Extrasolar planets: constraints for planet formation models

Since 1995, more than 150 extrasolar planets have been discovered, most of them in orbits quite different from those of the giant planets in our own solar system. The number of discovered extrasolar planets demonstrates that planetary systems are common but also that they may possess a large variety of properties. As the number of detections grows, statistical studies of the properties of exoplanets and their host stars can be conducted to unravel some of the key physical and chemical processes leading to the formation of planetary systems.     

Evidence for dust grain growth in young circumstellar disks

Hundreds of circumstellar disks in the Orion nebula are being rapidly destroyed by the intense ultraviolet radiation produced by nearby bright stars. These young, million-year-old disks may not survive long enough to form planetary systems. Nevertheless, the first stage of planet formation-the growth of dust grains into larger particles-may have begun in these systems. Observational evidence for these large particles in Orion's disks is presented. A model of grain evolution in externally irradiated protoplanetary disks is developed and predicts rapid particle size evolution and sharp outer disk boundaries. We discuss implications for the formation rates of planetary systems.     

A gaseous metal disk around a white dwarf

The destiny of planetary systems through the late evolution of their host stars is very uncertain. We report a metal-rich gas disk around a moderately hot and young white dwarf. A dynamical model of the double-peaked emission lines constrains the outer disk radius to just 1.2 solar radii. The likely origin of the disk is a tidally disrupted asteroid, which has been destabilized from its initial orbit at a distance of more than 1000 solar radii by the interaction with a relatively massive planetesimal object or a planet. The white dwarf mass of 0.77 solar mass implies that planetary systems may form around high-mass stars.     

Runaway growth of planetary embryos facilitated by massive bodies in a protoplanetary disk

About 30% of detected extrasolar planets exist in multiple-star systems. The standard model of planet formation cannot easily accommodate such systems and has difficulty explaining the odd orbital characteristics of most extrasolar giant planets. We demonstrate that the formation of terrestrial-size planets may be insulated from these problems, enabling much of the framework of the standard model to be salvaged for use in complex systems. A type of runaway growth is identified that allows planetary embryos to form by a combination of nebular gas drag and perturbations from massive companions-be they giant planets, brown dwarfs, or other stars.     

Occurrence of Earth-like bodies in planetary systems

Present theories of terrestrial planet formation predict the rapid ;;runaway formation' of planetary embryos. The sizes of the embryos increase with heliocentric distance. These embryos then merge to form planets. In earlier Monte Carlo simulations of the merger of these embryos it was assumed that embryos did not form in the asteroid belt, but this assumption may not be valid. Simulations in which runaways were allowed to form in the asteroid belt show that, although the initial distributions of mass, energy, and angular momentum are different from those observed today, during the growth of the planets these distributions spontaneously evolve toward those observed, simply as a result of known solar system processes. Even when a large planet analogous to ;;Jupiter' does not form, an Earth-sized planet is almost always found near Earth's heliocentric distance. These results suggest that occurrence of Earth-like planets may be a common feature of planetary systems.     

Self-assembly at all scales

Self-assembly is the autonomous organization of components into patterns or structures without human intervention. Self-assembling processes are common throughout nature and technology. They involve components from the molecular (crystals) to the planetary (weather systems) scale and many different kinds of interactions. The concept of self-assembly is used increasingly in many disciplines, with a different flavor and emphasis in each.     

Infrared Stars: The interaction between stars and interstellar clouds produces "infrared stars" of two different kinds

Our searches for very cool stars have revealed three kinds of objects: very cool Mira stars, perhaps cooler than any of this type previously known; extremely dense interstellar clouds, more dense than any known heretofore; and, probably, cool circumstellar clouds that may be planetary systems in an early stage of formation.     

Neptune's partial rings: action of galatea on self-gravitating arc particles

Numerical simulations of Neptune's arcs show that self-gravity between macroscopic arc particles can prevent interparticle impacts and thereby stabilize their resonant confinement by Galatea, a satellite of Neptune. Stable subkilometer arc particles provide a source for replenishing the observed dust and explain the clumpy substructure seen in arcs. A few confining kilometer-sized particles between the major arc components can account for the observed arc widths spanning several resonance sites. The modeled distribution of dust is consistent with observations and helps to explain how embedded satellites may affect the structure and evolution of planetary ring systems.     

基于齿面闪温最低的行星斜齿轮优化设计

根据Block闪温理论,通过分别计算某单级行星斜齿轮太阳轮和行星轮及行星轮和内齿圈的闪温曲线,对比了改变压力角、螺旋角及太阳轮变位系数对轮齿最高闪温的影响。通过对比,选择最佳齿面闪温．找到了最佳的齿轮压力角、螺旋角及太阳轮变位系数．实现了提高行星齿轮的胶合承载能力,达到了优化行星齿轮的目的。     

基于拓扑图基尔霍夫定律在行星轮系运动中的应用

应用基尔霍夫定律建立基于拓扑图的行星轮系数学模型——回路方程组、切割方程组来分析、解决行星轮系运动学的有关问题，求解行星轮系的传动比和角速度以及行星轮系中各个构件之间的受力和力矩。     

Mars: a new core-crystallization regime

The evolution of the martian core is widely assumed to mirror the characteristics observed for Earth's core. Data from experiments performed on iron-sulfur and iron-nickel-sulfur systems at pressures corresponding to the center of Mars indicate that its core is presently completely liquid and that it will not form an outwardly crystallizing iron-rich inner core, as does Earth. Instead, planetary cooling will lead to core crystallization following either a "snowing-core" model, whereby iron-rich solids nucleate in the outer portions of the core and sink toward the center, or a "sulfide inner-core" model, where an iron-sulfide phase crystallizes to form a solid inner core.     

Organic molecules and water in the planet formation region of young circumstellar disks

The chemical composition of protoplanetary disks is expected to hold clues to the physical and chemical processes that influence the formation of planetary systems. However, characterizing the gas composition in the planet formation region of disks has been a challenge to date. We report here that the protoplanetary disk within 3 astronomical units of AA Tauri possesses a rich molecular emission spectrum in the mid-infrared, indicating a high abundance of simple organic molecules (HCN, C2H2, and CO2), water vapor, and OH. These results suggest that water is abundant throughout the inner disk and that the disk supports an active organic chemistry.     

NGW行星减速器的模糊优化设计

NGW行星减速器是机械工业领域内广泛应用的一种行星变速装置,其设计是比较复杂的.根据模糊数学原理,本文提出了它的模糊优化设计模型并进行了模糊优化设计.     

Discovery of a Jupiter/Saturn analog with gravitational microlensing

Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of approximately 0.71 and approximately 0.27 times the mass of Jupiter and orbital separations of approximately 2.3 and approximately 4.6 astronomical units orbiting a primary star of mass approximately 0.50 solar mass at a distance of approximately 1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common.     

Remote sensing of the magnetic moment of uranus: predictions for voyager

Power is supplied to a planet's magnetosphere from the kinetic energy of planetary spin and the energy flux of the impinging solar wind. A fraction of this power is available to drive numerous observable phenomena, such as polar auroras and planetary radio emissions. In this report our present understanding of these power transfer mechanisms is applied to Uranus to make specific predictions of the detectability of radio and auroral emissions by the planetary radio astronomy (PRA) and ultraviolet spectrometer (UVS) instruments aboard the Voyager spacecraft before its encounter with Uranus at the end of January 1986. The power available for these two phenomena is (among other factors) a function of the magnetic moment of Uranus. The date of earliest detectability also depends on whether the predominant power source for the magnetosphere is planetary spin or solar wind. The magnetic moment of Uranus is derived for each power source as a function of the date of first detection of radio emissions by the PRA instrument or auroral emissions by the UVS instrument. If we accept the interpretation of ultraviolet observations now available from the Earth-orbiting International Ultraviolet Explorer satellite, Uranus has a surface magnetic field of at least 0.6 gauss, and more probably several gauss, making it the largest or second-largest planetary magnetic field in the solar system.     

Astronomy. Laser telemetry from space

Space missions currently on the drawing boards are expected to gather data at rates exceeding the transmission capabilities of today's telemetry systems by many orders of magnitude. Even on current missions, onboard data compression techniques are being implemented to compensate for lack of transmission speed. But while data compression can minimize the loss of data, it is no substitute for transmitting all of the data through a faster communications link. The transmission problem will soon reach crisis proportions and will affect astronomical, Earth resources, geophysical, meteorological, planetary and other space science missions. To overcome this communications bottleneck, the authors advocate the implementation of telemetry systems based on near-infrared laser transmission techniques. The fiber-optics communications industry has developed most of the basic components required for signal transmission in this wavelength band, which should make such a system affordable on scales relevant to the cost of anticipated space science missions.     

Pressure enhancement of ion mobilities in liquid silicates from computer simulation studies to 800 kilobars

Ion dynamics computer simulation methods show that for many liquid silicates, like silica itself, the component diffusion coefficients show anomalous pressure dependences. This implies that their viscosities have negative pressure dependences. Overall, there is an interesting degree of analogy between the fundamental binary solutions of geochemistry and the aqueous solutions of common experience; however, due to the stronger bonding in silicate systems, the anomalies are much more persistent. Diffusivity maxima occur at pressures of 200 to 300 kilobars and are correlated with a prevalence of fivefold coordination of silicon ions. The relevance of these findings to planetary dynamics and thermophysical modeling problems is briefly considered.     

环面传动啮合原理分析

本文通过对一种新型行星环面蜗杆传动的啮合原理进行分析，得出了蜗杆传动时定子与行里轮的啮合方程和定子的齿面方程，为将此环面传动广泛地应用到工业领域打下了理论基础。     

Gamma-ray burst observations by pioneer venus orbiter

The Pioneer Venus orbiter gamma burst detector is an astrophysics experiment for monitoring cosmic gamma-ray bursts. It is included in this planetary mission to provide a long baseline for accurately locating the sources ofthese bursts in order to identify them with specific astronomical objects. Responses to 14 gammaray burst events were examined; these events were verified from data acquired by other systems. Preliminary locations are proposed for three events, based on data from the Pioneer Venus orbiter, ISEE C, and Vela spacecraft. These locations will be improved, and additional locations will be determined by including in the analyses data from Helios B and the Russian Venera 11, Venera 12, and Prognoz 7 spacecraft.     

Organic synthesis via irradiation and warming of ice grains in the solar nebula

Complex organic compounds, including many important to life on Earth, are commonly found in meteoritic and cometary samples, though their origins remain a mystery. We examined whether such molecules could be produced within the solar nebula by tracking the dynamical evolution of ice grains in the nebula and recording the environments to which they were exposed. We found that icy grains originating in the outer disk, where temperatures were less than 30 kelvin, experienced ultraviolet irradiation exposures and thermal warming similar to that which has been shown to produce complex organics in laboratory experiments. These results imply that organic compounds are natural by-products of protoplanetary disk evolution and should be important ingredients in the formation of all planetary systems, including our own.