas disc, the region beyond 0.5AU from the primary is hostile to planetesimal accretion. In this area, impact velocities between different-size bodies are increased, by the differential orbital phasing, to values too high to allow mutual accretion. For any realistic size distribution for the planetesimal population, this accretion-inhibiting effect is the dominant collision outcome and the accretion process is halted. Results are robust with respect to the profile and density of the gas disc: except for an unrealistic almost gas-free case, the inner accretion safe area never extends beyond 0.75AU. We conclude that planet formation is very difficult in the terrestrial region around alpha Cen A, unless it started from fast-formed very large (>30km) planetesimals. Notwithstanding these unlikely initial conditions, the only possible explanation for the presence of planets around 1 AU from the star would be the hypothetical outward migration of planets formed closer to the star or a different orbital configuration in the binary's early history. Our conclusions differ from those of several studies focusing on the later embryos-to-planets stage, confirming that the planetesimals-to-embryos phase is more affected by binary perturbations., Comment: accepted for publication in MNRAS (Note: abstract truncated. Full abstract in the pdf file)

Numerical simulations of planetesimal accretion in circumprimary and circumbinary orbits are described. The secular perturbations by the com- panion star and gas drag are included in our models. We derive limits on the parameters of the binary system for which accretion and then planetary forma- tion are possible. In the circumbinary case we also outline the radial distance from the baricenter of the stars beyond which accumulation always occurs. Hy- drodynamical simulations are also presented to validate our N-body approach based on the axisymmetric approximation for the gas of the disk., Comment: to appear in Extreme Solar Systems, ASP Conference Series, eds. Debra Fischer, Fred Rasio, Steve Thorsett and Alex Wolszczan

We numerically investigate the possibility of planetesimal accretion in circumbinary disks, under the coupled influence of both stars' secular perturbations and friction due to the gaseous component of the protoplanetary disk. We focus on one crucial parameter: the distribution of encounter velocities between planetesimals in the 0.5 to 100km size range. An extended range of binary systems with differing orbital parameters is explored. The resulting encounter velocities are compared to the threshold velocities below which the net outcome of a collision is accumulation into a larger body instead of mass erosion. For each binary configuration, we derive the critical radial distance from the binary barycenter beyond which planetesimal accretion is possible. This critical radial distance is smallest for equal-mass binaries on almost circular orbits. It shifts to larger values for increasing eccentricities and decreasing mass ratio. The importance of the planetesimals' orbital alignments of planetesimals due to gas drag effects is discussed., Comment: accepted for publication in MNRAS

We study to which extent planetesimal accretion is affected by the perturbing presence of a compagnon star. We concentrate on one crucial parameter: the distribution of encounter velocities within the planetesimal swarm. We numerically explore the evolution of this parameter taking into account the secular perturbations of the binary and friction due to the very likely presence of gas in the disk. $<\Delta v>$ maps are derived, for planetesimals of different sizes, for a total of 120 binary configurations (eccentricity eb and separation ab). We identify for each case 3 different accreting behaviours. 1) In regions where no significant dV increase is observed, ``classical'' (i.e., single-star) accretion is possible. 2) In regions where $dV>v_{ero}$, the threshold velocity above which all impacts are eroding, no accretion is possible and planet growth is stopped. 3) In between these 2 limiting behaviours, a large fraction of binary configurations leads to significant dV increase, but still below the erosion threshold. In this intermediate case, planetesimal growth can occur, but proceeds slower than in the single-star case, possibly following the so-called type II runaway groth mode., Comment: Chapter to appear in the book "Planets in Binary Star Systems," ed. Nader Haghighipour (Springer publishing company), 2007

We numerically investigate under which conditions the planet detected at 2.1 AU of Gamma-Cephei could form through the core-accretion scenario despite the perturbing presence of the highly eccentric companion star. We first show that the initial stage of runaway accretion of kilometer-sized planetesimals is possible within 2.5 AU from the central star only if large amounts of gas are present. In this case, gaseous friction induces periastron alignment of the orbits which reduces the otherwise high mutual impact velocities due to the companion's secular perturbations. The following stage of mutual accretion of large embryos is also modeled. According to our simulations, the giant impacts among the embryos always lead to a core of 10 Mearth within 10 Myr, the average lifetime of gaseous discs. However, the core always ends up within 1.5 AU from the central star. Either the core grows more quickly in the inner region of the disc, or it migrates inside by scattering the residual embryos, Comment: 8 pages, 12 figures to appear in Astronomy and Astrophysics (accepted 08/06/2004)