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Article

Probing the interiors of the gaseous giant planets in our solar system is not an easy task. It requires a set of accurate measurements combined with theoretical models that are used to infer the planetary composition and its depth dependence. The masses of Jupiter and Saturn are 317.83 and 95.16 Earth masses (M ⊕ ), respectively, and since a few decades, it has been known that they mostly consist of hydrogen and helium. The mass of heavy elements (all elements heavier than helium) is not well determined, nor are their distribution within the planets. While the heavy elements are not the dominating materials inside Jupiter and Saturn, they are the key to understanding the planets’ formation and evolutionary histories. The planetary internal structure is inferred from theoretical models that fit the available observational constraints by using theoretical equations of states (EOSs) for hydrogen, helium, their mixtures, and heavier elements (typically rocks and/or ices). However, there is no unique solution for determining the planetary structure and the results depend on the used EOSs as well as the model assumptions imposed by the modeler. Major model assumptions that can affect the derived internal structure include the number of layers, the heat transport mechanism within the planet (and its entropy), the nature of the core (compact vs. diluted), and the location (pressure) of separation between the two envelopes. Alternative structure models assume a less distinct division between the layers and /or a non-homogenous distribution of the heavy elements. The fact that the behavior of hydrogen at high pressures and temperatures is not perfectly known and that helium may separate from hydrogen at the deep interior add sources of uncertainty to structure models. In the 21st century, with accurate measurements of the gravitational fields of Jupiter and Saturn from the Juno and Cassini missions, structure models can be further constrained. At the same time, these measurements introduce new challenges for planetary modelers.

Article

In addition to ground-based observations beginning in the 1970s, NASA’s Voyager 2 spacecraft flew by Triton in 1989, and NASA’s New Horizons spacecraft flew by Pluto in 2015. Prior to the flyby of New Horizons, Pluto and Triton were termed “sister worlds” due to what appeared to be a high degree of similarity in solid-body density, surface ices, diameter, and surface pressures. Despite being small, cold, icy bodies, both Pluto and Triton have been found to have atmospheres that behave as a continuous fluid up to 300 km altitude above the surface and thereby have a defined temperature, surface pressure, and global general circulation (wind). The primary constituent of these atmospheres is molecular nitrogen, with methane and carbon monoxide comprising the largest abundances of trace gases. The surface pressure as measured in the 2010s on both worlds is of the order of 10 microbars (1 Pa = 10 µbar), for these exotic atmospheres exchange mass between sublimation of surface ice and deposition of nitrogen over the course of each body’s year. Ground-based stellar occultation measurements observed a dramatic change in surface pressure, which one study found was as much as a factor of two increase between 1988 and 2003 on Pluto, presumably due to Pluto’s seasonal volatile cycle. Voyager 2 observed plumes and surface “streaks” on Triton, while New Horizons observed dunes (indicating wind speeds of 1–10 m s−1) as well as streaks, evidently indicating the presence of surface and near-surface winds. While wind velocity aloft has not been directly measured on Pluto or Triton, 3-D general circulation modeling studies of both worlds have shown zonal (east–west) wind speeds of the order of 10 m/s, meridional (north–south) wind speeds of the order of 1 m/s, and extremely weak vertical wind speeds. In 2015, New Horizons showed that Pluto and Triton were much more different than previously thought. New Horizons uncovered many spectacular views of Pluto’s atmosphere. First, while hydrocarbon haze was observed on Triton, Pluto had multiple, very distinct stratified haze layers bearing a similar appearance to the layers of an onion. Second, Pluto’s surface elevation was found to be largely inhomogeneous (in contrast to Triton) in the form of a large depression (Sputnik Planitia). Third, the characteristics of the surface markings on Pluto were found to be different than the streaks observed on Triton, which has implications for surface wind patterns. Further major discoveries made by New Horizons included evidence for many hydrocarbon species in trace concentrations, a lower than expected surface pressure, which could previously only be indirectly ascertained from ground-based observations, and a higher mixing ratio of methane at higher altitudes than at lower due to gravitational diffusive separation. Using radio occultation experiments (not conducted by Voyager 2 at Triton), New Horizons confirmed the existence of a stratosphere (temperature increasing with height) extending to 25 km altitude at both the entry and exit locations. The entry location had a shallow troposphere (temperature decreasing with height) extending to 3.5 km altitude above the surface, while the exit location did not.

Article

Kevin Righter

Asteroids 1 Ceres and 4 Vesta are the two most massive asteroids in the asteroid belt, with mean diameters of 946 km and 525 km, respectively. Ceres was reclassified as a dwarf planet by the International Astronomical Union as a result of its new dwarf planet definition which is a body that (a) orbits the sun, (b) has enough mass to assume a nearly round shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a moon. Scientists’ understanding of these two bodies has been revolutionized in the past decade by the success of the Dawn mission that visited both bodies. Vesta is an example of a small body that has been heated substantially and differentiated into a metallic core, silicate mantle, and basaltic crust. Ceres is a volatile-rich rocky body that experienced less heating than Vesta and has differentiated into rock and ice. These two contrasting bodies have been instrumental in learning how inner solar system material formed and evolved.

Article

The great diversity of extrasolar planetary systems has challenged our understanding of how planets form. During the formation process their orbits are modified while the protoplanetary disk is present. After its dispersal orbits may also be modified as a result of mutual gravitational interactions leading to their currently observed configurations in the longer term. A number of potentially significant phenomena have been identified. These include radial migration of solids in the protoplanetary disk, radial migration of protoplanetary cores produced by disk-planet interaction and how it can be halted by protoplanet traps, formation of resonant systems and subsystems, and gravitational interactions among planets or between a planet and an external stellar companion. These interactions may cause excitation of orbital inclinations and eccentricities which in the latter case may attain values close to unity. When the eccentricity approaches unity, tidal interaction with the central star could lead to orbital circularization and a close orbiting Hot Jupiter, providing a competitive process to direct migration through the disk or in-situ formation. Long-term dynamical instability may also account for the relatively small number of observed compact systems of super-Earths and Neptune class planets that have attained and subsequently maintained linked commensurabilities in the long term.

Article

Elvira Mulyukova and David Bercovici

All the rocky planets in our solar system, including the Earth, initially formed much hotter than their surroundings and have since been cooling to space for billions of years. The resulting heat released from planetary interiors powers convective flow in the mantle. The mantle is often the most voluminous and/or stiffest part of a planet and therefore acts as the bottleneck for heat transport, thus dictating the rate at which a planet cools. Mantle flow drives geological activity that modifies planetary surfaces through processes such as volcanism, orogenesis, and rifting. On Earth, the major convective currents in the mantle are identified as hot upwellings such as mantle plumes, cold sinking slabs, and the motion of tectonic plates at the surface. On other terrestrial planets in our solar system, mantle flow is mostly concealed beneath a rocky surface that remains stagnant for relatively long periods. Even though such planetary surfaces do not participate in convective circulation, they deform in response to the underlying mantle currents, forming geological features such as coronae, volcanic lava flows, and wrinkle ridges. Moreover, the exchange of material between the interior and surface, for example through melting and volcanism, is a consequence of mantle circulation and continuously modifies the composition of the mantle and the overlying crust. Mantle convection governs the geological activity and the thermal and chemical evolution of terrestrial planets and understanding the physical processes of convection helps us reconstruct histories of planets over billions of years after their formation.

Article

Morris Podolak

Modern observational techniques are still not powerful enough to directly view planet formation, and so it is necessary to rely on theory. However, observations do give two important clues to the formation process. The first is that the most primitive form of material in interstellar space exists as a dilute gas. Some of this gas is unstable against gravitational collapse, and begins to contract. Because the angular momentum of the gas is not zero, it contracts along the spin axis, but remains extended in the plane perpendicular to that axis, so that a disk is formed. Viscous processes in the disk carry most of the mass into the center where a star eventually forms. In the process, almost as a by-product, a planetary system is formed as well. The second clue is the time required. Young stars are indeed observed to have gas disks, composed mostly of hydrogen and helium, surrounding them, and observations tell us that these disks dissipate after about 5 to 10 million years. If planets like Jupiter and Saturn, which are very rich in hydrogen and helium, are to form in such a disk, they must accrete their gas within 5 million years of the time of the formation of the disk. Any formation scenario one proposes must produce Jupiter in that time, although the terrestrial planets, which don’t contain significant amounts of hydrogen and helium, could have taken longer to build. Modern estimates for the formation time of the Earth are of the order of 100 million years. To date there are two main candidate theories for producing Jupiter-like planets. The core accretion (CA) scenario supposes that any solid materials in the disk slowly coagulate into protoplanetary cores with progressively larger masses. If the core remains small enough it won’t have a strong enough gravitational force to attract gas from the surrounding disk, and the result will be a terrestrial planet. If the core grows large enough (of the order of ten Earth masses), and the disk has not yet dissipated, then the planetary embryo can attract gas from the surrounding disk and grow to be a gas giant. If the disk dissipates before the process is complete, the result will be an object like Uranus or Neptune, which has a small, but significant, complement of hydrogen and helium. The main question is whether the protoplanetary core can grow large enough before the disk dissipates. A second scenario is the disk instability (DI) scenario. This scenario posits that the disk itself is unstable and tends to develop regions of higher than normal density. Such regions collapse under their own gravity to form Jupiter-mass protoplanets. In the DI scenario a Jupiter-mass clump of gas can form—in several hundred years which will eventually contract into a gas giant planet. The difficulty here is to bring the disk to a condition where such instabilities will form. Now that we have discovered nearly 3000 planetary systems, there will be numerous examples against which to test these scenarios.

Article

While the moon naturally featured in Mediterranean cultures from time immemorial, principally noted in the earliest literature as a marker of time, time-dependent constructs such as the calendar, and time-related activities, awareness and recognition of the five visible planets came relatively late to the Greeks and thence to the Romans. The moon underlies the local calendars of the Greeks, with documentary and literary evidence from the Late Bronze Age through the Imperial Roman period, and there are signs that the earliest Roman calendar also paid homage to the moon in its divisions of the month. However, although Homer in the 8th century BCE knows of a Morning and an Evening Star, he shows no indication of realizing that these are one and the same, the planet Venus. That particular identification may have come in the 6th century BCE, and it appears to have been not until the 4th century BCE that the Greeks recognized the other four planets visible to the naked eye—Saturn, Jupiter, Mars, and Mercury. This awareness probably came via contact with Babylonian astronomy and astrology, where identification and observations of the planets had figured from the 2nd millennium BCE and served as a basis for astrological prognostications. But it is time, not astrology, that lies at the heart of Greek and Roman concerns with the moon and the planets. Indeed, the need to tell time accurately has been regarded as the fundamental motivation of Greek astronomy. A major cultural issue that long engaged the Greeks was how to synchronize the incommensurate cycles of the moon and the sun for calendrical purposes. Given the apparent irregularities of their cycles, the planets might seem to offer no obvious help with regard to time measurement. Nonetheless they were included by Plato in the 4th century BCE in his cosmology, along with the sun and moon, as heavenly bodies created specifically to compute time. Astrology then provided a useful framework in which the sun, moon, planets, and stars all combined to enable the interpretation and forecasting of life events. It became necessary for the Greeks, and their successors the Romans, to be able to calculate as accurately as possible the positions of the heavenly bodies in order to determine readings of the past, present, and future. Greek astronomy had always had a speculative aspect, as philosophers strove to make sense of the visible cosmos. A deep-seated assumption held by Greek astronomers, that the heavenly bodies moved in uniform, circular orbits, lead to a desire over the centuries to account for or explain away the observed irregularities of planetary motions with their stations and retrogradations. This intention “to save the phenomena,”— that is, to preserve the fundamental circularity—was said to have originated with Plato. While arithmetical schemes had sufficed in Babylonia for such calculation, it was a Greek innovation to devise increasingly complex geometric theories of circular motions (eccentrics and epicycles) in an effort to understand how the sun, moon, and planets moved, so as to place them more precisely in time and space.

Article

Alessandro Morbidelli

In planetary science, accretion is the process in which solids agglomerate to form larger and larger objects, and eventually planets are produced. The initial conditions are a disc of gas and microscopic solid particles, with a total mass of about 1% of the gas mass. These discs are routinely detected around young stars and are now imaged with the new generation of instruments. Accretion has to be effective and fast. Effective, because the original total mass in solids in the solar protoplanetary disk was probably of the order of ~300 Earth masses, and the mass incorporated into the planets is ~100 Earth masses. Fast, because the cores of the giant planets had to grow to tens of Earth masses to capture massive doses of hydrogen and helium from the disc before the dispersal of the latter, in a few millions of years. The surveys for extrasolar planets have shown that most stars have planets around them. Accretion is therefore not an oddity of the solar system. However, the final planetary systems are very different from each other, and typically very different from the solar system. Observations have shown that more than 50% of the stars have planets that don’t have analogues in the solar system. Therefore the solar system is not the typical specimen. Models of planet accretion have to explain not only how planets form, but also why the outcomes of the accretion history can be so diverse. There is probably not one accretion process but several, depending on the scale at which accretion operates. A first process is the sticking of microscopic dust into larger grains and pebbles. A second process is the formation of an intermediate class of objects called planetesimals. There are still planetesimals left in the solar system. They are the asteroids orbiting between the orbits of Mars and Jupiter, the trans-Neptunian objects in the distant system, and other objects trapped along the orbits of the planets (Trojans) or around the giant planets themselves (irregular satellites). The Oort cloud, source of the long period comets, is also made of planetesimals ejected from the region of formation of the giant planets. A third accretion process has to lead from planetesimals to planets. Actually, several processes can be involved in this step, from collisional coagulation among planetesimals to the accretion of small particles under the effect of gas drag, to giant impacts between protoplanets. Adopting a historical perspective of all these processes provides details of the classic processes investigated in the past decades to those unveiled in the last years. The quest for planet formation is ongoing. Open issues remain, and exciting future developments are expected.

Article

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

John MacDonald

Inuit are an indigenous people traditionally inhabiting the Arctic and sub-Arctic regions of Greenland, Canada, Alaska, and parts of Russia’s Chukchi Peninsula. Across this vast region, Inuit society, while not entirely homogeneous either culturally or linguistically, nevertheless shares a fundamental cosmology, in part based on a common understanding of the sky and its contents. Traditionally, Inuit used prominent celestial objects—the sun, moon, and major circumpolar asterisms—as markers for estimating the passage of time, as wayfinding and directional aids, and, importantly, as the basis of several of the foundational myths and legends underpinning their society’s social order and mores. Random inquiries on Inuit astronomy made by European visitors after initial contact through the mid-18th and early 20th centuries were characteristically haphazard and usually peripheral to some other line of ethnological enquiry, such as folklore or mythology. In addition, the early accounts of Inuit star lore were often prone to misrepresentation due to several factors, including European cultural bias, translation inadequacies, a deficiency of general astronomical knowledge on the part of most commentators, and, most significantly, a failure—sometimes due to lack of opportunity—to conduct systematic observations of the sky in the presence of Inuit knowledge holders. Early accounts therefore tended to diminish the cultural significance of Inuit astronomy, almost to the point of insignificance. Unfortunately, by the time systematic fieldwork began on the topic, in the mid-1980s, unalloyed information on Inuit astronomical knowledge was already elusive, more and more compromised by European acculturation and substitution and, notably, by light pollution—a consequence of the increasing urbanization of Inuit communities beginning in the late 1950s. For the residents of most Arctic settlements, street lights reflecting off the snow have virtually eliminated the evocative phenomenon of the “polar night.” For several reasons, the role of planets in Inuit astronomy is difficult to determine, due, in part, to the characteristics of the planets themselves. Naked-eye differentiation between the major visible planets is by no means straightforward, and for observers living north of the Arctic Circle, the continuous or semicontinuous periods of daylight/twilight obtaining throughout the late spring, summer, and early fall effectively prevent year-round viewing of the night sky, making much planetary movement unobservable, far less an appreciation of the planets’ predictable synodic and sidereal periods. Mitigating against the significant use of planets in Inuit culture is also the principle that their applied astronomy, along with its cosmology and mythologies depend principally on—apart from the sun and the moon—the predictability of the “fixed stars.” Inuit of course did see the major planets and took note of them when they moved through their familiar asterisms or appeared, irregularly, as markers of solstice, or harbingers of daylight after winter’s dark. Generally, however, planets seem to have been little regarded until after the introduction of Christianity, when, in parts of the Canadian eastern Arctic, Venus, in particular, became associated with Christmas. While there are anecdotal accounts that some of the planets, again especially Venus, may have had a place in Greenlandic mythology, this assertion is far from certain. Furthermore, reports from Alaska and Greenland suggesting that the appearance of Venus was a regular marker of the new year, or a predictor of sun’s return, need qualification, given the apparent irregularity of Venus’s appearances above the horizon. A survey of relevant literature, including oral history, pertaining either directly or peripherally to Inuit astronomical traditions, reveals few bona fide mention of planets. References to planets in Inuit mythology and astronomy are usually speculative, typically lacking supportive or corroborative information. It can therefore be reasonably inferred that, with the qualified exception of Venus, planets played little part in Inuit astronomy and cosmology despite their being, on occasion, the brightest objects in the Northern celestial sphere. This being the case, there is a certain irony in NASA’s recently bestowing Inuit mythological names on a group of Saturn’s moons—Saturn being a planet the Inuit themselves, as far as can be determined, did not note or recognize.

Article

Will Grundy

Pluto orbits the Sun at a mean distance of 39.5 AU (astronomical units; 1 AU is the mean distance between the Earth and the Sun), with an orbital period of 248 Earth years. Its orbit is just eccentric enough to cross that of Neptune. They never collide thanks to a 2:3 mean-motion resonance: Pluto completes two orbits of the Sun for every three by Neptune. The Pluto system consists of Pluto and its large satellite Charon, plus four small satellites: Styx, Nix, Kerberos, and Hydra. Pluto and Charon are spherical bodies, with diameters of 2,377 and 1,212 km, respectively. They are tidally locked to one another such that each spins about its axis with the same 6.39-day period as their mutual orbit about their common barycenter. Pluto’s surface is dominated by frozen volatiles nitrogen, methane, and carbon monoxide. Their vapor pressure supports an atmosphere with multiple layers of photochemical hazes. Pluto’s equator is marked by a belt of dark red maculae, where the photochemical haze has accumulated over time. Some regions are ancient and cratered, while others are geologically active via processes including sublimation and condensation, glaciation, and eruption of material from the subsurface. The surfaces of the satellites are dominated by water ice. Charon has dark red polar stains produced from chemistry fed by Pluto’s escaping atmosphere. The existence of a planet beyond Neptune had been postulated by Percival Lowell and William Pickering in the early 20th century to account for supposed clustering in comet aphelia and perturbations of the orbit of Uranus. Both lines of evidence turned out to be spurious, but they motivated a series of searches that culminated in Clyde Tombaugh’s discovery of Pluto in 1930 at the observatory Lowell had founded in Arizona. Over subsequent decades, basic facts about Pluto were hard-won through application of technological advances in astronomical instrumentation. During the progression from photographic plates through photoelectric photometers to digital array detectors, space-based telescopes, and ultimately, direct exploration by robotic spacecraft, each revealed more about Pluto. A key breakthrough came in 1978 with the discovery of Charon by Christy and Harrington. Charon’s orbit revealed the mass of the system. Observations of stellar occultations constrained the sizes of Pluto and Charon and enabled the detection of Pluto’s atmosphere in 1988. Spectroscopic instruments revealed Pluto’s volatile ices. In a series of mutual events from 1985 through 1990, Pluto and Charon alternated in passing in front of the other as seen from Earth. Observations of these events provided additional constraints on their sizes and albedo patterns and revealed their distinct compositions. The Hubble Space Telescope’s vantage above Earth’s atmosphere enabled further mapping of Pluto’s albedo patterns and the discovery of the small satellites. NASA’s New Horizons spacecraft flew through the system in 2015. Its instruments mapped the diversity and compositions of geological features on Pluto and Charon and provided detailed information on Pluto’s atmosphere and its interaction with the solar wind.