The Formation of Gas Giant Planets Through Slow Pebble Accretion

Growing the gas-giant planets by the gradual accumulation of pebbles

Authors:

Levison et al

Abstract:

It is widely held that the first step in forming the gas giant planets, such as Jupiter and Saturn, is to form solid `cores' of roughly 10 M⊕. Getting the cores to form before the solar nebula dissipates (∼1−10Myr) has been a major challenge for planet formation models. Recently models have emerged in which `pebbles' (centimeter- to meter-size objects) are first concentrated by aerodynamic drag and then gravitationally collapse to form 100 --- 1000 km objects. These `planetesimals' can then efficiently accrete leftover pebbles and directly form the cores of giant planets. This model known as `pebble accretion', theoretically, can produce 10 M⊕ cores in only a few thousand years. Unfortunately, full simulations of this process show that, rather than creating a few 10 M⊕ cores, it produces a population of hundreds of Earth-mass objects that are inconsistent with the structure of the Solar System. Here we report that this difficulty can be overcome if pebbles form slowly enough to allow the planetesimals to gravitationally interact with one another. In this situation the largest planetesimals have time to scatter their smaller siblings out of the disk of pebbles, thereby stifling their growth. Our models show that, for a large, and physically reasonable region of parameter space, this typically leads to the formation of one to four gas giants between 5 and 15 AU in agreement with the observed structure of the Solar System.

Forced Libration of Tidally Synchronized ExoPlanets & ExoMoons

Forced libration of tidally synchronized planets and moons

Authors:

Makarov et al

Abstraction:

Tidal dissipation of kinetic energy, when it is strong enough, tends to synchronize the rotation of planets and moons with the mean orbital motion, or drive it into long-term stable spin-orbit resonances. As the orbital motion undergoes periodic acceleration due to a finite orbital eccentricity, the spin rate oscillates around the equilibrium mean value too, giving rise to the forced, or eccentricity-driven, librations. Both the shape and amplitude of forced librations of synchronous viscoelastic planets and moons are defined by a combination of two different types of perturbative torque, the tidal torque and the triaxial torque. Consequently, forced librations can be tidally dominated (e.g., Io and possibly Titan) or deformation-dominated (e.g., the Moon) depending on a set of orbital, rheological, and other physical parameters. With small eccentricities, for the former kind, the largest term in the libration angle can be minus cosine of the mean anomaly, whereas for the latter kind, it is minus sine of the mean anomaly. The shape and the amplitude of tidal forced librations determine the rate of orbital evolution of synchronous planets and moons, i.e., the rate of dissipative damping of semimajor axis and eccentricity. The known super-Earth exoplanets can exhibit both kinds of libration, or a mixture thereof, depending on, for example, the effective Maxwell time of their rigid mantles. Our approach can be extended to estimate the amplitudes of other libration harmonics, as well as the forced libration in non-synchronous spin-orbit resonances.

Tidal Effects Disputed as Source of Exoplanet Spin Orbit Misalignment

Are Tidal Effects Responsible for Exoplanetary Spin-Orbit Alignment?

Authors:

Li et al

Abstract:

The obliquities of planet-hosting stars are clues about the formation of planetary systems. Previous observations led to the hypothesis that for close-in giant planets, spin-orbit alignment is enforced by tidal interactions. Here, we examine two problems with this hypothesis. First, Mazeh and coworkers recently used a new technique -- based on the amplitude of starspot-induced photometric variability -- to conclude that spin-orbit alignment is common even for relatively long-period planets, which would not be expected if tides were responsible. We re-examine the data and find a statistically significant correlation between photometric variability and planetary orbital period that is qualitatively consistent with tidal interactions. However it is still difficult to explain quantitatively, as it would require tides to be effective for periods as long as tens of days. Second, Rogers and Lin argued against a particular theory for tidal re-alignment by showing that initially retrograde systems would fail to be re-aligned, in contradiction with the observed prevalence of prograde systems. We investigate a simple model that overcomes this problem by taking into account the dissipation of inertial waves and the equilibrium tide, as well as magnetic braking. We identify a region of parameter space where re-alignment can be achieved, but it only works for close-in giant planets, and requires some fine tuning. Thus, while we find both problems to be more nuanced than they first appeared, the tidal model still has serious shortcomings.

Inferring Planetary Obliquity Using Rotational & Orbital Photometry

Inferring Planetary Obliquity Using Rotational & Orbital Photometry

Authors:

Schwartz et al

Abstract:

The obliquity of a terrestrial planet is an important clue about its formation and critical to its climate. Previous studies using simulated photometry of Earth show that continuous observations over most of a planet's orbit can be inverted to infer obliquity. We extend this approach to single-epoch observations for planets with arbitrary albedo maps. For diffuse reflection, the flux seen by a distant observer is the product of the planet's albedo map, the host star's illumination, and the observer's visibility of different planet regions. It is useful to treat the product of illumination and visibility as the kernel of a convolution; this kernel is unimodal and symmetric. For planets with unknown obliquity, the kernel is not known a priori, but could be inferred by fitting a rotational light curve. We analyze this kernel under different viewing geometries, finding it well described by its longitudinal width and latitudinal position. We use Monte Carlo simulation to estimate uncertainties on these kernel characteristics from variations in a planet's apparent albedo. We demonstrate that the kernel properties are functions of obliquity and axial orientation, which may both be inferred even if planets are A) East-West uniform or spinning rapidly, or B) North-South uniform. We consider degeneracies in these inferences with a case study, and describe how to tell prograde from retrograde rotation for inclined, oblique planets. This approach could be used to estimate obliquities of terrestrial planets with modest time investment from flagship direct-imaging missions.

WFIRST Gets an Early Start

In a clean room at the Goddard Space Flight Center in Maryland, the light-collecting heart of NASA’s next great space telescope is finally coming together. For the last several weeks, technicians, aided by a robotic arm, have been putting hexagonal mirror segments into a structure for the James Webb Space Telescope (JWST). As of last week, 13 of the 18 mirror segments were in place, with all 18 expected to be in position by the end of February.

The assembly of JWST’s primary mirror is just one aspect of the telescope’s construction. Elsewhere, the telescope’s instruments are being tested while the spacecraft bus and its deployable sunshade, the size of a tennis court, are put together. Several years after a critical “replan” of the observatory, years behind its original schedule and billions of dollars over its original budget, NASA says JWST remains on track for launch on an Ariane 5 in October 2018.

That means that spending on JWST—$620 million for the 2016 fiscal year—will soon ramp down. For several years, NASA had been anticipating the “wedge” in the budget this would create and started planning for the next large space observatory beyond JWST that wedge of funding would enable. The leading candidate for that mission has been a concept called the Wide Field Infrared Survey Telescope (WFIRST), one endorsed by astronomers as their top priority large mission in their latest decadal survey in 2010.

WFIRST, as it turns out, will start even sooner that NASA expected. At the 227th Meeting of the American Astronomical Society (AAS) held last week in Florida, agency officials announced that WFIRST will “enter formulation” in February. That milestone, also known in NASA’s project management terminology as “Key Decision Point A,” sets WFIRST on course for a launch in the mid-2020s.

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The Host Stars of Keplers Habitable Exoplanets: Superflares, Rotation and Activity

The Host Stars of Keplers Habitable Exoplanets: Superflares, Rotation and Activity

Authors:

Armstrong et al

Abstract:

We embark on a detailed study of the lightcurves of Keplers most Earth-like exoplanet host stars using the full length of Kepler data. We derive rotation periods, photometric activity indices, flaring energies, mass loss rates, gyrochronological ages, X-ray luminosities and consider implications for the planetary magnetospheres and habitability. Furthermore, we present the detection of superflares in the lightcurve of Kepler-438, the exoplanet with the highest Earth Similarity Index to date. Kepler-438b orbits at a distance of 0.166AU to its host star, and hence may be susceptible to atmospheric stripping. Our sample is taken from the Habitable Exoplanet Catalogue, and consists of the stars Kepler-22, Kepler-61, Kepler-62, Kepler-174, Kepler-186, Kepler-283, Kepler-296, Kepler-298, Kepler-438, Kepler-440, Kepler-442, Kepler-443 and KOI-4427, between them hosting 15 of the most habitable transiting planets known to date from Kepler.

Effects of stellar cosmic rays on M Dwarf Earth-like Exoplanets' Atmospheres

Atmospheric effects of stellar cosmic rays on Earth-like exoplanets orbiting M-dwarfs

Authors:

Tabataba-Vakili et al

Abstract:

M-dwarf stars are generally considered favourable for rocky planet detection. However, such planets may be subject to extreme conditions due to possible high stellar activity. The goal of this work is to determine the potential effect of stellar cosmic rays on key atmospheric species of Earth-like planets orbiting in the habitable zone of M-dwarf stars and show corresponding changes in the planetary spectra. We build upon the cosmic rays model scheme of Grenfell et al. (2012), who considered cosmic ray induced NOx production, by adding further cosmic ray induced production mechanisms (e.g. for HOx) and introducing primary protons of a wider energy range (16 MeV - 0.5 TeV). Previous studies suggested that planets in the habitable zone that are subject to strong flaring conditions have high atmospheric methane concentrations, while their ozone biosignature is completely destroyed. Our current study shows, however, that adding cosmic ray induced HOx production can cause a decrease in atmospheric methane abundance of up to 80\%. Furthermore, the cosmic ray induced HOx molecules react with NOx to produce HNO3, which produces strong HNO3 signals in the theoretical spectra and reduces NOx-induced catalytic destruction of ozone so that more than 25\% of the ozone column remains. Hence, an ozone signal remains visible in the theoretical spectrum (albeit with a weaker intensity) when incorporating the new cosmic ray induced NOx and HOx schemes, even for a constantly flaring M-star case. We also find that HNO3 levels may be high enough to be potentially detectable. Since ozone concentrations, which act as the key shield against harmful UV radiation, are affected by cosmic rays via NOx-induced catalytic destruction of ozone, the impact of stellar cosmic rays on surface UV fluxes is also studied.