# Posts Tagged solar system

## Today's Postings

### $f(T)$ gravity: effects on astronomical observation and Solar System experiments and upper-bounds

As an extension of a previous work in which perihelion advances are considered only and as an attempt to find more stringent constraints on its parameters, we investigate effects on astronomical observation and experiments conducted in the Solar System due to the $f(T)$ gravity which contains a quadratic correction of $\alpha T^2$ ($\alpha$ is a model parameter) and the cosmological constant $\Lambda$. Using a spherical solution describing the Sun’s gravitational field, the resulting secular evolution of planetary orbital motions, light deflection, gravitational time delay and frequency shift are calculated up to the leading contribution. Among them, we find qualitatively that the light deflection holds a unique bound on $\alpha$, without dependence on $\Lambda$, and the time delay experiments during inferior conjunction impose a clean constraint on $\Lambda$, regardless of $\alpha$. Based on observation and experiments, especially the supplementary advances in the perihelia provided by the INPOP10a ephemeris, we obtain the upper-bounds quantitatively: $|\alpha| \le 1.2 \times 10^{2}$ m${}^2$ and $|\Lambda| \le 1.8 \times 10^{-43}$ m${}^{-2}$, at least 10 times tighter than the previous result.

### Terrestrial Planet Formation in a protoplanetary disk with a local mass depletion: A successful scenario for the formation of Mars

Models of terrestrial planet formation for our solar system have been successful in producing planets with masses and orbits similar to those of Venus and Earth. However, these models have generally failed to produce Mars-sized objects around 1.5 AU. The body that is usually formed around Mars’ semimajor axis is, in general, much more massive than Mars. Only when Jupiter and Saturn are assumed to have initially very eccentric orbits (e $\sim$ 0.1), which seems fairly unlikely for the solar system, or alternately, if the protoplanetary disk is truncated at 1.0 AU, simulations have been able to produce Mars-like bodies in the correct location. In this paper, we examine an alternative scenario for the formation of Mars in which a local depletion in the density of the protosolar nebula results in a non-uniform formation of planetary embryos and ultimately the formation of Mars-sized planets around 1.5 AU. We have carried out extensive numerical simulations of the formation of terrestrial planets in such a disk for different scales of the local density depletion, and for different orbital configurations of the giant planets. Our simulations point to the possibility of the formation of Mars-sized bodies around 1.5 AU, specifically when the scale of the disk local mass-depletion is moderately high (50-75%) and Jupiter and Saturn are initially in their current orbits. In these systems, Mars-analogs are formed from the protoplanetary materials that originate in the regions of disk interior or exterior to the local mass-depletion. Results also indicate that Earth-sized planets can form around 1 AU with a substantial amount of water accreted via primitive water-rich planetesimals and planetary embryos. We present the results of our study and discuss their implications for the formation of terrestrial planets in our solar system.

### A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge

We present the first microlensing candidate for a free-floating exoplanet-exomoon system, MOA-2011-BLG-262, with a primary lens mass of M_host ~ 4 Jupiter masses hosting a sub-Earth mass moon. The data are well fit by this exomoon model, but an alternate star+planet model fits the data almost as well. Nevertheless, these results indicate the potential of microlensing to detect exomoons, albeit ones that are different from the giant planet moons in our solar system. The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product M pi_rel, where M is the lens system mass and pi_rel is the lens-source relative parallax. If the lens system is nearby (large pi_rel), then M is small (a few Jupiter masses) and the companion is a sub-Earth-mass exomoon. The best-fit solution has a large lens-source relative proper motion, mu_rel = 19.6 +- 1.6 mas/yr, which would rule out a distant lens system unless the source star has an unusually high proper motion. However, data from the OGLE collaboration nearly rule out a high source proper motion, so the exoplanet+exomoon model is the favored interpretation for the best fit model. However, the alternate solution has a lower proper motion, which is compatible with a distant (so stellar) host. A Bayesian analysis does not favor the exoplanet+exomoon interpretation, so Occam’s razor favors a lens system in the bulge with host and companion masses of M_host = 0.12 (+0.19 -0.06) M_solar and m_comp = 18 (+28 -100 M_earth, at a projected separation of a_perp ~ 0.84 AU. The existence of this degeneracy is an unlucky accident, so current microlensing experiments are in principle sensitive to exomoons. In some circumstances, it will be possible to definitively establish the low mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.

### Enhanced term of order $G^3$ in the light travel time: discussion for some solar system experiments

It is generally believed that knowing the light travel time up to the post-post-Minkowskian level (terms in $G^2$) is sufficient for modelling the most accurate experiments designed to test general relativity in a foreseeable future. However, we have recently brought a rigorous justification of the existence of an enhanced term of order $G^3$ which becomes larger than some first-order contributions like the gravitomagnetic effect due to the rotation of the Sun or the solar quadrupole moment for light rays almost grazing the solar surface. We show that this enhanced term must be taken into account in solar system experiments aiming to reach an accuracy less than $10^{-7}$ in measuring the post-Newtonian parameter $\gamma$.

### The Structure of Exoplanets

The hundreds of exoplanets that have been discovered in the past two decades offer a new perspective on planetary structure. Instead of being the archetypal examples of planets, those of our Solar System are merely possible outcomes of planetary system formation and evolution, and conceivably not even terribly common outcomes (although this remains an open question). Here, we review the diverse range of interior structures that are known to, and speculated to, exist in exoplanetary systems — from mostly degenerate objects that are more than 10 times as massive as Jupiter, to intermediate-mass Neptune-like objects with large cores and moderate hydrogen/helium envelopes, to rocky objects with roughly the mass of the Earth.

### On future opportunities to observe gravitational scattering of main belt asteroids into NEO source regions

Orbital resonances are believed to be responsible for the delivery of main belt asteroids to the inner Solar System. Several possibilities have been suggested to transport asteroids and their fragments into mean motion and secular resonances including non-gravitational forces and gravitational scattering. We investigate future resonance crossings of known asteroids that occur in the main belt over the next century. Our goal is to identify potentially observable injections of asteroids into source regions for Near Earth Objects (NEOs) as well as to determine the role of close encounters among main belt asteroids in this process.

### Astrometric tests of General Relativity in the Solar System: mathematical and computational scenarios [Cross-Listing]

We review the mathematical models available for relativistic astrometry, discussing the different approaches and their accuracies in the context of the modern experiments from space like Gaia and GAME, and we show how these models can be applied to the real world, and their consequences from the mathematical and numerical point of view, with specific reference to the case of Gaia, whose launch is due before the end of the year.

### Astrometric tests of General Relativity in the Solar System: mathematical and computational scenarios

We review the mathematical models available for relativistic astrometry, discussing the different approaches and their accuracies in the context of the modern experiments from space like Gaia and GAME, and we show how these models can be applied to the real world, and their consequences from the mathematical and numerical point of view, with specific reference to the case of Gaia, whose launch is due before the end of the year.

### Les Houches Lectures on Physics Beyond the Standard Model of Cosmology [Cross-Listing]

In these Lectures, I review various extensions of the Lambda-Cold Dark Matter model, characterized by additional light degrees of freedom in the dark sector. In order to reproduce the successful phenomenology of GR in the solar system, these fields must effectively decouple from matter on solar system/laboratory scales. This is achieved through screening mechanisms, which rely on the interplay between self-interactions and coupling to matter to suppress deviations from standard gravity. The manifestation of the new degrees of freedom depends sensitively on their environment, which in turn leads to striking experimental signatures.

### Les Houches Lectures on Physics Beyond the Standard Model of Cosmology

In these Lectures, I review various extensions of the Lambda-Cold Dark Matter model, characterized by additional light degrees of freedom in the dark sector. In order to reproduce the successful phenomenology of GR in the solar system, these fields must effectively decouple from matter on solar system/laboratory scales. This is achieved through screening mechanisms, which rely on the interplay between self-interactions and coupling to matter to suppress deviations from standard gravity. The manifestation of the new degrees of freedom depends sensitively on their environment, which in turn leads to striking experimental signatures.

### Terrestrial Planet Formation at Home and Abroad

We review the state of the field of terrestrial planet formation with the goal of understanding the formation of the inner Solar System and low-mass exoplanets. We review the dynamics and timescales of accretion from planetesimals to planetary embryos and from embryos to terrestrial planets. We discuss radial mixing and water delivery, planetary spins and the importance of parameters regarding the disk and embryo properties. Next, we connect accretion models to exoplanets. We first explain why the observed hot Super Earths probably formed by in situ accretion or inward migration. We show how terrestrial planet formation is altered in systems with gas giants by the mechanisms of giant planet migration and dynamical instabilities. Standard models of terrestrial accretion fail to reproduce the inner Solar System. The "Grand Tack" model solves this problem using ideas first developed to explain the giant exoplanets. Finally, we discuss whether most terrestrial planet systems form in the same way as ours, and highlight the key ingredients missing in the current generation of simulations.

### The Long-Term Dynamical Evolution of Planetary Systems

This chapter concerns the long-term dynamical evolution of planetary systems from both theoretical and observational perspectives. We begin by discussing the planet-planet interactions that take place within our own Solar System. We then describe such interactions in more tightly-packed planetary systems. As planet-planet interactions build up, some systems become dynamically unstable, leading to strong encounters and ultimately either ejections or collisions of planets. After discussing the basic physical processes involved, we consider how these interactions apply to extrasolar planetary systems and explore the constraints provided by observed systems. The presence of a residual planetesimal disc can lead to planetary migration and hence cause instabilities induced by resonance crossing; however, such discs can also stabilise planetary systems. The crowded birth environment of a planetary system can have a significant impact: close encounters and binary companions can act to destabilise systems, or sculpt their properties. In the case of binaries, the Kozai mechanism can place planets on extremely eccentric orbits which may later circularise to produce hot Jupiters.

### Rotational properties of Maria asteroid family

Maria family is regarded as an old-type (~3 +/- 1 Gyr) asteroid family which has experienced substantial collisional and dynamical evolution in the Main-belt. It is located nearby the 3:1 Jupter mean motion resonance area that supplies Near-Earth asteroids (NEAs) to the inner Solar System. We carried out observations of Maria family asteroids during 134 nights from 2008 July to 2013 May, and derived synodic rotational periods for 51 objects, including newly obtained periods of 34 asteroids. We found that there is a significant excess of fast and slow rotators in observed rotation rate distribution. The two-sample Kolmogorov-Smirnov test confirms that the spin rate distribution is not consistent with a Maxwellian at a 92% confidence level. From correlations among rotational periods, amplitudes of lightcurves, and sizes, we conclude that the rotational properties of Maria family asteroids have been changed considerably by non-gravitational forces such as the YORP effect. Using a lightcurve inversion method (Kaasalainen & Torppa 2001; Kaasalainen et al. 2001), we successfully determined the pole orientations for 13 Maria members, and found an excess of prograde versus retrograde spins with a ratio (N_p/N_r) of 3. This implies that the retrograde rotators could have been ejected by the 3:1 resonance into the inner Solar System since the formation of Maria family. We estimate that approximately 37 to 75 Maria family asteroids larger than 1 km have entered the near-Earth space every 100 Myr.

### Large-scale alignments from WMAP and Planck

We revisit the alignments of the largest structures observed in the cosmic microwave background (CMB) using the seven and nine-year WMAP and first-year Planck data releases. The observed alignments — the quadrupole with the octopole and their joint alignment with the direction of our motion with respect to the CMB (the dipole direction) and the geometry of the Solar System (defined by the Ecliptic plane) — are generally in good agreement with results from the previous WMAP data releases. However, a closer look at full-sky data on the largest scales reveals discrepancies between the earlier WMAP data releases (three to seven-year) and the final nine-year release. There are also discrepancies between all the WMAP data releases and the first-year Planck release. Nevertheless, both the WMAP and Planck data confirm the alignments of the largest observable CMB modes in the Universe. In particular, the p-values for the mutual alignment between the quadrupole and octopole, and the alignment of the plane defined by the two with the dipole direction, are both at the greater than 3-sigma level for all three Planck maps studied. We also calculate conditional statistics on the various alignments and find that it is currently difficult to unambiguously identify a leading anomaly that causes the others or even to distinguish correlation from causation.

### A possible mechanism of origin of heavy elements in the solar system

We advance a hypothesis that a collision of a neutron-rich compact object (NRCO) with a massive dense object of the early solar system was responsible for the heavy element enrichment of the system and for the formation of the terrestrial planets.

### A possible mechanism of origin of heavy elements in the solar system [Replacement]

We advance a hypothesis that a collision of a neutron-rich compact object (NRCO) with a massive dense object of the early solar system was responsible for the heavy element enrichment of the system and for the formation of the terrestrial planets.

### Silicon isotopic abundance toward evolved stars and its application for presolar grains

Galactic chemical evolution (GCE) is important for understanding the composition of the present-day interstellar medium (ISM) and of our solar system. In this paper, we aim to track the GCE by using the 29Si/30Si ratios in evolved stars and tentatively relate this to presolar grain composition. We used the APEX telescope to detect thermal SiO isotopologue emission toward four oxygen-rich M-type stars. Together with the data retrieved from the Herschel science archive and from the literature, we were able to obtain the 29Si/30Si ratios for a total of 15 evolved stars inferred from their optically thin 29SiO and 30SiO emission. These stars cover a range of masses and ages, and because they do not significantly alter 29Si/30Si during their lifetimes, they provide excellent probes of the ISM metallicity (or 29Si/30Si ratio) as a function of time. The 29Si/30Si ratios inferred from the thermal SiO emission tend to be lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya), and close to an interstellar or solar value of 1.5 for the higher-mass carbon star IRC+10216 and two red supergiants. There is a tentative correlation between the 29Si/30Si ratios and the mass-loss rates of evolved stars, where we take the mass-loss rate as a proxy for the initial stellar mass or current stellar age. This is consistent with the different abundance ratios found in presolar grains. We found that older objects (up to possibly 10 Gyr old) in our sample trace a previous, lower 29Si/30Si value of about 1. Material with this isotopic ratio is present in two subclasses of presolar grains, providing independent evidence of the lower ratio. Therefore, the 29Si/30Si ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains.

### Secular chaos and its application to Mercury, hot Jupiters, and the organization of planetary systems

In the inner solar system, the planets’ orbits evolve chaotically, driven primarily by secular chaos. Mercury has a particularly chaotic orbit, and is in danger of being lost within a few billion years. Just as secular chaos is reorganizing the solar system today, so it has likely helped organize it in the past. We suggest that extrasolar planetary systems are also organized to a large extent by secular chaos. A hot Jupiter could be the end state of a secularly chaotic planetary system reminiscent of the solar system. But in the case of the hot Jupiter, the innermost planet was Jupiter- (rather than Mercury-) sized, and its chaotic evolution was terminated when it was tidally captured by its star. In this contribution, we review our recent work elucidating the physics of secular chaos and applying it to Mercury and to hot Jupiters. We also present new results comparing the inclinations of hot Jupiters thus produced with observations.

### Si isotope homogeneity of the solar nebula

The presence or absence of variations in the mass-independent abundances of Si isotopes in bulk meteorites provides important clues concerning the evolution of the early solar system. No Si isotopic anomalies have been found within the level of analytical precision of 15 ppm in 29Si/28Si across a wide range of inner solar system materials, including terrestrial basalts, chondrites, and achondrites. A possible exception is the angrites, which may exhibit small excesses of 29Si. However, the general absence of anomalies suggests that primitive meteorites and differentiated planetesimals formed in a reservoir that was isotopically homogenous with respect to Si. Furthermore, the lack of resolvable anomalies in the Calcium-Aluminum-rich Inclusion measured here suggests that any nucleosynthetic anomalies in Si isotopes were erased through mixing in the solar nebula prior to the formation of refractory solids. The homogeneity exhibited by Si isotopes may have implications for the distribution of Mg isotopes in the solar nebula. Based on supernova nucleosynthetic yield calculations, the expected magnitude of heavy-isotope overabundance is larger for Si than for Mg, suggesting that any potential Mg heterogeneity, if present, exists below the 15 ppm level.

### Habitability in the Solar System and New Planetary Missions [Cross-Listing]

Definition of habitability depends on the organisms under consideration. One way to determine habitability of some environment is to compare its certain parameters to environments where extremophilic micro-organisms thrive on Earth. We can also define more common habitability criteria from the life as we know it. These criteria include basic elements, liquid water and an energy source. We know that some locations in our Solar System provide at least some of these limits and criteria. This article describes the aims and technical specifications of some planetary missions, such as NASAs MSL in 2012, ESAs ExoMars missions in 2016 and 2018, and JUICE in 2033. These missions will explore habitability of Mars, Europa, Ganymede and Callisto. Here we compare defined habitability criteria to instrumentation documentation to determine whether these missions could validate the habitability of Mars and those Jovian moons. These missions have about 13 habitability assessment related instruments for Mars, 3 for Europa, 5 for Ganymede and Callisto. I conclude that these missions will yield important information for habitability assessment of these targets.

### Habitability in the Solar System and New Planetary Missions [Replacement]

Definition of habitability depends on the organisms under consideration. One way to determine habitability of some environment is to compare its certain parameters to environments where extremophilic micro-organisms thrive on Earth. We can also define more common habitability criteria from the life as we know it. These criteria include basic elements, liquid water and an energy source. We know that some locations in our Solar System provide at least some of these limits and criteria. This article describes the aims and technical specifications of some planetary missions, such as NASAs MSL in 2012, ESAs ExoMars missions in 2016 and 2018, and JUICE in 2033. These missions will explore habitability of Mars, Europa, Ganymede and Callisto. Here we compare defined habitability criteria to instrumentation documentation to determine whether these missions could validate the habitability of Mars and those Jovian moons. These missions have about 13 habitability assessment related instruments for Mars, 3 for Europa, 5 for Ganymede and Callisto. I conclude that these missions will yield important information for habitability assessment of these targets.

### The Planetary System to KIC 11442793: A Compact Analogue to the Solar System [Replacement]

We announce the discovery of a planetary system with 7 transiting planets around a Kepler target, a current record for transiting systems. Planets b, c, e and f are reported for the first time in this work. Planets d, g and h were previously reported in the literature (Batalha et al. 2013), although here we revise their orbital parameters and validate their planetary nature. Planets h and g are gas giants and show strong dynamical interactions. The orbit of planet g is perturbed in such way that its orbital period changes by 25.7h between two consecutive transits during the length of the observations, which is the largest such perturbation found so far. The rest of the planets also show mutual interactions: planets d, e and f are super-Earths close to a mean motion resonance chain (2:3:4), and planets b and c, with sizes below 2 Earth radii, are within 0.5% of the 4:5 mean motion resonance. This complex system presents some similarities to our Solar System, with small planets in inner orbits and gas giants in outer orbits. It is, however, more compact. The outer planet has an orbital distance around 1 AU, and the relative position of the gas giants is opposite to that of Jupiter and Saturn, which is closer to the expected result of planet formation theories. The dynamical interactions between planets are also much richer.

### The Planetary System to KIC 11442793: A Compact Analogue to the Solar System

We announce the discovery of 7 transiting planets around a Kepler target, a current record for transiting systems. Planets b, c, e and f are reported for the first time in this work. Planets d, g and h were previously reported in the literature (Batalha et al. 2013), although here we revise their orbital parameters and confirm their planetary nature. Planets h and g are gas giants and show strong dynamical interactions. The orbit of planet g is perturbed in such way that its orbital period changes by 25.7h between two consecutive transits during the length of the observations, which is the largest such perturbation found so far. The rest of the planets also show mutual interactions: planets d, e and f are super-Earths close to a mean motion resonance chain (2:3:4), and planets b and c, with sizes below 2 Earth radii, are within 0.5% of the 4:5 mean motion resonance. This complex system presents some similarities to our Solar System, with small planets in inner orbits and gas giants in outer orbits. It is, however, more compact. The outer planet has an orbital distance around 1 AU, and the relative position of the gas giants is opposite to that of Jupiter and Saturn, which is closer to the expected result of planet formation theories. The dynamical interactions between planets are also much richer.

### Chameleons in the Early Universe: Kicks, Rebounds, and Particle Production

Chameleon gravity is a scalar-tensor theory that includes a non-minimal coupling between the scalar field and the matter fields and yet mimics general relativity in the Solar System. The scalar degree of freedom is hidden in high-density environments because the effective mass of the chameleon scalar depends on the trace of the stress-energy tensor. In the early Universe, when the trace of the matter stress-energy tensor is nearly zero, the chameleon is very light, and Hubble friction prevents it from reaching the minimum of its effective potential. Whenever a particle species becomes non-relativistic, however, the trace of the stress-energy tensor is temporarily nonzero, and the chameleon begins to roll. We show that these "kicks" to the chameleon field have catastrophic consequences for chameleon gravity. The velocity imparted to the chameleon by the kick is sufficiently large that the chameleon’s mass changes rapidly as it slides past its potential minimum. This nonadiabatic evolution shatters the chameleon field by generating extremely high-energy perturbations through quantum particle production. If the chameleon’s coupling to matter is slightly stronger than gravitational, the excited modes have trans-Planckian momenta. The production of modes with momenta exceeding 1e7 GeV can only be avoided for small couplings and finely tuned initial conditions. These quantum effects also significantly alter the background evolution of the chameleon field, and we develop new analytic and numerical techniques to treat quantum particle production in the regime of strong dissipation. This analysis demonstrates that chameleon gravity cannot be treated as a classical field theory at the time Big Bang Nucleosynthesis and casts doubt on chameleon gravity’s viability as an alternative to general relativity.

### Spherical Cows in the Sky with Fab Four [Cross-Listing]

We explore spherically symmetric static solutions in a subclass of unitary scalar-tensor theories of gravity, called the Fab Four’ models. The weak field large distance solutions may be phenomenologically viable, but only if the Gauss-Bonnet term is negligible. Only in this limit will the Vainshtein mechanism work consistently. Further, classical constraints and unitarity bounds constrain the models quite tightly. Nevertheless, in the limits where the range of individual terms at large scales is respectively Kinetic Braiding, Horndeski, and Gauss-Bonnet, the horizon scale effects may occur while the theory satisfies Solar system constraints and, marginally, unitarity bounds. On the other hand, to bring the cutoff down to below a millimeter constrains all the couplings scales such that Fab Fours’ can’t be heard outside of the Solar system.

### Spherical Cows in the Sky with Fab Four [Cross-Listing]

We explore spherically symmetric static solutions in a subclass of unitary scalar-tensor theories of gravity, called the Fab Four’ models. The weak field large distance solutions may be phenomenologically viable, but only if the Gauss-Bonnet term is negligible. Only in this limit will the Vainshtein mechanism work consistently. Further, classical constraints and unitarity bounds constrain the models quite tightly. Nevertheless, in the limits where the range of individual terms at large scales is respectively Kinetic Braiding, Horndeski, and Gauss-Bonnet, the horizon scale effects may occur while the theory satisfies Solar system constraints and, marginally, unitarity bounds. On the other hand, to bring the cutoff down to below a millimeter constrains all the couplings scales such that Fab Fours’ can’t be heard outside of the Solar system.

### Measurement of the 33S(\alpha,p)36Cl cross section: Implications for production of 36Cl in the early Solar System [Cross-Listing]

Short-lived radionuclides (SLRs) with lifetimes \tau < 100 Ma are known to have been extant when the Solar System formed over 4.5 billion years ago. Identifying the sources of SLRs is important for understanding the timescales of Solar System formation and processes that occurred early in its history. Extinct 36Cl (t_1/2 = 0.301 Ma) is thought to have been produced by interaction of solar energetic particles (SEPs), emitted by the young Sun, with gas and dust in the nascent Solar System. However, models that calculate SLR production in the early Solar System (ESS) lack experimental data for the 36Cl production reactions. We present here the first measurement of the cross section of one of the main 36Cl production reactions, 33S(\alpha,p)36Cl, in the energy range 0.70 – 2.42 MeV/A. The cross section measurement was performed by bombarding a target and collecting the recoiled 36Cl atoms produced in the reaction, chemically processing the samples, and measuring the 36Cl/Cl ratio of the activated samples with accelerator mass spectrometry (AMS). The experimental results were found to be systematically higher than the cross sections used in previous local irradiation models and other Hauser-Feshbach calculated predictions. However, the effects of the experimentally measured cross sections on the modeled production of 36Cl in the early Solar System were found to be minimal. Reactions channels involving S targets dominate 36Cl production, but the astrophysical event parameters can dramatically change each reactions’ relative contribution.

### Probing Oort Cloud and local ISM properties via dust produced in cometary collisions

The Oort Cloud remains one of the most poorly explored regions of the Solar System. We propose that its properties can be constrained by detecting and studying from space a population of dust grains produced in collisions of comets in the outer Solar System. We explore the dynamics of micron-size grains outside the heliosphere (beyond ~250 AU), which are affected predominantly by the magnetic field of the interstellar medium (ISM) flow past the Sun. We derive analytic models for the production and motion of small particles as a function of their birth location in the Cloud and calculate particle flux and velocity distribution in the inner Solar System. These models are verified by direct numerical simulations. We show that grains originating in the Oort Cloud have a unique distribution of arrival directions (mainly perpendicular to both the ISM wind velocity and the ISM magnetic field), which should easily distinguish them from both interplanetary and interstellar dust populations. We also demonstrate that the distribution of particle arrival velocities is uniquely related to the spatial distribution of the dust production inside the Cloud. The latter is, in turn, determined both by the mass distribution in the Cloud and the physical properties of comets. Cometary collisions within the Oort Cloud are expected to produce a flux of micron-size grains in the inner Solar System of up to several m^{-2} yr^{-1}. The next-generation dust detectors may be sensitive enough to detect and constrain this dust population, which will illuminate us about the Oort Cloud’s properties. We also show that the recently-detected mysterious population of large (micron-size) unbound particles, which seems to arrive with the ISM flow is unlikely to be of a cometary origin.

### On the VLBI measurement of the Solar System acceleration

We propose new estimates of the secular aberration drift, mainly due to the rotation of the Solar System about the Galactic center, based on up-to-date VLBI observations and and improved method of outlier elimination. We fit degree-2 vector spherical harmonics to extragalactic radio source proper motion field derived from geodetic VLBI observations spanning 1979-2013. We pay particular attention to the outlier elimination procedure to remove outliers from (i) radio source coordinate time series and (ii) the proper motion sample. We obtain more accurate values of the Solar system acceleration compared to those in our previous paper. The acceleration vector is oriented towards the Galactic center within 7 deg. The component perpendicular to the Galactic plane is statistically insignificant. We show that an insufficient cleaning of the data set can lead to strong variations in the dipole amplitude and orientation, and statistically biased results.

### A study of the high-inclination population in the Kuiper belt -- 1. The Plutinos

The dynamics of the high-inclination Plutinos is systematically studied. We first present the peculiar features of the 2:3 Neptune mean motion resonance (NMMR) for inclined orbits, especially for the correlation of resonant amplitude A_{\sigma} with inclination i. Using the numerical integrations for the age of the Solar system, the dynamical structure of the 2:3 NMMR is mapped out on the plane of semi-major axis versus i for different eccentricities. We have shown that i of stable resonant orbits could be as high as 90 deg; and the stable region is roughly surrounded by the contours of A_{\sigma} = 120 deg. These new findings allow us to further explore the 2:3 NMMR capture and retention of planetesimals with initial inclinations i0 =< 90 deg in the frame of the planet migration model. We find that the outward transportation of Plutinos is possible for any inclined or even perpendicular orbits. The role of i0 in the formation of Plutinos during Neptune’s migration is highlighted and interesting results are obtained: (1) The capture efficiency of the 2:3 NMMR decreases drastically first with the increase of i0, but it then raises instead when i0 exceeds ~ 50 deg; (2) The magnitude of i-variation is limited to less than 5 deg for any i0, and moreover, for Plutinos with i > 48 deg, their i are forced to decrease throughout the outward migration; (3) Plutinos with i > 48 deg are certainly outside the Kozai mechanism, since an inclination increase is prohibited by the migrating 2:3 NMMR; (4) The 7:11 inclination-type NMMR could be responsible for nearly-circular Plutinos, and a minimum i0 ~ 15 deg is required to intrigue this mechanism.

### The effects of stellar winds and magnetic fields on exoplanets

The great majority of exoplanets discovered so far are orbiting cool, low-mass stars whose properties are relatively similar to the Sun. However, the stellar magnetism of these stars can be significantly different from the solar one, both in topology and intensity. In addition, due to the present-day technology used in exoplanetary searches, most of the currently known exoplanets are found orbiting at extremely close distances to their host stars ($< 0.1$ au). The dramatic differences in stellar magnetism and orbital radius can make the interplanetary medium of exoplanetary systems remarkably distinct from that of the Solar System. To constrain interactions between exoplanets and their host-star’s magnetised winds and to characterise the interplanetary medium that surrounds exoplanets, more realistic stellar wind models, which account for factors such as stellar rotation and the complex stellar magnetic field configurations of cool stars, must be employed. Here, I briefly review the latest progress made in data-driven modelling of magnetised stellar winds. I also show that the interaction of the stellar winds with exoplanets can lead to several observable signatures, some of which that are absent in our own Solar System.

### Constraints on the conservation-law/preferred-frame {alpha}3 parameter from orbital motions [Replacement]

We analytically calculate some orbital effects induced by the Lorentz-invariance/momentum-conservation PPN parameter $\alpha_3$ in a gravitationally bound binary system made of a compact primary orbited by a test particle. We neither restrict ourselves to any particular orbital configuration nor to specific orientations of the primary’s spin axis. We use our results to put constraints on $|\alpha_3|$ in the weak-field regime by using the latest data from Solar System planetary dynamics. From the supplementary perihelion precessions determined with the EPM2011 ephemerides, we preliminarily infer $|\alpha_3|<= 9 x 10^{-11}$, which is about 3 orders of magnitude better than the previous weak-field constraints existing in the literature. The wide pulsar-white dwarf binary PSR J0407+1607 yields an upper bound on the strong-field version of the Lorentz-invariance/momentum-conservation PPN parameter ranging from 3 x 10^-17 up to to 2 x 10^-7 depending on the unknown values of the pulsar’s spin axis orientation and of the orbital node and inclination. We do not recur to statistical arguments involving more than one pulsar.

### Constraints on the conservation-law/preferred-frame {alpha}3 parameter from orbital motions [Replacement]

We analytically calculate some orbital effects induced by the Lorentz-invariance/momentum-conservation PPN parameter $\alpha_3$ in a gravitationally bound binary system made of a compact primary orbited by a test particle. We neither restrict ourselves to any particular orbital configuration nor to specific orientations of the primary’s spin axis. We use our results to put constraints on $|\alpha_3|$ in the weak-field regime by using the latest data from Solar System planetary dynamics. From the supplementary perihelion precessions determined with the EPM2011 ephemerides, we preliminarily infer $|\alpha_3|<= 9 x 10^{-11}$, which is about 3 orders of magnitude better than the previous weak-field constraints existing in the literature. The wide pulsar-white dwarf binary PSR J0407+1607 yields an upper bound on the strong-field version of the Lorentz-invariance/momentum-conservation PPN parameter ranging from 3 x 10^-17 up to to 2 x 10^-7 depending on the unknown values of the pulsar’s spin axis orientation and of the orbital node and inclination. We do not recur to statistical arguments involving more than one pulsar.

### 26Al in the Early Solar System: Not so Unusual After All [Replacement]

Recently acquired evidence shows that extrasolar asteroids exhibit over a factor of 100 variation in the iron to aluminum abundance ratio. This large range likely is a consequence of igneous differentiation that resulted from heating produced by radioactive decay of 26Al with an abundance comparable to that in the solar system’s protoplanetary disk at birth. If so, the conventional view that our solar system began with an unusually high amount of 26Al should be discarded.

### A Proposal for New Definitions of Solar System Bodies - Planet, Moon, and Satellite [Replacement]

A new classification system for Solar System bodies is proposed which takes into account both physical and dynamical perspectives as well as critiques of the IAU resolutions for the definitions of planet, dwarf planet, and small solar system bodies. In this paper definitions are proposed for the terms planet, moon, and satellite and a classification system is presented with four verified classes of planets in the Solar System: Terrestrial, Cerian, Jovian, and Kuiperian. Specific physical criteria are proposed for identifying an object as a planet and the classification scheme addresses additional dynamical and physical criteria with subclasses. The four planet classes naturally address dynamical concerns and generally correspond with environments and processes of planet formation within the Solar System. This proposal addresses the problem of how to classify the growing list of satellites discovered orbiting the Jovian planets by defining two classes of satellite: moons and satellites. Scientific, pedagogical, and cultural advantages of this new proposal over the current IAU definitions are discussed.

### Preliminary bounds of the gravitational Local Position Invariance from Solar System planetary precessions [Replacement]

In the framework of the Parameterized Post-Newtonian (PPN) formalism, we calculate the long-term Preferred Location (PL) effects, proportional to the Whitehead parameter $\xi$, affecting all the Keplerian orbital elements of a localized two-body system, apart from the semimajor axis $a$. They violate the gravitational Local Position Invariance (LPI), fulfilled by General Relativity (GR). We obtain preliminary bounds on $\xi$ by using the latest results in the field of the Solar System planetary ephemerides. The non-detection of any anomalous perihelion precession for Mars allows us to indirectly infer $|\xi|\leq 5.8\times 10^{-6}$. \textcolor{black}{Such a bound is close to the constraint, of the order of $10^{-6}$, expected from the future BepiColombo mission to Mercury. As a complementary approach, the PL effects should be explicitly included in the dynamical models fitted to planetary data sets to estimate $\xi$ in a least-square fashion in a dedicated ephemerides orbit solution.} The ratio of the anomalous perihelion precessions for Venus and Jupiter, determined with the EPM2011 ephemerides at the $<3\sigma$ level, if confirmed as genuine physical effects needing explanation by future studies, rules out the hypothesis $\xi \neq 0$. A critical discussion of the $|\xi| \lesssim 10^{-6}-10^{-7}$ upper bounds obtained in the literature from the close alignment of the Sun’s spin axis and the total angular momentum of the Solar System is presented.

### Preliminary bounds of the gravitational Local Position Invariance from Solar System planetary precessions [Replacement]

In the framework of the Parameterized Post-Newtonian (PPN) formalism, we calculate the long-term Preferred Location (PL) effects, proportional to the Whitehead parameter $\xi$, affecting all the Keplerian orbital elements of a localized two-body system, apart from the semimajor axis $a$. They violate the gravitational Local Position Invariance (LPI), fulfilled by General Relativity (GR). We obtain preliminary bounds on $\xi$ by using the latest results in the field of the Solar System planetary ephemerides. The non-detection of any anomalous perihelion precession for Mars allows us to indirectly infer $|\xi|\leq 5.8\times 10^{-6}$. \textcolor{black}{Such a bound is close to the constraint, of the order of $10^{-6}$, expected from the future BepiColombo mission to Mercury. As a complementary approach, the PL effects should be explicitly included in the dynamical models fitted to planetary data sets to estimate $\xi$ in a least-square fashion in a dedicated ephemerides orbit solution.} The ratio of the anomalous perihelion precessions for Venus and Jupiter, determined with the EPM2011 ephemerides at the $<3\sigma$ level, if confirmed as genuine physical effects needing explanation by future studies, rules out the hypothesis $\xi \neq 0$. A critical discussion of the $|\xi| \lesssim 10^{-6}-10^{-7}$ upper bounds obtained in the literature from the close alignment of the Sun’s spin axis and the total angular momentum of the Solar System is presented.

### Expanded solar-system limits on violations of the equivalence principle [Replacement]

Most attempts to unify general relativity with the standard model of particle physics predict violations of the equivalence principle associated in some way with the composition of the test masses. We test this idea by using observational uncertainties in the positions and motions of solar-system bodies to set upper limits on possible differences $\Delta$ between the gravitational and inertial mass of each body. For suitable pairs of objects, it is possible to constrain three different linear combinations of $\Delta$ using Kepler’s third law, the migration of stable Lagrange points, and orbital polarization (the Nordtvedt effect). Limits of order $10^{-10}-10^{-6}$ on $\Delta$ for individual bodies can then be derived from planetary and lunar ephemerides, Cassini observations of the Saturn system, and observations of Jupiter’s Trojan asteroids as well as recently discovered Trojan companions around the Earth, Mars, Neptune, and Saturnian moons. These results can be combined with models for elemental abundances in each body to test for composition-dependent violations of the universality of free fall in the solar system. The resulting limits are weaker than those from laboratory experiments, but span a larger volume in composition space.

### Expanded solar-system limits on violations of the equivalence principle [Replacement]

Most attempts to unify general relativity with the standard model of particle physics predict violations of the equivalence principle associated in some way with the composition of the test masses. We test this idea by using observational uncertainties in the positions and motions of solar-system bodies to set upper limits on possible differences $\Delta$ between the gravitational and inertial mass of each body. For suitable pairs of objects, it is possible to constrain three different linear combinations of $\Delta$ using Kepler’s third law, the migration of stable Lagrange points, and orbital polarization (the Nordtvedt effect). Limits of order $10^{-10}-10^{-6}$ on $\Delta$ for individual bodies can then be derived from planetary and lunar ephemerides, Cassini observations of the Saturn system, and observations of Jupiter’s Trojan asteroids as well as recently discovered Trojan companions around the Earth, Mars, Neptune, and Saturnian moons. These results can be combined with models for elemental abundances in each body to test for composition-dependent violations of the universality of free fall in the solar system. The resulting limits are weaker than those from laboratory experiments, but span a larger volume in composition space.

### Galileon forces in the Solar System [Replacement]

We consider the challenging problem of obtaining an analytic understanding of realistic astrophysical dynamics in the presence of a Vainshtein screened fifth force arising from infrared modifications of General Relativity. In particular, we attempt to solve — within the most general flat spacetime galileon model — the scalar force law between well separated bodies located well within the Vainshtein radius of the Sun. To this end, we derive the exact static Green’s function of the galileon wave equation linearized about the background field generated by the Sun, for the minimal cubic and maximally quartic galileon theories, and then introduce a method to compute the general leading order force law perturbatively away from these limits. We also show that the same nonlinearities which produce the Vainshtein screening effect present obstacles to an analytic calculation of the galileon forces between closely bound systems within the solar system, such as that of the Earth and Moon. Within the test mass approximation, we deduce that a large enough quartic galileon interaction would suppress the effect on planetary perihelion precession below the level detectable by even the next-generation experiments.

### Galileon forces in the Solar System [Replacement]

We consider the challenging problem of obtaining an analytic understanding of realistic astrophysical dynamics in the presence of a Vainshtein screened fifth force arising from infrared modifications of General Relativity. In particular, we attempt to solve — within the most general flat spacetime galileon model — the scalar force law between well separated bodies located well within the Vainshtein radius of the Sun. To this end, we derive the exact static Green’s function of the galileon wave equation linearized about the background field generated by the Sun, for the minimal cubic and maximally quartic galileon theories, and then introduce a method to compute the general leading order force law perturbatively away from these limits. We also show that the same nonlinearities which produce the Vainshtein screening effect present obstacles to an analytic calculation of the galileon forces between closely bound systems within the solar system, such as that of the Earth and Moon. Within the test mass approximation, we deduce that a large enough quartic galileon interaction would suppress the effect on planetary perihelion precession below the level detectable by even the next-generation experiments.

### Constraining the Preferred-Frame $\alpha_1$, $\alpha_2$ parameters from Solar System planetary precessions [Replacement]

Analytical expressions for the orbital precessions affecting the relative motion of the components of a local binary system induced by Lorentz-violating Preferred Frame Effects (PFE) are explicitly computed in terms of the PPN parameters $\alpha_1$, $\alpha_2$. A linear combination of the supplementary perihelion precessions of all the inner planets of the Solar System, able to remove the a-priori bias of unmodelled/mismodelled standard effects such as the general relativistic Lense-Thirring precessions and the classical rates due to the Sun’s oblateness $J_2$, allows to infer $|\alpha_1| \leq 6\times 10^{-6}, |\alpha_2| \leq 3.5\times 10^{-5}$. Such bounds should be improved in the near future after processing the data that are being collected by the MESSENGER spacecraft, currently orbiting Mercury. Further improvements may come in the mid-future from the approved BepiColombo mission to Mercury. The constraint $|\alpha_2|\leq 10^{-7}$ existing in the literature is critically discussed (Abridged).

### Constraining the Preferred-Frame $\alpha_1$, $\alpha_2$ parameters from Solar System planetary precessions [Replacement]

Analytical expressions for the orbital precessions affecting the relative motion of the components of a local binary system induced by Lorentz-violating Preferred Frame Effects (PFE) are explicitly computed in terms of the PPN parameters $\alpha_1$, $\alpha_2$. A linear combination of the supplementary perihelion precessions of all the inner planets of the Solar System, able to remove the a-priori bias of unmodelled/mismodelled standard effects such as the general relativistic Lense-Thirring precessions and the classical rates due to the Sun’s oblateness $J_2$, allows to infer $|\alpha_1| \leq 6\times 10^{-6}, |\alpha_2| \leq 3.5\times 10^{-5}$. Such bounds should be improved in the near future after processing the data that are being collected by the MESSENGER spacecraft, currently orbiting Mercury. Further improvements may come in the mid-future from the approved BepiColombo mission to Mercury. The constraint $|\alpha_2|\leq 10^{-7}$ existing in the literature is critically discussed (Abridged).

### Fermi LAT study of cosmic-rays and the interstellar medium in nearby molecular clouds

We report an analysis of the interstellar gamma-ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ~300 pc from the solar system. The gamma-ray emission produced by interactions of cosmic-rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained gamma-ray emissivities above 250 MeV are (5.9 +/- 0.1(stat) (+0.9/-1.0)(sys)), (10.2 +/- 0.4(stat) (+1.2/-1.7)(sys)), and (9.1 +/- 0.3(stat) (+1.5/-0.6)(sys)) x10^(-27) photons s^(-1) sr^(-1) H-atom^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ~20% in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, Xco = N(H2)/Wco, is found to be (0.96 +/- 0.06(stat) (+0.15/-0.12)(sys)), (0.99 +/- 0.08(stat) (+0.18/-0.10)(sys)), and (0.63 +/- 0.02(stat) (+0.09/-0.07)(sys)) x10^20 H2-molecule cm^(-2) (K km s^(-1))^(-1) for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of Xco in the vicinity of the solar system. From the obtained values of Xco, the masses of molecular gas traced by Wco in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ~5×10^3, ~10^3, and ~3.3×10^4 Msolar, respectively. A comparable amount of gas not traced well by standard HI and CO surveys is found in the regions investigated.

### Alignment of the stellar spin with the orbits of a three-planet system

The Sun’s equator and the planets’ orbital planes are nearly aligned, which is presumably a consequence of their formation from a single spinning gaseous disk. For exoplanetary systems this well-aligned configuration is not guaranteed: dynamical interactions may tilt planetary orbits, or stars may be misaligned with the protoplanetary disk through chaotic accretion, magnetic interactions or torques from neighbouring stars. Indeed, isolated ‘hot Jupiters’ are often misaligned and even orbiting retrograde. Here we report an analysis of transits of planets over starspots on the Sun-like star Kepler-30, and show that the orbits of its three planets are aligned with the stellar equator. Furthermore, the orbits are aligned with one another to within a few degrees. This configuration is similar to that of our Solar System, and contrasts with the isolated hot Jupiters. The orderly alignment seen in the Kepler-30 system suggests that high obliquities are confined to systems that experienced disruptive dynamical interactions. Should this be corroborated by observations of other coplanar multi-planet systems, then star-disk misalignments would be ruled out as the explanation for the high obliquities of hot Jupiters, and dynamical interactions would be implicated as the origin of hot Jupiters.

### Supernova-Triggered Molecular Cloud Core Collapse and the Rayleigh-Taylor Fingers that Polluted the Solar Nebula

A supernova is a likely source of short-lived radioisotopes (SLRIs) that were present during the formation of the earliest solar system solids. A suitably thin and dense supernova shock wave may be capable of triggering the self-gravitational collapse of a molecular cloud core while simultaneously injecting SLRIs. Axisymmetric hydrodynamics models have shown that this injection occurs through a number of Rayleigh-Taylor (RT) rings. Here we use the FLASH adaptive mesh refinement (AMR) hydrodynamics code to calculate the first fully three dimensional (3D) models of the triggering and injection process. The axisymmetric RT rings become RT fingers in 3D. While ~ 100 RT fingers appear early in the 3D models, only a few RT fingers are likely to impact the densest portion of the collapsing cloud core. These few RT fingers must then be the source of any SLRI spatial heterogeneity in the solar nebula inferred from isotopic analyses of chondritic meteorites. The models show that SLRI injection efficiencies from a supernova several pc away fall at the lower end of the range estimated for matching SLRI abundances, perhaps putting them more into agreement with recent reassessments of the level of 60Fe present in the solar nebula.

### Observational constraints on Kaluza-Klein models with $d$-dimensional spherical compactification [Cross-Listing]

We investigate Kaluza-Klein models in the case of spherical compactification of the internal space with an arbitrary number of dimensions. The gravitating source has the dust-like equation of state in the external/our space and an arbitrary equation of state (with the parameter $\Omega$) in the internal space. We get the perturbed (up to $O(1/c^2)$) metric coefficients. For the external space, these coefficients consist of two parts: the standard general relativity expressions plus the admixture of the Yukawa interaction. This admixture takes place only for some certain condition which is equivalent to the condition for the internal space stabilization. We demonstrate that the mass of the Yukawa interaction is defined by the mass of the gravexciton/radion. In the Solar system, the Yukawa mass is big enough for dropping the admixture of this interaction and getting good agreement with the gravitational tests for any value of $\Omega$. However, the gravitating body acquires the effective relativistic pressure in the external space which vanishes only in the case of tension $\Omega=-1/2$ in the internal space.

### Axion as a cold dark matter candidate: low-mass case

Axion as a coherently oscillating scalar field is known to behave as a cold dark matter in all cosmologically relevant scales. For conventional axion mass with 10^{-5} eV, the axion reveals a characteristic damping behavior in the evolution of density perturbations on scales smaller than the solar system size. The damping scale is inversely proportional to the square-root of the axion mass. We show that the axion mass smaller than 10^{-24} eV induces a significant damping in the baryonic density power spectrum in cosmologically relevant scales, thus deviating from the cold dark matter in the scale smaller than the axion Jeans scale. With such a small mass, however, our basic assumption about the coherently oscillating scalar field is broken in the early universe. This problem is shared by other dark matter models based on the Bose-Einstein condensate and the ultra-light scalar field. We introduce a simple model to avoid this problem by introducing evolving axion mass in the early universe, and present observational effects of present-day low-mass axion on the baryon density power spectrum, the cosmic microwave background radiation (CMB) temperature power spectrum, and the growth rate of baryon density perturbation. In our low-mass axion model we have a characteristic small-scale cutoff in the baryon density power spectrum below the axion Jeans scale. The small-scale deviations from the cold dark matter model in both matter and CMB power spectra clearly differ from the ones expected in the cold dark matter model mixed with the massive neutrinos as a hot dark matter component.

### Signatures of Modified Gravity on the 21-cm Power Spectrum at Reionisation [Replacement]

Scalar modifications of gravity have an impact on the growth of structure. Baryon and Cold Dark Matter (CDM) perturbations grow anomalously for scales within the Compton wavelength of the scalar field. In the late time Universe when reionisation occurs, the spectrum of the 21cm brightness temperature is thus affected. We study this effect for chameleon-f(R) models, dilatons and symmetrons. Although the f(R) models are more tightly constrained by solar system bounds, and effects on dilaton models are negligible, we find that symmetrons where the phase transition occurs before z_* ~ 12 will be detectable for a scalar field range as low as 5 kpc. For all these models, the detection prospects of modified gravity effects are higher when considering modes parallel to the line of sight where very small scales can be probed. The study of the 21 cm spectrum thus offers a complementary approach to testing modified gravity with large scale structure surveys. Short scales, which would be highly non-linear in the very late time Universe when structure forms and where modified gravity effects are screened, appear in the linear spectrum of 21 cm physics, hence deviating from General Relativity in a maximal way.

### Signatures of Modified Gravity on the 21-cm Power Spectrum at Reionisation

Scalar modifications of gravity have an impact on the growth of structure. Baryon and Cold Dark Matter (CDM) perturbations grow anomalously for scales within the Compton wavelength of the scalar field. In the late time Universe when reionisation occurs, the spectrum of the 21cm brightness temperature is thus affected. We study this effect for chameleon-f(R) models, dilatons and symmetrons. Although the f(R) models are more tightly constrained by solar system bounds, and effects on dilaton models are negligible, we find that symmetrons where the phase transition occurs before z_* ~ 2 will be detectable for a scalar field range as low as 5 kpc. For all these models, the detection prospects of modified gravity effects are higher when considering modes parallel to the line of sight where very small scales can be probed. The study of the 21 cm spectrum thus offers a complementary approach to testing modified gravity with large scale structure surveys. Short scales, which would be highly non-linear in the very late time Universe when structure forms and where modified gravity effects are screened, appear in the linear spectrum of 21 cm physics, hence deviating from General Relativity in a maximal way.