Below One Earth: The Detection, Formation, and Properties of Subterrestrial Worlds

The Solar System includes two planets—Mercury and Mars—significantly less massive than Earth, and all evidence indicates that planets of similar size orbit many stars. In fact, one of the first exoplanets to be discovered is a lunar-mass planet around a millisecond pulsar. Novel classes of exoplanets have inspired new ideas about planet formation and evolution, and these “sub-Earths” should be no exception: they include planets with masses between Mars and Venus for which there are no Solar System analogs. Advances in astronomical instrumentation and recent space missions have opened the sub-Earth frontier for exploration: the Kepler mission has discovered dozens of confirmed or candidate sub-Earths transiting their host stars. It can detect Mars-size planets around its smallest stellar targets, as well as exomoons of comparable size. Although the application of the Doppler method is currently limited by instrument stability, future spectrographs may detect equivalent planets orbiting close to nearby bright stars. Future space-based microlensing missions should be able to probe the sub-Earth population on much wider orbits. A census of sub-Earths will complete the reconnaissance of the exoplanet mass spectrum and test predictions of planet formation models, including whether low-mass M dwarf stars preferentially host the smallest planets. The properties of sub-Earths may reflect their low gravity, diverse origins, and environment, but they will be elusive: Observations of eclipsing systems by the James Webb Space Telescope may give us our first clues to the properties of these small worlds.     

From Disks to Planets: The Making of Planets and Their Early Atmospheres. An Introduction

This paper is an introduction to volume 56 of the Space Science Series of ISSI, “From disks to planets—the making of planets and their proto-atmospheres”, a key subject in our quest for the origins and evolutionary paths of planets, and for the causes of their diversity. Indeed, as exoplanet discoveries progressively accumulated and their characterization made spectacular progress, it became evident that the diversity of observed exoplanets can in no way be reduced to the two classes of planets that we are used to identify in the solar system, namely terrestrial planets and gas or ice giants: the exoplanet reality is just much broader. This fact is no doubt the result of the exceptional diversity of the evolutionary paths linking planetary systems as a whole as well as individual exoplanets and their proto-atmospheres to their parent circumstellar disks: this diversity and its causes are exactly what this paper explores. For each of the main phases of the formation and evolution of planetary systems and of individual planets, we summarize what we believe we understand and what are the important open questions needing further in-depth examination, and offer some suggestions on ways towards solutions.We start with the formation mechanisms of circumstellar disks, with their gas and disk components in which chemical composition plays a very important role in planet formation. We summarize how dust accretion within the disk generates planet cores, while gas accretion on these cores can lead to the diversity of their fluid envelopes. The temporal evolution of the parent disk itself, and its final dissipation, put strong constraints on how and how far planetary formation can proceed. The radiation output of the central star also plays an important role in this whole story. This early phase of planet evolution, from disk formation to dissipation, is characterized by a co-evolution of the disk and its daughter planets. During this co-evolution, planets and their protoatmospheres not only grow, but they also migrate radially as a result of their interaction with the disk, thus moving progressively from their distance of formation to their final location. The formation of planetary fluid envelopes (proto-atmospheres and oceans), is an essential product of this planet formation scenario which strongly constrains their possible evolution towards habitability. We discuss the effects of the initial conditions in the disk, of the location, size and mass of the planetary core, of the disk lifetime and of the radiation output and activity of the central star, on the formation of these envelopes and on their relative extensions with respect to the planet core. Overall, a fraction of the planets retain the primary proto-atmosphere they initially accreted from the gas disk. For those which lose it in this early evolution, outgassing of volatiles from the planetary core and mantle, together with some contributions of volatiles from colliding bodies, give them a chance to form a “secondary” atmosphere, like that of our own Earth.When the disk finally dissipates, usually before 10 Million years of age, it leaves us with the combination of a planetary system and a debris disk, each with a specific radial distribution with respect to their parent star(s). Whereas the dynamics of protoplanetary disks is dominated by gas-solid dynamical coupling, debris disks are dominated by gravitational dynamics acting on diverse families of planetesimals. Solid-body collisions between them and giant impacts on young planetary surfaces generate a new population of gas and dust in those disks. Synergies between solar system and exoplanet studies are particularly fruitful and need to be stimulated even more, because they give access to different and complementary components of debris disks: whereas the different families of planetesimals can be extensively studied in the solar system, they remain unobserved in exoplanet systems. But, in those systems, long-wavelength telescopic observations of dust provide a wealth of indirect information about the unobserved population of planetesimals. Promising progress is being currently made to observe the gas component as well, using millimetre and sub-millimetre giant radio interferometers.Within planetary systems themselves, individual planets are the assembly of a solid body and a fluid envelope, including their planetary atmosphere when there is one. Their characteristics range from terrestrial planets through sub-Neptunes and Neptunes and to gas giants, each type covering most of the orbital distances probed by present-day techniques. With the continuous progress in detection and characterization techniques and the advent of major providers of new data like the Kepler mission, the architecture of these planetary systems can be studied more and more accurately in a statistically meaningful sense and compared to the one of our own solar system, which does not appear to be an exceptional case. Finally, our understanding of exoplanets atmospheres has made spectacular advances recently using the occultation spectroscopy techniques implemented on the currently operating space and ground-based observing facilities.The powerful new observing facilities planned for the near and more distant future will make it possible to address many of the most challenging current questions of the science of exoplanets and their systems. There is little doubt that, using this new generation of facilities, we will be able to reconstruct more and more accurately the complex evolutionary paths which link stellar genesis to the possible emergence of habitable worlds.     

The Interior Structure, Composition, and Evolution of Giant Planets

We discuss our current understanding of the interior structure and thermal evolution of giant planets. This includes the gas giants, such as Jupiter and Saturn, that are primarily composed of hydrogen and helium, as well as the “ice giants,” such as Uranus and Neptune, which are primarily composed of elements heavier than H/He. The effect of different hydrogen equations of state (including new first-principles computations) on Jupiter’s core mass and heavy element distribution is detailed. This variety of the hydrogen equations of state translate into an uncertainty in Jupiter’s core mass of 18M _⊕ . For Uranus and Neptune we find deep envelope metallicities up to 0.95, perhaps indicating the existence of an eroded core, as also supported by their low luminosity. We discuss the results of simple cooling models of our solar system’s planets, and show that more complex thermal evolution models may be necessary to understand their cooling history. We review how measurements of the masses and radii of the nearly 50 transiting extrasolar giant planets are changing our understanding of giant planets. In particular a fraction of these planets appear to be larger than can be accommodated by standard models of planetary contraction. We review the proposed explanations for the radii of these planets. We also discuss very young giant planets, which are being directly imaged with ground- and space-based telescopes.     

Photoemission Phenomena in the Solar System

Much of what we know about the atmospheres of the planets and other bodies in the solar system comes from detection of photons over a wide wavelength range, from X-rays to radio waves. In this chapter, we present current information in various categories—measurements of the airglows of the terrestrial planets, the dayglows of the outer planets and satellites, aurora throughout the solar system, observations of cometary spectra, and the emission of X-rays from a variety of planetary bodies.     

Dynamo Models for Planets Other Than Earth

Observations from planetary spacecraft missions have demonstrated a spectrum of dynamo behaviour in planets. From currently active dynamos, to remanent crustal fields from past dynamo action, to no observed magnetization, the planets and moons in our solar system offer magnetic clues to their interior structure and evolution. Here we review numerical dynamo simulations for planets other than Earth. For the terrestrial planets and satellites, we discuss specific magnetic field oddities that dynamo models attempt to explain. For the giant planets, we discuss both non-magnetic and magnetic convection models and their ability to reproduce observations of surface zonal flows and magnetic field morphology. Future improvements to numerical models and new missions to collect planetary magnetic data will continue to improve our understanding of the magnetic field generation process inside planets.     

Searching for Signs of Life in the Reflected Light from Exoplanets: A Catalog of Nearby Target Stars

Which stars are the best stars to search for habitable planets and signs of life? This is a trick question, because it depends not only on the kind of circumstellar environment we think is likely to be supportive to life as we know it, but it depends also on the technique being used to do the search. For example, the Catalog of Nearby Habitable Stellar Systems was designed for SETI, a search for technological signals. Because this search strategy relies on life forms out-shining their star (at least at certain frequencies), target selection is not complicated by the need to spatially resolve the habitable planets on which these life forms presumably live. On the other hand, because the life forms being sought are technologically advanced, it seems reasonable to assume that their planet had to be continuously habitable for long enough to evolve such biological complexity. Thus the deciding factor for SETI is that of long term habitability. Meanwhile, other missions to directly detect habitable planets (e.g., NASA’s TPF and ESA’s Darwin) are less worried about long term habitability but must struggle with the competing factors of planet separation from the star and planet brightness relative to the star. This paper outlines a variety of challenges in the search for simple and complex life in the Solar Neighborhood.     

Water, Life, and Planetary Geodynamical Evolution

In our search for life on other planets over the past decades, we have come to understand that the solid terrestrial planets provide much more than merely a substrate on which life may develop. Large-scale exchange of heat and volatile species between planetary interiors and hydrospheres/atmospheres, as well as the presence of a magnetic field, are important factors contributing to the habitability of a planet. This chapter reviews these processes, their mutual interactions, and the role life plays in regulating or modulating them.     

Feedstocks of the Terrestrial Planets

The processes of planet formation in our Solar System resulted in a final product of a small number of discreet planets and planetesimals characterized by clear compositional distinctions. A key advance on this subject was provided when nucleosynthetic isotopic variability was discovered between different meteorite groups and the terrestrial planets. This information has now been coupled with theoretical models of planetesimal growth and giant planet migration to better understand the nature of the materials accumulated into the terrestrial planets. First order conclusions include that carbonaceous chondrites appear to contribute a much smaller mass fraction to the terrestrial planets than previously suspected, that gas-driven giant planet migration could have pushed volatile-rich material into the inner Solar System, and that planetesimal formation was occurring on a sufficiently rapid time scale that global melting of asteroid-sized objects was instigated by radioactive decay of ²⁶Al. The isotopic evidence highlights the important role of enstatite chondrites, or something with their mix of nucleosynthetic components, as feedstock for the terrestrial planets. A common degree of depletion of moderately volatile elements in the terrestrial planets points to a mechanism that can effectively separate volatile and refractory elements over a spatial scale the size of the whole inner Solar System. The large variability in iron to silicon ratios between both different meteorite groups and between the terrestrial planets suggests that mechanisms that can segregate iron metal from silicate should be given greater importance in future investigations. Such processes likely include both density separation of small grains in the nebula, but also preferential impact erosion of either the mantle or core from differentiated planets/planetesimals. The latter highlights the important role for giant impacts and collisional erosion during the late stages of planet formation.     

Environments in the Outer Solar System

The outer planets of our solar system Jupiter, Saturn, Uranus, and Neptune are fascinating objects on their own. Their intrinsic magnetic fields form magnetic environments (so called magnetospheres) in which charged and neutral particles and dust are produced, lost or being transported through the system. These magnetic environments of the gas giants can be envisaged as huge plasma laboratories in space in which electromagnetic waves, current systems, particle transport mechanisms, acceleration processes and other phenomena act and interact with the large number of moons in orbit around those massive planets. In general it is necessary to describe and study the global environments (magnetospheres) of the gas giants, its global configuration with its large-scale transport processes; and, in combination, to study the local environments of the moons as well, e.g. the interaction processes between the magnetospheric plasma and the exosphere/atmosphere/magnetosphere of the moon acting on time scales of seconds to days. These local exchange processes include also the gravity, shape, rotation, astrometric observations and orbital parameters of the icy moons in those huge systems. It is the purpose of this chapter of the book to describe the variety of the magnetic environments of the outer planets in a broad overview, globally and locally, and to show that those exchange processes can dramatically influence the surfaces and exospheres/atmospheres of the moons and they can also be used as a tool to study the overall physics of systems as a whole.     

Large-Scale Structure and Dynamics of the Magnetotails of Mercury,Earth, Jupiter and Saturn

Spacecraft observations have established that all known planets with an internal magnetic field, as part of their interaction with the solar wind, possess well-developed magnetic tails, stretching vast distances on the nightside of the planets. In this review paper we focus on the magnetotails of Mercury, Earth, Jupiter and Saturn, four planets which possess well-developed tails and which have been visited by several spacecraft over the years. The fundamental physical processes of reconnection, convection, and charged particle acceleration are common to the magnetic tails of Mercury, Earth, Jupiter and Saturn. The great differences in solar wind conditions, planetary rotation rates, internal plasma sources, ionospheric properties, and physical dimensions from Mercury’s small magnetosphere to the giant magnetospheres of Jupiter and Saturn provide an outstanding opportunity to extend our understanding of the influence of such factors on basic processes. In this review article, we study the four planetary environments of Mercury, Earth, Jupiter and Saturn, comparing their common features and contrasting their unique dynamics.     

Origin of Comet Nuclei and Dynamics

We present a review of the main physical features of comet nuclei, their birthplaces and the dynamical processes that allow some of them to reach the Sun’s neighborhood and become potentially detectable. Comets are thought to be the most primitive bodies of the solar system although some processing—for instance, melting water ice in their interiors and collisional fragmentation and reaccumulation—could have occurred after formation to alter their primordial nature. Their estimated low densities (a few tenths g?cm^?3) point to a very fluffy, porous structure, while their composition rich in water ice and other highly volatile ices point to a formation in the region of the Jovian planets, or the trans-neptunian region. The main reservoir of long-period comets is the Oort cloud, whose visible radius is ～3.3×10⁴ AU. Yet, the existence of a dense inner core cannot be ruled out, a possibility that would have been greatly favored if the solar system formed in a dense galactic environment. The trans-neptunian object Sedna might be the first discovered member that belongs to such a core. The trans-neptunian population is the main source of Jupiter family comets, and may be responsible for a large renovation of the Oort cloud population.     

Formation of the Outer Planets

Models of the origins of gas giant planets and ‘ice’ giant planets are discussed and related to formation theories of both smaller objects (terrestrial planets) and larger bodies (stars). The most detailed models of planetary formation are based upon observations of our own Solar System, of young stars and their environments, and of extrasolar planets. Stars form from the collapse, and sometimes fragmentation, of molecular cloud cores. Terrestrial planets are formed within disks around young stars via the accumulation of small dust grains into larger and larger bodies until the planetary orbits become well enough separated that the configuration is stable for the lifetime of the system. Uranus and Neptune almost certainly formed via a bottom-up (terrestrial planet-like) mechanism; such a mechanism is also the most likely origin scenario for Saturn and Jupiter.     

Infrared Spectroscopy of Solar-System Planets

Most of our knowledge regarding planetary atmospheric composition and structure has been achieved by remote sensing spectroscopy. Planetary spectra strongly differ from one planet to another. CO₂ signatures dominate on Mars, and even more on Venus (where the thermal component is detectable down to 1 μm on the dark side). Spectroscopic monitoring of Venus, Earth and Mars allows us to map temperature fields, wind fields, clouds, aerosols, surface mineralogy (in the case of the Earth and Mars), and to study the planets’ seasonal cycles. Spectra of giant planets are dominated by H₂, CH₄ and other hydrocarbons, NH₃, PH₃ and traces of other minor compounds like CO, H₂O and CO₂. Measurements of the atmospheric composition of giant planets have been used to constrain their formation scenario.     

The New Horizons Pluto Kuiper Belt Mission: An Overview with Historical Context

NASA’s New Horizons (NH) Pluto–Kuiper Belt (PKB) mission was selected for development on 29 November 2001 following a competitive selection resulting from a NASA mission Announcement of Opportunity. New Horizons is the first mission to the Pluto system and the Kuiper belt, and will complete the reconnaissance of the classical planets. New Horizons was launched on 19 January 2006 on a Jupiter Gravity Assist (JGA) trajectory toward the Pluto system, for a 14 July 2015 closest approach to Pluto; Jupiter closest approach occurred on 28 February 2007. The ～400 kg spacecraft carries seven scientific instruments, including imagers, spectrometers, radio science, a plasma and particles suite, and a dust counter built by university students. NH will study the Pluto system over an 8-month period beginning in early 2015. Following its exploration of the Pluto system, NH will go on to reconnoiter one or two 30–50 kilometer diameter Kuiper Belt Objects (KBOs) if the spacecraft is in good health and NASA approves an extended mission. New Horizons has already demonstrated the ability of Principal Investigator (PI) led missions to use nuclear power sources and to be launched to the outer solar system. As well, the mission has demonstrated the ability of non-traditional entities, like the Johns Hopkins Applied Physics Laboratory (JHU/APL) and the Southwest Research Institute (SwRI) to explore the outer solar system, giving NASA new programmatic flexibility and enhancing the competitive options when selecting outer planet missions. If successful, NH will represent a watershed development in the scientific exploration of a new class of bodies in the solar system—dwarf planets, of worlds with exotic volatiles on their surfaces, of rapidly (possibly hydrodynamically) escaping atmospheres, and of giant impact derived satellite systems. It will also provide other valuable contributions to planetary science, including: the first dust density measurements beyond 18 AU, cratering records that shed light on both the ancient and present-day KBO impactor population down to tens of meters, and a key comparator to the puzzlingly active, former dwarf planet (now satellite of Neptune) called Triton which is in the same size class as the small planets Eris and Pluto.     

察言观色看透小“心机”

察颜观色,通过对方一个不经意的小表情、小动作破解其深藏的内心奥秘,是行走于百态人问人们的一个普遍心愿。世界太复杂,准不想拥有一双火眼金睛,探知动作、表情背后潜藏的危机呢?美国电视剧《Lie to Me》(别对我说谎)的热播,更是点燃了人们破解微动作、微表情的热情火药。一时间各路"神仙"纷纷著书立说,鱼龙混杂,那么究竟人们的表情、动作能否透露内心深藏的秘密?中国"应激心理微反应"学科带头人姜振宇,在《姜振宇教你察言观色》一书中破除了诸多不靠谱的"读心"神话,带领读者从科学的角度出发,一步步破解人类的丛林基因密码,帮助读者分析     

New Horizons: Anticipated Scientific Investigations at the Pluto System

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25–2.50 micron spectral images for studying surface compositions, and measurements of Pluto’s atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to achieve the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth–moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N₂:CH₄ atmospheres (such as Titan, Triton, and the early Earth).     

A gentle process for the formation of Algols

This work is concerned with binary systems that we call ‘moderately close’. These are systems in which the primary (by which we mean the initially more massive star) fills its Roche lobe when it is on the giant branch with a deep convective envelope but before helium ignition (late case B). We find that if the mass ratio q(= M ₁/M ₂) < q _crit = 0.7 when the primary fills its Roche lobe positive feedback will lead to a rapid hydrodynamic phase of mass transfer which will probably lead to common envelope evolution and thence to either coalescence or possibly to a close binary in a planetary nebula. Although most Algols have probably filled their Roche lobes before evolving off the main-sequence we find that some could not have and are therefore ‘moderately close’. Since rapid overflow is unlikely to lead to an Algol-like system there must be some way of avoiding it. The most likely possibility is that the primary can lose sufficient mass to reduce q below q _crit before overflow begins. Ordinary mass loss rates are insufficient but evidence that enhanced mass loss does take place is provided by RS CVn systems that have inverted mass ratios but have not yet begun mass transfer. We postulate that the cause of enhanced mass loss lies in the heating of the corona by by magnetic fields maintained by an α — ω dynamo which is enhanced by tidal effects associated with corotation. In order to model the the effects of enhanced mass loss we ignore the details and adopt an empirical approach calibrating a simple formula with the RS CVn system Z Her. Using further empirical relations (deduced from detailed stellar models) that describe the evolution of red giants we have investigated the effect on a large number of systems of various initial mass ratios and periods. These are notable in that some systems can now enter a much gentler Algol-like overflow phase and others are prevented from transferring mass altogether. We have also investigated the effects of enhanced angular momentum loss induced by corotation of the wind in the strong magnetic fields and consider this in relation to observed period changes. We find that a typical ‘moderately close’ Algol-like system evolves through an RS CVn like system and then possibly a symbiotic state before becoming an Algol and then goes on through a red giant-white dwarf state which may become symbiotic before ending up as a double white dwarf system in either a close or wide orbit depending on how much mass is lost before the secondary fills its Roche lobe.     

Ground-Based and Space-Based Radio Observations of Planetary Lightning

We review radio detection of planetary lightning performed by Voyager, Galileo (including in-situ probe measurements), Cassini, and other spacecraft, and compare the information on the underlying physics derived from these observations—especially the discharge duration, at Jupiter and Saturn—with our knowledge of terrestrial lightning. The controversial evidence at Venus is discussed, as well as the prospects for lightning-like discharges in Martian dust-storms (and studies on terrestrial analogues). In addition, lightning sources provide radio beacons that allow us to probe planetary ionospheres. Ground-based observations of Saturn’s lightning have been attempted several times in the past and have been recently successful. They will be the subject of observations by the new generation of giant radio arrays. We review past results and future studies, focussing on the detection challenges and on the interest of ground-based radio monitoring, in conjunction with spacecraft observations or in standalone mode.     

Low-Mass Pre–Main-Sequence Stars in the Magellanic Clouds

The stellar Initial Mass Function (IMF) suggests that stars with sub-solar mass form in very large numbers. Most attractive places for catching low-mass star formation in the act are young stellar clusters and associations, still (half-)embedded in star-forming regions. The low-mass stars in such regions are still in their pre–main-sequence (PMS) evolutionary phase, i.e., they have not started their lives on the main-sequence yet. The peculiar nature of these objects and the contamination of their samples by the fore- and background evolved populations of the Galactic disk impose demanding observational techniques, such as X-ray surveying and optical spectroscopy of large samples for the detection of complete numbers of PMS stars in the Milky Way. The Magellanic Clouds, the metal-poor companion galaxies to our own, demonstrate an exceptional star formation activity. The low extinction and stellar field contamination in star-forming regions of these galaxies imply a more efficient detection of low-mass PMS stars than in the Milky Way, but their distance from us make the application of the above techniques unfeasible. Nonetheless, imaging with the Hubble Space Telescope within the last five years yield the discovery of solar and sub-solar PMS stars in the Magellanic Clouds from photometry alone. Unprecedented numbers of such objects are identified as the low-mass stellar content of star-forming regions in these galaxies, changing completely our picture of young stellar systems outside the Milky Way, and extending the extragalactic stellar IMF below the persisting threshold of a few solar masses. This review presents the recent developments in the investigation of the PMS stellar content of the Magellanic Clouds, with special focus on the limitations by single-epoch photometry that can only be circumvented by the detailed study of the observable behavior of these stars in the color-magnitude diagram. The achieved characterization of the low-mass PMS stars in the Magellanic Clouds allowed thus a more comprehensive understanding of the star formation process in our neighboring galaxies.     

Neutral Atmospheres

This paper summarizes the understanding of aeronomy of neutral atmospheres in the solar system, discussing most planets as well as Saturn’s moon Titan and comets. The thermal structure and energy balance is compared, highlighting the principal reasons for discrepancies amongst the atmospheres, a combination of atmospheric composition, heliocentric distance and other external energy sources not common to all. The composition of atmospheres is discussed in terms of vertical structure, chemistry and evolution. The final section compares dynamics in the upper atmospheres of most planets and highlights the importance of vertical dynamical coupling as well as magnetospheric forcing in auroral regions, where present. It is shown that a first order understanding of neutral atmospheres has emerged over the past decades, thanks to the combined effects of spacecraft and Earth-based observations as well as advances in theoretical modeling capabilities. Key gaps in our understanding are highlighted which ultimately call for a more comprehensive programme of observation and laboratory measurements.