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Register a new userSolar System Formation in the Context of Extra-Solar Planets
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Exoplanet surveys have confirmed one of humanitys (and all teenagers) worst fears: we are weird. If our Solar System were observed with present-day Earth technology -- to put our system and exoplanets on the same footing -- Jupiter is the only planet that would be detectable. The statistics of exo-Jupiters indicate that the Solar System is unusual at the ~1% level among Sun-like stars (or ~0.1% among all stars). But why are we different? Successful formation models for both the Solar System and exoplanet systems rely on two key processes: orbital migration and dynamical instability. Systems of close-in super-Earths or sub-Neptunes require substantial radial inward motion of solids either as drifting mm- to cm-sized pebbles or migrating Earth-mass or larger planetary embryos. We argue that, regardless of their formation mode, the late evolution of super-Earth systems involves migration into chains of mean motion resonances, generally followed by instability when the disk dissipates. This pattern is likely also ubiquitous in giant planet systems. We present three models for inner Solar System formation -- the low-mass asteroid belt, Grand Tack, and Early Instability models -- each invoking a combination of migration and instability. We identify bifurcation points in planetary system formation. We present a series of events to explain why our Solar System is so weird. Jupiters core must have formed fast enough to quench the growth of Earths building blocks by blocking the flux of inward-drifting pebbles. The large Jupiter/Saturn mass ratio is rare among giant exoplanets but may be required to maintain Jupiters wide orbit. The giant planets instability must have been gentle, with no close encounters between Jupiter and Saturn, also unusual in the larger (exoplanet) context. Our Solar System system is thus the outcome of multiple unusual, but not unheard of, events.
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All extra-solar planet masses that have been derived spectroscopically are lower limits since the inclination of the orbit to our line-of-sight is unknown except for transiting systems. It is, however, possible to determine the inclination angle, i, between the rotation axis of a star and an observers line-of-sight from measurements of the projected equatorial velocity (v sin i), the stellar rotation period (P_rot) and the stellar radius (R_star). This allows the removal of the sin i dependency of spectroscopically derived extra-solar planet masses under the assumption that the planetary orbits lie perpendicular to the stellar rotation axis. We have carried out an extensive literature search and present a catalogue of v sin i, P_rot, and R_star estimates for exoplanet host stars. In addition, we have used Hipparcos parallaxes and the Barnes-Evans relationship to further supplement the R_star estimates obtained from the literature. Using this catalogue, we have obtained sin i estimates using a Markov-chain Monte Carlo analysis. This allows proper 1-sigma two-tailed confidence limits to be placed on the derived sin is along with the transit probability for each planet to be determined. While a small proportion of systems yield sin is significantly greater than 1, most likely due to poor P_rot estimations, the large majority are acceptable. We are further encouraged by the cases where we have data on transiting systems, as the technique indicates inclinations of ~90 degrees and high transit probabilities. In total, we estimate the true masses of 133 extra-solar planets. Of these, only 6 have revised masses that place them above the 13 Jupiter mass deuterium burning limit. Our work reveals a population of high-mass planets with low eccentricities and we speculate that these may represent the signature of different planetary formation mechanisms at work.
In the last few years, the so-called Nice model has got a significant importance in the study of the formation and evolution of the solar system. According to this model, the initial orbital configuration of the giant planets was much more compact than the one we observe today. We study the formation of the giant planets in connection with some parameters that describe the protoplanetary disk. The aim of this study is to establish the conditions that favor their simultaneous formation in line with the initial configuration proposed by the Nice model. We focus in the conditions that lead to the simultaneous formation of two massive cores, corresponding to Jupiter and Saturn, able to achieve the cross-over mass (where the mass of the envelope of the giant planet equals the mass of the core, and gaseous runway starts) while Uranus and Neptune have to be able to grow to their current masses. We compute the in situ planetary formation, employing the numerical code introduced in our previous work, for different density profiles of the protoplanetary disk. Planetesimal migration is taken into account and planetesimals are considered to follow a size distribution between $r_p^{min}$ (free parameter) and $r_p^{max}= 100$ km. The cores growth is computed according to the oligarchic growth regime. The simultaneous formation of the giant planets was successfully completed for several initial conditions of the disk. We find that for protoplanetary disks characterized by a power law ($Sigma propto r^{-p}$), smooth surface density profiles ($p leq 1.5$) favor the simultaneous formation. However, for steep slopes ($psim 2$, as previously proposed by other authors) the simultaneous formation of the solar system giant planets is unlikely ...
Fewer giants planets are found around M dwarfs than around more massive stars, and this dependence of planetary characteristics on the mass of the central star is an important observational diagnostic of planetary formation theories. In part to improve on those statistics, we are monitoring the radial velocities of nearby M dwarfs with the HARPS spectrograph on the ESO 3.6 m telescope. We present here the detection of giant planets around two nearby M0 dwarfs: planets, with minimum masses of respectively 5 Jupiter masses and 1 Saturn mass, orbit around Gl 676A and HIP 12961. The latter is, by over a factor of two, the most massive planet found by radial velocity monitoring of an M dwarf, but its being found around an early M-dwarf is in approximate line with the upper envelope of the planetary vs stellar mass diagram. HIP 12961 ([Fe/H]=-0.07) is slightly more metal-rich than the average solar neighborhood ([Fe/H]=-0.17), and Gl 676A ([Fe/H=0.18) significantly so. The two stars together therefore reinforce the growing trend for giant planets being more frequent around more metal-rich M dwarfs, and the 5~Jupiter mass Gl 676Ab being found around a metal-rich star is consistent with the expectation that the most massive planets preferentially form in disks with large condensate masses.
The cloud formation process starts with the formation of seed particles, after which, surface chemical reactions grow or erode the cloud particles. We investigate which materials may form cloud condensation seeds in the gas temperature and pressure regimes (T$_{rm gas}$ = 100-2000 K, p$_{rm gas}$ = 10$^{-8}$-100 bar) expected to occur in planetary and brown dwarf atmospheres. We apply modified classical nucleation theory which requires surface tensions and vapour pressure data for each solid species, which are taken from the literature. We calculate the seed formation rates of TiO$_{2}$[s] and SiO[s] and find that they efficiently nucleate at high temperatures of T$_{rm gas}$ = 1000-1750 K. Cr[s], KCl[s] and NaCl[s] are found to efficiently nucleate across an intermediate temperature range of T$_{rm gas}$ = 500-1000 K. We find CsCl[s] may serve as the seed particle for the water cloud layers in cool sub-stellar atmospheres. Four low temperature ice species, H$_{2}$O[s/l], NH$_{3}$[s], H$_{2}$S[s/l] and CH$_{4}$[s], nucleation rates (T$_{rm gas}$ = 100-250 K) are also investigated for the coolest sub-stellar/planetary atmospheres. Our results suggest a possibly, (T$_{rm gas}$, p$_{rm gas}$) distributed hierarchy of seed particle formation regimes throughout the sub-stellar and planetary atmospheric temperature-pressure space. In order to improve the accuracy of the nucleation rate calculation, further research into the small cluster thermochemical data for each cloud species is warranted. The validity of these seed particle scenarios will be tested by applying it to more complete cloud models in the future.
The Solar system was once rich in the short-lived radionuclide (SLR) $^{26}$Al, but deprived in $^{60}$Fe. Several models have been proposed to explain these anomalous abundances in SLRs, but none has been set within a self-consistent framework of the evolution of the Solar system and its birth environment. The anomalous abundance in $^{26}$Al may have originated from the accreted material in the wind of a massive $apgt 20$,$M_odot$ Wolf-Rayet star, but the star could also have been a member of the parental star-cluster instead of an interloper or an older generation that enriched the proto-solar nebula. The protoplanetary disk at that time was already truncated around the Kuiper-cliff (at $45$ au) by encounters with another cluster members before it was enriched by the wind of the nearby Wolf-Rayet star. The supernova explosion of a nearby star, possibly but not necessarily the exploding Wolf-Rayet star, heated the disk to $apgt 1500$K, melting small dust grains and causing the encapsulation and preservation of $^{26}$Al into vitreous droplets. This supernova, and possibly several others, caused a further abrasion of the disk and led to its observed tilt of $5.6pm1.2^circ$ with respect to the Suns equatorial plane. The abundance of $^{60}$Fe originates from a supernova shell, but its preservation results from a subsequent supernova. At least two supernovae are needed (one to deliver $^{60}$Fe, and one to preserve it in the disk) to explain the observed characteristics of the Solar system. The most probable birth cluster then has $N = 2500pm300$ stars and a radius of $r_{rm vir} = 0.75pm0.25$ pc. We conclude that Solar systems equivalent systems form in the Milky Way Galaxy at a rate of about 30 per Myr, in which case approximately 36,000 Solar system analogues roam the Milky Way.
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