Aims: Finding potential life harboring exo-Earths with future telescopes is one of the aims of exoplanetary science. Detecting signatures of life in exoplanets will likely first be accomplished by determining the bulk composition of the planetary atmosphere via reflected or transmitted spectroscopy. However, a complete understanding of the habitability conditions will surely require mapping the presence of liquid water, continents, and/or clouds. Spin-orbit tomography is a technique that allows us to obtain maps of the surface of exoplanets around other stars using the light scattered by the planetary surface.
Methods: We leverage the enormous potential of deep learning, and propose a mapping technique for exo-Earths in which the regularization is learned from mock surfaces. The solution of the inverse mapping problem is posed as a deep neural network that can be trained end-to-end with suitable training data. Since we still lack observational data of the surface albedo of exoplanets, in this work we propose methods based on the procedural generation of planets, inspired by what we have found on Earth. We also consider mapping the recovery of surfaces and the presence of persistent clouds in cloudy planets, a much more challenging problem.
Results: We show that reliable mapping can be carried out with our approach, producing very compact continents, even when using single-passband observations. More importantly, if exoplanets are partially cloudy like the Earth is, we show that it is possible to map the distribution of persistent clouds that always occur in the same position on the surface (associated with orography and sea surface temperatures) together with nonpersistent clouds that move across the surface. This will become the first test to perform on an exoplanet for the detection of an active climate system. For small rocky planets in the habitable zone of their stars, this weather system will be driven by water, and the detection can be considered a strong proxy for truly habitable conditions.