Stellar encounters as the origin of distant Solar System objects in highly eccentric orbits

The Kuiper belt extends from the orbit of Neptune at 30 au to an abrupt outer edge about 50 au from the Sun. Beyond the edge is a sparse population of objects with large orbital eccentricities. Neptune shapes the dynamics of most Kuiper belt objects, but the recently discovered planet 2003 VB12 (Sedna) has an eccentric orbit with a perihelion distance of 70 au, far beyond Neptune's gravitational influence. Although influences from passing stars could have created the Kuiper belt's outer edge and could have scattered objects into large, eccentric orbits, no model currently explains the properties of Sedna. Here we show that a passing star probably scattered Sedna from the Kuiper belt into its observed orbit. The likelihood that a planet at 60-80 au can be scattered into Sedna's orbit is about 50 per cent; this estimate depends critically on the geometry of the fly-by. Even more interesting is the approximately 10 per cent chance that Sedna was captured from the outer disk of the passing star. Most captures have very high inclination orbits; detection of such objects would confirm the presence of extrasolar planets in our own Solar System.     

Crystalline water ice on the Kuiper belt object (50000) Quaoar

The Kuiper belt is a disk-like structure consisting of solid bodies orbiting the Sun beyond Neptune. It is the source of the short-period comets and the likely repository of the Solar System's most primitive materials. Surface temperatures in the belt are low ( approximately 50 K), suggesting that ices trapped at formation should have been preserved over the age of the Solar System. Unfortunately, most Kuiper belt objects are too faint for meaningful compositional study, even with the largest available telescopes. Water ice has been reported in a handful of objects, but most appear spectrally featureless. Here we report near-infrared observations of the large Kuiper belt object (50000) Quaoar, which reveal the presence of crystalline water ice and ammonia hydrate. Crystallinity indicates that the ice has been heated to at least 110 K. Both ammonia hydrate and crystalline water ice should be destroyed by energetic particle irradiation on a timescale of about 10(7) yr. We conclude that Quaoar has been recently resurfaced, either by impact exposure of previously buried (shielded) ices or by cryovolcanic outgassing, or by a combination of these processes.     

Formation of Kuiper-belt binaries by dynamical friction and three-body encounters

The Kuiper belt is a disk of icy bodies that orbit the Sun beyond Neptune; the largest known members are Pluto and its companion Charon. A few per cent of Kuiper-belt bodies have recently been found to be binaries with wide separations and mass ratios of the order of unity. Collisions were too infrequent to account for the observed number of binaries, implying that these binaries formed through collisionless interactions mediated by gravity. These interactions are likely to have been most effective during the period of runaway accretion, early in the Solar System's history. Here we show that a transient binary forms when two large bodies penetrate one another's Hill sphere (the region where their mutual forces are larger than the tidal force of the Sun). The loss of energy needed to stabilize the binary orbit can then occur either through dynamical friction from surrounding small bodies, or through the gravitational scattering of a third large body. Our estimates slightly favour the former mechanism. We predict that five per cent of Kuiper-belt objects are binaries with apparent separations greater than 0.2 arcsec, and that most are in tighter binaries or systems of higher multiplicity.     

A low mass for Mars from Jupiter's early gas-driven migration

Jupiter and Saturn formed in a few million years (ref. 1) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ～100,000 years (ref. 2). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later, and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 au is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 au, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 au; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.     

The evolution of comets in the Oort cloud and Kuiper belt

Comets are remnants from the time when the outer planets formed, approximately 4-4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.     

The formation of Kuiper-belt binaries through exchange reactions

Recent observations have revealed that an unexpectedly high fraction--a few per cent--of the trans-Neptunian objects (TNOs) that inhabit the Kuiper belt are binaries. The components have roughly equal masses, with very eccentric orbits that are wider than a hundred times the radius of the primary. Standard theories of binary asteroid formation tend to produce close binaries with circular orbits, so two models have been proposed to explain the unique characteristics of the TNOs. Both models, however, require extreme assumptions regarding the size distribution of the TNOs. Here we report a mechanism that is capable of producing binary TNOs with the observed properties during the early stages of their formation and growth. The only required assumption is that the TNOs were initially formed through gravitational instabilities in the protoplanetary dust disk. The basis of the mechanism is an exchange reaction in which a binary whose primary component is much more massive than the secondary interacts with a third body, whose mass is comparable to that of the primary. The low-mass secondary component is ejected and replaced by the third body in a wide but eccentric orbit.     

Kuiper带天体轨道的观测特性

对当前已观测到的具有可靠轨道根数、位于30～50AU的Kuiper主带和50AU以外Kuiper散射带的轨道半长径、轨道偏心率和倾角的分布特性分别进行了统计作图分析．结果显示：轨道半长径在40～50AU之间的KBOs的数量呈近似正态分布,KBOs在共振区出现聚集,并且共振区天体普遍具有较大的轨道偏心率;经典KBOs的轨道倾角的分布范围比共振KBOs的还大;散射KBOs的近日点距离大多在30～45AU之间．     

Voyager 1 exited the solar wind at a distance of approximately 85 Au from the Sun

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow. Theory predicts that a termination shock marks this boundary, with locations ranging from a few to over 100 au (1 Au approximately 1.5 x 10(8) km, the distance from Earth to the Sun). 'Pick-up ions' that originate as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays. Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of approximately 85 Au (heliolatitude approximately 34 degrees N), then re-entered the supersonic solar wind about 200 days later at approximately 87 au from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.     

A collisional family of icy objects in the Kuiper belt

The small bodies in the Solar System are thought to have been highly affected by collisions and erosion. In the asteroid belt, direct evidence of the effects of large collisions can be seen in the existence of separate families of asteroids--a family consists of many asteroids with similar orbits and, frequently, similar surface properties, with each family being the remnant of a single catastrophic impact. In the region beyond Neptune, in contrast, no collisionally created families have hitherto been found. The third largest known Kuiper belt object, 2003 EL61, however, is thought to have experienced a giant impact that created its multiple satellite system, stripped away much of an overlying ice mantle, and left it with a rapid rotation. Here we report the discovery of a family of Kuiper belt objects with surface properties and orbits that are nearly identical to those of 2003 EL61. This family appears to be fragments of the ejected ice mantle of 2003 EL61.     

The trans-neptunian object UB313 is larger than Pluto

The most distant known object in the Solar System, 2003 UB313 (97 au from the Sun), was recently discovered near its aphelion. Its high eccentricity and inclination to the ecliptic plane, along with its perihelion near the orbit of Neptune, identify it as a member of the 'scattered disk'. This disk of bodies probably originates in the Kuiper belt objects, which orbit near the ecliptic plane in circular orbits between 30 and 50 au, and may include Pluto as a member. The optical brightness of 2003 UB313, if adjusted to Pluto's distance, is greater than that of Pluto, which suggested that it might be larger than Pluto. The actual size, however, could not be determined from the optical measurements because the surface reflectivity (albedo) was unknown. Here we report observations of the thermal emission of 2003 UB313 at a wavelength of 1.2 mm, which in combination with the measured optical brightness leads to a diameter of 3,000 +/- 300 +/- 100 km. Here the first error reflects measurement uncertainties, while the second derives from the unknown object orientation. This makes 2003 UB313 the largest known trans-neptunian object, even larger than Pluto (2,300 km). The albedo is 0.60 +/- 0.10 +/- 0.05, which is strikingly similar to that of Pluto, suggesting that the methane seen in the optical spectrum causes a highly reflective icy surface.     

对Kuiper带中3:2共振天体的轨道根数的分析研究

在太阳系最外层的Kuiper带中，有许多天体处于海王星3：2平运动共振中．对这些天体的主要轨道根数的观测数据分析发现，与其它Kuiper带天体比较，它们普遍具有高偏心率、高倾角和相对稳定的轨道．     

The binary Kuiper-belt object 1998 WW31

The recent discovery of a binary asteroid during a spacecraft fly-by generated keen interest, because the orbital parameters of binaries can provide measures of the masses, and mutual eclipses could allow us to determine individual sizes and bulk densities. Several binary near-Earth, main-belt and Trojan asteroids have subsequently been discovered. The Kuiper belt-the region of space extending from Neptune (at 30 astronomical units) to well over 100 AU and believed to be the source of new short-period comets-has become a fascinating new window onto the formation of our Solar System since the first member object, not counting Pluto, was discovered in 1992 (ref. 13). Here we report that the Kuiper-belt object 1998 WW31 is binary with a highly eccentric orbit (eccentricity e approximately 0.8) and a long period (about 570 days), very different from the Pluto/Charon system, which was hitherto the only previously known binary in the Kuiper belt. Assuming a density in the range of 1 to 2 g cm-3, the albedo of the binary components is between 0.05 and 0.08, close to the value of 0.04 generally assumed for Kuiper-belt objects.     

海王星特洛伊小天体动力学

特洛伊小天体与行星同享一个轨道,并与太阳、行星在空间构成等边三角形,最早为人们所知的特洛伊小天体是位于木星轨道上并位于木星前(后)方60°的两群小天体.而海王星特洛伊小天体则是近20年来太阳系内最重要的发现之一.观测证据表明海王星特洛伊小天体的总数量和总质量远超过木星特洛伊小天体和主带小行星,是太阳系内仅次于柯伊伯带的第二大小天体集群.它们一方面具有独特的轨道特征,另一方面又联系着海王星轨道内、外的空间,自然而然地成为检验太阳系起源与演化的试金石.我们简要介绍了对海王星特洛伊小天体的观测结果、对它们的轨道动力学和起源研究的进展.     

Rotational breakup as the origin of small binary asteroids

Asteroids with satellites are observed throughout the Solar System, from subkilometre near-Earth asteroid pairs to systems of large and distant bodies in the Kuiper belt. The smallest and closest systems are found among the near-Earth and small inner main-belt asteroids, which typically have rapidly rotating primaries and close secondaries on circular orbits. About 15 per cent of near-Earth and main-belt asteroids with diameters under 10 km have satellites. The mechanism that forms such similar binaries in these two dynamically different populations was hitherto unclear. Here we show that these binaries are created by the slow spinup of a 'rubble pile' asteroid by means of the thermal YORP (Yarkovsky-O'Keefe-Radzievskii-Paddack) effect. We find that mass shed from the equator of a critically spinning body accretes into a satellite if the material is collisionally dissipative and the primary maintains a low equatorial elongation. The satellite forms mostly from material originating near the primary's surface and enters into a close, low-eccentricity orbit. The properties of binaries produced by our model match those currently observed in the small near-Earth and main-belt asteroid populations, including 1999 KW(4) (refs 3, 4).     

Compositional homogeneity in the fragmented comet 73P/Schwassmann-Wachmann 3

The remarkable compositional diversity of volatile ices within comets can plausibly be attributed to several factors, including differences in the chemical, thermal and radiation environments in comet-forming regions, chemical evolution during their long storage in reservoirs far from the Sun, and thermal processing by the Sun after removal from these reservoirs. To determine the relevance of these factors, measurements of the chemistry as a function of depth in cometary nuclei are critical. Fragmenting comets expose formerly buried material, but observational constraints have in the past limited the ability to assess the importance of formative conditions and the effects of evolutionary processes on measured composition. Here we report the chemical composition of two distinct fragments of 73P/Schwassmann-Wachmann 3. The fragments are remarkably similar in composition, in marked contrast to the chemical diversity within the overall comet population and contrary to the expectation that short-period comets should show strong compositional variation with depth in the nucleus owing to evolutionary processing from numerous close passages to the Sun. Comet 73P/Schwassmann-Wachmann 3 is also depleted in the most volatile ices compared to other comets, suggesting that the depleted carbon-chain chemistry seen in some comets from the Kuiper belt reservoir is primordial and not evolutionary.     

A planetary system as the origin of structure in Fomalhaut's dust belt

The Sun and >15 per cent of nearby stars are surrounded by dusty disks that must be collisionally replenished by asteroids and comets, as the dust would otherwise be depleted on timescales <10(7) years (ref. 1). Theoretical studies show that the structure of a dusty disk can be modified by the gravitational influence of planets, but the observational evidence is incomplete, at least in part because maps of the thermal infrared emission from the disks have low linear resolution (35 au in the best case). Optical images provide higher resolution, but the closest examples (AU Mic and beta Pic) are edge-on, preventing the direct measurement of the azimuthal and radial disk structure that is required for fitting theoretical models of planetary perturbations. Here we report the detection of optical light reflected from the dust grains orbiting Fomalhaut (HD 216956). The system is inclined 24 degrees away from edge-on, enabling the measurement of disk structure around its entire circumference, at a linear resolution of 0.5 au. The dust is distributed in a belt 25 au wide, with a very sharp inner edge at a radial distance of 133 au, and we measure an offset of 15 au between the belt's geometric centre and Fomalhaut. Taken together, the sharp inner edge and offset demonstrate the presence of planetary-mass objects orbiting Fomalhaut.     

A collimated jet of molecular gas from a star on the asymptotic giant branch

Evolved stars of about one solar mass are in general spherically symmetric, yet the planetary nebulae that they produce in the next phase of their evolution tend not to exhibit such symmetry. Collimated 'jets' and outflows of material have been observed up to approximately 0.3 parsec from the central stars of planetary nebulae, and precession of those jets has been proposed to explain the observed asymmetries. Moreover, it has recently been shown theoretically that magnetic fields could launch and collimate such jets. Here we report the detection of a collimated and precessing jet of molecular gas that is traced by water-vapour maser spots approximately 500 astronomical units (au) from the star W43A in Aquila. We conclude that the jet is formed in the immediate vicinity of the star, and infer that elongated planetary nebulae are formed by jets during the short period, of less than 1,000 years, when the star makes its transition through the proto-planetary nebula phase to become a planetary nebula.     

对Kuiper带天体动力学演化的数值模拟

鉴于无法找到描述Kuiper带天体动力学演化的分析解,本文采用数值模拟的方法,即通过数值积分,得到由太阳、海王星和Kuiper带天体组成的限制性三体问题中Kuiper带天体随时间演化的数值解.结果表明,经长期演化后,共振带中的一些天体的偏心率明显被激励;Kuiper带天体的轨道偏心率和倾角的激励程度和天体的初始轨道半长径、偏心率、倾角有关.     

2018年深空探测热点回眸

 深空探测指人类航天器离开近地轨道、进入太阳系空间和宇宙空间，对地球以外天体（月球及以远天体）或空间环境开展的科学探测。2018年，国际深空探测叠彩纷呈：中国“嫦娥四号”成功实现国际首次月球背面软着陆并将开展巡视勘察；美国“洞察号”探测器登陆火星；向太阳系空间进发，朝向日心方向，欧洲空间局和日本合作研制的BeipiColombo探测器正飞向水星、美国“帕克号”探测器开启“史诗级”旅行去“触摸太阳”，远离日心方向，“新视野号”成功飞掠柯伊伯带的小行星“天涯海角”、“旅行者2号”突破日球层顶；美国“奥西里斯-REx”和日本“隼鸟2号”顺利抵达各自目标小行星执行采样任务。2018年，月球表面存在水冰、火星发现有机分子、太阳系边际再抵近等发现或突破对于探寻生命起源、太阳系起源和演化，拓展人类知识体系具有重要意义。     

Discovery of two new satellites of Pluto

Pluto's first known satellite, Charon, was discovered in 1978. It has a diameter (approximately 1,200 km) about half that of Pluto, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful, but they were not sensitive to objects less, similar150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of approximately 38 days (P1) and approximately 25 days (P2).