Parameterizing pressure-temperature profiles of exoplanet atmospheres
  with neural networks
Timothy D. Gebhard Daniel Angerhausen Björn S. Konrad Eleonora Alei Sascha P. Quanz Bernhard Schölkopf
Atmospheric retrievals (AR) of exoplanets typically rely on a combination of
a Bayesian inference technique and a forward simulator to estimate atmospheric
properties from an observed spectrum. A key component in simulating spectra is
the pressure-temperature (PT) profile, which describes the thermal structure of
the atmosphere. Current AR pipelines commonly use ad hoc fitting functions here
that limit the retrieved PT profiles to simple approximations, but still use a
relatively large number of parameters. In this work, we introduce a
conceptually new, data-driven parameterization scheme for physically consistent
PT profiles that does not require explicit assumptions about the functional
form of the PT profiles and uses fewer parameters than existing methods. Our
approach consists of a latent variable model (based on a neural network) that
learns a distribution over functions (PT profiles). Each profile is represented
by a low-dimensional vector that can be used to condition a decoder network
that maps $P$ to $T$. When training and evaluating our method on two publicly
available datasets of self-consistent PT profiles, we find that our method
achieves, on average, better fit quality than existing baseline methods,
despite using fewer parameters. In an AR based on existing literature, our
model (using two parameters) produces a tighter, more accurate posterior for
the PT profile than the five-parameter polynomial baseline, while also speeding
up the retrieval by more than a factor of three. By providing parametric access
to physically consistent PT profiles, and by reducing the number of parameters
required to describe a PT profile (thereby reducing computational cost or
freeing resources for additional parameters of interest), our method can help
improve AR and thus our understanding of exoplanet atmospheres and their
habitability.