Posts Tagged solar system

Recent Postings from solar system

A statistical search for a population of Exo-Trojans in the Kepler dataset

Trojans are small bodies in planetary Lagrangian points. In our solar system, Jupiter has the largest number of such companions. Their existence is assumed for exoplanetary systems as well, but none has been found so far. We present an analysis by super-stacking $\sim4\times10^4$ Kepler planets with a total of $\sim9\times10^5$ transits, searching for an average trojan transit dip. Our result gives an upper limit to the average Trojan transiting area (per planet) corresponding to one body of radius $<460$km at $2\sigma$ confidence. We find a significant Trojan-like signal in a sub-sample for planets with more (or larger) Trojans for periods $>$60 days. Our tentative results can and should be checked with improved data from future missions like PLATO2.0, and can guide planetary formation theories.

Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence

Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

A Bayesian estimation of the helioseismic solar age

The helioseismic determination of the solar age has been a subject of several studies because it provides us with an independent estimation of the age of the solar system. We present the Bayesian estimates of the helioseismic age of the Sun, which are determined by means of calibrated solar models that employ different equations of state and nuclear reaction rates. We use 17 frequency separation ratios $r_{02}(n)=(\nu_{n,l=0}-\nu_{n-1,l=2})/(\nu_{n,l=1}-\nu_{n-1,l=1})$ from 8640 days of low-$\ell$ BiSON frequencies and consider three likelihood functions that depend on the handling of the errors of these $r_{02}(n)$ ratios. Moreover, we employ the 2010 CODATA recommended values for Newton’s constant, solar mass, and radius to calibrate a large grid of solar models spanning a conceivable range of solar ages. It is shown that the most constrained posterior distribution of the solar age for models employing Irwin EOS with NACRE reaction rates leads to $t_\odot = 4.587 \pm 0.007$ Gyr, while models employing the Irwin EOS and Adelberger, et al., Reviews of Modern Physics, 83, 195 (2011) reaction rate have $t_\odot = 4.569 \pm 0.006 $ Gyr. Implementing OPAL EOS in the solar models results in reduced evidence ratios (Bayes factors) and leads to an age that is not consistent with the meteoritic dating of the solar system. An estimate of the solar age that relies on an helioseismic age indicator such as $r_{02}(n)$ turns out to be essentially independent of the type of likelihood function. However, with respect to model selection, abandoning any information concerning the errors of the $r_{02}(n)$ ratios leads to inconclusive results, and this stresses the importance of evaluating the trustworthiness of error estimates.

Physical properties of the extreme centaur and super-comet candidate 2013 AZ60

We present estimates of the basic physical properties — including size and albedo — of the extreme Centaur 2013 AZ60. These properties have been derived from optical and thermal infrared measurements. Our optical measurements revealed a likely full period of ~9.4 h with a shallow amplitude of 4.5%. By combining optical brightness information and thermal emission data, we are able to derive a diameter of 62.3 +/- 5.3 km and a geometric albedo of 2.9% — corresponding to an extremely dark surface. Additionally, our finding of ~> 50 Jm^{-2}K^{-1}s^{-1/2} for the thermal inertia is also noticeably for objects in such a distance. The results of dynamical simulations yield an unstable orbit, with a 50% probability that the target will be ejected from the Solar System within 700,000 years. The current orbit of this object as well as its instability could imply a pristine cometary surface. This possibility is in agreement with the observed low geometric albedo and red photometric colour indices for the object, which are a good match for the surface of a dormant comet — as would be expected for a long-period cometary body approaching perihelion. Despite the fact it was approaching ever closer to the Sun, however, the object exhibited star-like profiles in each of our observations, lacking any sign of cometary activity. By the albedo, 2013 AZ60 is a candidate for the darkest body among the known TNOs.

From Cosmic Birth to Living Earths: The Future of UVOIR Space Astronomy

For the first time in history, humans have reached the point where it is possible to construct a revolutionary space-based observatory that has the capability to find dozens of Earth-like worlds, and possibly some with signs of life. This same telescope, designed as a long-lived facility, would also produce transformational scientific advances in every area of astronomy and astrophysics from black hole physics to galaxy formation, from star and planet formation to the origins of the Solar System. The Association of Universities for Research in Astronomy (AURA) commissioned a study on a next-generation UVOIR space observatory with the highest possible scientific impact in the era following JWST. This community-based study focuses on the future space-based options for UV and optical astronomy that significantly advance our understanding of the origin and evolution of the cosmos and the life within it. The committee concludes that a space telescope equipped with a 12-meter class primary mirror can find and characterize dozens of Earth-like planets and make fundamental advances across nearly all fields of astrophysics. The concept is called the High Definition Space Telescope (HDST). The telescope would be located at the Sun-Earth L2 point and would cover a spectral range that, at a minimum, runs from 0.1 to 2 microns. Unlike JWST, HDST will not need to operate at cryogenic temperatures. HDST can be made to be serviceable on orbit but does not require servicing to complete its primary scientific objectives. We present the scientific and technical requirements for HDST and show that it could allow us to determine whether or not life is common outside the Solar System. We do not propose a specific design for such a telescope, but show that designing, building and funding such a facility is feasible beginning in the next decade – if the necessary strategic investments in technology begin now.

From Cosmic Birth to Living Earths: The Future of UVOIR Space Astronomy [Replacement]

For the first time in history, humans have reached the point where it is possible to construct a revolutionary space-based observatory that has the capability to find dozens of Earth-like worlds, and possibly some with signs of life. This same telescope, designed as a long-lived facility, would also produce transformational scientific advances in every area of astronomy and astrophysics from black hole physics to galaxy formation, from star and planet formation to the origins of the Solar System. The Association of Universities for Research in Astronomy (AURA) commissioned a study on a next-generation UVOIR space observatory with the highest possible scientific impact in the era following JWST. This community-based study focuses on the future space-based options for UV and optical astronomy that significantly advance our understanding of the origin and evolution of the cosmos and the life within it. The committee concludes that a space telescope equipped with a 12-meter class primary mirror can find and characterize dozens of Earth-like planets and make fundamental advances across nearly all fields of astrophysics. The concept is called the High Definition Space Telescope (HDST). The telescope would be located at the Sun-Earth L2 point and would cover a spectral range that, at a minimum, runs from 0.1 to 2 microns. Unlike JWST, HDST will not need to operate at cryogenic temperatures. HDST can be made to be serviceable on orbit but does not require servicing to complete its primary scientific objectives. We present the scientific and technical requirements for HDST and show that it could allow us to determine whether or not life is common outside the Solar System. We do not propose a specific design for such a telescope, but show that designing, building and funding such a facility is feasible beginning in the next decade – if the necessary strategic investments in technology begin now.

The Changing Perception of the Solar System

The solar system has changed dramatically since its birth, and so did our understanding of it. A considerable research effort has been invested in the past decade in an attempt to reconstruct the solar system history, including the earliest stages some 4.5 billion years ago. The results indicate how several processes, such as planetary migration and dynamical instabilities, acted to relax the orbital spacing of the outer planets, and provided the needed perturbation to explain the present planetary orbits that are not precisely circular and coplanar. Here we highlight this work and illustrate the key results in a computer simulation that unifies several recently developed theories. The emerging view represents another step away from the initial perception of the solar system as part of unchanging heavens.

A Venus-Mass Planet Orbiting a Brown Dwarf: Missing Link between Planets and Moons

The co-planarity of solar-system planets led Kant to suggest that they formed from an accretion disk, and the discovery of hundreds of such disks around young stars as well as hundreds of co-planar planetary systems by the {\it Kepler} satellite demonstrate that this formation mechanism is extremely widespread. Many moons in the solar system, such as the Galilean moons of Jupiter, also formed out of the accretion disks that coalesced into the giant planets. We report here the discovery of an intermediate system OGLE-2013-BLG-0723LB/Bb composed of a Venus-mass planet orbiting a brown dwarf, which may be viewed either as a scaled down version of a planet plus star or as a scaled up version of a moon plus planet orbiting a star. The latter analogy can be further extended since they orbit in the potential of a larger, stellar body. For ice-rock companions formed in the outer parts of accretion disks, like Uranus and Callisto, the scaled masses and separations of the three types of systems are similar, leading us to suggest that formation processes of companions within accretion disks around stars, brown dwarfs, and planets are similar.

A Venus-Mass Planet Orbiting a Brown Dwarf: Missing Link between Planets and Moons [Replacement]

The co-planarity of solar-system planets led Kant to suggest that they formed from an accretion disk, and the discovery of hundreds of such disks around young stars as well as hundreds of co-planar planetary systems by the Kepler satellite demonstrate that this formation mechanism is extremely widespread. Many moons in the solar system, such as the Galilean moons of Jupiter, also formed out of the accretion disks that coalesced into the giant planets. We report here the discovery of an intermediate system OGLE-2013-BLG-0723LB/Bb composed of a Venus-mass planet orbiting a brown dwarf, which may be viewed either as a scaled down version of a planet plus star or as a scaled up version of a moon plus planet orbiting a star. The latter analogy can be further extended since they orbit in the potential of a larger, stellar body. For ice-rock companions formed in the outer parts of accretion disks, like Uranus and Callisto, the scaled masses and separations of the three types of systems are similar, leading us to suggest that formation processes of companions within accretion disks around stars, brown dwarfs, and planets are similar.

The composition of the protosolar disk and the formation conditions for comets

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today. This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.

The unstable CO2 feedback cycle on ocean planets

Ocean planets are volatile rich planets, not present in our Solar System, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO2). In our study, we focus on the CO2 cycle between the atmosphere and the ocean which determines the atmospheric CO2 content. The atmospheric amount of CO2 is a fundamental quantity for assessing the potential habitability of the planet’s surface because of its strong greenhouse effect, which determines the planetary surface temperature to a large degree. In contrast to the stabilising carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth atmosphere, we find that the CO2 cycle feedback on ocean planets is negative and has strong destabilising effects on the planetary climate. By using a chemistry model for oceanic CO2 dissolution and an atmospheric model for exoplanets, we show that the CO2 feedback cycle can severely limit the extension of the habitable zone for ocean planets.

The unstable CO2 feedback cycle on ocean planets [Replacement]

Ocean planets are volatile rich planets, not present in our Solar System, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO2). In our study, we focus on the CO2 cycle between the atmosphere and the ocean which determines the atmospheric CO2 content. The atmospheric amount of CO2 is a fundamental quantity for assessing the potential habitability of the planet’s surface because of its strong greenhouse effect, which determines the planetary surface temperature to a large degree. In contrast to the stabilising carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth atmosphere, we find that the CO2 cycle feedback on ocean planets is negative and has strong destabilising effects on the planetary climate. By using a chemistry model for oceanic CO2 dissolution and an atmospheric model for exoplanets, we show that the CO2 feedback cycle can severely limit the extension of the habitable zone for ocean planets.

Solar System Constraints on Disformal Gravity Theories [Cross-Listing]

Disformal theories of gravity are scalar-tensor theories where the scalar couples derivatively to matter via the Jordan frame metric. These models have recently attracted interest in the cosmological context since they admit accelerating solutions. We derive the solution for a static isolated mass in generic disformal gravity theories and transform it into the parameterised post-Newtonian form. This allows us to investigate constraints placed on such theories by local tests of gravity. The tightest constraints come from preferred-frame effects due to the motion of the Solar System with respect to the evolving cosmological background field. The constraints we obtain improve upon the previous solar system constraints by two orders of magnitude, and constrain the scale of the disformal coupling for generic models to $\mathcal{M} \gtrsim 100$ eV. These constraints render all disformal effects irrelevant for cosmology.

Solar System Constraints on Disformal Gravity Theories

Disformal theories of gravity are scalar-tensor theories where the scalar couples derivatively to matter via the Jordan frame metric. These models have recently attracted interest in the cosmological context since they admit accelerating solutions. We derive the solution for a static isolated mass in generic disformal gravity theories and transform it into the parameterised post-Newtonian form. This allows us to investigate constraints placed on such theories by local tests of gravity. The tightest constraints come from preferred-frame effects due to the motion of the Solar System with respect to the evolving cosmological background field. The constraints we obtain improve upon the previous solar system constraints by two orders of magnitude, and constrain the scale of the disformal coupling for generic models to $\mathcal{M} \gtrsim 100$ eV. These constraints render all disformal effects irrelevant for cosmology.

Solar System Constraints on Disformal Gravity Theories [Cross-Listing]

Disformal theories of gravity are scalar-tensor theories where the scalar couples derivatively to matter via the Jordan frame metric. These models have recently attracted interest in the cosmological context since they admit accelerating solutions. We derive the solution for a static isolated mass in generic disformal gravity theories and transform it into the parameterised post-Newtonian form. This allows us to investigate constraints placed on such theories by local tests of gravity. The tightest constraints come from preferred-frame effects due to the motion of the Solar System with respect to the evolving cosmological background field. The constraints we obtain improve upon the previous solar system constraints by two orders of magnitude, and constrain the scale of the disformal coupling for generic models to $\mathcal{M} \gtrsim 100$ eV. These constraints render all disformal effects irrelevant for cosmology.

Solar System Constraints on Disformal Gravity Theories [Cross-Listing]

Disformal theories of gravity are scalar-tensor theories where the scalar couples derivatively to matter via the Jordan frame metric. These models have recently attracted interest in the cosmological context since they admit accelerating solutions. We derive the solution for a static isolated mass in generic disformal gravity theories and transform it into the parameterised post-Newtonian form. This allows us to investigate constraints placed on such theories by local tests of gravity. The tightest constraints come from preferred-frame effects due to the motion of the Solar System with respect to the evolving cosmological background field. The constraints we obtain improve upon the previous solar system constraints by two orders of magnitude, and constrain the scale of the disformal coupling for generic models to $\mathcal{M} \gtrsim 100$ eV. These constraints render all disformal effects irrelevant for cosmology.

Solar and Heliospheric Physics with the Square Kilometre Array

The fields of solar radiophysics and solar system radio physics, or radio heliophysics, will benefit immensely from an instrument with the capabilities projected for SKA. Potential applications include interplanetary scintillation (IPS), radio-burst tracking, and solar spectral radio imaging with a superior sensitivity. These will provide breakthrough new insights and results in topics of fundamental importance, such as the physics of impulsive energy releases, magnetohydrodynamic oscillations and turbulence, the dynamics of post-eruptive processes, energetic particle acceleration, the structure of the solar wind and the development and evolution of solar wind transients at distances up to and beyond the orbit of the Earth. The combination of the high spectral, time and spatial resolution and the unprecedented sensitivity of the SKA will radically advance our understanding of basic physical processes operating in solar and heliospheric plasmas and provide a solid foundation for the forecasting of space weather events.

Dynamic stability of the Solar System: Statistically inconclusive results from ensemble integrations

Due to the chaotic nature of the Solar System, the question of its long-term stability can only be answered in a statistical sense, for instance, based on numerical ensemble integrations of nearby orbits. Destabilization of the inner planets, leading to close encounters and/or collisions can be initiated through a large increase in Mercury’s eccentricity, with a currently assumed likelihood of ~1%. However, little is known at present about the robustness of this number. Here I report ensemble integrations of the full equations of motion of the eight planets and Pluto over 5 Gyr, including contributions from general relativity. The results show that different numerical algorithms lead to statistically different results for the evolution of Mercury’s eccentricity (eM). For instance, starting at present initial conditions (eM ~= 0.21), Mercury’s maximum eccentricity achieved over 5 Gyr is on average significantly higher in symplectic ensemble integrations using heliocentricthan Jacobi coordinates and stricter error control. In contrast, starting at a possible future configuration (eM ~= 0.53), Mercury’s maximum eccentricity achieved over the subsequent 500 Myr is on average significantly lower using heliocentric than Jacobi coordinates. For example, the probability for eM to increase beyond 0.53 over 500 Myr is >90% (Jacobi) vs. only 40-55% (heliocentric). This poses a dilemma as the physical evolution of the real system – and its probabilistic behavior – cannot depend on the coordinate system or numerical algorithm chosen to describe it. Some tests of the numerical algorithms suggest that symplectic integrators using heliocentric coordinates underestimate the odds for destabilization of Mercury’s orbit at high initial eM.

HST observations of planetary aurorae, a unique tool to study giant magnetospheres

Ultraviolet (UV) planetary astronomy is a unique tool to probe planetary environments of the solar system and beyond. But despite a rising interest for new generation giant UV telescopes regularly proposed to international agencies, none has been selected yet, leaving the Hubble Space Telescope (HST) as the most powerful UV observatory in activity. HST regularly observed the auroral emissions of the Jupiter, Saturn and Uranus systems, leading to significant discoveries and achievements. This rich legacy remains of high interest for further statistical and long-term studies, but new observations are necessary to comparatively tackle pending questions, under varying solar or seasonal cycles.

The Compositional Structure of the Asteroid Belt

The past decade has brought major improvements in large-scale asteroid discovery and characterization with over half a million known asteroids and over 100,000 with some measurement of physical characterization. This explosion of data has allowed us to create a new global picture of the Main Asteroid Belt. Put in context with meteorite measurements and dynamical models, a new and more complete picture of Solar System evolution has emerged. The question has changed from "What was the original compositional gradient of the Asteroid Belt?" to "What was the original compositional gradient of small bodies across the entire Solar System?" No longer is the leading theory that two belts of planetesimals are primordial, but instead those belts were formed and sculpted through evolutionary processes after Solar System formation. This article reviews the advancements on the fronts of asteroid compositional characterization, meteorite measurements, and dynamical theories in the context of the heliocentric distribution of asteroid compositions seen in the Main Belt today. This chapter also reviews the major outstanding questions relating to asteroid compositions and distributions and summarizes the progress and current state of understanding of these questions to form the big picture of the formation and evolution of asteroids in the Main Belt. Finally, we briefly review the relevance of asteroids and their compositions in their greater context within our Solar System and beyond.

Oxygen Isotopic Composition of Coarse- and Fine-grained Material from Comet 81P/Wild 2

Individual particles from comet 81P/Wild 2 collected by NASA’s Stardust mission vary in size from small sub-$\mu$m fragments found in the walls of the aerogel tracks, to large fragments up to tens of $\mu$m in size found towards the termini of tracks. The comet, in an orbit beyond Neptune since its formation, retains an intact a record of early-Solar-System processes that was compromised in asteroidal samples by heating and aqueous alteration. We measured the O isotopic composition of seven Stardust fragments larger than $\sim$2 $\mu$m extracted from five different Stardust aerogel tracks, and 63 particles smaller than $\sim$2 $\mu$m from the wall of a Stardust track. The larger particles show a relatively narrow range of O isotopic compositions that is consistent with $^{16}$O-poor phases commonly seen in meteorites. Many of the larger Stardust fragments studied so far have chondrule-like mineralogy which is consistent with formation in the inner Solar System. The fine-grained material shows a very broad range of O isotopic compositions ($-70<\Delta^{17}$O$<+60$) suggesting that Wild 2 fines are either primitive outer-nebula dust or a very diverse sampling of inner Solar System compositional reservoirs that accreted along with a large number of inner-Solar-System rocks to form comet Wild 2.

Distribution of CO2 ice on the large moons of Uranus and evidence for compositional stratification of their near-surfaces

The surfaces of the large Uranian satellites are characterized by a mixture of H2O ice and a dark, potentially carbon-rich, constituent, along with CO2 ice. At the mean heliocentric distance of the Uranian system, native CO2 ice should be removed on timescales shorter than the age of the Solar System. Consequently, the detected CO2 ice might be actively produced. Analogous to irradiation of icy moons in the Jupiter and Saturn systems, we hypothesize that charged particles caught in Uranus’ magnetic field bombard the surfaces of the Uranian satellites, driving a radiolytic CO2 production cycle. To test this hypothesis, we investigated the distribution of CO2 ice by analyzing near-infrared (NIR) spectra of these moons, gathered using the SpeX spectrograph at NASA’s Infrared Telescope Facility (IRTF) (2000 – 2013). Additionally, we made spectrophotometric measurements using images gathered by the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope (2003 – 2005). We find that the detected CO2 ice is primarily on the trailing hemispheres of the satellites closest to Uranus, consistent with other observations of these moons. Our band parameter analysis indicates that the detected CO2 ice is pure and segregated from other constituents. Our spectrophotometric analysis indicates that IRAC is not sensitive to the CO2 ice detected by SpeX, potentially because CO2 is retained beneath a thin surface layer dominated by H2O ice that is opaque to photons over IRAC wavelengths. Thus, our combined SpeX and IRAC analyses suggest that the near-surfaces (i.e., top few 100 microns) of the Uranian satellites are compositionally stratified. We briefly compare the spectral characteristics of the CO2 ice detected on the Uranian moons to icy satellites elsewhere, and we also consider the most likely drivers of the observed distribution of CO2 ice.

How Sedna and family were captured in a close encounter with a solar sibling

The discovery of 2012VP_113 initiated the debate on the origin of the Sedna family of planetesimals in orbit around the Sun. Sednitos roam the outer regions of the Solar System between the Egeworth–Kuiper belt and the Oort cloud, in extraordinary wide (a>150 au) orbits with a large perihelion distance of q>30 au compared to the Earth’s (a=1 au and eccentricity e=(1-q/a)~0.0167 or q~1 au). This population is composed of a dozen objects, which we consider a family because they have similar perihelion distance and inclination with respect to the ecliptic i=10–30 deg. They also have similar argument of perihelion of (340+/-55) deg. There is no ready explanation for their origin. Here we show that these orbital parameters are typical for a captured population from the planetesimal disk of another star. Using the orbital parameters of the Sednitos we reconstruct the encounter that led to their capture. We conclude that they might have been captured in a near miss with a 1.8 M_Sun star that impacted the Sun at ~340 au at an inclination with respect to the ecliptic of 17–34 deg with a relative velocity at infinity of ~4.3 km/s. We predict that the Sednitos-region is populated by 930 planetesimals and the inner Oort cloud acquired about 440 planetesimals through the same encounter.

Water delivery in the Early Solar System

As part of the national scientific network ‘Pathways to Habitable Worlds’ the delivery of water onto terrestrial planets is a key question since water is essential for the development of life as we know it. After summarizing the state of the art we show some first results of the transport of water in the early Solar System for scattered main belt objects. Hereby we investigate the questions whether planetesimals and planetesimal fragments which have gained considerable inclination due to the strong dynamical interactions in the main belt region around 2 AU can be efficient water transporting vessels. The Hungaria asteroid group is the best example that such scenarios are realistic. Assuming that the gas giants and the terrestrial planets are already formed, we monitor the collisions of scattered small bodies containing water (in the order of a few percent) with the terrestrial planets. Thus we are able to give a first estimate concerning the respective contribution of such bodies to the actual water content in the crust of the Earth.

The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores

The basic structure of the solar system is set by the presence of low-mass terrestrial planets in its inner part and giant planets in its outer part. This is the result of the formation of a system of multiple embryos with approximately the mass of Mars in the inner disk and of a few multi-Earth-mass cores in the outer disk, within the lifetime of the gaseous component of the protoplanetary disk. What was the origin of this dichotomy in the mass distribution of embryos/cores? We show in this paper that the classic processes of runaway and oligarchic growth from a disk of planetesimals cannot explain this dichotomy, even if the original surface density of solids increased at the snowline. Instead, the accretion of drifting pebbles by embryos and cores can explain the dichotomy, provided that some assumptions hold true. We propose that the mass-flow of pebbles is two-times lower and the characteristic size of the pebbles is approximately ten times smaller within the snowline than beyond the snowline (respectively at heliocentric distance $r<r_{ice}$ and $r>r_{ice}$, where $r_{ice}$ is the snowline heliocentric distance), due to ice sublimation and the splitting of icy pebbles into a collection of chondrule-size silicate grains. In this case, objects of original sub-lunar mass would grow at drastically different rates in the two regions of the disk. Within the snowline these bodies would reach approximately the mass of Mars while beyond the snowline they would grow to $\sim 20$ Earth masses. The results may change quantitatively with changes to the assumed parameters, but the establishment of a clear dichotomy in the mass distribution of protoplanets appears robust, provided that there is enough turbulence in the disk to prevent the sedimentation of the silicate grains into a very thin layer.

The Complex History of Trojan Asteroids

The Trojan asteroids provide a unique perspective on the history of Solar System. As a large population of small bodies, they record important gravitational interactions and dynamical evolution of the Solar System. In the past decade, significant advances have been made in understanding physical properties, and there has been a revolution in thinking about the origin of Trojans. The ice and organics generally presumed to be a significant part of Trojan compositions have yet to be detected directly, though low density of the binary system Patroclus (and possibly low density of the binary/moonlet system Hektor) is consistent with an interior ice component. By contrast, fine-grained silicates that appear to be similar to cometary silicates in composition have been detected, and a color bimodality may indicate distinct compositional groups among the Trojans. Whereas Trojans had traditionally been thought to have formed near 5 AU, a new paradigm has developed in which the Trojans formed in the proto-Kuiper Belt, and they were scattered inward and captured in the Trojan swarms as a result of resonant interactions of the giant planets. Whereas the orbital and population distributions of current Trojans are consistent with this origin scenario, there are significant differences between current physical properties of Trojans and those of Kuiper Belt objects. These differences may be indicative of surface modification due to the inward migration of objects that became the Trojans, but understanding of appropriate modification mechanisms is poor and would benefit from additional laboratory studies. Many open questions remain, and the future promises significant strides in our understanding of Trojans. The time is ripe for a spacecraft mission to the Trojans, to turn these objects into geologic worlds that can be studied in detail to unravel their complex history.

The formation of the Galilean moons and Titan in the Grand Tack scenario

In the "Grand Tack" (GT) scenario for the young solar system, Jupiter formed beyond 3.5 AU from the Sun and migrated as close as 1.5 AU until it encountered an orbital resonance with Saturn. Both planets then supposedly migrated outward for several $10^5$ yr, with Jupiter ending up at ~5 AU. The initial conditions of the GT and the timing between Jupiter’s migration and the formation of the Galilean satellites remain unexplored. We study the formation of Ganymede and Callisto, both of which consist of ~50% water and rock, respectively, in the GT scenario. We examine why they lack dense atmospheres, while Titan is surrounded by a thick nitrogen envelope. We model an axially symmetric circumplanetary disk (CPD) in hydrostatic equilibrium around Jupiter. The CPD is warmed by viscous heating, Jupiter’s luminosity, accretional heating, and the Sun. The position of the water ice line in the CPD, which is crucial for the formation of massive moons, is computed at various solar distances. We assess the loss of Galilean atmospheres due to high-energy radiation from the young Sun. Ganymede and Callisto cannot have accreted their water during Jupiter’s supposed GT, because its CPD (if still active) was too warm to host ices and much smaller than Ganymede’s contemporary orbit. From a thermal perspective, the Galilean moons might have had significant atmospheres, but these would probably have been eroded during the GT in < $10^5$ yr by solar XUV radiation. Jupiter and the Galilean moons formed beyond 4.5 (+/-0.5) AU and prior to the proposed GT. Thereafter, Jupiter’s CPD would have been dry, and delayed accretion of planetesimals should have created water-rich Io and Europa. While Galilean atmospheres would have been lost during the GT, Titan would have formed after Saturn’s own tack, because Saturn still accreted substantially for ~$10^6$ yr after its closest solar approach, ending up at about 7 AU.

The formation of the Galilean moons and Titan in the Grand Tack scenario [Replacement]

In the Grand Tack (GT) scenario for the young solar system, Jupiter formed beyond 3.5 AU from the Sun and migrated as close as 1.5 AU until it encountered an orbital resonance with Saturn. Both planets then supposedly migrated outward for several $10^5$ yr, with Jupiter ending up at ~5 AU. The initial conditions of the GT and the timing between Jupiter’s migration and the formation of the Galilean satellites remain unexplored. We study the formation of Ganymede and Callisto, both of which consist of ~50% H$_2$O and rock, in the GT scenario. We examine why they lack dense atmospheres, while Titan is surrounded by a thick N$_2$ envelope. We model an axially symmetric circumplanetary disk (CPD) in hydrostatic equilibrium around Jupiter. The CPD is warmed by viscous heating, Jupiter’s luminosity, accretional heating, and the Sun. The position of the H$_2$O ice line in the CPD, which is crucial for the formation of massive moons, is computed at various solar distances. We assess the loss of Galilean atmospheres due to high-energy radiation from the young Sun. Ganymede and Callisto cannot have accreted their H$_2$O during Jupiter’s supposed GT, because its CPD (if still active) was too warm to host ices and much smaller than Ganymede’s contemporary orbit. From a thermal perspective, the Galilean moons might have had significant atmospheres, but these would probably have been eroded during the GT in < $10^5$ yr by solar XUV radiation. Jupiter and the Galilean moons formed beyond 4.5 (+/- 0.5) AU and prior to the proposed GT. Thereafter, Jupiter’s CPD would have been dry, and delayed accretion of planetesimals should have created water-rich Io and Europa. While Galilean atmospheres would have been lost during the GT, Titan would have formed after Saturn’s own tack, because Saturn still accreted substantially for ~$10^6$ yr after its closest solar approach, ending up at about 7 AU.

Mercury's resonant rotation from secular orbital elements [Replacement]

We used recently produced Solar System ephemerides, which incorporate two years of ranging observations to the MESSENGER spacecraft, to extract the secular orbital elements for Mercury and associated uncertainties. As Mercury is in a stable 3:2 spin-orbit resonance these values constitute an important reference for the planet’s measured rotational parameters, which in turn strongly bear on physical interpretation of Mercury’s interior structure. In particular, we derive a mean orbital period of 87.96934962 $\pm$ 0.00000037 days and (assuming a perfect resonance) a spin rate of 6.138506839 $\pm$ 0.000000028 degree/day. The difference between this rotation rate and the currently adopted rotation rate (Archinal et al, 2011) corresponds to a longitudinal displacement of approx. 67 m per year at the equator. Moreover, we present a basic approach for the calculation of the orientation of the instantaneous Laplace and Cassini planes of Mercury. The analysis allows us to assess the uncertainties in physical parameters of the planet when derived from observations of Mercury’s rotation.

Mercury's resonant rotation from secular orbital elements

We used recently produced Solar System ephemeris, which incorporate two years of ranging observations to the MESSENGER spacecraft, to extract the secular orbital elements for Mercury and associated uncertainties. As Mercury is in a stable 3:2 spin-orbit resonance these values constitute an important reference for the planet’s measured rotational parameters, which in turn strongly bear on physical interpretation of Mercury’s interior structure. In particular, we derive an mean orbital period of 87.96934962 $\pm$ 0.00000037 days and (assuming the perfect resonance) a spin rate of 6.138506839 $\pm$ 0.000000028 degree/day. The difference between this rotation rate and the currently adopted rotation rate (Archinal et al, 2011) corresponds to a longitudinal displacement of approx. 67 m per year at the equator. Moreover, we present a basic approach for the calculation of the orientation of the instantaneous Laplace and Cassini planes of Mercury. The analysis allows us to assess the uncertainties in physical parameters of the planet when derived from observations of Mercury’s rotation.

Mercury's resonant rotation from secular orbital elements [Replacement]

We used recently produced Solar System ephemerides, which incorporate two years of ranging observations to the MESSENGER spacecraft, to extract the secular orbital elements for Mercury and associated uncertainties. As Mercury is in a stable 3:2 spin-orbit resonance these values constitute an important reference for the planet’s measured rotational parameters, which in turn strongly bear on physical interpretation of Mercury’s interior structure. In particular, we derive a mean orbital period of 87.96934962 $\pm$ 0.00000037 days and (assuming a perfect resonance) a spin rate of 6.138506839 $\pm$ 0.000000028 degree/day. The difference between this rotation rate and the currently adopted rotation rate (Archinal et al, 2011) corresponds to a longitudinal displacement of approx. 67 m per year at the equator. Moreover, we present a basic approach for the calculation of the orientation of the instantaneous Laplace and Cassini planes of Mercury. The analysis allows us to assess the uncertainties in physical parameters of the planet when derived from observations of Mercury’s rotation.

NEOWISE: Observations of the Irregular Satellites of Jupiter and Saturn

We present thermal model fits for 11 Jovian and 3 Saturnian irregular satellites based on measurements from the WISE/NEOWISE dataset. Our fits confirm spacecraft-measured diameters for the objects with in situ observations (Himalia and Phoebe) and provide diameters and albedo for 12 previously unmeasured objects, 10 Jovian and 2 Saturnian irregular satellites. The best-fit thermal model beaming parameters are comparable to what is observed for other small bodies in the outer Solar System, while the visible, W1, and W2 albedos trace the taxonomic classifications previously established in the literature. Reflectance properties for the irregular satellites measured are similar to the Jovian Trojan and Hilda Populations, implying common origins.

The role of dynamics on the habitability of an Earth-like planet

From the numerous detected planets outside the Solar system, no terrestrial planet comparable to our Earth has been discovered so far. The search for an Exo-Earth is certainly a big challenge which may require the detections of planetary systems resembling our Solar system in order to find life like on Earth. However, even if we find Solar system analogues, it is not certain that a planet in Earth position will have similar circumstances as those of Earth. Small changes in the architecture of the giant planets can lead to orbital perturbations which may change the conditions of habitability for a terrestrial planet in the habitable zone (HZ). We present a numerical investigation where we first study the motion of test-planets in a particular Jupiter-Saturn configuration for which we can expect strong gravitational perturbations on the motion at Earth position according to a previous work. In this study, we show that these strong perturbations can be reduced significantly by the neighboring planets of Earth. In the second part of our study we investigate the motion of test-planets in inclined Jupiter-Saturn systems where we analyze changes in the dynamical behavior of the inner planetary system. Moderate values of inclination seem to counteract the perturbations in the HZ while high inclinations induce more chaos in this region. Finally, we carry out a stability study of the actual orbits of Venus, Earth and Mars moving in the inclined Jupiter-Saturn systems for which we used the Solar system parameters. This study shows that the three terrestrial planets will only move in low-eccentric orbits if Saturn’s inclination is <=10{\deg}. Therefore, it seems that it is advantageous for the habitability of Earth when all planets move nearly in the same plane.

Comet 67P/Churyumov-Gerasimenko: Constraints on its origin from OSIRIS observations

One of the main aims of the ESA Rosetta mission is to study the origin of the solar system by exploring comet 67P/Churyumov-Gerasimenko at close range. In this paper we discuss the origin and evolution of comet 67P/Churyumov-Gerasimenko in relation to that of comets in general and in the framework of current solar system formation models. We use data from the OSIRIS scientific cameras as basic constraints. In particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. We also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar system. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. The lobes are separate bodies, though sufficiently similar to have formed in the same environment. An estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67P/Churyumov-Gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded. A collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. We thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. We point to the primordial signatures exhibited by 67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the collisional evolution.

Constraining f(T) gravity in the Solar System [Cross-Listing]

In the framework of $f(T)$ theories of gravity, we solve the field equations for $f(T)=T+\alpha T^{n}$, in the weak-field approximation and for spherical symmetry spacetime. Since $f(T)=T$ corresponds to Teleparallel Gravity, which is equivalent to General Relativity, the non linearity of the Lagrangian are expected to produce perturbations of the general relativistic solutions, parameterized by $\alpha$. Hence, we use the $f(T)$ solutions to model the gravitational field of the Sun, and exploit data from accurate tracking of spacecrafts orbiting Mercury and Saturn to infer preliminary insights on what could be obtained about the model parameter $\alpha$ and the cosmological constant $\Lambda$. It turns out that improvements of about one-three orders with respect to the present-day constraints in the literature of magnitude seem possible.

Constraining f(T) gravity in the Solar System

In the framework of $f(T)$ theories of gravity, we solve the field equations for $f(T)=T+\alpha T^{n}$, in the weak-field approximation and for spherical symmetry spacetime. Since $f(T)=T$ corresponds to Teleparallel Gravity, which is equivalent to General Relativity, the non linearity of the Lagrangian are expected to produce perturbations of the general relativistic solutions, parameterized by $\alpha$. Hence, we use the $f(T)$ solutions to model the gravitational field of the Sun, and exploit data from accurate tracking of spacecrafts orbiting Mercury and Saturn to infer preliminary insights on what could be obtained about the model parameter $\alpha$ and the cosmological constant $\Lambda$. It turns out that improvements of about one-three orders with respect to the present-day constraints in the literature of magnitude seem possible.

Constraining f(T) gravity in the Solar System [Replacement]

In the framework of $f(T)$ theories of gravity, we solve the field equations for $f(T)=T+\alpha T^{n}$, in the weak-field approximation and for spherical symmetry spacetime. Since $f(T)=T$ corresponds to Teleparallel Gravity, which is equivalent to General Relativity, the non linearity of the Lagrangian are expected to produce perturbations of the general relativistic solutions, parameterized by $\alpha$. Hence, we use the $f(T)$ solutions to model the gravitational field of the Sun, and exploit data from accurate tracking of spacecrafts orbiting Mercury and Saturn to infer preliminary insights on what could be obtained about the model parameter $\alpha$ and the cosmological constant $\Lambda$. It turns out that improvements of about one-three orders with respect to the present-day constraints in the literature of magnitude seem possible.

Constraining f(T) gravity in the Solar System [Replacement]

In the framework of $f(T)$ theories of gravity, we solve the field equations for $f(T)=T+\alpha T^{n}$, in the weak-field approximation and for spherical symmetry spacetime. Since $f(T)=T$ corresponds to Teleparallel Gravity, which is equivalent to General Relativity, the non linearity of the Lagrangian are expected to produce perturbations of the general relativistic solutions, parameterized by $\alpha$. Hence, we use the $f(T)$ solutions to model the gravitational field of the Sun, and exploit data from accurate tracking of spacecrafts orbiting Mercury and Saturn to infer preliminary insights on what could be obtained about the model parameter $\alpha$ and the cosmological constant $\Lambda$. It turns out that improvements of about one-three orders with respect to the present-day constraints in the literature of magnitude seem possible.

Cosmic-ray diffusion in magnetized turbulence

The problem of cosmic-ray scattering in the turbulent electromagnetic fields of the interstellar medium and the solar wind is of great importance due to the variety of applications of the resulting diffusion coefficients. Examples are diffusive shock acceleration, cosmic-ray observations, and, in the solar system, the propagation of coronal mass ejections. In recent years, it was found that the simple diffusive motion that had been assumed for decades is often in disagreement both with numerical and observational results. Here, an overview is given of the interaction processes of cosmic rays and turbulent electromagnetic fields. First, the formation of turbulent fields due to plasma instabilities is treated, where especially the non-linear behavior of the resulting unstable wave modes is discussed. Second, the analytical and the numerical side of high-energy particle propagation will be reviewed by presenting non-linear analytical theories and Monte-Carlo simulations. For the example of the solar wind, the impact of anisotropic and dynamical turbulence models will be discussed. In addition, it will be shown how further complications can be treated that arise from the large-scale magnetic field geometry and turbulent electric fields. The transport properties of energetic particles can thus be calculated for current turbulence models so that they withstand a comparison with measurements taken in the solar wind.

A FIR-Survey of TNOs and Related Bodies

The small solar-system bodies that reside between 30 and 50 AU are often referred to as the Trans Neptunian Objects, or TNOs. A far-infrared (FIR) mission with survey capabilities, like the prospective Cryogenic Aperture Large Infrared Space Telescope Observatory (CALISTO; Goldsmith et al. 2008), offers the potential for the first time of really probing the population of TNOs, and related populations, down to moderates sizes, and out to distances exceeding 100 AU from the Sun.

Towards a dynamics-based estimate of the extent of HR 8799's unresolved warm debris belt

In many ways, the HR8799 system resembles our Solar system more closely than any other discovered to date – albeit on a larger, younger, and more dramatic scale – featuring four giant planets and two debris belts. The first belt lies beyond the orbit of the outer planet, and mirrors our Solar system’s Edgeworth-Kuiper belt. The second belt lies interior to the orbit of the inner planet, HR8799e, and is analogous to our Asteroid Belt. With such a similar architecture, the system is a valuable laboratory for examining exoplanet dynamics, and the interaction between debris disks and planets. In recent years, HR8799′s outer disk has been relatively well characterised, primarily using the Herschel Space Observatory. In contrast, the inner disk, too close to HR8799 to be spatially resolved by Herschel, remains poorly understood. This leaves significant questions over both the location of the planetesimals responsible for producing the observed dust, and the physical properties of those grains. We have performed extensive simulations of HR8799′s inner, unresolved debris belt, using UNSW Australia’s supercomputing facility, Katana. Here, we present the results of integrations following the evolution of a belt of dynamically hot debris interior to the orbit of HR8799e, for a period of 60 Myr, using an initial population of 500,000 massless test particles. These simulations have enable the characterisation of the extent and structure of the inner belt, revealing that its outer edge must lie interior to the 3:1 mean-motion resonance with HR8799, at approximately 7.5au, and highlighting the presence of fine structure analogous to the Solar system’s Kirkwood gaps. In the future, out results will allow us to calculate a first estimate of the small-body impact rate and water delivery prospects for any potential terrestrial planet(s) that might lurk, undetected, in the inner system.

Relationship between key events in Earth history

A model of cyclical (sinusoidal) motion of the solar system, intercepting event lines distributed at fixed intervals, explains the pattern of timings of mass extinctions, earlier glaciations, largest impact craters and the largest known extrusions of magma in the history of the Earth. The model reveals links between several sets of key events, including the end-Cretaceous and end-Ordovician extinctions with the Marinoan glaciation, and the end-Permian with the end-Serpukhovian extinctions. The model is supported by significant clusters of events and a significant reduction of impact crater size with position (sine value). The pattern of event lines is sustained to the earliest-dated impact craters (2023 and 1849 Ma) and to the origin of the solar system, close to 4567.4 Ma. The implication is that, for the entirety of its existence, the solar system has passed in a consistent manner through a predictably structured galaxy. Dark matter is a possible contender for the structure determining the event lines.

New Paradigms For Asteroid Formation

Asteroids and meteorites provide key evidence on the formation of planetesimals in the Solar System. Asteroids are traditionally thought to form in a bottom-up process by coagulation within a population of initially km-scale planetesimals. However, new models challenge this idea by demonstrating that asteroids of sizes from 100 to 1000 km can form directly from the gravitational collapse of small particles which have organised themselves in dense filaments and clusters in the turbulent gas. Particles concentrate passively between eddies down to the smallest scales of the turbulent gas flow and inside large-scale pressure bumps and vortices. The streaming instability causes particles to take an active role in the concentration, by piling up in dense filaments whose friction on the gas reduces the radial drift compared to that of isolated particles. In this chapter we review new paradigms for asteroid formation and compare critically against the observed properties of asteroids as well as constraints from meteorites. Chondrules of typical sizes from 0.1 to 1 mm are ubiquitous in primitive meteorites and likely represent the primary building blocks of asteroids. Chondrule-sized particles are nevertheless tightly coupled to the gas via friction and are therefore hard to concentrate in large amounts in the turbulent gas. We review recent progress on understanding the incorporation of chondrules into the asteroids, including layered accretion models where chondrules are accreted onto asteroids over millions of years. We highlight in the end ten unsolved questions in asteroid formation where we expect that progress will be made over the next decade.

Measurement of the radial velocity of the Sun as a star by means of a reflecting solar system body. The effect of the body rotation

Minor bodies of the solar system can be used to measure the spectrum of the Sun as a star by observing sunlight reflected by their surfaces. To perform an accurate measurement of the radial velocity of the Sun as a star by this method, it is necessary to take into account the Doppler shifts introduced by the motion of the reflecting body. Here we discuss the effect of its rotation. It gives a vanishing contribution only when the inclinations of the body rotation axis to the directions of the Sun and of the Earth observer are the same. When this is not the case, the perturbation of the radial velocity does not vanish and can reach up to about 2.4 m/s for an asteroid such as 2 Pallas that has an inclination of the spin axis to the plane of the ecliptic of about 30 degrees. We introduce a geometric model to compute the perturbation in the case of a uniformly reflecting body of spherical or triaxial ellipsoidal shape and provide general results to easily estimate the magnitude of the effect.

Thorium Abundances in Solar Twins and Analogues: Implications for the Habitability of Extrasolar Planetary Systems

We present the first investigation of Th abundances in Solar twins and analogues to understand the possible range of this radioactive element and its effect on rocky planet interior dynamics and potential habitability. The abundances of the radioactive elements Th and U are key components of a planet’s energy budget, making up 30% to 50% of the Earth’s (Korenaga 2008; All\`egre et al. 2001; Schubert et al. 1980; Lyubetskaya & Korenaga 2007; The KamLAND Collaboration 2011; Huang et al. 2013). Radiogenic heat drives interior mantle convection and surface plate tectonics, which sustains a deep carbon and water cycle and thereby aides in creating Earth’s habitable surface. Unlike other heat sources that are dependent on the planet’s specific formation history, the radiogenic heat budget is directly related to the mantle concentration of these nuclides. As a refractory element, the stellar abundance of Th is faithfully reflected in the terrestrial planet’s concentration. We find that log eps Th varies from 59% to 251% that of Solar, suggesting extrasolar planetary systems may possess a greater energy budget with which to support surface to interior dynamics and thus increase their likelihood to be habitable compared to our Solar System.

Does the Newton's gravitational constant vary sinusoidally with time? An independent test with planetary orbital motions

A sinusoidally time-varying pattern for the values of the Newton’s constant of gravitation $G$ measured in Earth-based laboratories over the latest decades has been recently reported in the literature. Its amplitude and period amount to $A_G=1.619\times 10^{-14} \textrm{kg}^{-1} \textrm{m}^3 \textrm{s}^{-2}, P_G=5.899 \textrm{yr}$, respectively. Given the fundamental role played by $G$ in the currently accepted theory of gravitation and the attempts to merge it with quantum mechanics, it is important to put to the test the hypothesis that the aforementioned harmonic variation may pertain $G$ itself in a direct and independent way. The bounds on $\dot G/G$ existing in the literature may not be extended straightforwardly to the present case since they were inferred by considering just secular variations. Thus, we numerically integrated the ad-hoc modified equations of motion of the major bodies of the Solar System by finding that the orbits of the planets would be altered by an unacceptably larger amount in view of the present-day high accuracy astrometric measurements. In the case of Saturn, its geocentric right ascension $\alpha$, declination $\delta$ and range $\rho$ would be affected up to $10^4-10^5$ milliarcseconds and $10^5$ km, respectively; the present-day residuals of such observables are as little as about $4$ milliarcseconds and $10^{-1}$ km, respectively.

Does the Newton's gravitational constant vary sinusoidally with time? An independent test with planetary orbital motions [Cross-Listing]

A sinusoidally time-varying pattern for the values of the Newton’s constant of gravitation $G$ measured in Earth-based laboratories over the latest decades has been recently reported in the literature. Its amplitude and period amount to $A_G=1.619\times 10^{-14} \textrm{kg}^{-1} \textrm{m}^3 \textrm{s}^{-2}, P_G=5.899 \textrm{yr}$, respectively. Given the fundamental role played by $G$ in the currently accepted theory of gravitation and the attempts to merge it with quantum mechanics, it is important to put to the test the hypothesis that the aforementioned harmonic variation may pertain $G$ itself in a direct and independent way. The bounds on $\dot G/G$ existing in the literature may not be extended straightforwardly to the present case since they were inferred by considering just secular variations. Thus, we numerically integrated the ad-hoc modified equations of motion of the major bodies of the Solar System by finding that the orbits of the planets would be altered by an unacceptably larger amount in view of the present-day high accuracy astrometric measurements. In the case of Saturn, its geocentric right ascension $\alpha$, declination $\delta$ and range $\rho$ would be affected up to $10^4-10^5$ milliarcseconds and $10^5$ km, respectively; the present-day residuals of such observables are as little as about $4$ milliarcseconds and $10^{-1}$ km, respectively.

Cosmological Tests of Gravity [Cross-Listing]

Einstein’s theory of General Relativity (GR) is tested accurately within the local universe i.e., the Solar System, but this leaves open the possibility that it is not a good description at the largest scales in the Universe. The standard model of cosmology assumes GR as the theory to describe gravity on all scales. In 1998, astronomers made the surprising discovery that the expansion of the Universe is accelerating, not slowing down. This late-time acceleration of the Universe has become the most challenging problem in theoretical physics. Within the framework of GR, the acceleration would originate from an unknown dark energy. Alternatively, it could be that there is no dark energy and GR itself is in error on cosmological scales. The standard model of cosmology is based on a huge extrapolation of our limited knowledge of gravity. This discovery of the late time acceleration of the Universe may require us to revise the theory of gravity and the standard model of cosmology based on GR. We will review recent progress in constructing modified gravity models as an alternative to dark energy and developing cosmological tests of gravity.

Cosmological Tests of Gravity [Cross-Listing]

Einstein’s theory of General Relativity (GR) is tested accurately within the local universe i.e., the Solar System, but this leaves open the possibility that it is not a good description at the largest scales in the Universe. The standard model of cosmology assumes GR as the theory to describe gravity on all scales. In 1998, astronomers made the surprising discovery that the expansion of the Universe is accelerating, not slowing down. This late-time acceleration of the Universe has become the most challenging problem in theoretical physics. Within the framework of GR, the acceleration would originate from an unknown dark energy. Alternatively, it could be that there is no dark energy and GR itself is in error on cosmological scales. The standard model of cosmology is based on a huge extrapolation of our limited knowledge of gravity. This discovery of the late time acceleration of the Universe may require us to revise the theory of gravity and the standard model of cosmology based on GR. We will review recent progress in constructing modified gravity models as an alternative to dark energy and developing cosmological tests of gravity.

Cosmological Tests of Gravity

Einstein’s theory of General Relativity (GR) is tested accurately within the local universe i.e., the Solar System, but this leaves open the possibility that it is not a good description at the largest scales in the Universe. The standard model of cosmology assumes GR as the theory to describe gravity on all scales. In 1998, astronomers made the surprising discovery that the expansion of the Universe is accelerating, not slowing down. This late-time acceleration of the Universe has become the most challenging problem in theoretical physics. Within the framework of GR, the acceleration would originate from an unknown dark energy. Alternatively, it could be that there is no dark energy and GR itself is in error on cosmological scales. The standard model of cosmology is based on a huge extrapolation of our limited knowledge of gravity. This discovery of the late time acceleration of the Universe may require us to revise the theory of gravity and the standard model of cosmology based on GR. We will review recent progress in constructing modified gravity models as an alternative to dark energy and developing cosmological tests of gravity.

 

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