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The inner solar system possesses a unique orbital structure in which there are no planets inside the Mercury orbit and the mass is concentrated around the Venus and Earth orbits. The origins of these features still remain unclear. We propose a novel concept that the building blocks of the inner solar system formed at the dead-zone inner edge in the early phase of the protosolar disk evolution, where the disk is effectively heated by the disk accretion. First, we compute the dust evolution in a gas disk with a dead zone and obtain the spatial distribution of rocky planetesimals. The disk is allowed to evolve both by a viscous diffusion and magnetically-driven winds. We find that the rocky planetesimals are formed in concentrations around $\sim$ 1 au with a total mass comparable to the mass of the current inner solar system in the early phase of the disk evolution within $\lesssim0.1$ Myr. Based on the planetesimal distribution and the gas disk structure, we subsequently perform \textit{N}-body simulations of protoplanets to investigate the dynamical configuration of the planetary system. We find that the protoplanets can grow into planets without significant orbital migration because of the rapid clearing of the inner disk by the magnetically-driven disk winds. Our model can explain the origins of the orbital structure of the inner solar system. Several other features such as the rocky composition can also be explained by the early formation of rocky planetesimals., Comment: 8 pages, 6 figures, accepted for publication in ApJL

Super-Earths possess low-mass H$_2$/He atmospheres (typically less than 10% by mass). However, the origins of super-Earth atmospheres have not yet been ascertained. We investigate the role of rapid disk clearing by photoevaporation during the formation of super-Earths and their atmospheres. We perform unified simulations of super-Earth formation and atmospheric evolution in evolving disks that consider both photoevaporative winds and magnetically driven disk winds. For the growth mode of planetary cores, we consider two cases in which planetary embryos grow with and without pebble accretion. Our main findings are summarized as follows. (i) The time span of atmospheric accretion is shortened by rapid disk dissipation due to photoevaporation, which prevents super-Earth cores from accreting massive atmospheres. (ii) Even if planetary cores grow rapidly by embryo accretion in the case without pebble accretion, the onset of runaway gas accretion is delayed because the isolation mass for embryo accretion is small. Together with rapid disk clearing, the accretion of massive atmospheres can be avoided. (iii) After rapid disk clearing, a number of high-eccentricity embryos can remain in outer orbits. Thereafter, such embryos may collide with the super-Earths, leading to efficient impact erosion of accreted atmospheres. We, therefore, find that super-Earths with low-mass H$_2$/He atmospheres are naturally produced by N-body simulations that consider realistic disk evolution., 17 pages, 11 figures, accepted for publication in ApJ

We propose a pebble-driven planet formation scenario to form giant planets with high multiplicity and large orbital distances in the early gas disk phase. We perform N-body simulations to investigate the growth and migration of low-mass protoplanets in the disk with inner viscously heated and outer stellar irradiated regions. The key feature of this model is that the giant planet cores grow rapidly by a combination of pebble accretion and planet-planet collisions. This consequently speeds up their gas accretion. Because of efficient growth, the planet transitions from rapid type I migration to slow type II migration early, reducing the inward migration substantially. Multiple giant planets can sequentially form in this way with increasing semimajor axes. Both mass growth and orbital retention are more pronounced when a large number of protoplanets are taken into account compared to the case of single planet growth. Eventually, a few numbers of giant planets form with orbital distances of a few to a few tens of AUs within $1.5{-}3$ Myr after the birth of the protoplanets. The resulting simulated planet populations could be linked to the substructures exhibited in disk observations as well as large orbital distance exoplanets observed in radial velocity and microlensing surveys., MNRAS accepted, 12 pages, 5 figures

Recently, transmission spectroscopy in the atmospheres of the TRAPPIST-1 planets revealed flat and featureless absorption spectra, which rule out cloud-free hydrogen-dominated atmospheres. Earth-sized planets orbiting TRAPPIST-1 likely have either a clear or a cloudy/hazy hydrogen-poor atmosphere. In this paper, we investigate whether a proposed formation scenario is consistent with expected atmospheric compositions of the TRAPPIST-1 planets. We examine the amount of a hydrogen-rich gas that TRAPPIST-1-like planets accreted from the ambient disk until disk dispersal. Since TRAPPIST-1 planets are trapped into a resonant chain, we simulate disk gas accretion onto a migrating TRAPPIST-1-like planet. We find that the amount of an accreted hydrogen-rich gas is as small as 10$^{-2}$ wt% and 0.1 wt% for TRAPPIST-1b and 1c, 10$^{-2}$ wt% for 1d, 1 wt% for 1e, a few wt% for 1f and 1g and 1 wt% for 1h, respectively. We also calculate a long-term thermal evolution of TRAPPIST-1-like planets after disk dissipation and estimate the mass loss of their hydrogen-rich atmospheres driven by a stellar X-ray and UV irradiation. We find that all the accreted hydrogen-rich atmospheres can be lost via hydrodynamic escape. Therefore, we conclude that TRAPPIST-1 planets should have no primordial hydrogen-rich gases but secondary atmospheres such as a Venus-like one and water vapor, if they currently retain atmospheres., Comment: 10 pages, 3 figures, accepted for publication in ApJ

A substantial number of super-Earths have been discovered, and atmospheres of transiting super-Earths have also been observed by transmission spectroscopy. Several lines of observational evidence indicate that most super-Earths do not possess massive H$_2$/He atmospheres. However, accretion and retention of less massive atmospheres on super-Earths challenge planet formation theory. We consider the following three mechanisms: (i) envelope heating by pebble accretion, (ii) mass loss during giant impacts, and (iii) atmospheric loss by stellar X-ray and EUV photoevaporation. We investigate whether these mechanisms influence the amount of the atmospheres that form around super-Earths. We develop a code combining an N-body simulation of pebble-driven planetary formation and an atmospheric evolution simulation. We demonstrate that the observed orbital properties of super-Earths are well reproduced by the results of our simulations. However, (i) heating by pebble accretion ceases prior to disk dispersal, (ii) the frequency of giant impact events is too low to sculpt massive atmospheres, and (iii) many super-Earths having H$_2$/He atmospheres of $\gtrsim 10$ wt% survive against stellar irradiations for 1 Gyr. Therefore, it is likely that other mechanisms such as suppression of gas accretion are required to explain less massive atmospheres ($\lesssim 10$ wt%) of super-Earths., 17 pages, 10 figures, accepted for publication in ApJ

The core-accretion model predicts that planetary cores as massive as super-Earths undergo runaway gas accretion to become gas giants. However, the exoplanet census revealed the prevalence of super-Earths close to their host stars, which should have avoided runaway gas accretion. In fact, mass-radius relationships of transiting planets suggest that some close-in super-Earths possess H$_2$/He atmospheres of ~ 0.1-10% by mass. Previous studies indicated that properties of a disk gas such as metallicity and the inflow/outflow cycle of a disk gas around a super-Earth can regulate accumulation of a H$_2$/He atmosphere onto itself. In this paper, we propose a new mechanism that radial mass accretion in a disk can limit the gas accretion onto super-Earth cores. Recent magneto-hydrodynamic simulations found that magnetically driven disk winds can drive a rapid gas flow near the disk surface. Such a rapid gas flow may slip out of a planetary core and regulate gas supply to an accreting gas onto the core. We performed N-body simulations for formation of super-Earths with accretion of atmospheres in a viscous accretion disk including effects of wind-driven accretion. We found that even super-Earth cores can avoid triggering runaway gas accretion if the inflow of a disk gas toward the cores is limited by viscous accretion. Our model predicts that super-Earths having H$_2$/He atmosphere of ~ 0.1-10 wt % form within $\lesssim$ 1 au of the central star, whereas gas giants are born in the outer region. This mechanism can explain the radial dependence of observed giant planets beyond the solar system., 10 pages, 9 figures, accepted for publication in ApJ

The Infrared Doppler (IRD) instrument is a fiber-fed high-resolution NIR spectrometer for the Subaru telescope covering the Y,J,H-bands simultaneously with a maximum spectral resolution of 70,000. The main purpose of IRD is a search for Earth-mass planets around nearby M-dwarfs by precise radial velocity measurements, as well as a spectroscopic characterization of exoplanet atmospheres. We report the current status of the instrument, which is undergoing commissioning at the Subaru Telescope, and the first light observation successfully done in August 2017. The general description of the instrument will be given including spectrometer optics, fiber injection system, cryogenic system, scrambler, and laser frequency comb. A large strategic survey mainly focused on late-type M-dwarfs is planned to start from 2019.

No planets exist inside the orbit of Mercury and the terrestrial planets of the solar system exhibit a localized configuration. According to thermal structure calculation of protoplanetary disks, a silicate condensation line (~ 1300 K) is located around 0.1 au from the Sun except for the early phase of disk evolution, and planetesimals could have formed inside the orbit of Mercury. A recent study of disk evolution that includes magnetically driven disk winds showed that the gas disk obtains a positive surface density slope inside ~ 1 au from the central star. In a region with positive midplane pressure gradient, planetesimals undergo outward radial drift. We investigate the radial drift of planetesimals and type I migration of planetary embryos in a disk that viscously evolves with magnetically driven disk winds. We show a case in which no planets remain in the close-in region. Radial drifts of planetesimals are simulated using a recent disk evolution model that includes effects of disk winds. The late stage of planet formation is also examined by performing N-body simulations of planetary embryos. We demonstrate that in the middle stage of disk evolution, planetesimals can undergo convergent radial drift in a magnetorotational instability (MRI)-inactive disk, in which the pressure maximum is created, and accumulate in a narrow ring-like region with an inner edge at ~ 0.7 au from the Sun. We also show that planetary embryos that may grow from the narrow planetesimal ring do not exhibit significant type I migration in the late stage of disk evolution. The origin of the localized configuration of the terrestrial planets of the solar system, in particular the deficit of close-in planets, can be explained by the convergent radial drift of planetesimals in disks with a positive pressure gradient in the close-in region., 5 pages, 4 figures, accepted for publication in A&A Letters

Magnetically-driven disk winds would alter the surface density slope of gas in the inner region of a protoplanetary disk $(r \lesssim 1 {\rm au})$. This in turn affects planet formation. Recently, the effect of disk wind torque has been considered with the suggestion that it would carve out the surface density of the disk from inside and would induce global gas flows (wind-driven accretion). We aim to investigate effects of global gas flows on type I migration and also examine planet formation. A simplified approach was taken to address this issue, and N-body simulations with isolation-mass planets were also performed. In previous studies, the effect of gas flow induced by turbulence-driven accretion has been taken into account for its desaturation effect of the corotation torque. If more rapid gas flows (e.g., wind-driven accretion) are considered, the desaturation effect can be modified. In MRI-inactive disks, in which the wind-driven accretion dominates the disk evolution, the gas flow at the midplane plays an important role. If this flow is fast, the corotation torque is efficiently desaturated. Then, the fact that the surface density slope can be positive in the inner region due to the wind torque can generate an outward migration region extended to super-Earth mass planets. In this case, we observe that no planets fall onto the central star in N-body simulations with migration forces imposed to reproduce such migration pattern. We also see that super-Earth mass planets can undergo outward migration. Relatively rapid gas flows affects type I migration and thus the formation of close-in planets., 9 pages, 9 figures, accepted for publication in A&A

"Hot super-Earths" (or "Mini-Neptunes") between 1 and 4 times Earth's size with period shorter than 100 days orbit 30-50\% of Sun-like type stars. Their orbital configuration -- measured as the period ratio distribution of adjacent planets in multi-planet systems -- is a strong constraint for formation models. Here we use N-body simulations with synthetic forces from an underlying evolving gaseous disk to model the formation and long-term dynamical evolution of super-Earth systems. While the gas disk is present, planetary embryos grow and migrate inward to form a resonant chain anchored at the inner edge of the disk. These resonant chains are far more compact than the observed super-Earth systems. Once the gas dissipates resonant chains may become dynamically unstable. They undergo a phase of giant impacts that spreads the systems out. Disk turbulence has no measurable effect on the outcome. Our simulations match observations if a small fraction of resonant chains remain stable, while most super-Earths undergo a late dynamical instability. Our statistical analysis restricts the contribution of stable systems to less than $25\%$. Our results also suggest that the large fraction of observed single planet systems does not necessarily imply any dichotomy in the architecture of planetary systems. Finally, we use the low abundance of resonances in Kepler data to argue that, in reality, the survival of resonant chains happens likely only in $\sim 5\%$ of the cases. This leads to a mystery: in our simulations only 50-60\% of resonant chains became unstable whereas at least 75\% (and probably 90-95\%) must be unstable to match observations., Accepted in MNRAS - Kepler dichotomy section substantially updated

Aims: We investigate the evolution of protoplanetary discs (PPDs hereafter) with magnetically driven disc winds and viscous heating. Methods: We consider an initially massive disc with ~0.1 Msun to track the evolution from the early stage of PPDs. We solve the time evolution of surface density and temperature by taking into account viscous heating and the loss of the mass and the angular momentum by the disc winds within the framework of a standard alpha model for accretion discs. Our model parameters, turbulent viscosity, disc wind mass loss, and disc wind torque, which are adopted from local magnetohydrodynamical simulations and constrained by the global energetics of the gravitational accretion, largely depends on the physical condition of PPDs, particularly on the evolution of the vertical magnetic flux in weakly ionized PPDs. Results: Although there are still uncertainties concerning the evolution of the vertical magnetic flux remaining, surface densities show a large variety, depending on the combination of these three parameters, some of which are very different from the surface density expected from the standard accretion. When a PPD is in a "wind-driven accretion" state with the preserved vertical magnetic field, the radial dependence of the surface density can be positive in the inner region, 16 pages, 11 figures embedded, accepted by A&A (comments are welcome)

We have investigated accretion of terrestrial planets from planetesimals around M dwarfs through N-body simulations including the effect of tidal interaction with disk gas. Because of low luminosity of M dwarfs, habitable zones around them are located near the disk inner edge. Planetary embryos undergo type-I migration and pile up near the disk inner edge. We found that after repeated close scatterings and occasional collisions, three or four planets eventually remain in stable orbits in their mean motion resonances. Furthermore, large amount of water-rich planetesimals rapidly migrate to the terrestrial planet regions from outside of the snow line, so that formed planets in these regions have much more water contents than those around solar-type stars.

Recent three-dimensional magnetohydrodynamical simulations have identified a disk wind by which gas materials are lost from the surface of a protoplanetary disk, which can significantly alter the evolution of the inner disk and the formation of terrestrial planets. A simultaneous description of the realistic evolution of the gaseous and solid components in a disk may provide a clue for solving the problem of the mass concentration of the terrestrial planets in the solar system. We simulate the formation of terrestrial planets from planetary embryos in a disk that evolves via magnetorotational instability and a disk wind. The aim is to examine the effects of a disk wind on the orbital evolution and final configuration of planetary systems. We perform N-body simulations of sixty 0.1 Earth-mass embryos in an evolving disk. The evolution of the gas surface density of the disk is tracked by solving a one-dimensional diffusion equation with a sink term that accounts for the disk wind. We find that even in the case of a weak disk wind, the radial slope of the gas surface density of the inner disk becomes shallower, which slows or halts the type I migration of embryos. If the effect of the disk wind is strong, the disk profile is significantly altered (e.g., positive surface density gradient, inside-out evacuation), leading to outward migration of embryos inside ~ 1 AU. Disk winds play an essential role in terrestrial planet formation inside a few AU by changing the disk profile. In addition, embryos can undergo convergent migration to ~ 1 AU in certainly probable conditions. In such a case, the characteristic features of the solar system's terrestrial planets (e.g., mass concentration around 1 AU, late giant impact) may be reproduced., 8 pages, 4 figures, accepted for publication in A&A

IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. It is a relatively compact (~1m size) spectrometer with a new echelle-grating and Volume-Phase Holographic gratings covering 1-2 micron wavelengths combined with an original frequency comb using optical pulse synthesizer. The spectrometer will employ a 4096x4096-pixel HgCdTe array under testing at IfA, University of Hawaii. Both the telescope/Adaptive Optics and comb beams are fed to the spectrometer via optical fibers, while the instrument is placed at the Nasmyth platform of the Subaru telescope. Expected accuracy of the Doppler-shifted velocity measurements is about 1 m s-1. Helped with the large collecting area and high image quality of the Subaru telescope, IRD can conduct systematic radial velocity surveys of nearby middle-to-late M stars aiming for down to one Earth-mass planet. Systematic observational and theoretical studies of M stars and their planets for the IRD science are also ongoing. We will report the design and preliminary development progresses of the whole and each component of IRD.

We have investigated planetary accretion from planetesimals in terrestrial planet regions inside the ice line around M dwarf stars through N-body simulations including tidal interactions with disk gas. Because of low luminosity of M dwarfs, habitable zones (HZs) are located in inner regions. In the close-in HZ, type-I migration and the orbital decay induced by eccentricity damping are efficient according to the high disk gas density in the small orbital radii. In the case of full efficiency of type-I migration predicted by the linear theory, we found that protoplanets that migrate to the vicinity of the host star undergo close scatterings and collisions, and 4 to 6 planets eventually remain in mutual mean motion resonances and their orbits have small eccentricities and they are stable both before and after disk gas decays. In the case of slow migration, the resonant capture is so efficient that densely-packed ~ 40 small protoplanets remain in mutual mean motion resonances. In this case, they start orbit crossing, after the disk gas decays and eccentricity damping due to tidal interaction with gas is no more effective. Through merging of the protoplanets, several planets in widely-separated non-resonant orbits with relatively large eccentricities are formed. Thus, the final orbital configurations of the terrestrial planets around M dwarfs sensitively depend on strength of type-I migration. We also found that large amount of water-ice is delivered by type-I migration from outer regions and final planets near the inner disk edge around M dwarfs are generally abundant in water-ice except for the innermost one that is shielded by the outer planets, unless type-I migration speed is reduced by a factor of more than 100 from that predicted by the linear theory., Accepted to ApJ. 33 pages, 9 figures, 1 table

We present a model for the formation of non‐resonant multiple short‐period Earths/super‐Earths, based on direct N‐body simulations and population synthesis calculations. We investigated the distributions of mass, semimajor axis and orbital eccentricities of solid planets; in particular, those of rocky planets in inner disk regions, in the absence of effects of gas giant planets. In the inner regions, embryos accrete most of the planetesimals by disk gas depletion. Coupled effects of Type I migration of embryos, termination of the migration near the inner disk edge, and resonant trapping between embryos all produce a resonantly trapped cluster of embryos that extends from the disk inner edge of ∼0.04 AU to beyond 0.1 AU. After tidal eccentricity damping due to the gas disk declines, the embryos undergo orbit crossing and their eccentricities are pumped up through close scattering between them. They collide with each other (giant impacts) to increase mass by an order of magnitude over 100 Myr. They form multiple Earths or super‐Earths in non‐resonant orbits around ∼0.1 AU with moderate eccentricities of 0.01–0.1 in the disks with masses comparable to or a few times larger than the minimum‐mass solar nebula model. If the disk‐star magnetic coupling is weaker than a critical value, the short‐period multiple Earths or super‐Earths would be engulfed by their host stars and the inner regions at ≲1 AU would become clear. The variation of strength of the disk‐star magnetic coupling may produce a diversity of distributions of short‐period terrestrial planets.

We have investigated planetary accretion from planetesimals in terrestrial planet regions around M dwarf stars through N‐body simulations including tidal interactions with disk gas. Because of the low luminosity of M dwarfs, habitable zones (HZs) are located in inner regions. In the close‐in region, type‐I migration is efficient according to the high disk gas density, although there is a large uncertainty in migration speed. Through a series of N‐body simulations, we found that the final orbital configurations of the close‐in terrestrial planets sensitively depend on the strength of type‐I migration.

A large fraction of stars host one or multiple close-in super-Earth planets. There is an active debate about whether these planets formed in situ or at greater distances from the central star and migrated to their current position. It has been shown that part of their observed properties (e.g., eccentricity distribution) can be reproduced by N-body simulations of in situ formation starting with a population of protoplanets of high masses and neglecting the effects of the disk gas. We plan to reassess the in situ formation of close-in super-Earths through more complete simulations. We performed N-body simulations of a population of small planetary embryos and planetesimals that include the effects of disk-planet interactions (e.g., eccentricity damping, type I migration). In addition, we also consider the accretion of a primitive atmosphere from a protoplanetary disk. We find that planetary embryos grow very quickly well before the gas dispersal, and thus undergo rapid inward migration, which means that one cannot neglect the effects of a gas disk when considering the in-situ formation of close-in super-Earths. Owing to their rapid inward migration, super-Earths reach a compact configuration near the disk's inner edge whose distribution of orbital parameters matches the observed close-in super-Earths population poorly. On the other hand, simulations including eccentricity damping, but no type I migration, reproduce the observed distributions better. Including the accretion of an atmosphere does not help reproduce the bulk architecture of observations. Interestingly, we find that the massive embryos can migrate inside the disk edge while capturing only a moderately massive hydrogen/helium atmosphere. By this process they avoid becoming giant planets. The bulk of close-in super-Earths cannot form in situ, unless type I migration is suppressed in the entire disk inside 1 AU., Comment: 9 pages, 7 figures, accepted for publication in A&A