HD 155358's Interacting Gas Giants

The orbital configuration of the two interacting Jupiters in HD 155358 system
Authors:

Ma et al

Abstract:
Recent observation reveals two interacting Jupiters possibly trapped in a 2:1 mean motion resonance (MMR) around the star HD 155358. For the 2:1 MMR, Beaugé et al. found that as long as the orbital decay was sufficiently slow, the trapped planets should also be in apsidal co-rotation. So it is very interesting to explore whether HD 155358 did undergo such an evolution and presents an apsidal co-rotation. Based on the existing results of spectroscopic orbital determination, the global dynamics of the system shows that the two planets are in an apsidal co-rotation if the eccentricity of the outer planet (ec) takes values very close to the lower limit of its 1σ confidence interval. This makes us conjecture that the globally minimizing solution could be missed in the previous orbital determination. Using an efficient global optimization method, we do find a better solution, reducing χ2 from 1.4 to 1.2. This new solution is significantly different from the previous one, and in particular, with smaller ec. However, the increased possibility for the system to be trapped in a 2:1 MMR with apsidal co-rotation is still not high. A set of simulations of the adiabatic convergent migration process are then performed. The results consistently indicate that the 2:1 MMR forms before apsidal co-rotation. Finally, the long-term stability of the formed system and of its resonant structure are extensively explored, and the resulting statistics are given. A conclusion is that the 2:1 MMR with apsidal co-rotation is a very stable structure.

More Data Needed to Understand hot Rocky SuperEarth Atmospheres

Laboratory spectra of hot molecules: data needs for hot super-Earth exoplanets 

Authors:

Tennyson et al

Abstract:
The majority of stars are now thought to support exoplanets. Many of those exoplanets discovered thus far are categorized as rocky objects with an atmosphere. Most of these objects are however hot due to their short orbital period. Models suggest that water is the dominant species in their atmospheres. The hot temperatures are expected to turn these atmospheres into a (high pressure) steam bath containing remains of melted rock. The spectroscopy of these hot rocky objects will be very different from that of cooler objects or hot gas giants. Molecules suggested to be important for the spectroscopy of these objects are reviewed together with the current status of the corresponding spectroscopic data. Perspectives of building a comprehensive database of linelist/cross sections applicable for atmospheric models of rocky super-Earths as part of the ExoMol project are discussed. The quantum-mechanical approaches used in linelist productions and their challenges are summarized.

Multiple Disk Gaps and Rings Generated by a Single Super-Earth

Multiple Disk Gaps and Rings Generated by a Single Super-Earth
Authors:

Dong et al

Abstract:
We investigate the observational signatures of super-Earths (i.e., Earth-to-Neptune mass planets) in their natal disks of gas and dust. Combining two-fluid global hydrodynamics simulations with a radiative transfer code, we calculate the distributions of gas and of sub-mm-sized dust in a disk perturbed by a super-Earth, synthesizing images in near-infrared scattered light and the mm-wave thermal continuum for direct comparison with observations. In low viscosity gas (α≲10−4), a super-Earth opens two annular gaps to either side of its orbit by the action of Lindblad torques. This double gap and its associated gas pressure gradients cause dust particles to be dragged by gas into three rings: one ring sandwiched between the two gaps, and two rings located at the gap edges farthest from the planet. Depending on system parameters, additional rings may manifest for a single planet. A double gap located at tens of AUs from a host star in Taurus can be detected in the dust continuum by the Atacama Large Millimeter Array (ALMA) at an angular resolution of ~0".03 after two hours of integration. Ring and gap features persist in a variety of background disk profiles, last for thousands of orbits, and change their relative positions and dimensions depending on the speed and direction of planet migration. Candidate double gaps have been observed by ALMA in systems like HL Tau (D5 and D6) and TW Hya (at 37 and 43 AU); we submit that each double gap is carved by one super-Earth in nearly inviscid gas.

Simulations Fail to Explain SuperEarth Resonant Chains

Breaking the Chains: Hot Super-Earth systems from migration and disruption of compact resonant chains
Authors:

Izidoro et al

Abstract:
“Hot super-Earths” (or “Mini-Neptunes”) between 1 and 4 times Earth’s size with period shorter than 100 days orbit 30-50% of Sun-like type stars. Their orbital configuration – measured as the period ratio distribution of adjacent planets in multi-planet systems – is a strong constraint for formation models. Here we use N-body simulations with synthetic forces from an underlying evolving gaseous disk to model the formation and long-term dynamical evolution of super-Earth systems. While the gas disk is present, planetary embryos grow and migrate inward to form a resonant chain anchored at the inner edge of the disk. These resonant chains are far more compact than the observed super-Earth systems. Once the gas dissipates resonant chains may become dynamically unstable. They undergo a phase of giant impacts that spreads the systems out. Disk turbulence has no measurable effect on the outcome. Our simulations match observations if a small fraction of resonant chains remain stable, while most super-Earths undergo a late dynamical instability. Our statistical analysis restricts the contribution of stable systems to less than 25%. Our results also suggest that the large fraction of observed single planet systems does not necessarily imply any dichotomy in the architecture of planetary systems. Finally, we use the low abundance of resonances in Kepler data to argue that, in reality, the survival of resonant chains happens likely only in ∼5% of the cases. This leads to a mystery: in our simulations only 50-60% of resonant chains became unstable whereas at least 75% (and probably 90-95%) must be unstable to match observations.

Stellar Cosmic Rays in a Habitable Zone

Stellar Cosmic Rays in a Habitable Zone 

Authors: 

Struminsky et al 

Abstract: 

According to recent observations relative number of flare stars does not change very much from cool dwarfs to hot A stars. Flare energies are strongly correlated with stellar luminosity and radius. Whence it follows that the typical magnetic field associated with a flare is several tens gauss and the typical flare loop length-scales are parts of the stellar radius. Flares on O--B stars were not observed, but they are possible, since strong magnetic fields are detected on O--B stars. Therefore stars of O--M spectral classes are potential sources of cosmic rays. Energy estimates of a magnetic field strength in a tube in photospheres of O--M stars are performed. Basing on their values possible flare energies and numbers of accelerated protons are estimated. The values obtained for the Sun correspond to observations by order of magnitude that justify estimates for other stars. Values of magnetic field strength in a tube differ less than five times for O and M flares (700 and 3500~G), but corresponding flare energies and numbers of accelerated protons for O stars are greater by five orders. Contrary fluencies of stellar protons appear to be five orders less.

The Inner Most TRAPPIST-1 Exoplanets may Have Lost Their Atmospheres

Atmospheric escape from the TRAPPIST-1 planets and implications for habitability 

Authors: 

Dong et al 

Abstract: 

The presence of an atmosphere over sufficiently long timescales is widely regarded as one of the most prominent criteria associated with planetary surface habitability. We address the crucial question as to whether the seven Earth-sized planets transiting the recently discovered ultracool dwarf star TRAPPIST-1 are capable of retaining their atmospheres. To this effect, we carry out numerical simulations to characterize the stellar wind of TRAPPIST-1 and the atmospheric ion escape rates for all the seven planets. We also estimate the escape rates analytically and demonstrate that they are in good agreement with the numerical results. We conclude that the outermost planets of the TRAPPIST-1 system are capable of retaining their atmospheres over billion-year timescales, and are thus likely to be habitable. The consequences arising from our results are also explored in the context of abiogenesis, biodiversity, and searches for future exoplanets. In light of the many unknowns involved, these conclusions must be interpreted with due caution.

A Review of Potential Biosignature Detection Methods

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life 

Authors: 

Schwieterman et al 

Abstract: 

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has imprinted evidence of its presence upon the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earths biosphere, both in the present and through geologic time, for informative analogies of what signatures to search for elsewhere. Here we have compiled an overview of our current understanding of potential exoplanet biosignatures including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well-known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required for a given atmospheric signature. We focus particularly on advances made since the seminal review by Des Marais et al. (2002). The purpose of this work is not to propose new biosignatures strategies, a goal left to companion papers in this series, but to review the current literature and provide a foundation for a path forward.