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1.
系外行星的探测是近年来炙手可热的话题,尤其是类地行星的探测.随着观测数据的不断积累,以及NASA的Kepler卫星的升空,越来越多的系外行星系统和类地行星被探测到,这将极大地丰富系外行星和系外行星系统的样本,为我们提供更多的素材,使我们对系外行星的形成、演化等过程有更加深刻的认识. 相似文献
2.
天文学的重要使命是探讨人类地球在宇宙中的位置、人和宇宙的关系,宇宙的构造和演化发展以及为宇航事业的拓展服务,其中也包括探索系外行星、系外类地行星和搜索外星人——具有智慧生命的地外文明。 相似文献
3.
《天文学报》2017,(6)
系外类地行星空间探测计划(Search for Terrestrial Exo-Planets,以下简称"STEP")采用天体测量法和微像素级焦平面定标测量技术,设计望远镜焦平面检测精度达到1μas.在假定焦平面设计能达到检测精度的前提条件下,系统分析了恒星自行、视差、卫星速度和位置、光学系统的光心等关键因素对检测系外行星的影响.有别于传统的窄视场照相底片常数法,提出了一种恒星相对角距测量方法,以检测由于可能存在的系外行星而引起星对角距变化的非线性项,消除了传统窄视场天体测量中参考星位置和自行精度对检测系外行星的直接影响.针对同一天区内的8颗参考星和1颗具有行星系统的待测星,分别模拟出5 yr内的观测数据,利用最小二乘法进行处理,发现基于STEP自身1μas的观测精度,在这种情况下是可以观测到类地行星的. 相似文献
4.
M型恒星(M dwarf)是主序星中质量较小的恒星,也是银河系中数量最多的恒星类型,在其周围形成的行星通常距离主星较近,宜居带也比F、G、K型恒星更靠近主星,更有利于发现系外宜居行星.研究表明, M型恒星周围平均存在2.5颗小质量行星,约为F、 G、 K型恒星的3.5倍,但M型恒星周围巨行星的出现率(occurrence rate)则比F、 G、K型小一个量级.基于M型恒星周围发现的401颗行星的参数开展了统计研究,发现质量越大的行星平均轨道半长径越大.类地行星约占行星总数的74%,且轨道半长径均小于1 au,其中28颗行星具有潜在宜居性.根据行星质量-半径关系,在质量等于4倍地球质量(M⊕)处存在一拐点,除少数几颗行星外,大部分小于该质量的行星可能都是由约65%的硅酸盐和约35%的铁组成,大于该质量的行星半径则随质量增加而迅速增大.约60%的M型恒星周围的行星位于多行星系统且轨道分布紧密,相邻行星轨道在3:2、5:3及2:1等平运动共振位置处存在峰值. M型恒星的多行星系统形成与演化等问题对现今的行星形成理论提出了新挑战. 相似文献
5.
根据太阳系行星物质的主要性态和大小,人们通常将其分成行星（包括类地行星和类木行星）、卫星、小行星、彗星和流星体。类地行星包括水星、金星、地球和火星;类木行星包括木星、土星、天王星和海王星;质量较大的小行星和卫星的内部结构与类地行星相似,质量较小的小行星和卫星以及流星体主要由岩石和金属组成;彗星是含有太阳系形成时期物质且没有经过太多物理和化学演化的冰态小天体。 相似文献
6.
2009年美国东部时间3月6日22时50分（北京时间7日11时50分）,美国航空航天局（NASA）用德尔他－2火箭发射了第一个专门用于寻找系外类地行星的空间望远镜——“开普勒”（Kepler）。65分钟后,它进入了距地球大约721千米高的预定轨道,并将在这一轨道试运行两个月,随后正式开始执行探索任务。 相似文献
7.
为什么要观测凌星？
当代天文学家对于在茫茫银河系内的千亿颗恒星周围寻找太阳系之外的行星（称“太阳系外行星”,后文简称“系外行星”）,尤其是类似地球这样可能孕育生命的星球（类地行星）有着极高的兴趣和热情。这一方面得益于人类对于探索地外生命与文明的不懈追求,另一方面对这些系外行星的探测及其物理性质的分析,对深入理解行星系统（尤其是我们所处的太阳系）的形成与演化机制也有着重要作用。 相似文献
8.
《天文研究与技术》2020,(1)
太阳系外类地行星直接成像探测极具挑战性。在可见光短波段,行星信号主要来自恒星经由行星大气的反射光,两者对比度在10~(-10)量级,导致微弱的系外行星光被淹没。因此,有效抑制来自恒星的强光是实现类地行星探测的关键。提出了基于光瞳振幅调制和相位校正的技术方案,以实现全区域高对比度成像。针对上述方案,进行了数值模拟分析,说明其可行性及潜在性能。数值模拟基于点扩散函数的能量分布,评价系统的目标成像对比度,并采用随机并行梯度下降迭代算法进行优化,最终成像对比度在360°全区域内达到10~(-10)量级。研究表明,该方案有望用于未来空间类地行星直接成像探测。 相似文献
9.
截止到2014年4月21日,已发现了1490多颗系外行星和3705颗Kepler候选体。这从观测角度证明了行星在银河系中是普遍存在的。对系外行星的研究丰富并加深了人们对行星形成与演化的认识。另外,新的观测与发现也不断提出新的科学问题。本论文开展了类地行星的形成演化、内部结构以及大气逃逸的研究。 相似文献
10.
在．广袤茫茫的大宇宙中寻找类地行星无异于大海捞针,特别是地面观测受地球大气条件。限制,观测手段存在种种弊端,其困难程度可想而知。系外行星的探索任务如何展开？到底从哪个天空区域开始搜索？等等,这些问题一直困扰着天文学家。 相似文献
11.
It is generally supposed that the atmospheres of the terrestrial planets were formed by secondary degassing processes. We propose, instead, that they are of primary origin, forming as an immediate and necessary consequence of the final stages of planetary accretion. Once the planetary embryo reached a critical size, the impacting material began to vaporize. The atmosphere, so created, then decelerated other impacting material, thus limiting the rate of atmospheric growth. We show that, given reasonable assumptions concerning the chemical composition of the impacting material, an acceptable model for the early atmosphere of the Earth, and the present atmospheres of Venus and Mars results.A discussion of the noble gas data for the terrestrial atmosphere indicates that these can be readily reconciled with an impact origin. 相似文献
12.
Helmut Lammer Aubrey L. Zerkle Stefanie Gebauer Nicola Tosi Lena Noack Manuel Scherf Elke Pilat-Lohinger Manuel Güdel John Lee Grenfell Mareike Godolt Athanasia Nikolaou 《Astronomy and Astrophysics Review》2018,26(1):2
We review the origin and evolution of the atmospheres of Earth, Venus and Mars from the time when their accreting bodies were released from the protoplanetary disk a few million years after the origin of the Sun. If the accreting planetary cores reached masses \(\ge 0.5 M_\mathrm{Earth}\) before the gas in the disk disappeared, primordial atmospheres consisting mainly of H\(_2\) form around the young planetary body, contrary to late-stage planet formation, where terrestrial planets accrete material after the nebula phase of the disk. The differences between these two scenarios are explored by investigating non-radiogenic atmospheric noble gas isotope anomalies observed on the three terrestrial planets. The role of the young Sun’s more efficient EUV radiation and of the plasma environment into the escape of early atmospheres is also addressed. We discuss the catastrophic outgassing of volatiles and the formation and cooling of steam atmospheres after the solidification of magma oceans and we describe the geochemical evidence for additional delivery of volatile-rich chondritic materials during the main stages of terrestrial planet formation. The evolution scenario of early Earth is then compared with the atmospheric evolution of planets where no active plate tectonics emerged like on Venus and Mars. We look at the diversity between early Earth, Venus and Mars, which is found to be related to their differing geochemical, geodynamical and geophysical conditions, including plate tectonics, crust and mantle oxidation processes and their involvement in degassing processes of secondary \(\hbox {N}_2\) atmospheres. The buildup of atmospheric \(\hbox {N}_2\), \(\hbox {O}_2\), and the role of greenhouse gases such as \(\hbox {CO}_2\) and \(\hbox {CH}_4\) to counter the Faint Young Sun Paradox (FYSP), when the earliest life forms on Earth originated until the Great Oxidation Event \(\approx \) 2.3 Gyr ago, are addressed. This review concludes with a discussion on the implications of understanding Earth’s geophysical and related atmospheric evolution in relation to the discovery of potential habitable terrestrial exoplanets. 相似文献
13.
We have tested the implications and limitations of Program ACRETE, a scheme based merely on Newtonian physics and accretion with unit sticking efficiency, devised by Dole in 1970 to simulate the origin of the planets. The dependence of the results on a variety of radial and vertical density distribution laws, on the ratio of gas to dust in the solar nebula, on the total nebula mass, and on the orbital eccentricity, ?, of the accreting grains are explored. Only for a small subset of conceivable cases are planetary systems closely like our own generated. Many models have tendencies toward one of two preferred configurations: multiple-star systems, or planetary systems in which Jovian planets either have substantially smaller masses than in our system or are absent altogether. But for a wide range of cases recognizable planetary systems are generated, ranging from multiple-star systems with accompanying planets, to systems with Jovian planets at several hundred astronomical units, to single stars surrounded only by asteroids. Many systems exhibit planets like Pluto and objects of asteroidal mass, in addition to usual terrestrial and Jovian planets. No terrestrial planets were generated more massive than five Earth masses. The number of planets per system is for most cases of order 10, and, roughly, inversely proportional to ?. All systems generated obey a relation of the Titius-Bode variety for relative planetary spacing. The case with which planetary systems are generated using such elementary and incomplete physical assumptions supports the idea of abundant and morphologically diverse planetary systems throughout the Galaxy. 相似文献
14.
Origin of the atmospheres of the terrestrial planets 总被引：1，自引：0，他引：1
A.G.W. Cameron 《Icarus》1983,56(2):195-201
The monotonic decrease in the atmospheric abundance of 36Ar per gram of planet in the sequence, Venus, Earth, and Mars has been assumed to reflect some conditions in the primitive solar nebula at the time of formation of the planetary atmospheres, having to do either with the composition of the nebula itself or the composition of the trapped gases in small solid bodies in the nebula. Behind such hypotheses lies the assumption that planetary atmospheres steadily gain components. However, not only can gases enter atmospheres; they may also be lost from atmospheres both by adsorption into the planetary interior and by loss into space as a result of collisions with minor and major planetesimals. In this paper a necessarily qualitative discussion is given of the problem of collisions with minor planetesimals, a process called atmospheric cratering or atmospheric erosion, and a discussion is given of atmospheric loss accompanying collision of a planet with a major planetesimal, such as may have produced the Earth's Moon. 相似文献
15.
Stuart Ross TAYLOR 《Meteoritics & planetary science》1999,34(3):317-329
Abstract— Here I discuss the series of events that led to the formation and evolution of our planet to examine why the Earth is unique in the solar system. A multitude of factors are involved: These begin with the initial size and angular momentum of the fragment that separated from a molecular cloud; such random factors are crucial in determining whether a planetary system or a double star develops from the resulting nebula. Another requirement is that there must be an adequate concentration of heavy elements to provide the 2% “rock” and “ice” components of the original nebula. An essential step in forming rocky planets in the inner nebula is the loss of gas and depletion of volatile elements, due to early solar activity that is linked to the mass of the central star. The lifetime of the gaseous nebula controls the formation of gas giants. In our system, fine timing was needed to form the gas giant, Jupiter, before the gas in the nebula was depleted. Although Uranus and Neptune eventually formed cores large enough to capture gas, they missed out and ended as ice giants. The early formation of Jupiter is responsible for the existence of the asteroid belt (and our supply of meteorites) and the small size of Mars, whereas the gas giant now acts as a gravitational shield for the terrestrial planets. The Earth and the other inner planets accreted long after the giant planets, from volatile-depleted planetesimals that were probably already differentiated into metallic cores and silicate mantles in a gas-free, inner nebula. The accumulation of the Earth from such planetesimals was essentially a stochastic process, accounting for the differences among the four rocky inner planets—including the startling contrast between those two apparent twins, Earth and Venus. Impact history and accretion of a few more or less planetesimals were apparently crucial. The origin of the Moon by a single massive impact with a body larger than Mars accounts for the obliquity (and its stability) and spin of the Earth, in addition to explaining the angular momentum, orbital characteristics, and unique composition of the Moon. Plate tectonics (unique among the terrestrial planets) led to the development of the continental crust on the Earth, an essential platform for the evolution of Homo sapiens. Random major impacts have punctuated the geological record, accentuating the directionless course of evolution. Thus a massive asteroidal impact terminated the Cretaceous Period, resulted in the extinction of at least 70% of species living at that time, and led to the rise of mammals. This sequence of events that resulted in the formation and evolution of our planet were thus unique within our system. The individual nature of the eight planets is repeated among the 60-odd satellites—no two appear identical. This survey of our solar system raises the question whether the random sequence of events that led to the formation of the Earth are likely to be repeated in detail elsewhere. Preliminary evidence from the “new planets” is not reassuring. The discovery of other planetary systems has removed the previous belief that they would consist of a central star surrounded by an inner zone of rocky planets and an outer zone of giant planets beyond a few astronomical units (AU). Jupiter-sized bodies in close orbits around other stars probably formed in a similar manner to our giant planets at several astronomical units from their parent star and, subsequently, migrated inwards becoming stranded in close but stable orbits as “hot Jupiters”, when the nebula gas was depleted. Such events would prevent the formation of terrestrial-type planets in such systems. 相似文献
16.
Using Ockham's razor as a guide, we have tried to find the simplest model for the formation of giant planets that can explain current observations of atmospheric composition. While this “top-down” approach is far from sufficient to define such models, it establishes a set of boundary conditions whose satisfaction is necessary. Using Jupiter as the prototype, we find that a simple model for giant planet formation that begins with a solar nebula of uniform composition and relies on accretion of low temperature icy planetesimals plus collapse of surrounding solar nebula gas supplies that satisfaction. We compare the resulting predictions of elemental abundances and isotope ratios in the atmospheres of the other giants with those from contrasting models and suggest some key measurements to make further progress. 相似文献
17.
The final stage in the formation of terrestrial planets consists of the accumulation of ∼1000-km “planetary embryos” and a swarm of billions of 1-10 km “planetesimals.” During this process, water-rich material is accreted by the terrestrial planets via impacts of water-rich bodies from beyond roughly 2.5 AU. We present results from five high-resolution dynamical simulations. These start from 1000-2000 embryos and planetesimals, roughly 5-10 times more particles than in previous simulations. Each simulation formed 2-4 terrestrial planets with masses between 0.4 and 2.6 Earth masses. The eccentricities of most planets were ∼0.05, lower than in previous simulations, but still higher than for Venus, Earth and Mars. Each planet accreted at least the Earth's current water budget. We demonstrate several new aspects of the accretion process: (1) The feeding zones of terrestrial planets change in time, widening and moving outward. Even in the presence of Jupiter, water-rich material from beyond 2.5 AU is not accreted for several millions of years. (2) Even in the absence of secular resonances, the asteroid belt is cleared of >99% of its original mass by self-scattering of bodies into resonances with Jupiter. (3) If planetary embryos form relatively slowly, then the formation of embryos in the asteroid belt may have been stunted by the presence of Jupiter. (4) Self-interacting planetesimals feel dynamical friction from other small bodies, which has important effects on the eccentricity evolution and outcome of a simulation. 相似文献
18.
We explore the cross section of giant planet envelopes at capturing planetesimals of different sizes. For this purpose we employ two sets of realistic planetary envelope models (computed assuming for the protoplanetary nebula masses of 10 and 5 times the mass of the minimum mass solar nebula), account for drag and ablation effects and study the trajectories along which planetesimals move. The core accretion of these models has been computed in the oligarchic growth regime [Fortier, A., Benvenuto, O.G., Brunini, A., 2007. Astron. Astrophys. 473, 311-322], which has also been considered for the velocities of the incoming planetesimals. This regime predicts velocities larger that those used in previous studies of this problem. As the rate of ablation is dependent on the third power of velocity, ablation is more important in the oligarchic growth regime. We compute energy and mass deposition, fractional ablated masses and the total cross section of planets for a wide range of values of the critical parameter of ablation. In computing the total cross section of the planet we have included the contributions due to mass deposited by planetesimals moving along unbound orbits. Our results indicate that, for the case of small planetary cores and low velocities for the incoming planetesimals, ablation has a negligible impact on the capture cross section in agreement with the results presented in Inaba and Ikoma [Inaba, S., Ikoma, M., 2003. Astron. Astrophys. 410, 711-723]. However for the case of larger cores and high velocities of the incoming planetesimals as predicted by the oligarchic growth regime, we find that ablation is important in determining the planetary cross section, being several times larger than the value corresponding ignoring ablation. This is so regardless of the size of the incoming planetesimals. 相似文献
19.
The origin of water in the inner Solar System is not well understood. It is believed that temperatures were too high in the accretion disk in the region of the terrestrial planets for hydrous phases to be thermodynamically stable. Suggested sources of water include direct adsorption of hydrogen from the nebula into magma oceans after the terrestrial planets formed, and delivery of asteroidal or cometary material from beyond the zone of the terrestrial planets. We explore a new idea, direct adsorption of water onto grains prior to planetary accretion. This hypothesis is motivated by the observation that the accretion disk from which our planetary system formed was composed of solid grains bathed in a gas dominated by hydrogen, helium, and oxygen. Some of that hydrogen and oxygen combined to make water vapor. We examine quantitatively adsorption of water onto grains in the inner Solar System accretion disk by exploring the adsorption dynamics of water molecules onto forsterite surfaces via kinetic Monte Carlo simulations. We conclude that many Earth oceans of water could be adsorbed. 相似文献
20.
Earth and Titan are two planetary bodies formed far from each other. Nevertheless the chemical composition of their atmospheres exhibits common indications of being produced by the accretion, plus ulterior in-situ processing of cometary materials. This is remarkable because while the Earth formed in the inner part of the disk, presumably from the accretion of rocky planetesimals depleted in oxygen and exhibiting a chemical similitude with enstatite chondrites, Titan formed within Saturn's sub-nebula from oxygen- and volatile-rich bodies, called cometesimals. From a cosmochemical and astrobiological perspective, the study of the H, C, N, and O isotopes on Earth and Titan could be the key to decipher the processes occurred in the early stages of formation of both planetary bodies. The main goal of this paper is to quantify the presumable ways of chemical evolution of both planetary bodies, in particular the abundance of CO and N2 in their early atmospheres. In order to do that the primeval atmospheres and evolution of Titan and Earth have been analyzed from a thermodynamic point of view. The most relevant chemical reactions involving these species and presumably important at their early stages are discussed. Then, we have interpreted the results of this study in light of the results obtained by the Cassini–Huygens mission on these species and their isotopes. Given that H, C, N, and O were preferentially depleted from inner disk materials that formed our planet, the observed similitude of their isotopic fractionation, and subsequent close evolution of Earth's and Titan's atmospheres points towards a cometary origin of Earth atmosphere. Consequently, our scenario also supports the key role of late veneers (comets and water-rich carbonaceous asteroids) enriching the volatile content of the Earth at the time of the late heavy bombardment of terrestrial planets. 相似文献