Spectral retrieval is a key computational tool for interpreting spectroscopic observations of exoplanets. As data quality has dramatically improved with the launch of JWST, so the complexity of retrieval models has also increased. Due to the lack of ground truth and prior knowledge for exoplanets, retrievals adopt algorithms capable of exploring a wide parameter space, such as MCMC or Nested Sampling, which become increasingly inefficient for higher dimensionality problems. Tuning retrieval complexity for these scenarios requires a delicate compromise between sufficient physical accuracy and pragmatic approximations. Retrievals can be applied to many different types of observation, with specific challenges associated with each. In the current environment, as we begin characterize smaller, temperate planets that may potentially be habitable, interpretation of retrievals and calculation of detection significance must be approached with special care. In this review, we summarize the current computational challenges in all aspects of the retrieval process: atmospheric modelling; data preparation; inversion and likelihood calculation; and interpretation, for a range of observational strategies. We conclude with a series of recommendations for scientists approaching retrievals for the first time in this new data-rich era for exoplanet science.
Today, more than 6000 exoplanets are known after the first detection of an extrasolar planet around a main sequence star in 1995. Among them are representatives of new planetary classes such as Super-Earths and Mini-Neptunes, in addition to those already known from the solar system: rocky planets (Mercury, Earth, Venus, Mars), gas giants (Jupiter, Saturn), and ice giants (Uranus, Neptune). In order to understand the formation of planets, their interior structure and composition, thermal evolution and magnetic field, precise data on the equation of state, the transport coefficients, and the optical properties of the most abundant planetary materials are necessary. Relevant temperatures and densities cover a wide range from cold and dilute molecular clouds or planetary atmospheres up to warm and dense planetary cores. Ab initio simulations allow to determine thermophysical properties of warm dense matter with high precision. In this review we give an introduction into the ab initio methods and demonstrate their capacity for planetary physics by showing exemplary results for the thermophysical properties of key materials such as H-He (gas giants) and H-C-N-O mixtures (ice giants). Based on these results, advanced models for the interior structure of planets, their thermal evolution, and their magnetic field can be developed.
Stars are born with a surrounding disk made up of gas entrained by dusty and icy particles, which are the building material for planetary systems. The evolution from small grains towards planetesimals and embryos is regulated by turbulence-driven transport and collisions. Likewise, the migration and growth of planets depends on the gas turbulence. In order to make interpretations of disk observations, one needs to model the turbulent state and the dynamic evolution in these disks. Typical observations of molecular lines provide us with information about the velocity deviation from laminar rotation, and continuum observations of emitted and scattered light helps us determine the spatial and size distribution of dust. The purpose of this review is to provide a guide book on how to set up hydrodynamic and radiation hydrodynamic simulations of a disk around a young star reflecting the current state of code development. These models can then be used to study the effect of turbulence on the distribution and evolution of dust up to planetesimals and planetary embryos, as well as to study the interaction of planets with the disk gas. We will focus on the effects of radiation transport on dynamic stability in global three-dimensional simulations and, therefore, neglect the effects of self-gravity, particle feedback and magnetic fields to some later iteration of the topic.
Multiphase gas-ranging from cold molecular clouds ( ≲ 100 K) to hot, diffuse plasma ( ≳ 10^6 K) is a defining feature of the interstellar, circumgalactic, intracluster, and intergalactic media. Accurately simulating its dynamics is critical to improving our understanding of galaxy formation and evolution, however, due to their multi-scale and multi-physics nature, multiphase systems are highly challenging to model. In this review, we provide a comprehensive overview of numerical simulations of multiphase gas in and around galaxies. We begin by outlining the environments where multiphase gas arises and the physical and computational challenges associated with its modeling. Key quantities that characterize multiphase gas dynamics are discussed, followed by an in-depth look at idealized setups such as turbulent mixing layers, cloud-wind interactions, thermal instability, and turbulent boxes. The review then transitions to less idealized and/or larger-scale simulations, covering radiative supernovae bubbles, tall box simulations, isolated galaxy models including dwarf and Milky Way-mass systems, and cosmological zoom-in simulations, with a particular focus on simulations that enhance resolution in the halo. Throughout, we emphasize the importance of connecting scales, extracting robust diagnostics, and comparing simulations to observations. We conclude by outlining persistent challenges and promising directions for future work in simulating the multiphase Universe.
Most of the physical information about astrophysical objects is obtained via the analysis of their electromagnetic spectra. Observed data coupled with radiation transfer models in physical conditions representative of stars, planets, kilonovae, and ISM, yield constrains on their physical structure, gas flow dynamics at the surface, mass loss, and detailed chemical composition of the systems. All these core astrophysical parameters are only as reliable as the physical quality of the models that are employed for simulations of radiation transfer. Recent advances in multi-dimensional transfer modelling with Non-Local Thermodynamic Equilibrium (NLTE) in inhomogeneous time-dependent systems revealed systematic shortcomings of canonical models. Owing to major complexities of solving coupled multi-frequency RT equations in 3D geometry, a number of approximations have been introduced. This review presents an overview of the physical problem, standard solutions, and recent methodological advances. We also provide an overview of main results in the area of 3D NLTE radiation transfer and its applications to modelling diverse astrophysical environments, including FGKM type- and OBA-type stars, multi-epoch spectra of kilonovae, and atmospheres of rocky and gaseous exoplanets.
