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Register a new userExploring terrestrial lightning parameterisations for exoplanets and brown dwarfs
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Added by
Gabriella Hodos\\'an
Publication date
2021
fields
Physics
and research's language is
English
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Observations and models suggest that the conditions to develop lightning may be present in cloud-forming extrasolar planetary and brown dwarf atmospheres. Whether lightning on these objects is similar to or very different from what is known from the Solar System awaits answering as lightning from extrasolar objects has not been detected yet. We explore terrestrial lightning parameterisations to compare the energy radiated and the total radio power emitted from lightning discharges for Earth, Jupiter, Saturn, extrasolar giant gas planets and brown dwarfs. We find that lightning on hot, giant gas planets and brown dwarfs may have energies of the order of $10^{11}$--$10^{17}$ J, which is two to eight orders of magnitude larger than the average total energy of Earth lightning ($10^9$ J), and up to five orders of magnitude more energetic than lightning on Jupiter or Saturn ($10^{12}$ J), affirming the stark difference between these atmospheres. Lightning on exoplanets and brown dwarfs may be more energetic and release more radio power than what has been observed from the Solar System. Such energies would increase the probability of detecting lightning-related radio emission from an extrasolar body.
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More than 4000 planet are known that orbit stars other than our Sun. Many harbor a dynamic atmosphere that is cold enough that cloud particles can form in abundance. The diversity of exoplanets leads to differences in cloud coverage depending on global system parameters. Some planets will be fully covered in clouds, some have clouds on the nightside but are largely cloud-free on the dayside. These cloud particles can easily be charged and lightning discharges will occur in cloudy, dynamic exoplanet atmosphere. Lightning supports a Global Electric Circuit (GCE) on Earth and we argue that exoplanet may develop a GCE in particular if parts of the exoplanet atmospheres can remain cloud free.
We present an auto-differentiable spectral modeling of exoplanets and brown dwarfs. This model enables a fully Bayesian inference of the high-dispersion data to fit the ab initio line-by-line spectral computation to the observed spectrum by combining it with the Hamiltonian Monte Carlo in recent probabilistic programming languages. An open source code, exojax, developed in this study, was written in Python using the GPU/TPU compatible package for automatic differentiation and accelerated linear algebra, JAX (Bradbury et al. 2018). We validated the model by comparing it with existing opacity calculators and a radiative transfer code and found reasonable agreements of the output. As a demonstration, we analyzed the high-dispersion spectrum of a nearby brown dwarf, Luhman 16 A and found that a model including water, carbon monoxide, and $mathrm{H_2/He}$ collision induced absorption was well fitted to the observed spectrum ($R=10^5$ and $2.28-2.30 mumathrm{m}$). As a result, we found that $T_0 = 1295 pm 14 mathrm{K}$ at 1 bar and $mathrm{C/O} = 0.62 pm 0.01$, which is slightly higher than the solar value. This work demonstrates the potential of full Bayesian analysis of brown dwarfs and exoplanets as observed by high-dispersion spectrographs and also directly-imaged exoplanets as observed by high-dispersion coronagraphy.
We explore the prospects for the detection of giant circumbinary exoplanets and brown dwarfs (BDs) orbiting Galactic double white dwarfs binaries (DWDs) with the Laser Interferometer Space Antenna (LISA). By assuming an occurrence rate of 50%, motivated by white dwarf pollution observations, we built a Galactic synthetic population of P-type giant exoplanets and BDs orbiting DWDs. We carried this out by injecting different sub-stellar populations, with various mass and orbital separation characteristics, into the DWD population used in the LISA mission proposal. We then performed a Fisher matrix analysis to measure how many of these three-body systems show a periodic Doppler-shifted gravitational wave perturbation detectable by LISA. We report the number of circumbinary planets (CBPs) and (BDs) that can be detected by LISA for various combinations of mass and semi-major axis distributions. We identify pessimistic and optimistic scenarios corresponding, respectively, to 3 and 83 (14 and 2218) detections of CBPs (BDs), observed during the length of the nominal LISA mission. These detections are distributed all over the Galaxy following the underlying DWD distribution, and they are biased towards DWDs with higher LISA signal-to-noise ratio and shorter orbital period. Finally, we show that if LISA were to be extended for four more years, the number of systems detected will be more than doubled in both the optimistic and pessimistic scenarios. Our results present promising prospects for the detection of post-main sequence exoplanets and BDs, showing that gravitational waves can prove the existence of these populations over the totality of the Milky Way. Detections by LISA will deepen our knowledge on the life of exoplanets subsequent to the most extreme evolution phases of their hosts, clarifying whether new phases of planetary formation take place later in the life of the stars.
(abridged) We calculate near-infrared thermal emission spectra using a doubling-adding radiative transfer code, which includes scattering by clouds and haze. Initial temperature profiles and cloud optical depths are taken from the drift-phoenix brown dwarf model. As is well known, cloud particles change the spectrum compared to when clouds are ignored. The clouds reduce fluxes in the near-infrared spectrum and make it redder than for the clear sky case. We also confirm that not including scattering in the spectral calculations can result in errors on the spectra of many tens of percent, both in magnitude and in variations with wavelength. This is especially apparent for particles that are larger than the wavelength and only have little iron in them. Scattering particles will show deeper absorption features than absorbing (e.g. iron) particles and particle size will also affect the calculated infrared colours. Large particles also tend to be strongly forward-scattering, and we show that assuming isotropic scattering in this case also leads to very large errors in the spectrum. Thus, care must be taken in the choice of radiative transfer method for heat balance or spectral calculations when clouds are present in the atmosphere. Besides the choice of radiative transfer method, the type of particles that are predicted by models will change conclusions about e.g. infrared colours and trace gas abundances. As a result, knowledge of the scattering properties of the clouds is essential when deriving temperature profiles or gas abundances from direct infrared observations of exoplanets or brown dwarfs and from secondary eclipse measurements of transiting exoplanets, since scattering clouds will change the depth of gas absorption features, among other things. Thus, ignoring the presence of clouds can yield retrieved properties that differ significantly from the real atmospheric properties.
In recent years there have been many attempts to characterize the occurrence of stellar, BD and planetary-mass companions to solar-type stars, with the aim of constraining formation mechanisms. From RV observations a dearth of companions with masses between 10-40 MJup has been noticed at close separations, suggesting the possibility of a distinct formation mechanism for objects above and below this range. We present a model for the substellar companion mass function (CMF). It consists of the superposition of the planet and BD companion mass distributions, assuming that we can extrapolate the RV measured companion mass function for planets to larger separations and the stellar companion mass-ratio distribution over all separations into the BD mass regime. By using both the results of the VLT/NaCo large program and the complementary archive datasets that probe the occurrence of planets and BDs on wide orbits around solar-type stars, we place some constraints on the planet and BD distributions. We developed a MC simulation tool to predict the outcome of a given survey, depending on the shape of the orbital parameter distributions. Comparing the predictions with the results of the observations, we calculate how likely different models are and which can be ruled out. Current observations are consistent with the proposed model for the CMF, as long as a sufficiently small outer truncation radius is introduced for the planet separation distribution. The results of the direct imaging surveys searching for substellar companions around Sun-like stars are consistent with a combined substellar mass spectrum of planets and BDs. This mass distribution has a minimum between 10 and 50 MJup, in agreement with RV measurements. The dearth of objects in this mass range would naturally arise from the shape of the mass distribution, without the introduction of any distinct formation mechanism for BDs.
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