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Planetary Aurorae  

Steve Miller

Planetary aurorae are some of the most iconic and brilliant (in all senses of that word) indicators not only of the interconnections on Planet Earth, but that these interconnections pertain throughout the entire Solar System as well. They are testimony to the centrality of the Sun, not just in providing the essential sunlight that drives weather systems and makes habitability possible, but also in generating a high velocity wind of electrically charged particles—known as the Solar Wind—that buffets each of the planets in turn as it streams outward through interplanetary space. Aurorae are created when electrically charged particles—predominantly negatively charged electrons or positive ions such as protons, the nuclei of hydrogen—crash into the atoms and molecules of a planetary or lunar atmosphere. Such particles can excite the electrons in atoms and molecules from their ground state to higher levels. The atoms and molecules that have been excited by these high-energy collisions can then relax; the emitted radiation is at certain well-defined wavelengths, giving characteristic colors to the aurorae. Just how many particles, how much atmosphere, and what strength of magnetic field are required to create aurorae is an open question. But giant planets like Jupiter and Saturn have aurorae, as does Earth. Some moons also show these emissions. Overall, the aurorae of the Solar System are very varied, variable, and exciting.

Article

Detection and Characterization Methods of Exoplanets  

Nuno C. Santos, Susana C.C. Barros, Olivier D.S. Demangeon, and João P. Faria

Is the Solar System unique, or are planets ubiquitous in the universe? The answer to this long-standing question implies the understanding of planet formation, but perhaps more relevant, the observational assessment of the existence of other worlds and their frequency in the galaxy. The detection of planets orbiting other suns has always been a challenging task. Fortunately, technological progress together with significant development in data reduction and analysis processes allowed astronomers to finally succeed. The methods used so far are mostly based on indirect approaches, able to detect the influence of the planets on the stellar motion (dynamical methods) or the planet’s shadow as it crosses the stellar disk (transit method). For a growing number of favorable cases, direct imaging has also been successful. The combination of different methods also allowed probing planet interiors, composition, temperature, atmospheres, and orbital architecture. Overall, one can confidently state that planets are common around solar-type stars, low mass planets being the most frequent among them. Despite all the progress, the discovery and characterization of temperate Earth-like worlds, similar to the Earth in both mass and composition and thus potential islands of life in the universe, is still a challenging task. Their low amplitude signals are difficult to detect and are often submerged by the noise produced by different instrumentation sources and astrophysical processes. However, the dawn of a new generation of ground and space-based instruments and missions is promising a new era in this domain.

Article

Exoplanets: Atmospheres of Hot Jupiters  

Dmitry V. Bisikalo, Pavel V. Kaygorodov, and Valery I. Shematovich

The history of exoplanetary atmospheres studies is strongly based on the observations and investigations of the gaseous envelopes of hot Jupiters—exoplanet gas giants that have masses comparable to the mass of Jupiter and orbital semi-major axes shorter than 0.1 AU. The first exoplanet around a solar-type star was a hot Jupiter discovered in 1995. Researchers found an object that had completely atypical parameters compared to planets known in the solar system. According to their estimates, the object might have a mass about a half of the Jovian mass and a very short orbital period (four days), which means that it has an orbit roughly corresponding to the orbit of Mercury. Later, many similar objects were discovered near different stars, and they acquired a common name—hot Jupiters. It is still unclear what the mechanism is for their origin, because generally accepted theories of planetary evolution predict the formation of giant planets only at large orbital distances, where they can accrete enough matter before the protoplanetary disc disappears. If this is true, before arriving at such low orbits, hot Jupiters might have a long migration path, caused by interactions with other massive planets and/or with the gaseous disc. In favor of this model is the discovery of many hot Jupiters in elliptical and highly inclined orbits, but on the other hand several observed hot Jupiters have circular orbits with low inclination. An alternative hypothesis is that the cores of future hot Jupiters are super-Earths that may later intercept matter from the protoplanetary disk falling on the star. The scientific interest in hot Jupiters has two aspects. The first is the peculiarity of these objects: they have no analogues in the solar system. The second is that, until recently, only for hot Jupiters was it possible to obtain observational characteristics of their atmospheres. Many of the known hot Jupiters are eclipsing their host stars, so, from their light curve and spectral data obtained during an eclipse, it became possible to obtain information about their shape and their atmospheric composition. Thus it is possible to conclude that hot Jupiters are a common type of exoplanet, having no analogues in the solar system. Many aspects of their evolution and internal structure remain unclear. Being very close to their host stars, hot Jupiters must interact with the stellar wind and stellar magnetic field, as well as with stellar flares and coronal mass ejections, allowing researchers to gather information about them. According to UV observations, at least a fraction of hot Jupiters have extended gaseous envelopes, extending far beyond of their upper atmospheres. The envelopes are observable with current astronomical instruments, so it is possible to develop their astrophysical models. The history of hot Jupiter atmosphere studies during the past 20 years and the current status of modern theories describing the extended envelopes of hot Jupiters are excellent examples of the progress in understanding planetary atmospheres formation and evolution both in the solar system and in the extrasolar planetary systems.

Article

The Formation and Evolution of the Solar System  

Mikhail Marov

The formation and evolution of our solar system (and planetary systems around other stars) are among the most challenging and intriguing fields of modern science. As the product of a long history of cosmic matter evolution, this important branch of astrophysics is referred to as stellar-planetary cosmogony. Interdisciplinary by way of its content, it is based on fundamental theoretical concepts and available observational data on the processes of star formation. Modern observational data on stellar evolution, disc formation, and the discovery of extrasolar planets, as well as mechanical and cosmochemical properties of the solar system, place important constraints on the different scenarios developed, each supporting the basic cosmogony concept (as rooted in the Kant-Laplace hypothesis). Basically, the sequence of events includes fragmentation of an original interstellar molecular cloud, emergence of a primordial nebula, and accretion of a protoplanetary gas-dust disk around a parent star, followed by disk instability and break-up into primary solid bodies (planetesimals) and their collisional interactions, eventually forming a planet. Recent decades have seen major advances in the field, due to in-depth theoretical and experimental studies. Such advances have clarified a new scenario, which largely supports simultaneous stellar-planetary formation. Here, the collapse of a protosolar nebula’s inner core gives rise to fusion ignition and star birth with an accretion disc left behind: its continuing evolution resulting ultimately in protoplanets and planetary formation. Astronomical observations have allowed us to resolve in great detail the turbulent structure of gas-dust disks and their dynamics in regard to solar system origin. Indeed radio isotope dating of chondrite meteorite samples has charted the age and the chronology of key processes in the formation of the solar system. Significant progress also has been made in the theoretical study and computer modeling of protoplanetary accretion disk thermal regimes; evaporation/condensation of primordial particles depending on their radial distance, mechanisms of clustering, collisions, and dynamics. However, these breakthroughs are yet insufficient to resolve many problems intrinsically related to planetary cosmogony. Significant new questions also have been posed, which require answers. Of great importance are questions on how contemporary natural conditions appeared on solar system planets: specifically, why the three neighbor inner planets—Earth, Venus, and Mars—reveal different evolutionary paths.

Article

Planet Formation Through Gravitational Instabilities  

Ken Rice

It is now widely accepted that planets form in discs around young stars, with the most widely accepted planet formation scenario being a bottom-up process typically referred to as “core accretion.” The basic process involves a core growing through the accumulation of solids and, if it gets massive enough while there is still gas present in the disc, undergoing a runaway gas accretion phase to form a Jupiter-like gas giant. However, early models of this process suggested that the formation timescale for a Jupiter-like gas giant exceeded the lifetime of the gas disc, suggesting that massive, gas giant planets form via some alternative process. One possibility is that they form via direct gravitational collapse. During the earliest stages of star formation, the disc around a young star can have a mass that is comparable to that of the central protostar and can be susceptible to the growth of a gravitational instability. One outcome of such an instability is that the disc fragments into bound objects that can then contract to become gas giant planets. This would happen very early in the star formation process and is very rapid, overcoming the timescale problem. Subsequent work has, however, both illustrated that core accretion may operate on timescales shorter than disc lifetimes and that disc fragmentation is very unlikely to operate in the inner parts of planet-forming discs. Hence, it is very unlikely that disc fragmentation plays a role in the direct formation of close-in exoplanets. However, disc fragmentation may operate at large orbital radii and is expected to preferentially form either massive gas giant planets or brown dwarfs. Therefore, it is intriguing that exactly such objects are starting to be directly imaged at orbital radii where disc fragmentation may operate. Additionally, even if a self-gravitating phase doesn’t play a direct role in the formation of gas giant planets, it may play an indirect role in the planet formation process. The spiral density waves that develop due to the gravitational instability can act to enhance the local density of solids, potentially accelerating their collisional growth or leading to the direct gravitational collapse of the solid component of the disc. This could then provide some of the building blocks for planets that later form via core accretion.

Article

Tidal Interactions Between Planets and Host Stars  

Gordon Ogilvie

Hundreds of planets are already known to have orbits only a few times wider than the stars that host them. The tidal interaction between a planet and its host star is one of the main agents shaping the observed distributions of properties of these systems. Tidal dissipation in the planet tends make the orbit circular, as well as synchronizing and aligning the planet’s spin with the orbit, and can significantly heat the planet, potentially affecting its size and structure. Dissipation in the star typically leads to inward orbital migration of the planet, accelerating the star’s rotation, and in some cases destroying the planet. Some essential features of tidal evolution can be understood from the basic principles that angular momentum and energy are exchanged between spin and orbit by means of a gravitational field and that energy is dissipated. For example, most short-period exoplanetary systems have too little angular momentum to reach a tidal equilibrium state. Theoretical studies aim to explain tidal dissipation quantitatively by solving the equations of fluid and solid mechanics in stars and planets undergoing periodic tidal forcing. The equilibrium tide is a nearly hydrostatic bulge that is carried around the body by a large-scale flow, which can be damped by convection or hydrodynamic instability, or by viscoelastic dissipation in solid regions of planets. The dynamical tide is an additional component that generally takes the form of internal waves restored by Coriolis and buoyancy forces in a rotating and stratified fluid body. It can lead to significant dissipation if the waves are amplified by resonance, are efficiently damped when they attain a very short wavelength, or break because they exceed a critical amplitude. Thermal tides are excited in a planetary atmosphere by the variable heating by the star’s radiation. They can oppose gravitational tides and prevent tidal locking, with consequences for the climate and habitability of the planet. Ongoing observations of transiting exoplanets provide information on the orbital periods and eccentricities as well as the obliquity (spin–orbit misalignment) of the star and the size of the planet. These data reveal several tidal processes at work and provide constraints on the efficiency of tidal dissipation in a variety of stars and planets.