Machine learning and advanced statistical analysis
Quantifying the similarity between planetary systems for unsupervised machine learning
Comparing results of single planet formation models with observations is relatively easy. For example, use the Euclidean distance in the space of features of planets (e.g. mass and period). In the case of planetary systems with more than one planet, this is more difficult, since a planetary system is formally a set of points in the feature space. In order to circumvent this difficulty, we have developed a new distance in the space of planetary system in a soon to be published paper by Yann Alibert. The distance can be used in a variety of unsupervised machine learning algorithms, as for example visualisation techniques.
Dimensionality reduction and T-SNE visualization
We have use T-SNE in order to visualize results of planet formation models. An example is provided on the video below which shows the T-SNE minimisation, and where each point represents a planetary system with up to 10 planets. On this visualization, the distance between two points is related to the similarity between planetary systems. The similarity between systems is correlated with the number of planets in the system (a quantity that is not known by the visualization algorithm).
The similarity between planetary systems should be related to the similarity of the disks in which they form. To check this, run the video below! The location of points (each of them representing a planetary system) will evolve. It reflects at the beginning the similarity between systems, and at the end of the video the similarity between disks in which the systems were formed. The color of the points is kept unchanged during the video. At the end of the video, the similarity of colors reflects the similarity of disks, whereas the similarity of location reflects the similarity of disks. As can be seen, both are correlated, meaning similar protoplanetary disks give birth to similar planetary systems.
Using Deep Learning to compute planetary structure
One key step of planet formation model is the computation of the mass of the planetary envelope. This mass depends on a variety of thermodynamical conditions, and must be computed by solving a set of differential equations. Yann Alibert and Julia Venturini, we have trained a Deep Neural Network (DNN) in order to predict the mass of planetary envelopes, without having to solve these differential equations. By training the DNN on tens of millions of internal structure calculations, we have been able to reach a very good precision accuracy (of the order of percents). This is much better than the analytical relations one can find in the literature, and allows computing very rapidly planetary envelop mass with a very good accuracy.
The figure below shows the comparison between the exact results (red), the results of the DNN (blue), and the results obtained using the analytical formulas from Ikoma et al. (2000) and Bitsch et al. (2015) for a planet forming in different conditions (see title of the graphs).
Advanced statistical tools: internal structure of low-mass planets
In preparation of CHEOPS data, we have, with Jonas Haldemann, developed advanced statistical methods to infer the internal composition of low-mass planets. For this, we have improved the MCMC scheme developed by Caroline Dorn (project 2.4 – see this recent paper and references therein) to speed up the calculations. Part of this effort is to develop a neural network to compute the internal structure of planets, in a way similar to what is presented above.
Predictions for PLATO
We use results of planet formation models coupled with the expected PLATO observational bias, as well as expected radial velocity performances, in order to predict the yield of PLATO. This project is developed with Thibaut Roger, and the figure below shows a preliminary result.